Predicting selection–response gradients of heat tolerance in a widespread reef-building coral

Ponchanok, Weeriyanun,; B., Collins, Rachael; Alex, Macadam,; Hugo, Kiff,; L., Randle, Janna; M., Quigley, Kate

doi:10.1242/jeb.243344

Predicting selection–response gradients of heat tolerance in a widespread reef-building coral

Ponchanok, Weeriyanun,;B., Collins, Rachael;Alex, Macadam,;Hugo, Kiff,;L., Randle, Janna;M., Quigley, Kate 2022-03-08 00:00:00 © 2022. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 RESEARCH ARTICLE Predicting selection–response gradients of heat tolerance in a widespread reef-building coral 1,2, 1,3, 1 4 1 Ponchanok Weeriyanun *, Rachael B. Collins *, Alex Macadam , Hugo Kiff , Janna L. Randle and 1,‡ Kate M. Quigley ABSTRACT ecological functioning via trophic interactions, with downstream impacts on fish and other invertebrate species (Jones et al., 2004; Ocean temperatures continue to rise owing to climate change, but it Komyakova et al., 2013; Nelson et al., 2016). Coral reefs are is unclear whether heat tolerance of marine organisms will keep pace biodiversity hotspots that provide access to food, act as natural with warming. Understanding how tolerance scales from individuals barriers against storm damage and benefit the economy through to species and quantifying adaptive potentials is essential to tourism and other recreational activities (O’Mahony et al., 2017). forecasting responses to warming. We reproductively crossed Coral reef ecosystems such as the Great Barrier Reef (GBR) corals from a globally distributed species (Acropora tenuis) on the contribute, on average, $6.4 billion (AUD) to the Australian Great Barrier Reef (Australia) from three thermally distinct reefs to economy per annum (O’Mahony et al., 2017). Consequently, the create 85 offspring lineages. Individuals were experimentally loss of these crucial habitats will have devastating economic and exposed to temperatures (27.5, 31 and 35.5°C) in adult and two social-ecological effects on coastal communities. critical early life stages (larval and settlement) to assess acquired The upper thermal thresholds of reef-building corals generally heat tolerance via outcrossing of offspring phenotypes by comparing reside ∼1°C higher than their local summer maxima (Schoepf et al., five physiological responses (photosynthetic yields, bleaching, 2019). Once temperatures exceed this threshold, corals may lose necrosis, settlement and survival). Adaptive potentials and their symbiotic dinoflagellates (Symbiodiniaceae) through a process physiological reaction norms were calculated across three stages to known as bleaching, which is typically experienced by corals during integrate heat tolerance at different biological scales. Selective marine heatwaves if warming persists for extended periods of time breeding improved larval survival to heat by 1.5–2.5× but did not (Hoegh-Guldberg, 1999). Extreme bleaching (defined as >60% of result in substantial enhancement of settlement, although population bleached corals present within a reef) often leads to high mortality crosses were significantly different. Under heat stress, adults were (Hughes et al., 2017). At least four mass bleaching events have been less variable compared with larval responses in warmer reefs than in recorded on the GBR in the last century – 1998, 2002, 2016 and the cooler reef. Adults and offspring also differed in their mean 2017 – generally increasing in intensity and severity (Hughes et al., population responses, likely underpinned by heat stress imposing 2017), and have led to large-scale losses in hard coral cover. Rates strong divergent selection on adults. These results have implications of decline vary by region, but overall, declines have rapidly for downstream selection during reproduction, evidenced by increased per year since 2006 (De’Ath et al., 2012), although there variability in a conserved heat tolerance response across offspring have been recent signs of recovery. Therefore, bleaching-related lineages. These results inform our ability to forecast the impacts of stress has not been equal across the GBR. This trend in decline is climate change on wild populations of corals and will aid in developing reflected globally, with SSTs continuing to exceed previously held novel conservation tools such as the assisted evolution of at-risk records (Heron et al., 2016), suggesting that corals may soon reach species. their physiological limits to cope with increasing heat (Matz et al., KEY WORDS: Thermal tolerance, Heritability, Coral reefs, Larvae, 2017). Settlement, Survival Changes in temperature have the potential to impact organismal life stages differently, in which larvae, recruitment stages and adults INTRODUCTION can all vary in their thresholds to climate extremes. Despite this, Ecosystems globally continue to suffer from the adverse effects of information on larval physiological responses is relatively scarce for anthropogenic climate change. Coral reef ecosystems in particular coral early life-history stages (McLachlan et al., 2020), with 95% of are declining as sea surface temperatures (SSTs) have risen studies focusing on adult responses and only 2% and 1% on pre- dramatically (Hughes et al., 2017). Coral reefs are some of the settled and settlement stages, respectively (McLachlan et al., 2020). most productive habitats in the world, and the continued loss of In corals, sexually mature adults release their gametes within a corals will likely impact reef community dynamics by changing temperature range of ∼28–30°C, with larvae of some coral species able to survive 2–5°C above this range (Heyward and Negri, 2010). 1 2 However, coral spawning typically occurs during the warmest Australian Institute of Marine Science, Townsville 4810, Australia. Ghent University, Sint-Pietersnieuwstraat 33, 9000 Gent, Belgium. University of Plymouth, summer months, when spikes in SSTs are most likely to occur Plymouth PL4 8AA, UK. Liverpool John Moores University, Liverpool L3 3AF, UK. (Keith et al., 2016). As a result, coral larvae are potentially subjected *These authors contributed equally to this work. to much greater temperatures, increasing their risk of mortality and Author for correspondence (katemarie.quigley@my.jcu.edu.au) reducing recruitment and settlement success (Heyward and Negri, 2010). After fertilization, larvae undergo a pre-competency period R.B.C., 0000-0002-2827-9037; J.L.R., 0000-0003-1467-2505; K.M.Q., 0000- where they develop and disperse throughout the water column. Once 0001-5558-1904 competent, they respond to biophysical and chemical cues to find Received 31 August 2021; Accepted 7 January 2022 optimal settling conditions (Doropoulos et al., 2018). Increased Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 SSTs may induce premature metamorphosis by increasing larval coefficients (S; relative fitness of a phenotypic trait; van metabolic activity and development rates, thereby reducing pre- Tienderen and de Jong, 1994) and responses to selection (R)are competency periods (Heyward and Negri, 2010). This reduction commonly measured (Falconer and Makcay, 1996), and allow may influence subsequent larval dispersal distances and result in for the prediction of organismal responses to future stressors. Thus settlement occurring in suboptimal conditions (Edmunds et al., far, models incorporating the ‘breeder’s equation’ (R=h S; used 2001). Therefore, it is important to incorporate these responses into to predict the effect of selection pressures on phenotypic traits; predictive models of ecosystem change given their flow-on effects Falconer and Makcay, 1996) have demonstrated the utility of into key demographic and population-level dynamics. evolutionary modelling for predicting the potential of the coral’s Reductions in dispersal distances have the potential to reduce algal symbionts to confer increased survival (Quigley et al., 2018). gene flow between populations, thereby potentially accelerating Similarly, these principles can be applied to the coral host to population differentiation and local adaption (Bassim et al., 2002). quantify adaptive potentials, and this information can then be used Additionally, some coral populations rely on the dispersal of larvae to harness the existing adaptive potential of coral populations for from neighbouring populations for the addition of beneficial genes/ conservation and restoration. genotypes to their gene pool (Munday et al., 2009; Quigley et al., Coral restoration interventions, some of which can be classified 2019a,b). With reduced reef connectivity, populations may not be as assisted evolution methods (van Oppen et al., 2015), aim to able to adapt rapidly enough to cope with increasing SSTs (Quigley increase the adaptive potential of organisms by targeting the et al., 2019a,b) or may struggle to regenerate following mass acceleration of adaptation through various genetic mechanisms. bleaching and mortality (Bassim et al., 2002). There is scope for This may include selective breeding for the selection of desirable adaptation of corals to increasing temperatures (Matz et al., 2018 phenotypes (van Oppen et al., 2015). Assisted gene flow (AGF) is preprint). Further, the temperatures at which bleaching is occurring one intervention that utilises the differing thermal tolerances of have increased by ∼0.5°C from 1998 to 2017, suggesting an parental colonies combined with the intentional movement of increase in more heat-adapted genotypes within coral populations recombinant offspring, by reproductively mixing populations (Sully et al., 2019), potentially through processes such as selective sourced from different environmental conditions within the same sweeps (Quigley et al., 2019a). Locally, there have been reported species to prepare future populations for increased warming. This increases in coral cover in both the central and southern GBR strategy is based on increasing gene flow between populations that (AIMS, 2020), with evidence to suggest that warm or ‘extreme’ have a beneficial phenotype(s) with those of a target population(s) habitats harbour an increased number of thermally adapted via the creation and subsequent introduction of new, more resilient genotypes with the potential to transmit heat tolerance (Quigley genotypes (Aitken and Whitlock, 2013). Selective breeding through et al., 2020a; Schoepf et al., 2019). However, current estimates the outcrossing of gametes between these reefs of distinct thermal of the rate of SST increases suggest that these increases may exceed profiles assumes that populations exposed to climates of similar the potential rate of fixation of beneficial genetic variants given temperatures to those predicted to arise from anthropogenic factors such as currents and reef topology (Quigley et al., 2019a). climate change may be better adapted, and therefore better able to Additionally, the annual increase in SSTs has extended the period cope with warming (Aitken and Whitlock, 2013). The inheritance at which ‘summer’ temperatures occur, further increasing global of these beneficial alleles by populations that evolutionarily bleaching by reducing potential recovery periods that occur when have experienced cooler temperatures should increase the overall coral populations are exposed to cooler ‘winter’ temperatures resilience of the species via increased population persistence (Heron et al., 2016). Taken together, this suggests that corals (National Academies of Science, Engineering and Medicine, 2019). adaptive potentials may be constrained. Here, we compare the physiological responses at heat stress of Variability in heat tolerance has been observed at many selectively bred Acropora tenuis larvae and newly settled recruits ecological levels, including between individuals, populations and and their parental colonies collected from three sites along the GBR species, and by reef region. For example, differences in the heat – Davies and Esk reefs (central GBR) and in the Keppels (southern tolerance of corals have been observed across reefs globally, with GBR). Percentage larval survival and settlement were recorded coral populations present along the Persian Gulf having following exposure to control (27°C) and heat stress conditions demonstrated very high thermal thresholds of ∼4°C above mean (35.5–36°C) and compared with adult heat tolerance at 31°C, as monthly temperatures (Kirk et al., 2018; Moghaddam et al., 2021; measured in bleaching score, photosynthetic quantum yields, Savary et al., 2021). Northern GBR corals also demonstrate higher percent necrosis and percent survival. We then compared these thermal tolerances compared with some central populations, in responses across multiple biological scales (individuals to which heritable host genetic mechanisms play a role in the thermal populations) to calculate adaptive potentials from data collected resistance of these northern corals (Dixon et al., 2015; Quigley et al., across each life stage. This work contributes to our understanding of 2020a). This geographically distinct habitat could contribute to sub- the potential to increase heat tolerance and for the transgenerational speciation, seen in other marine invertebrates that have different transfer of this tolerance using the selective breeding of coral thermal thresholds, such as subspecies of Crassostrea gigas populations sourced from along the GBR. (Ghaffari et al., 2019). However, it is less clear whether similar patterns in heat tolerance at the individual coral genotype scale to MATERIALS AND METHODS the population level, a trend seen in fish, but not insect or plant Coral colony collections species collected across temperature clines (Payne et al., 2021; Reproductively mature Acropora tenuis (Dana 1846) colonies were Rezende and Bozinovic, 2019). Quantifying the scaling of heat collected in early November 2019 from three sites on the GBR tolerance across different biological levels will therefore help to in Australia, including two central reefs: Davies (18°49.881′S, elucidate the baseline potential of wild populations to adapt. 147°37.953′E) and Esk (18°45.892′S, 146°31.219′E), and one Further, evolutionary models incorporating metrics such narrow- southern reef in the Keppel islands (23°03.856′S, 150°57.095′E). sense heritability (h ; the phenotypic traits within an organism that Corals were identified as individual genotypes during collection by arise from allele inheritance; Evans et al., 2018), selection removing colonies approximately 10 m apart if possible. Genotypes Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 were not known a priori. Esk was on average the warmest reef surface of the water. The eggs and sperm in the bundles were location, followed by Davies and then the Keppels (Fig. 1A). The separated by washing for ∼2 min with FSW through a 120 μm mesh mean maximum monthly temperature of each site was as follows: filter. The number of sperm was quantified using a Computer Esk 26.26±2.30°C, Davies 26.06±1.80°C and the Keppels 24.62 Assisted Semen Analyser (CASA) (CEROS II software, Hamilton ±2.30°C (Fig. 1B). Corals were transported by boat to the National Thorne). The sperm from one parental colony was then added at a Sea Simulator at the Australian Institute of Marine Science (AIMS). concentration of 10 ml to isolated and cleaned eggs from another Colonies from each location were acclimated in separate outdoor parental colony to create 85 distinct coral families with at least one holding tanks under constant 0.2 μm filtered seawater (FSW) flow- cross per family. These families, observed as biological replicates, through conditions, maintained at 27.5°C. comprise intrapopulation (within the same reef) and interpopulation Corals were collected under the following permit numbers: G12/ (between different reefs) crosses, where both are generally 35236.1 (Davies and Esk) and G19/43024.1 (Keppels) to AIMS. referred to here as population crosses (Table S2). Population-level crosses are referred to as follows, with the maternal colony first Coral spawning, selective breeding and larval rearing and then paternal colony, in which intrapopulation crosses (filled Prior to spawning, each A. tenuis colony was given an identification circles; Fig. 1D) are Esk×Esk (E×E), Keppels×Keppels (K×K) number (Table S1). Individual colonies were isolated in separate and Davies×Davies (D×D), and interpopulation crosses are bins following signs of spawning imminence, which includes the Esk×Keppels (E×K), Esk×Davies (E×D), Keppels×Esk (K×E), appearance of egg–sperm bundles under the oral disc and ‘setting’ Keppels×Davies (K×D), Davies×Esk (D×E) and Davies×Keppels (polyps extended but tentacles retracted). Gamete bundles were (D×K) (open circles; Fig. 1D). released between 19:00 and 19:40 h on 17 and 18 November 2019 Eggs were allowed to fertilize in separate bowls per cross for 3 h. and collected by gently skimming and collecting the bundles off the Every hour, aliquots of developing embryos were taken from each Spawning Induce settlement Short-term Short-term Longer-term heat stress heat stress heat stress Northern Cross F0 Control Heat F1 Esk Central EskEsk (EE) Davies EskDavies (ED) EskKeppels (EK) Keppels DaviesDavies (DD) Southern DaviesEsk (DE) DaviesKeppels (DK) 142 147 KeppelsKeppels (KK) KeppelsDavies (KD) Longitude (°E) KeppelsEsk (KE) B E Short-term heat stress 27.5 Long-term heat stress 25.0 32 Life stage 22.5 Adult Larvae Settlers 13 5 7 9 11 0 48 96 144 192 240 288 Month Hours Fig. 1. Summary of experimental design for determining variability in heat tolerance across coral life-history stages. (A) Map of the Great Barrier Reef showing the three sites of adult coral collection, including Esk (central inshore, black), Davies (central mid-shelf, tan) and Keppels (southern inshore, maroon) reefs. (B) Mean±s.e.m. monthly sea surface temperature profiles at each reef of adult coral collection. (C) A graphic showing when the three life stages were experimentally exposed to heat stress. (D) Summary of the population crosses produced from the three coral populations. Filled circles represent intrapopulation crosses, whereas open circles represent interpopulation crosses. (E) Figure indicating the ramping time to reach each experimental temperature and the length that it was held. Circles indicate the end of each experimental time point, triangles indicate specific points along the experimental time frame. Mean sea surface temperature (°C) Latitude (°S) 20 13 Temperature (°C) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 family (hereafter referred to as crosses; Fig. 1C,D) to visually population (28 fragments). Each fragment was glued to a calcium inspect fertilization success and cell division under magnification. carbonate plug using epoxy glue, placed into PVC plug holders Once fertilization was confirmed, embryos from each separate bowl (hereafter referred to as ‘sticks’), and allowed to acclimate at control were transferred into separate 15 l constant flow-through conical temperatures before being moved to the experimental tanks. This tanks with 0.2 μm FSW in a temperature-controlled room, such that acclimation period attempts to minimize the effects of sampling and the temperature of each cone was maintained at 27.5°C, P handling before the experiment, where all coral fragments were CO 400±60 ppm, ambient light (i.e. non-photosynthetic) and salinity allowed to acclimate and recover from the fragmentation stress for of 35 psu. Each flow-through cross culture had an outflow covered 1 week. During this time and over the course of the experiments, −1 by a 10 μm filter and air curtain of bubbles to prevent larvae from corals were fed once a day at 16:30 h with 0.5 individuals ml 6 −1 collecting on the outflow filter. By days three and four post- of Artemia sp. and 5×10 cells ml of microalgae (Parmelia fertilization, larvae were ciliated and motile, consistent with the sulcata, T-ISO, Chaetoceros muelleri, Nannochloropsis oceania 96-h stage of larval development. and Dunnaiella sp.). Upon moving the corals to experimental tanks, the position Larval heat stress experiment of each fragment was randomized in each tank and across all Thirty larvae from each of the total 85 crosses were placed into tank replicates, such that each tank included all genotypes from floating net-wells (n=3 replicates per cross, per temperature), each population. Experimental tanks were filled with 45 l of separated into two holding tanks. One tank was set at 27.5°C either 27.5°C FSW (control treatment) or 31°C (heat treatment) (control treatment) and the other at 35.5°C (heat treatment). To on constant flow through. Each tank was equipped with a tungsten achieve the latter temperature, the heat treatment was ramped up to aquarium pump for constant aeration and mixture of water. The 35.5°C in hourly increments of 0.5°C from 27.5°C. Once 35.5°C heat treatment corals were ramped at a rate of 0.5°C until 31°C was reached, survival assessments began, in which the number of was reached. The light cycle for each tank was set on a 12 h:12 h larvae that were alive in each net-well (of the total n=30) was regime and light intensity at 171 PAR. Sunrise was set at 09:00 h. counted twice daily to estimate survival rates per replicate. The final The coral fragments underwent these experimental conditions for survival counts occurred once ∼50% of the larval crosses reached 14 days. Temperatures for larval and adult experiments were chosen 50% survival in the heat treatment. No crosses perished post- in order to compare with previous work performed in the same fertilization before sampling took place. region using a cross design (Dixon et al., 2015; Quigley et al., 2020b). Settlement experiment Coral colour, as a proxy for bleaching, was determined using an Following the larval heat stress experiment, only 69 of the original underwater colour reference card for corals (Coral Watch Card; 85 crosses contained sufficient larval stock for further experimental Siebeck et al., 2008). Images with the bleaching card were taken at use. These 69 crosses were used to investigate the effect of thermal least twice a week, and fragments were compared with the brown hue stress on larval settlement behaviour. Larvae were sampled 35 days (D1–D6). ImageJ (Schneider et al., 2012) was used to create a red- after spawning and transferred to sterile six-well plates (n=10 larvae green-blue standard curve for coral colour as per Quigley et al. per well, n=3 wells per cross, n=2 crosses per sterile six-well plate) (2019b). Survival of each coral fragment was noted from the images. containing 10 ml of 0.2 μm FSW. Crustose coralline algae (CCA) Coral death was determined by the presence of microalgae growing rubble was freshly cut into 3×3 mm chips using bone cutters, and on the bare skeleton on each coral fragment. The percentage of tissue placed into each well to induce larval settlement. The plates were necrosis was measured using the ImageJ surface area tool (Schneider then placed into plastic bags and sealed to prevent evaporation and et al., 2012) per coral fragment, per tank, per time point. Effective replicates were transferred into two incubators with photosynthetic quantum yield of photosystem II (ΔF/F ′), which is the efficiency of lights (12 h:12 h light:dark cycle, 170 to 180 PAR, Steridium E-500 photon absorption, was measured using the Diving PAM (DIVING- Sylvania FHO24W/T5/865 and Innova 4230, Sylvania F15W/865) PAM-II, Walz, Germany). PAM measurements of each fragment set at 27–27.5°C (control treatment) and 35.5–36°C (heat were taken twice per week at 10:00 h. treatment). The number of settled larvae (metamorphosed and attached to substrate) were counted for 17, 24 and 48 h after Statistical analysis incubation at both temperatures and the number of settled larvae was Larval heat stress experiment recorded. Settled larvae were defined as those that were attached to After testing data normality and homogeneity of variance with a the well or CCA and deposited a basal plate that was visible after diagnostic plot using the ‘stats’ package in R (https://www.r-project. metamorphosis. This was distinguished from only metamorphosed org/), differences in larval survival were assessed using the non- larvae (metamorphosis but no attachment to substrate), which were parametric ‘wilcox.test’ from the package ‘ggpubr’ (https://rpkgs. excluded from this analysis. datanovia.com/ggpubr/) to determine whether the mean values between each cross at the control and heat treatments (two Adult heat stress experiment independent groups) were statistically different. Colonies were kept in outdoor aquaria under the following conditions before fragmentation: 0.2 μm FSW, 27.5°C, P Settlement experiment CO 400±60 ppm and salinity of 35 psu. Three to five colonies The percentage of settled larvae was first analyzed using the base representing different genotypes for each population were ‘stats’ package in R to test the normality and homogeneity of the fragmented using a ‘diamond-tipped’ bandsaw (Table S1). Each values. If non-normal distributions were present, as shown through colony was divided into a minimum of six fragments, with each diagnostic plots, the random factors cross and plate were tested for placed into one of six experimental tanks (n=3 control tanks, n=3 their contribution to variability in settlement. The ‘ggpubr’ package heat tanks). For the Davies population, five genotypes were selected and Wilcoxon’s test (Wilcoxon, 1945) were used to statistically and cut into 82 fragments; three genotypes were used from the Esk compare the median percentage settlement between heat and control population (51 fragments); and three genotypes from the Keppels treatments at each time point. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Adult heat stress experiment previously been used in corals (Quigley et al., 2018) to determine Several coral traits have been identified as important biometrics for how selection on host–symbiont communities may adapt to restoration, including partial mortality and bleaching, in conjunction different selective environments. Contour plots of R, S and h for with classic response measurements such as survival (Baums et al., larvae grouped by maternal reef of origin were made using the 2019). Here bleaching was assessed by the change in colour from packages ‘plotly’ (https://plotly.com/) and ‘dplyr’ (https://CRAN.R- photographs, where a 6 is indicative of a non-bleached, healthy project.org/package=dplyr). fragment and a 0 is a white, bleached fragment. Differences in the photophysiological responses of the algal symbionts within corals RESULTS (ΔF/F ′), percent necrosis, bleaching and survival were evaluated Differences in larval survival under heat stress with respect to temperature treatment and coral population using The influence of both temperature treatments on larval survival linear models implemented in the ‘lme4’ package (Bates, 2005). varied by population cross, where larval lineages produced from The metric ΔF/F ′ and the percentage of necrosis were treated as Davies, Esk and Keppels corals displayed both high and low continuous variables, and temperature treatment and population were survival under control and heat conditions (Fig. 2A). Both median set as fixed factors in each model. Replicate tank and fragment holder survival and the variance around the median under both control and (‘sticks’) were set as random effects. These factors were not heat conditions varied across the lineages. Overall, the Keppels significant and were dropped from the final model (Table S3). intrapopulation larvae survived better under heat stress compared Once these random effects were dropped, the linear model was re- with interpopulation larvae, whilst the Esk and Davies larvae fitted, and all model assumptions were checked (linearity, normality survived better when crossed with either of the other two reefs as and homogeneity) using diagnostic plots in the ‘stats’ package in R. interpopulation crosses. The overall ‘winners’ under heat stress were Finally, a negative binomial generalized linear model (nbGLM) Davies×Esk and Esk×Keppels (which also had high survival under was used for bleaching, a linear model (LM) for ΔF/F ′ and control conditions), while the overall losers were Keppels×Davies the percentage of necrosis, and a generalized linear model (GLM) for and Keppels×Esk. survival. The function ‘anova’ was used to calculate model P-values, When scaled to the population level, Davies purebred larvae used for interpreting the significant difference among means. Post and larvae produced using Davies eggs were only significantly hoc pairwise comparisons of population and temperature treatment different in their survival between temperatures for one of the three −2 −2 were then run on the model outputs (Tukey, 1977). For survival data, lineages (P=0.71, 2.50×10 and 9.70×10 for Davies×Davies, a binomial distribution was used to determine whether there was a Davies×Esk and Davies×Keppels, respectively; Fig. 2B). Survival significant difference between populations and temperature under heat stress was higher in the interpopulation crosses compared treatments. For each trait, the statistical difference in median values with the purebred larvae (median survival: Davies×Davies= was assessed using the packages ‘ggplot2’ and ‘plyr’ (Wickham, 56.70%, Davies×Esk=75.00%, Davies×Keppels=62.30%). Again, 2011, 2016), with Wilcoxon’s test (Wilcoxon, 1945). The percentage Esk purebred larvae and larvae produced with Esk eggs were only change of adult responses by latitude was calculated and plotted using significantly different in their survival between temperatures for one the ‘ggplot2’ package (Wickham, 2016). All analyses were carried of the three lineages, with the same (reciprocal) cross significantly −2 out using RStudio, version 2.13.2 (https://www.rstudio.com/). different (P=4.10×10 , 0.53 and 0.29 for Esk×Davies, Esk×Esk and Esk×Keppels, respectively). Survival under heat stress was Predicting response gradients to selection higher for interpopulation crosses than for the purebred Esk larvae The density plots of adult and larval survival were made using the (Esk×Davies=61.70%, Esk×Esk=38.30%, Esk×Keppels=81.70%). ‘ggplot2’ package (Wickham, 2016). The breeder’s equation, The variation in median survival was also greater in purebred Esk R=h S (Falconer and Makcay, 1996) was used to calculate larvae compared with other lineages under heat stress. Keppels expected responses to selection (R) and potential constraints to the interpopulation larvae all had lower survival under heat stress −2 −8 evolution of heat tolerance given selective potentials (S) and compared with control conditions (P=3.20×10 and 2.30×10 for narrow-sense heritability for this trait (h ). This approach has Keppels×Davies and Keppels×Esk) whilst purebred larvae had A B DD DE DK ED EE EK KD KE KK * *** 50 100 150 27.5 35.5 Temperature (°C) Cumulative hours Fig. 2. Larval survival under experimental stress. (A) Percentage survival over time of n=85 different Acropora tenuis lineages of larvae exposed to control (27.5°C) and heat stress (35.5°C) conditions. (B) Percentage survival of each cross type over time of larvae exposed to control (27.5°C) and heat stress (35.5°C) conditions. The coloured boxes around the crosses indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Wilcoxon’s test was performed to analyze statistical differences. Survival (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 approximately equal survival between treatments (P=0.61). Median There was a significant effect of temperature treatment −16 survival was lower in the interpopulation larvae compared with (F =887.50, P=2.00×10 ) and heat exposure time 1,1633 −16 purebred larvae (Keppels×Keppels=65.60%, (F =120.70, P=2.00×10 ) on the percentage of settled 3,1631 Keppels×Davies=46.70%, Keppels×Esk=36.70%). larvae, and a significant interaction between temperature treatment −16 and heat exposure time (F =225.42, P=2.00×10 ), 3,1564 Influence of heat stress on larval settlement rates temperature treatment and population-level cross (F =5.30, 8,1564 −6 When exposed to heat, larvae from all crosses significantly P=1.39×10 ). There was no significant interaction between heat decreased in their settlement behaviour (attachment and exposure time and population-level cross (F =1.06, 24,1564 −1 metamorphosis) relative to the control temperature after 17 h P=3.86×10 ), or between temperature treatment, heat exposure −16 −1 (F =370.30, P=2.00×10 ), 24 h (6/9 crosses with 0.00% time and population-level cross (F =1.26, P=1.82×10 ). This 1,433 23,1564 −16 settlement; F =916.50, P=2.00×10 ) and 48 h (7/9 crosses significant decrease in settlement at 24 and 48 h occurred regardless 1,430 −16 −1 with 0.00% settlement; F =722.80, P=2.00×10 ; Fig. 3A–C). of larval cross (F =1.583, P=1.25×10 ) or reef of origin of the 1,334 8,1626 −1 Specifically, larvae from all crosses settled significantly less under maternal coral (F =0.133, P=8.75×10 ). 2,1632 heat stress compared with at control temperatures, regardless of After 17 h of temperature incubation, a median value of 0.00% population cross (P-values in Table S4). After 48 h, Davies×Esk larval settlement was observed for all crosses in the heat treatment and Esk×Keppels showed the greatest percentage settlement and was significantly lower compared with the control temperature compared with the other crosses (median settlement under heat (median 70.00%) across all larval lineages (Wilcoxon’s test, stress=both 0.00%, control=60.00 and 75.00%, respectively), in P<0.001; Fig. 3A). In the control treatment, the median settlement which Davies×Esk was the only population cross that settled under percentage was highest in Davies×Davies and Keppels×Esk, with a heat stress (upper quartile=100%). median of 80.00% settlement. The lowest median percentage Fig. 3. Larval settlement under experimental stress. DD DE DK ED EE EK KD KE KK Percentage of median settlement of n=69 different 100 Acropora tenuis lineages of larvae exposed to control * * * * * * * * * (blue) or heat stress (red) conditions after (A) 17 h, (B) 24 h and (C) 48 h. The coloured boxes around the crosses 75 indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Wilcoxon’s test was performed to analyze statistical differences. DD DE DK ED EE EK KD KE KK * * * * * * * * * DD DE DK ED EE EK KD KE KK * * * * * * * * 27.5 35.5 Temperature (°C) Settlement (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 settlement was observed in the Esk×Esk crosses, with 55.50% Photophysiology settlement and correspondingly high variability. The photophysiological responses, as measured by effective quantum After 24 h (Fig. 3B), median settlement remained at 70.00% in yield (ΔF/F ′), showed that ΔF/F ′ significantly decreased in the m m −15 the control treatment and ∼0.00% in the heat treatment, with all heat compared with the control treatment (LM, P=1.09×10 ; crosses settling significantly less under heat stress compared Fig. 4B) and showed significant differences between population −8 with controls (Wilcoxon’s test, P<0.001). At this time point, origins (LM, P=6.80×10 ). Davies and Keppels fragments reported Keppels×Davies showed the highest percentage settlement the highest yields at control temperatures (mean and median ΔF/F ′ (80.00%) under control conditions, followed by Davies×Keppels >0.60), whereas Esk fragments were extremely low (0.12±0.05, and Keppels×Keppels (75.00%). Finally, after 48 h (Fig. 3C), the median=0.00). Mean ΔF/F ′ recorded from fragments in the median settlement percentage in the control treatment dropped heat treatment was highest in Keppels (0.22±0.07, median=0.00), slightly to 66.67%, where Keppels×Davies crosses again exhibited Davies (0.20±0.05, median=0.00) and finally Esk (0.12±0.05, the highest larval settlement (75.00%). At heat, the only cross to median=0.00), but very variable overall. Relative to control settle included some of the larval replicates within Davies×Esk. temperatures, pairwise comparisons showed that Keppels and Finally, some crosses also experienced mortality of recent recruits Davies fragments showed a significant decrease in ΔF/F ′ −4 from 17 to 48 h (Davies×Davies), whilst others continued to settle (Tukey’stest, P<0.001 for both, median Wilcoxon=2.40×10 for −12 (Davies×Esk). Keppels and P=2.70×10 for Davies), but not Esk (Tukey’stest, −1 P=5.68×10 , median Wilcoxon=0.16). Adult responses to heat stress at the population level Bleaching Necrosis There is a significant effect of temperature treatment (nbGLM, Partial mortality was assessed as the percentage of necrotic tissue −16 P=2.20×10 ; Fig. 4A) and population origin (nbGLM, relative to each fragment (Fig. 4C). Population origin had a −15 −8 P=3.56×10 ) on the median bleaching score of coral fragments significant effect on percentage necrosis (LM, P=3.25×10 ). There after 16 days. At the control temperature, fragments sourced from was no significant difference in percentage necrosis owing −2 Davies recorded a mean colour score of 4.75±0.14 (mean±s.e.m.; to temperature treatment (LM, P=5.50×10 ). At the control median=5.00) whilst fragments sourced from Keppels and Esk temperature, Davies and Keppels corals showed very little to scored 1.85±0.25 (median=2.00) and 1.35±0.39 (median=0.00), no necrosis (median=0.00%), whereas Esk fragments were slightly respectively (Fig. 4A). At heat, fragments from all populations necrotic (median 15.29%). After 16 days under heat stress, bleached heavily (all mean scores <1.00, all median=0.00). Relative fragments sourced from Esk lost, on average, approximately to the control temperature, Davies fragments bleached the most 46.04±9.89% (median=0.00%) of their tissue per fragment, (median percentage change 87.41±5.32% decrease in bleaching compared with 26.67±11.82% (median=0.00%) and 12.20±5.17% category), followed by Keppels (62.96±37.04%) and then Esk (median=0.00%) for Keppels and Davies fragments, respectively. fragments (28.57±28.57%). Pairwise comparisons showed that Pairwise comparisons showed that at the control temperature, Esk bleaching score was significantly different between the control lost, on average, significantly more tissue compared with Davies −3 and heat treatments in Davies (mean Tukey’s test, P<0.001, median (Tukey’s test, P<0.001) and Keppels (Tukey’s test, P=3.39×10 ). −15 Wilcoxon P=1.80×10 ), Esk (Tukey’s test, P<0.001, In the heat treatment, Esk also experienced significantly more −3 median Wilcoxon P=0.11) and Keppels (Tukey’s test, P=0.15, necrosis compared with Davies (Tukey’s test, P=2.76×10 ). −2 median Wilcoxon P=1.50×10 ). All other population comparisons were not significantly different A B E Davies Esk Keppels Davies Esk Keppels 6 * * Bleaching 0.8 ** Necrosis Survival 0.6 ΔF/F ’ 0.4 0.2 C Davies Esk Keppels D Davies Esk Keppels −100 ** 1.0 −19 −23 Latitude 0.8 Esk Davies Keppels 0.6 27.5 31 Temperature (°C) 0.4 0.2 Temperature (°C) Temperature (°C) Fig. 4. Median physiological responses in adult corals exposed to control and heat stress temperatures. (A) Bleaching category core, (B) effective quantum yield, (C) percent necrosis and (D) and percent survival of adult Acropora tenuis fragments. Colonies were collected from three sites on the Great Barrier Reef: Davies reef (tan outlines in A–D), Esk reef (black) and Keppels reef (maroon). (E) Mean percent change in each response by latitude of collection site. Wilcoxon’s test was performed to analyze statistical differences. Necrosis (%) Bleaching category Survival (%) Effective quantum yield Change control to heat (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 −1 (Esk–Keppels, Tukey’s test, P=5.35×10 , Keppels–Davies, had much lower survival at the control temperature, their overall −1 Tukey’s test, P=7.43×10 ). Relative to control temperatures, the mean survival under heat stress was roughly equivalent to Davies median percent necrosis was not significantly different for any and Keppels corals, suggesting that the selection differential for −2 population (median Wilcoxon P=0.09, 0.91 and 5.30×10 for survival under heat stress in each population was roughly equivalent Davies, Esk and Keppels). (horizontal dashed black lines), defined as the difference between a selected phenotypic scope (triangle) and the mean percent survival Survival under heat stress (vertical dashed red line). Temperature showed significant effect on survival (binomial LM, −8 P=1.19×10 ). In the control treatment, survivorship was highest Comparisons of purebred and hybrid larval responses to adults in the Keppels fragments (median=1.00), followed by Davies Mean percent survival for larvae in the control treatments were (median=1.00) and Esk (median=0.00) (Fig. 4D). In the heat higher (56.44–78.92%) compared with in the heat treatments treatment, survivorship was highest in the Keppels fragments (41.48–78.40%). As expected, based on similar adult selection (40.00±13.09%, median=0.00), followed by Davies (31.71±7.36%, differentials (Fig. 5A), purebred larvae from Davies×Davies, median=0.00) and Esk (20.00±8.16%, median=0.00). Compared Esk×Esk and Keppels×Keppels responded similarly in the between heat and control temperatures, fragments sourced breadth of larval responses between control and heat treatments from Davies and Keppels significantly decreased in survivorship (specifically, a small difference between treatment responses). In −3 (Tukey’s test, P<0.001 and 4.98×10 , median contrast, interpopulation crosses differed in their responses, in −3 −10 Wilcoxon=5.10×10 and 1.40×10 , respectively), but not Esk which Keppels×Davies and Keppels×Esk had the largest difference fragments, which also survived poorly at the control temperature in mean survival between control and heat treatments (17.14% and −1 treatment (Tukey’s test, P=7.55×10 , median Wilcoxon=0.25). 32.84%) and Esk×Keppels the smallest (0.52%). Given the significant population effect of temperature treatment for The Davies×Davies purebreds exhibited a 56.44±8.77% mean bleaching score, ΔF/F ′ and necrosis (but not survival), the relative survival in the heat treatment, almost 2× greater survival than the differences in responses were also calculated and compared across Davies adults in the heat treatment (31.71±7.36%; Fig. 5B). the latitudinal gradient of adult origin (Fig. 4D). There was no trend Davies×Davies larval crosses demonstrated high variability (e.g. in performance across traits by latitude. flat distribution) in responses in survival in the control treatment, compared with a unimodal response in the heat treatment, Predicting adult and offspring responses using gradients although the mean survival was roughly equal (56.44±8.77% and of selection 55.78±4.71%, respectively, dashed lines). When Davies eggs were Adult responses crossed with sperm from the other central reef, heat tolerance When survival was averaged at the population level, almost all adult increased by 14.46% compared with purebred larvae. Davies×Esk corals collected from Davies exhibited approximately equivalent larvae exhibited unimodal responses for both temperatures, with survival in the control treatment, but when exposed to the heat average survival at 78.38% in the control and 70.24% in the heat treatment, the population responses were generally bimodal, where treatment. Heat tolerance was slightly less for Davies×Keppels some individuals within each population exhibited high survival larvae (55.41%) compared with Davies×Davies, where larval and others low survival (Fig. 5A). Corals collected from Esk and the responses were more variable but still unimodal. Keppels also exhibited this bimodal response for both the heat and The Esk×Esk larval purebreds exhibited 48.29±7.15% survival control temperatures. For both Davies and Keppels corals, the mean in the heat treatment, ∼2.5× greater survival than the Esk adults in survival in the control treatment was high and there was a roughly the heat treatment (20.00±8.16%; Fig. 5C). Esk×Esk larvae in the equal decrease in survival in the heat treatment. Although Esk corals control and heat treatments were relatively flat, demonstrating that AB Davies Esk Keppels DD DE DK 2.5 2.0 1.5 ED EE EK 1.0 0.5 07 25 50 5 100 D KD KE KK Survival (%) 0.04 0.03 0.02 27.5 35.5 Temperature (°C) 0.01 0 25 50 75 100 Survival (%) Fig. 5. Percent survival density plot. Number of (A) adult colonies (y) from each population and (B–D) offspring of each population cross. Arrows indicate the selected phenotypic scope. Horizontal dashed lines represent the selection differential for each population. Vertical dashed lines represent mean percent survival per population. The coloured boxes around the crosses indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Median survivorship data are presented in Figs 2 and 4D, whereas survival data here are presented as the number of individual colonies per percent survival value. Survival at ambient and hot are shown together to aid in comparison. Density Density Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 both treatments consisted of crosses with high and low survival. produced from selective breeding methods onto cooler reefs to When Esk eggs were crossed with sperm from the other reefs, heat prepare them for warming (Quigley et al., 2018), a method that will tolerance increased by 8.40% and 30.11% in Esk×Davies and also increase the genetic diversity on reefs – fuel for natural Esk×Keppels, respectively, relative to Esk×Esk. Esk×Davis larval selection. Quantifying the feasibility for enhancing corals’ ability to survival responses were weakly unimodal with mean survival at survive further ocean warming is therefore vital for the conservation 70.29±3.47% and 56.68±4.61% for the control and heat treatments. of the world’s coral reefs. Alternatively, Esk×Keppels larvae exhibited strongly unimodal survival responses at 78.92±3.82% in the control and 78.40±2.59% Little variation in adult physiological responses to heat in the heat treatment. stress across three GBR populations The Keppels×Keppels purebreds exhibited 62.84±3.73% Phenotypic variation in organisms drives the capacity for plastic, survival in the heat treatment, ∼1.5× greater survival than adaptive responses to environmental pressure. This variation may be the Keppels adults in the heat treatment (40.00±13.09%; underpinned by genetic variation or by responses mediated by non- Fig. 5D). Keppels×Keppels larval responses formed relatively flat genetic mechanisms, such as changes in the microbiome (e.g. distributions in the control treatment, and unimodal responses in bacteria or Symbiodiniaceae), in which algal symbiont assemblages the heat treatment. When Keppels eggs were crossed with sperm may shape corals’ responses to heat stress (Berkelmans and van from the other reefs, heat tolerance decreased by 17.59% and Oppen, 2006). Understanding the scope for phenotypic variation to 21.35% in Keppels×Davies and Keppels×Esk, respectively. In heat stress at the adult stage is essential to evaluating the scope for Keppels×Davies, the distribution was flat and wide, indicating heritable diversity of heat tolerance at later life stages in corals. variation in survival. The Keppels×Esk density plot for larvae in the Overall, there was no difference in heat tolerance of adult control treatment exhibited a unimodal peak in survival, as well as in southern Keppels corals compared with central Davies corals, with the heat treatment. both populations suffering similar percent necrosis, drops in In the heat stress treatment when larvae were grouped by the photophysiology and lowered survival under heat stress. However, population identity of the maternal colony, larvae produced from the relative change in bleaching was greater for Keppels corals. The Davies and Keppels had selective landscapes that were wider magnitude of bleaching was also more severe in Davies compared compared with Esk larvae (Fig. 6A–C, Table S5), although Davies with Keppels corals. Acropora tenuis in both central and southern and Esk had overall higher selection coefficients compared with reefs generally hosts dominant abundances of Cladocopium Keppels larvae (S=Esk: 60–70, Davies: 54–70, Keppels 50–60; (Rocker et al., 2017; Ulstrup and van Oppen, 2003), which could Fig. 6A–C). Combined with narrow-sense heritability estimates, contribute to the similarity in their physiological performance, these differences resulted in overall higher selective responses (R) whereas symbionts from Davies reef or the host corals themselves for Davies and Esk (i.e. more values of R between 40 and 60) may have lower initial tolerances but are able to recover and survive compared with Keppels offspring (Fig. 6D–F). equally well. Taken together, these results suggest that the absolute heat tolerance of both populations was roughly equal. Finally, adult DISCUSSION coral fragments from the Esk population in heat treatment exhibited Mean maximum SSTs are expected to increase by between 2 and the highest bleaching severity, lowest effective quantum yield, 4°C by 2100 globally (IPCC, 2014). Without adaptation, this will highest percentage necrosis and lowest survivorship. It should be likely exceed the thermal thresholds of corals. Understanding the noted that although fragments sourced from the Esk population lost underlying adaptive capacity of wild populations is therefore critical the greatest overall percentage of tissue (necrosis) per population, to forecasting species persistence. Moreover, various conservation fragments in the control treatment were also highly necrotic, strategies are being considered worldwide to help corals withstand suggesting a compromised health state of corals from this increasing ocean temperatures whilst carbon emissions are curbed population, also reflected in the low survival and photosynthesis (National Academies of Science, Engineering and Medicine, 2019). in controls. Both Davies and Esk corals were collected during the This includes the introduction of more heat-tolerant offspring sample trip with the same level of handling. This suggests that Fig. 6. Adaptive landscape of selection for heat 1.0 A D tolerance. (A–C) Modelled selective values (S) and narrow-sense heritability (h ) values of larval survival at 0.5 heat grouped by the population identity of the maternal colony. (D–F) Estimated response to selection (R) for 0.0 h and S. 1.00 B E 0.75 0.50 0.25 0.00 C F Response (R) 60 60 0.5 40 40 20 20 0.0 -20 −20 -0.5 50 55 60 65 70 50 55 60 65 70 Selection (S) Heritability (h ) Keppels Esk Davies Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 transport issues were not the cause of their diminished health state, during transition phases of larvae to polyp (Randall and Szmant, and instead point towards population-level differences between 2009). The reduction in larval settlement could also be a these corals. consequence of energy deficiency. As cellular proteins unfold and The high overall fitness of adult Keppels corals under heat stress aggregate, HSP70, a heat shock protein that refolds degrading was surprising. This population exhibited the lowest bleaching proteins (Daugaard et al., 2007), has been found to be upregulated in severity, highest effective quantum yield and highest survival at Acropora millepora larvae to maintain normal cell function heat. Although enhanced heat tolerance in corals is generally (Rodriguez-Lanetty et al., 2009). This requires vast amounts of attributed to corals from warmer reefs exhibiting higher upper energy, that could potentially otherwise be used for settlement. thermal thresholds (Berkelmans, 2002; Howells et al., 2012; Ulstrup Higher respiration rates at heat also increase metabolic activity et al., 2006), the enhanced performance of Keppels corals may be (Edmunds et al., 2001), which, in turn, increases the amount of food attributed to the greater variability in their local thermal regime. The required to maintain these elevated levels. These factors could all increase in tolerance may also partly be attributed to the coral contribute to the reduced settlement of larvae measured here. symbionts (Howells et al., 2012; Thomas et al., 2018; Ulstrup et al., Overall, although our results showed a significant decrease in 2006) and their interaction with host genetics (Dixon et al., 2015; larval settlement at heat compared with control temperatures, this Smith-Keune and van Oppen, 2006; Thomas et al., 2018), in which did not correspond to reef of origin. Our findings only weakly allude complex holobiont interactions influence the overall heat stress to the potential for selectively bred coral larvae to settle at higher responses via gene regulation, symbiont density control and temperatures. For example, while our results do not demonstrate a assemblage shuffling (Cunning and Baker, 2020; Yuyama et al., significant increase, there was a higher percentage of settled larvae 2018). Our results indicate that the control of heat tolerance is whose maternal colony was from either Esk or Davies (the central, complex and that many factors, including local thermal regime, warmer, inshore reefs in this study) compared with those with a likely play a role. maternal colony from the southern, cooler Keppels reef, in contrast to the adult’s response in which Keppels had higher overall fitness Minimal improvement in larval settlement owing to selection than Davies and Esk. Previous research suggests that mitochondrial for heat tolerance DNA plays a large role in the thermal resistance of corals, alluding Previous breeding experiments have demonstrated the transfer of to a high maternal effect on the heat tolerance of coral offspring increased offspring survival from parents sourced from warm reefs (Dixon et al., 2015; Quigley et al., 2020a,b). There was when reproductively crossed with cooler reefs (Dixon et al., 2015), also some variation in settlement within population crosses under or at least one parent from warmer reefs (Quigley et al., 2020b), heat, demonstrating the potential for plasticity. This aligns with suggesting a genetic contribution to offspring. In this study, larval information that corals found in warmer environments or with high survival was high in the crosses with a maternal colony sourced daily temperature variability have greater genetic plasticity (Kenkel from either Esk or Davies. However, it is currently unknown and Matz, 2017) which can be passed onto offspring. However, the whether an increased propensity for settlement at high temperatures findings presented here are preliminary and it appears that the is also transferable using colonies sourced from warmer reefs to maximum thermal limits of parental corals and the resulting larvae achieve an enhancement in settlement success. Although settlement are not indicative of settlement success. As warming increases in is a heritable trait under control conditions (h =0.49; Meyer et al., severity, these processes may be the first to be disrupted (Radchuk 2009), the overall heritability is low relative to other fitness-related et al., 2019), and assessing the impacts on these and other traits. Moreover, it is well known that settlement in corals is fundamental processes such as recruitment will become negatively impacted by heat. For example, early life-stage A. tenuis increasingly important. settlement decreased by 100% when exposed to >5°C above The lack of a significant effect of parental colony from warmer ambient temperature (Humanes et al., 2016), and by 55% when reefs to enhance settlement at high temperatures may be due to combined with a suspended sediments treatment (Humanes et al., either experimental or biological factors. The high temperatures 2017). Diploria strigosa larvae demonstrated a decrease in chosen here may have surpassed the corals’ settlement ability at settlement behaviour at temperatures exceeding 30°C compared these temperature limits, in which temperatures exceeding 35.5°C with controls (Bassim and Sammarco, 2003), and Acropora were extreme compared with the mean monthly maximums of these palmata settlement decreased by 25% at 31.5°C compared with sites (all 24–27°C), resembling short-term acute heat stress 28°C (Randall and Szmant, 2009). The lack of strong differences in temperature range (Grottoli et al., 2021; McLachlan et al., 2020). settlement success between the crosses here may be reflected in the However, the experimental temperature of many studies does not roughly equal heat tolerance of both Davies and Keppels corals, exceed >5°C above the control temperature (Humanes et al., 2016, suggesting that both populations are roughly equivalent in tolerance 2017; McLachlan et al., 2020; Quigley et al., 2020b). Hence, our and therefore did not produce strong differences in settlement of result could reflect the contribution of higher-than-threshold larvae. Combined, these previous results suggest that selection temperature treatment. Alternatively, environmental factors may should act on this important trait over time if oceans continue to contribute a greater influence in determining settlement compared warm. with host genetics. Specifically, settlement deficiency at high The results from this study show that, when exposed to heat, temperature may be driven by the disruption of the microbial larvae from all crosses significantly decreased in their settlement biofilm needed to induce metamorphosis and settlement, where it is behaviour relative to the control temperature. During periods of well established that settlement is induced by the presence of CCA increased temperature, the cellular processes within larvae become and microbial biofilms (Webster et al., 2004). During periods of compromised, including disruption in the repair of cellular proteins increased temperature, chemical cues released by CCA can be and enzymes (Negri et al., 2007). Consequently, abnormalities weakened and microbial cells present in biofilms (on the CCA) can develop, and the rate of cell cleavage rapidly increases. This cellular become damaged (Randall and Szmant, 2009). Therefore, this impairment could prevent the transition from larvae to recruit by reduction in biochemical cues could be the main contributing factor preventing attachment to substrata or an increase in hypersensitivity to the reduction in larval settlement at higher temperatures. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 In summary, larvae from intrapopulation and interpopulation responses suggests a divergence between adult population mean crosses demonstrated a general inability to settle at high (and responses and interpopulation offspring crosses. The divergence perhaps extreme) temperatures, suggesting that improvement in between thermal responses of different life stages within species has larval settlement responses owing to selection for heat tolerance been demonstrated in other marine organisms such as mollusks may be challenging as a result of the competing influence of (Truebano et al., 2018), in which larval forms are often more or less environmental effects. Combined with information that behavioural vulnerable to heat compared with adults. In this case, mollusks or morphological traits generally respond less to selection compared reported a 1.7–2.1× difference in responses between larvae and with life-history traits (Mousseau and Roff, 1987) and the potential adults, which mirrors reports seen in other invertebrates such as governing importance of environmental factors (e.g. bacterial brine shrimp (2.7–4.9×; Norouzitallab et al., 2014), copepods communities), this suggests that the enhancement of this trait (Tangwancharoen, 2014) and others (Pandori and Sorte, 2019). under heat stress may require the selection through microbial Finally, adaptive response surfaces revealed that when larvae were community contribution more than through processes targeting host grouped by their maternal populations, Davies and Esk offspring genetics. generally had higher selective responses (R) compared with Keppels offspring. Taken together, this suggests that although adult Predicting responses to heat using selection differentials populations may respond similarly to heat, the overall potential of and gradients of selection offspring responses to selection in warmer populations of corals Genetic variation underpins the potential and speed for adaptation from Davies and Esk is greater compared with cooler populations in through natural selection (Falconer and Makcay, 1996). Warming the Keppels. These findings have important implications for influences traits differentially, with morphological traits generally forecasting the impacts of climate change on wild populations of less impacted compared with phenological traits (Radchuk et al., corals and for the development of novel conservation tools such as 2019). Survival was chosen as the trait of interest to examine the assisted evolution of at-risk populations. gradients of selection given the importance of survival and other life- history traits compared with behavioural or morphological traits Conclusions (Mousseau and Roff, 1987). Measures such as narrow-sense As climate change accelerates ecosystem change, critical heritability (h ), selection (S) and responses to selection (R)are information on how important fitness traits will vary in the future useful for quantitatively assessing the ability of organisms to respond is essential to move from understanding impacts to predicting and to their environment, especially future stressors. Narrow-sense forecasting those impacts. This comparative physiological dataset heritability ranges from 0 to 1, where 0 is indicative of no genetic across different life-history stages in one important coral species contribution to trait variance and 1 is indicative of complete provides key mechanistic and adaptive insights into how corals may dominance of genetics in determining trait variance (Falconer and function under heat stress caused by warming oceans, and in Makcay, 1996). Measurements derived from corals suggest that they particular, the heritability of heat tolerance. do have a strong underlying capacity to respond adaptively to heat Acknowledgements either through host genetics (h =mean: 0.86, range: 0.48–0.93) The authors would like to thank the Traditional Owners from whose Sea Country (Dixon et al., 2015; Dziedzic et al., 2019; Kirk et al., 2018; Quigley these corals were collected. Specifically, we would like to thank the Woppaburra, et al., 2020b) or through changes to their symbiont communities Manbarra, Bindal and Wulgurukaba Traditional Owners. We would like to thank (Quigley et al., 2018). It might be expected that heat stress would Andrea Severati, Line Bay, Christine Giuliano and the spawning collections field crews in the central and southern Great Barrier Reef for assisting in the coral colony elicit directional selection through differential mortality of adults, collections. resulting in the survival of a subset of phenotypes at one end of the phenotypic distribution. However, the bimodal responses in survival Competing interests curves of adult corals suggest that heat stress manifests as disruptive The authors declare no competing or financial interests. selection, which may explain the high variability of heat responses across the numerous offspring lineages. Although the underlying Author contributions Conceptualization: K.M.Q.; Methodology: K.M.Q.; Formal analysis: P.W., K.M.Q.; mechanisms are unknown here, the drivers of bimodality may be Investigation: P.W., R.B.C., A.M., H.K., J.L.R., K.M.Q.; Resources: K.M.Q.; Data linked to biochemical complexity (Rezende and Bozinovic, 2019). curation: P.W., K.M.Q.; Writing - original draft: R.B.C., J.L.R., K.M.Q.; Writing - review Selection differentials depend on the heritability of the trait, & editing: P.W., R.B.C., A.M., K.M.Q.; Supervision: K.M.Q.; Funding acquisition: where heritability is generally equal to the slope of the response over K.M.Q. selection, as determined by the breeder’s equation. Interestingly, the Funding selection differentials at heat (i.e. the intensity of adaptive This study was supported by funding from the Australian Institute of Marine Science. responses) were similar across the three populations of adult P.W. was supported by a travel and study grant provided by Ghent University, corals. This mirrors the similar physiological responses of the adult Erasmus, Master of Science in Marine Biological Resources (IMBRSea). corals to heat stress. The selection differential between survival at high temperatures can be described as the difference between the Data availability Physiological data are available as datasets 1 and 2 in the supplementary material mean value measured (Davies and Keppels=∼40–30%, and Esk and analysis code from: https://github.com/LaserKate/KeppelsAGF19 ∼25%) and the desired mean value (e.g. ∼90%). Hence, the selection differential was approximately 55% (90 minus 35%) References across these three populations, which represents a desired 63% AIMS (2020). Initial recovery of the Great Barrier Reef threatened by the third mass increase in potential trait enhancement. When translated to bleaching event in five years. Australian Institute of Marine Science. Aitken, S. N. and Whitlock, M. C. (2013). Assisted gene flow to facilitate local estimated adaptive larval responses, these varied by population adaptation to climate change. Annu. Rev. Ecol. Evol. Syst. 44, 367-388. doi:10. cross. In comparison, responses to selection in well-studied systems 1146/annurev-ecolsys-110512-135747 such as aquaculture species, including fish and shellfish, averaged Bassim, K. and Sammarco, P. (2003). Effects of temperature and ammonium on ∼13% for growth and 4.9% for survival (Gjedrem and Rye, 2018). larval development and survivorship in a scleractinian coral (Diploria strigosa). Furthermore, our comparative analysis between adult and offspring Marine Biol. 142, 241-52. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Bassim, K. M., Sammarco, A. P. W. and Snell, A. T. L. (2002). Effects of Jones, G. P., Mccormick, M. I., Srinivasan, M. and Eagle, J. V. (2004). Coral temperature on success of (self and non-self) fertilization and embryogenesis in decline threatens fish biodiversity in marine reserves. Popul. Biol. 101, Diploria strigosa (Cnidaria, Scleractinia). Mar. Biol. 140, 479-488. doi:10.1007/ 8251-8253. s00227-001-0722-4 Keith, S. A., Maynard, J. A., Edwards, A. J., Guest, J. R., Bauman, A. G., van Bates, D. (2005). Fitting linear mixed models in R. Obes. Facts 3, 27-30. Hooidonk, R., Heron, S. F., Berumen, M. L., Bouwmeester, J., Baums, I. B., Baker, A. C., Davies, S. W., Grottoli, A. G., Kenkel, C. D., Piromvaragorn, S. et al. (2016). Coral mass spawning predicted by rapid seasonal rise in ocean temperature. Proc. R. Soc. B Biol. Sci. 283, 20160011. Kitchen, S. A., Kuffner, I. B., LaJeunesse, T. C., Matz, M. V., Miller, M. W. et al. (2019). Considerations for maximizing the adaptive potential of restored coral Kenkel, C. D. and Matz, M. V. (2017). Gene expression plasticity as a mechanism of coral adaptation to a variable environment. Nat. Ecol. Evol. 1, 1-6. doi:10.1038/ populations in the western Atlantic. Ecol. Appl. 29, 1-23. doi:10.1002/eap.1978 s41559-016-0014 Berkelmans, R. (2002). Time-integrated thermal bleaching thresholds of reefs and Kirk, N. L., Howells, E. J., Abrego, D., Burt, J. A. and Meyer, E. (2018). Genomic their variation on the Great Barrier Reef. Mar. Ecol. Prog. Ser. 237, 309-310. and transcriptomic signals of thermal tolerance in heat-tolerant corals (Platygyra Berkelmans, R. and van Oppen, M. J. H. (2006). The role of zooxanthellae in the daedalea) of the Arabian/Persian Gulf. Mol. Ecol. 27, 5180-5194. doi:10.1111/ thermal tolerance of corals: a “nugget of hope” for coral reefs in an era of climate mec.14934 change. Proc. R. Soc. B Biol. Sci. 273, 2305-2312. doi:10.1098/rspb.2006.3567 Komyakova, V., Munday, P. L. and Jones, G. P. (2013). Relative importance of Cunning, R. and Baker, A. C. (2020). Thermotolerant coral symbionts modulate coral cover, habitat complexity and diversity in determining the structure of reef heat stress-responsive genes in their hosts. Mol. Ecol. 29, 2940-2950. doi:10. fish communities. PLoS One 8, e83178. doi:10.1371/journal.pone.0083178 1111/mec.15526 Matz, M. V., Treml, E. A., Aglyamova, G. V. and Bay, L. K. (2017). Potential and Daugaard, M., Rohde, M. and Jaattela,M. (2007). The heat shock protein 70 ̈ ̈ ̈ limits for rapid genetic adaptation to warming in a Great Barrier Reef coral. PLoS family: highly homologous proteins with overlapping and distinct functions. FEBS Genet. 14, e1007220. doi:10.1371/journal.pgen.1007220 Lett. 581, 3702-3710. doi:10.1016/j.febslet.2007.05.039 Matz, M. V., Treml, E. A., Aglyamova, G. V., van Oppen, M. J. H. and Bay, L. K. De’Ath, G., Fabricius, K. E., Sweatman, H. and Puotinen, M. (2012). The 27-year (2018). Potential for rapid genetic adaptation to warming in a Great Barrier Reef decline of coral cover on the Great Barrier Reef and its causes. Proc. Natl. Acad. coral. bioRxiv. doi:10.1101/2021.10.06.463349 Sci. USA 109, 17995-17999. doi:10.1073/pnas.1208909109 McLachlan, R. H., Price, J. T., Solomon, S. L. and Grottoli, A. G. (2020). Thirty Dixon, G., Davies, S., Aglyamova, G., Meyer, E., Bay, L. and Matz, M. (2015). years of coral heat-stress experiments: a review of methods. Coral Reefs 39, Genomic determinants of coral heat tolerance across latitudes. Science 348, 885-902. doi:10.1007/s00338-020-01931-9 1460-1462. doi:10.1126/science.1261224 Meyer, E., Davies, S., Wang, S., Willis, B. L., Abrego, D., Juenger, T. E. and Doropoulos, C., Gómez-Lemos, L. A. and Babcock, R. C. (2018). Exploring Matz, M. V. (2009). Genetic variation in responses to a settlement cue and variable patterns of density-dependent larval settlement among corals with elevated temperature in the reef-building coral Acropora millepora. Mar. Ecol. distinct and shared functional traits. Coral Reefs 37, 25-29. doi:10.1007/s00338- Prog. Ser. 392, 81-92. doi:10.3354/meps08208 017-1629-y Moghaddam, S., Shokri, M. R. and Tohidfar, M. (2021). The enhanced expression Dziedzic, K. E., Elder, H., Tavalire, H. and Meyer, E. (2019). Heritable variation in of heat stress-related genes in scleractinian coral ‘Porites harrisoni’ during warm bleaching responses and its functional genomic basis in reef-building corals episodes as an intrinsic mechanism for adaptation in ‘the Persian Gulf’. Coral (Orbicella faveolata). Mol. Ecol. 28, 2238-2253. doi:10.1111/mec.15081 Reefs 40, 1013-1028. Edmunds, P., Gates, R. and Gleason, D. (2001). The biology of larvae from the reef Mousseau, T. A. and Roff, D. A. (1987). Natural selection and the heritability of coral Porites astreoides, and their response to temperature disturbances. Mar. fitness components. Heredity 59, 181-197. doi:10.1038/hdy.1987.113 Biol. 139, 981-989. doi:10.1007/s002270100634 Munday, P. L., Leis, J. M., Lough, J. M., Paris, C. B., Kingsford, M. J., Evans, L. M., Tahmasbi, R., Jones, M., Vrieze, S. I., Abecasis, G. R., Das, S., Berumen, M. L. and Lambrechts, J. (2009). Climate change and coral reef Bjelland, D. W., de Candia, T. R., Yang, J., Goddard, M. E. et al. (2018). Narrow- connectivity. Coral Reefs 28, 379-395. doi:10.1007/s00338-008-0461-9 sense heritability estimation of complex traits using identity-by-descent National Academies of Sciences, Engineering, and Medicine (2019). A research information. Heredity 121, 616-630. doi:10.1038/s41437-018-0067-0 review of interventions to increase the persistence and resilience of coral reefs. Falconer, D. S. and Makcay, T. F. C. (1996). Introduction to Quantitative Genetics, Washington, DC: The National Academies Press. 4th edn. London: Longman Group Ltd. Negri, A. P., Marshall, P. A. and Heyward, A. J. (2007). Differing effects of thermal Ghaffari, H., Wang, W., Li, A., Zhang, G. and Li, L. (2019). Thermotolerance stress on coral fertilization and early embryogenesis in four Indo Pacific species. divergence revealed by the physiological and molecular responses in two oyster Coral Reefs 26, 759-763. doi:10.1007/s00338-007-0258-2 subspecies of Crassostrea gigas in China. Front. Physiol. 10, 1137. doi:10.3389/ Nelson, H. R., Kuempel, C. D. and Altieri, A. H. (2016). The resilience of reef fphys.2019.01137 invertebrate biodiversity to coral mortality. Ecosphere 7, e01399. Gjedrem, T. and Rye, M. (2018). Selection response in fish and shellfish: a review. Norouzitallab, P., Baruah, K., Vandegehuchte, M., Van Stappen, G., Catania, F., Rev. Aquac. 10, 168-179. doi:10.1111/raq.12154 Bussche, J. V., Vanhaecke, L., Sorgeloos, P. and Bossier, P. (2014). Grottoli, A. G., Toonen, R. J., van Woesik, R., Vega Thurber, R., Warner, M. E., Environmental heat stress induces epigenetic transgenerational inheritance of McLachlan, R. H., Price, J. T., Bahr, K. D., Baums, I. B., Castillo, K. D. et al. robustness in parthenogenetic Artemia model. FASEB J. 28, 3552-3563. doi:10. (2021). Increasing comparability among coral bleaching experiments. Ecol. Appl. 1096/fj.14-252049 31, e02262. doi:10.1002/eap.2262 O’Mahony, J., Simes, R., Redhill, D., Heaton, K., Atkinson, C., Hayward, E. and Heron, S. F., Maynard, J. A., van Hooidonk, R. and Eakin, C. M. (2016). Warming Nguyen, M. (2017). At what price? The economic, social and icon value of the trends and bleaching stress of the world’s coral reefs 1985-2012. Sci. Rep. 6, 1-14. Great Barrier Reef. Deloitte Access Econ. 92, 1-92. doi:10.1038/s41598-016-0001-8 Pandori, L. L. M. and Sorte, C. J. B. (2019). The weakest link: sensitivity to climate Heyward, A. J. and Negri, A. P. (2010). Plasticity of larval pre-competency in extremes across life stages of marine invertebrates. Oikos 128, 621-629. doi:10. response to temperature: observations on multiple broadcast spawning coral 1111/oik.05886 species. Coral Reefs 29, 631-636. doi:10.1007/s00338-009-0578-5 Payne, N. L., Morley, S. A., Halsey, L. G., Smith, J. A., Stuart-Smith, R., Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the Waldock, C. and Bates, A. E. (2021). Fish heating tolerance scales similarly world’s coral reefs. Mar. Freshw. Res. 50, 839-866. across individual physiology and populations. Commun. Biol. 4, 1-5. doi:10.1038/ Howells, E. J., Beltran, V. H., Larsen, N. W., Bay, L. K., Willis, B. L. and van s42003-021-01773-3 Oppen, M. J. H. (2012). Coral thermal tolerance shaped by local adaptation of Quigley, K. M., Bay, L. K. and Willis, B. L. (2018). Leveraging new knowledge of photosymbionts. Nat. Clim. Chang. 2, 116-120. doi:10.1038/nclimate1330 Symbiodinium community regulation in corals for conservation and reef Hughes, T. P., Barnes, M. L., Bellwood, D. R., Cinner, J. E., Cumming, G. S., restoration. Mar. Ecol. Prog. Ser. 600, 245-253. doi:10.3354/meps12652 Jackson, J. B. C., Kleypas, J., van de Leemput, I. A., Lough, J. M., Morrison, Quigley, K. M., Bay, L. K. and van Oppen, M. J. H. (2019a). The active spread of T. H. et al. (2017). Coral reefs in the Anthropocene. Nature 546, 82-90. doi:10. adaptive variation for reef resilience. Ecol. Evol. 9, 1-14. doi:10.1002/ece3.5616 1038/nature22901 Quigley, K. M., Willis, B. L. and Kenkel, C. D. (2019b). Transgenerational Humanes, A., Noonan, S. H. C., Willis, B. L., Fabricius, K. E. and Negri, A. P. inheritance of shuffled symbiont communities in the coral Montipora digitata. Sci. (2016). Cumulative effects of nutrient enrichment and elevated temperature Rep. 9, 1-11. doi:10.1038/s41598-019-50045-y compromise the early life history stages of the coral Acropora tenuis. PLoS One Quigley, K. M., Bay, L. K. and van Oppen, M. J. H. (2020a). Genome-wide SNP 11, 1-23. doi:10.1371/journal.pone.0161616 analysis reveals an increase in adaptive genetic variation through selective Humanes, A., Ricardo, G. F., Willis, B. L., Fabricius, K. E. and Negri, A. P. (2017). breeding of coral. Mol. Ecol. 29, 2176-2188. doi:10.1111/mec.15482 Cumulative effects of suspended sediments, organic nutrients and temperature Quigley, K. M., Randall, C. J., van Oppen, M. J. H. and Bay, L. K. (2020b). stress on early life history stages of the coral Acropora tenuis. Sci. Rep. 7, 44101. Assessing the role of historical temperature regime and algal symbionts on the doi:10.1038/srep44101 heat tolerance of coral juveniles. Biol. Open 9, 1-11. IPCC (2014). Climate Change 2014: Synthesis Report. Contribution of Working Radchuk, V., Reed, T., Teplitsky, C., van de Pol, M., Charmantier, A., Hassall, C., Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel Adamık,́ P., Adriaensen, F., Ahola, M. P., Arcese, P. et al. (2019). Adaptive on Climate Change (ed. Core Writing Team, R. K. Pachauri and L.A. Meyer). responses of animals to climate change are most likely insufficient. Nat. Commun. Geneva: IPCC. 10, 1-14. doi:10.1038/s41467-019-10924-4 Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Randall, C. J. and Szmant, A. M. (2009). Elevated temperature reduces Thomas, L., Rose, N. H., Bay, R. A., López, E. H., Morikawa, M. K., Ruiz- survivorship and settlement of the larvae of the Caribbean scleractinian coral, Jones, L. and Palumbi, S. R. (2018). Mechanisms of thermal tolerance in reef- Favia fragum (Esper). Coral Reefs 28, 537-545. doi:10.1007/s00338-009-0482-z building corals across a fine-grained environmental mosaic: lessons from Ofu, Rezende, E. L. and Bozinovic, F. (2019). Thermal performance across levels of American Samoa. Front. Mar. Sci. 4, 434. biological organization. Philos. Trans. R. Soc. B Biol. Sci. 374, 20180549. do:10. Truebano, M., Fenner, P., Tills, O., Rundle, S. D. and Rezende, E. L. 1098/rstb.2018.0549 (2018). Thermal strategies vary with life history stage. J. Exp. Biol. 221, Rocker, M. M., Francis, D. S., Fabricius, K. E., Willis, B. L. and Bay, L. K. (2017). jeb171629. Variation in the health and biochemical condition of the coral Acropora tenuis Tukey, J. W. (1977). Exploratory Data Analysis. Reading, MA: Pearson. along two water quality gradients on the Great Barrier Reef, Australia. Mar. Pollut. Ulstrup, K. E. and van Oppen, M. J. H. (2003). Geographic and habitat partitioning Bull. 119, 106-119. doi:10.1016/j.marpolbul.2017.03.066 of genetically distinct zooxanthellae (Symbiodinium)in Acropora corals on the Rodriguez-Lanetty, M., Harii, S. and Hoegh-Guldberg, O. (2009). Early molecular Great Barrier Reef. Mol. Ecol. 12, 3477-3484. doi:10.1046/j.1365-294X.2003. responses of coral larvae to hyperthermal stress. Mol. Ecol. 18, 5101-5114. 01988.x doi:10.1111/j.1365-294X.2009.04419.x Ulstrup, K. E., Berkelmans, R., Ralph, P. J. and van Oppen, M. J. H. (2006). Savary, R., Barshis, D. J., Voolstra, C. R., Cardenas, A., Evensen, N. R., Variation in bleaching sensitivity of two coral species across a latitudinal gradient Banc-Prandi, G., Fine, M. and Meibom, A. (2021). Fast and pervasive on the Great Barrier Reef: the role of zooxanthellae. Mar. Ecol. Prog. Ser. 314, transcriptomic resilience and acclimation of extremely heat-tolerant coral 135-148. doi:10.3354/meps314135 holobionts from the northern Red Sea. Proc. Natl. Acad. Sci. 118, 1-9. doi:10. van Oppen, M. J. H., Oliver, J. K., Putnam, H. M. and Gates, R. D. (2015). Building 1073/pnas.2023298118 coral reef resilience through assisted evolution. Proc. Natl. Acad. Sci. USA 112, Schneider, C. A., Rasband, W. S. and Eliceiri, K. W. (2012). NIH Image to ImageJ: 2307-2313. doi:10.1073/pnas.1422301112 25 years of image analysis. Nat. Methods 9, 671-675. doi:10.1038/nmeth.2089 van Tienderen, P. H. and de Jong, G. (1994). A general model of the relation Schoepf, V., Carrion, S. A., Pfeifer, S. M., Naugle, M., Dugal, L., Bruyn, J. and between phenotypic selection and genetic response. J. Evol. Biol. 7, 1-12. doi:10. McCulloch, M. T. (2019). Stress-resistant corals may not acclimatize to ocean 1046/j.1420-9101.1994.7010001.x warming but maintain heat tolerance under cooler temperatures. Nat. Commun. Webster, N. S., Smith, L. D., Heyward, A. J., Watts, J. E. M., Webb, R. I., Blackall, 10, 4031. L. L. and Negri, A. P. (2004). Metamorphosis of a scleractinian coral in response Siebeck, U. E., Logan, D. and Marshall, N. J. (2008). CoralWatch - a flexible coral to microbial biofilms. Appl. Environ. Microbiol. 70, 1213-1221. doi:10.1128/AEM. bleaching monitoring tool for you and your group. Proc. 11th Int. Coral Reef Symp., 70.2.1213-1221.2004 p. 16. Wickham, H. (2011). The split-apply-combine strategy for data analysis. J. Stat. Smith-Keune, C. and van Oppen, M. (2006). Genetic structure of a reef-building Softw. 40, 1-29. coral from thermally distinct environments on the Great Barrier Reef. Coral Reefs Wickham, H. (2016). ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag, 25, 493-502. New York. Sully, S., Burkepile, D. E., Donovan, M. K., Hodgson, G. and van Woesik, R. Wilcoxon, F. (1945). Individual comparisons by ranking methods. Biometrics Bull. 1, (2019). A global analysis of coral bleaching over the past two decades. Nat. 80-83. doi:10.2307/3001968 Commun. 10, 1-5. doi:10.1038/s41467-019-09238-2 Yuyama, I., Ishikawa, M., Nozawa, M., Yoshida, M. and Ikeo, K. (2018). Tangwancharoen, S. (2014). Early life stages are not always the most sensitive: Transcriptomic changes with increasing algal symbiont reveal the detailed heat stress responses in the copepod Tigriopus californicus. Mar. Ecol. Prog. Ser. process underlying establishment of coral-algal symbiosis. Sci. Rep. 8, 16802. 517, 75-83. doi:10.3354/meps11013 doi:10.1038/s41598-018-34575-5 Journal of Experimental Biology http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Experimental Biology The Company of Biologists http://www.deepdyve.com/lp/the-company-of-biologists/predicting-selection-response-gradients-of-heat-tolerance-in-a-aQR3HzE1mB

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Publisher: The Company of Biologists
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DOI: 10.1242/jeb.243344
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Abstract

© 2022. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 RESEARCH ARTICLE Predicting selection–response gradients of heat tolerance in a widespread reef-building coral 1,2, 1,3, 1 4 1 Ponchanok Weeriyanun *, Rachael B. Collins *, Alex Macadam , Hugo Kiff , Janna L. Randle and 1,‡ Kate M. Quigley ABSTRACT ecological functioning via trophic interactions, with downstream impacts on fish and other invertebrate species (Jones et al., 2004; Ocean temperatures continue to rise owing to climate change, but it Komyakova et al., 2013; Nelson et al., 2016). Coral reefs are is unclear whether heat tolerance of marine organisms will keep pace biodiversity hotspots that provide access to food, act as natural with warming. Understanding how tolerance scales from individuals barriers against storm damage and benefit the economy through to species and quantifying adaptive potentials is essential to tourism and other recreational activities (O’Mahony et al., 2017). forecasting responses to warming. We reproductively crossed Coral reef ecosystems such as the Great Barrier Reef (GBR) corals from a globally distributed species (Acropora tenuis) on the contribute, on average, $6.4 billion (AUD) to the Australian Great Barrier Reef (Australia) from three thermally distinct reefs to economy per annum (O’Mahony et al., 2017). Consequently, the create 85 offspring lineages. Individuals were experimentally loss of these crucial habitats will have devastating economic and exposed to temperatures (27.5, 31 and 35.5°C) in adult and two social-ecological effects on coastal communities. critical early life stages (larval and settlement) to assess acquired The upper thermal thresholds of reef-building corals generally heat tolerance via outcrossing of offspring phenotypes by comparing reside ∼1°C higher than their local summer maxima (Schoepf et al., five physiological responses (photosynthetic yields, bleaching, 2019). Once temperatures exceed this threshold, corals may lose necrosis, settlement and survival). Adaptive potentials and their symbiotic dinoflagellates (Symbiodiniaceae) through a process physiological reaction norms were calculated across three stages to known as bleaching, which is typically experienced by corals during integrate heat tolerance at different biological scales. Selective marine heatwaves if warming persists for extended periods of time breeding improved larval survival to heat by 1.5–2.5× but did not (Hoegh-Guldberg, 1999). Extreme bleaching (defined as >60% of result in substantial enhancement of settlement, although population bleached corals present within a reef) often leads to high mortality crosses were significantly different. Under heat stress, adults were (Hughes et al., 2017). At least four mass bleaching events have been less variable compared with larval responses in warmer reefs than in recorded on the GBR in the last century – 1998, 2002, 2016 and the cooler reef. Adults and offspring also differed in their mean 2017 – generally increasing in intensity and severity (Hughes et al., population responses, likely underpinned by heat stress imposing 2017), and have led to large-scale losses in hard coral cover. Rates strong divergent selection on adults. These results have implications of decline vary by region, but overall, declines have rapidly for downstream selection during reproduction, evidenced by increased per year since 2006 (De’Ath et al., 2012), although there variability in a conserved heat tolerance response across offspring have been recent signs of recovery. Therefore, bleaching-related lineages. These results inform our ability to forecast the impacts of stress has not been equal across the GBR. This trend in decline is climate change on wild populations of corals and will aid in developing reflected globally, with SSTs continuing to exceed previously held novel conservation tools such as the assisted evolution of at-risk records (Heron et al., 2016), suggesting that corals may soon reach species. their physiological limits to cope with increasing heat (Matz et al., KEY WORDS: Thermal tolerance, Heritability, Coral reefs, Larvae, 2017). Settlement, Survival Changes in temperature have the potential to impact organismal life stages differently, in which larvae, recruitment stages and adults INTRODUCTION can all vary in their thresholds to climate extremes. Despite this, Ecosystems globally continue to suffer from the adverse effects of information on larval physiological responses is relatively scarce for anthropogenic climate change. Coral reef ecosystems in particular coral early life-history stages (McLachlan et al., 2020), with 95% of are declining as sea surface temperatures (SSTs) have risen studies focusing on adult responses and only 2% and 1% on pre- dramatically (Hughes et al., 2017). Coral reefs are some of the settled and settlement stages, respectively (McLachlan et al., 2020). most productive habitats in the world, and the continued loss of In corals, sexually mature adults release their gametes within a corals will likely impact reef community dynamics by changing temperature range of ∼28–30°C, with larvae of some coral species able to survive 2–5°C above this range (Heyward and Negri, 2010). 1 2 However, coral spawning typically occurs during the warmest Australian Institute of Marine Science, Townsville 4810, Australia. Ghent University, Sint-Pietersnieuwstraat 33, 9000 Gent, Belgium. University of Plymouth, summer months, when spikes in SSTs are most likely to occur Plymouth PL4 8AA, UK. Liverpool John Moores University, Liverpool L3 3AF, UK. (Keith et al., 2016). As a result, coral larvae are potentially subjected *These authors contributed equally to this work. to much greater temperatures, increasing their risk of mortality and Author for correspondence (katemarie.quigley@my.jcu.edu.au) reducing recruitment and settlement success (Heyward and Negri, 2010). After fertilization, larvae undergo a pre-competency period R.B.C., 0000-0002-2827-9037; J.L.R., 0000-0003-1467-2505; K.M.Q., 0000- where they develop and disperse throughout the water column. Once 0001-5558-1904 competent, they respond to biophysical and chemical cues to find Received 31 August 2021; Accepted 7 January 2022 optimal settling conditions (Doropoulos et al., 2018). Increased Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 SSTs may induce premature metamorphosis by increasing larval coefficients (S; relative fitness of a phenotypic trait; van metabolic activity and development rates, thereby reducing pre- Tienderen and de Jong, 1994) and responses to selection (R)are competency periods (Heyward and Negri, 2010). This reduction commonly measured (Falconer and Makcay, 1996), and allow may influence subsequent larval dispersal distances and result in for the prediction of organismal responses to future stressors. Thus settlement occurring in suboptimal conditions (Edmunds et al., far, models incorporating the ‘breeder’s equation’ (R=h S; used 2001). Therefore, it is important to incorporate these responses into to predict the effect of selection pressures on phenotypic traits; predictive models of ecosystem change given their flow-on effects Falconer and Makcay, 1996) have demonstrated the utility of into key demographic and population-level dynamics. evolutionary modelling for predicting the potential of the coral’s Reductions in dispersal distances have the potential to reduce algal symbionts to confer increased survival (Quigley et al., 2018). gene flow between populations, thereby potentially accelerating Similarly, these principles can be applied to the coral host to population differentiation and local adaption (Bassim et al., 2002). quantify adaptive potentials, and this information can then be used Additionally, some coral populations rely on the dispersal of larvae to harness the existing adaptive potential of coral populations for from neighbouring populations for the addition of beneficial genes/ conservation and restoration. genotypes to their gene pool (Munday et al., 2009; Quigley et al., Coral restoration interventions, some of which can be classified 2019a,b). With reduced reef connectivity, populations may not be as assisted evolution methods (van Oppen et al., 2015), aim to able to adapt rapidly enough to cope with increasing SSTs (Quigley increase the adaptive potential of organisms by targeting the et al., 2019a,b) or may struggle to regenerate following mass acceleration of adaptation through various genetic mechanisms. bleaching and mortality (Bassim et al., 2002). There is scope for This may include selective breeding for the selection of desirable adaptation of corals to increasing temperatures (Matz et al., 2018 phenotypes (van Oppen et al., 2015). Assisted gene flow (AGF) is preprint). Further, the temperatures at which bleaching is occurring one intervention that utilises the differing thermal tolerances of have increased by ∼0.5°C from 1998 to 2017, suggesting an parental colonies combined with the intentional movement of increase in more heat-adapted genotypes within coral populations recombinant offspring, by reproductively mixing populations (Sully et al., 2019), potentially through processes such as selective sourced from different environmental conditions within the same sweeps (Quigley et al., 2019a). Locally, there have been reported species to prepare future populations for increased warming. This increases in coral cover in both the central and southern GBR strategy is based on increasing gene flow between populations that (AIMS, 2020), with evidence to suggest that warm or ‘extreme’ have a beneficial phenotype(s) with those of a target population(s) habitats harbour an increased number of thermally adapted via the creation and subsequent introduction of new, more resilient genotypes with the potential to transmit heat tolerance (Quigley genotypes (Aitken and Whitlock, 2013). Selective breeding through et al., 2020a; Schoepf et al., 2019). However, current estimates the outcrossing of gametes between these reefs of distinct thermal of the rate of SST increases suggest that these increases may exceed profiles assumes that populations exposed to climates of similar the potential rate of fixation of beneficial genetic variants given temperatures to those predicted to arise from anthropogenic factors such as currents and reef topology (Quigley et al., 2019a). climate change may be better adapted, and therefore better able to Additionally, the annual increase in SSTs has extended the period cope with warming (Aitken and Whitlock, 2013). The inheritance at which ‘summer’ temperatures occur, further increasing global of these beneficial alleles by populations that evolutionarily bleaching by reducing potential recovery periods that occur when have experienced cooler temperatures should increase the overall coral populations are exposed to cooler ‘winter’ temperatures resilience of the species via increased population persistence (Heron et al., 2016). Taken together, this suggests that corals (National Academies of Science, Engineering and Medicine, 2019). adaptive potentials may be constrained. Here, we compare the physiological responses at heat stress of Variability in heat tolerance has been observed at many selectively bred Acropora tenuis larvae and newly settled recruits ecological levels, including between individuals, populations and and their parental colonies collected from three sites along the GBR species, and by reef region. For example, differences in the heat – Davies and Esk reefs (central GBR) and in the Keppels (southern tolerance of corals have been observed across reefs globally, with GBR). Percentage larval survival and settlement were recorded coral populations present along the Persian Gulf having following exposure to control (27°C) and heat stress conditions demonstrated very high thermal thresholds of ∼4°C above mean (35.5–36°C) and compared with adult heat tolerance at 31°C, as monthly temperatures (Kirk et al., 2018; Moghaddam et al., 2021; measured in bleaching score, photosynthetic quantum yields, Savary et al., 2021). Northern GBR corals also demonstrate higher percent necrosis and percent survival. We then compared these thermal tolerances compared with some central populations, in responses across multiple biological scales (individuals to which heritable host genetic mechanisms play a role in the thermal populations) to calculate adaptive potentials from data collected resistance of these northern corals (Dixon et al., 2015; Quigley et al., across each life stage. This work contributes to our understanding of 2020a). This geographically distinct habitat could contribute to sub- the potential to increase heat tolerance and for the transgenerational speciation, seen in other marine invertebrates that have different transfer of this tolerance using the selective breeding of coral thermal thresholds, such as subspecies of Crassostrea gigas populations sourced from along the GBR. (Ghaffari et al., 2019). However, it is less clear whether similar patterns in heat tolerance at the individual coral genotype scale to MATERIALS AND METHODS the population level, a trend seen in fish, but not insect or plant Coral colony collections species collected across temperature clines (Payne et al., 2021; Reproductively mature Acropora tenuis (Dana 1846) colonies were Rezende and Bozinovic, 2019). Quantifying the scaling of heat collected in early November 2019 from three sites on the GBR tolerance across different biological levels will therefore help to in Australia, including two central reefs: Davies (18°49.881′S, elucidate the baseline potential of wild populations to adapt. 147°37.953′E) and Esk (18°45.892′S, 146°31.219′E), and one Further, evolutionary models incorporating metrics such narrow- southern reef in the Keppel islands (23°03.856′S, 150°57.095′E). sense heritability (h ; the phenotypic traits within an organism that Corals were identified as individual genotypes during collection by arise from allele inheritance; Evans et al., 2018), selection removing colonies approximately 10 m apart if possible. Genotypes Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 were not known a priori. Esk was on average the warmest reef surface of the water. The eggs and sperm in the bundles were location, followed by Davies and then the Keppels (Fig. 1A). The separated by washing for ∼2 min with FSW through a 120 μm mesh mean maximum monthly temperature of each site was as follows: filter. The number of sperm was quantified using a Computer Esk 26.26±2.30°C, Davies 26.06±1.80°C and the Keppels 24.62 Assisted Semen Analyser (CASA) (CEROS II software, Hamilton ±2.30°C (Fig. 1B). Corals were transported by boat to the National Thorne). The sperm from one parental colony was then added at a Sea Simulator at the Australian Institute of Marine Science (AIMS). concentration of 10 ml to isolated and cleaned eggs from another Colonies from each location were acclimated in separate outdoor parental colony to create 85 distinct coral families with at least one holding tanks under constant 0.2 μm filtered seawater (FSW) flow- cross per family. These families, observed as biological replicates, through conditions, maintained at 27.5°C. comprise intrapopulation (within the same reef) and interpopulation Corals were collected under the following permit numbers: G12/ (between different reefs) crosses, where both are generally 35236.1 (Davies and Esk) and G19/43024.1 (Keppels) to AIMS. referred to here as population crosses (Table S2). Population-level crosses are referred to as follows, with the maternal colony first Coral spawning, selective breeding and larval rearing and then paternal colony, in which intrapopulation crosses (filled Prior to spawning, each A. tenuis colony was given an identification circles; Fig. 1D) are Esk×Esk (E×E), Keppels×Keppels (K×K) number (Table S1). Individual colonies were isolated in separate and Davies×Davies (D×D), and interpopulation crosses are bins following signs of spawning imminence, which includes the Esk×Keppels (E×K), Esk×Davies (E×D), Keppels×Esk (K×E), appearance of egg–sperm bundles under the oral disc and ‘setting’ Keppels×Davies (K×D), Davies×Esk (D×E) and Davies×Keppels (polyps extended but tentacles retracted). Gamete bundles were (D×K) (open circles; Fig. 1D). released between 19:00 and 19:40 h on 17 and 18 November 2019 Eggs were allowed to fertilize in separate bowls per cross for 3 h. and collected by gently skimming and collecting the bundles off the Every hour, aliquots of developing embryos were taken from each Spawning Induce settlement Short-term Short-term Longer-term heat stress heat stress heat stress Northern Cross F0 Control Heat F1 Esk Central EskEsk (EE) Davies EskDavies (ED) EskKeppels (EK) Keppels DaviesDavies (DD) Southern DaviesEsk (DE) DaviesKeppels (DK) 142 147 KeppelsKeppels (KK) KeppelsDavies (KD) Longitude (°E) KeppelsEsk (KE) B E Short-term heat stress 27.5 Long-term heat stress 25.0 32 Life stage 22.5 Adult Larvae Settlers 13 5 7 9 11 0 48 96 144 192 240 288 Month Hours Fig. 1. Summary of experimental design for determining variability in heat tolerance across coral life-history stages. (A) Map of the Great Barrier Reef showing the three sites of adult coral collection, including Esk (central inshore, black), Davies (central mid-shelf, tan) and Keppels (southern inshore, maroon) reefs. (B) Mean±s.e.m. monthly sea surface temperature profiles at each reef of adult coral collection. (C) A graphic showing when the three life stages were experimentally exposed to heat stress. (D) Summary of the population crosses produced from the three coral populations. Filled circles represent intrapopulation crosses, whereas open circles represent interpopulation crosses. (E) Figure indicating the ramping time to reach each experimental temperature and the length that it was held. Circles indicate the end of each experimental time point, triangles indicate specific points along the experimental time frame. Mean sea surface temperature (°C) Latitude (°S) 20 13 Temperature (°C) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 family (hereafter referred to as crosses; Fig. 1C,D) to visually population (28 fragments). Each fragment was glued to a calcium inspect fertilization success and cell division under magnification. carbonate plug using epoxy glue, placed into PVC plug holders Once fertilization was confirmed, embryos from each separate bowl (hereafter referred to as ‘sticks’), and allowed to acclimate at control were transferred into separate 15 l constant flow-through conical temperatures before being moved to the experimental tanks. This tanks with 0.2 μm FSW in a temperature-controlled room, such that acclimation period attempts to minimize the effects of sampling and the temperature of each cone was maintained at 27.5°C, P handling before the experiment, where all coral fragments were CO 400±60 ppm, ambient light (i.e. non-photosynthetic) and salinity allowed to acclimate and recover from the fragmentation stress for of 35 psu. Each flow-through cross culture had an outflow covered 1 week. During this time and over the course of the experiments, −1 by a 10 μm filter and air curtain of bubbles to prevent larvae from corals were fed once a day at 16:30 h with 0.5 individuals ml 6 −1 collecting on the outflow filter. By days three and four post- of Artemia sp. and 5×10 cells ml of microalgae (Parmelia fertilization, larvae were ciliated and motile, consistent with the sulcata, T-ISO, Chaetoceros muelleri, Nannochloropsis oceania 96-h stage of larval development. and Dunnaiella sp.). Upon moving the corals to experimental tanks, the position Larval heat stress experiment of each fragment was randomized in each tank and across all Thirty larvae from each of the total 85 crosses were placed into tank replicates, such that each tank included all genotypes from floating net-wells (n=3 replicates per cross, per temperature), each population. Experimental tanks were filled with 45 l of separated into two holding tanks. One tank was set at 27.5°C either 27.5°C FSW (control treatment) or 31°C (heat treatment) (control treatment) and the other at 35.5°C (heat treatment). To on constant flow through. Each tank was equipped with a tungsten achieve the latter temperature, the heat treatment was ramped up to aquarium pump for constant aeration and mixture of water. The 35.5°C in hourly increments of 0.5°C from 27.5°C. Once 35.5°C heat treatment corals were ramped at a rate of 0.5°C until 31°C was reached, survival assessments began, in which the number of was reached. The light cycle for each tank was set on a 12 h:12 h larvae that were alive in each net-well (of the total n=30) was regime and light intensity at 171 PAR. Sunrise was set at 09:00 h. counted twice daily to estimate survival rates per replicate. The final The coral fragments underwent these experimental conditions for survival counts occurred once ∼50% of the larval crosses reached 14 days. Temperatures for larval and adult experiments were chosen 50% survival in the heat treatment. No crosses perished post- in order to compare with previous work performed in the same fertilization before sampling took place. region using a cross design (Dixon et al., 2015; Quigley et al., 2020b). Settlement experiment Coral colour, as a proxy for bleaching, was determined using an Following the larval heat stress experiment, only 69 of the original underwater colour reference card for corals (Coral Watch Card; 85 crosses contained sufficient larval stock for further experimental Siebeck et al., 2008). Images with the bleaching card were taken at use. These 69 crosses were used to investigate the effect of thermal least twice a week, and fragments were compared with the brown hue stress on larval settlement behaviour. Larvae were sampled 35 days (D1–D6). ImageJ (Schneider et al., 2012) was used to create a red- after spawning and transferred to sterile six-well plates (n=10 larvae green-blue standard curve for coral colour as per Quigley et al. per well, n=3 wells per cross, n=2 crosses per sterile six-well plate) (2019b). Survival of each coral fragment was noted from the images. containing 10 ml of 0.2 μm FSW. Crustose coralline algae (CCA) Coral death was determined by the presence of microalgae growing rubble was freshly cut into 3×3 mm chips using bone cutters, and on the bare skeleton on each coral fragment. The percentage of tissue placed into each well to induce larval settlement. The plates were necrosis was measured using the ImageJ surface area tool (Schneider then placed into plastic bags and sealed to prevent evaporation and et al., 2012) per coral fragment, per tank, per time point. Effective replicates were transferred into two incubators with photosynthetic quantum yield of photosystem II (ΔF/F ′), which is the efficiency of lights (12 h:12 h light:dark cycle, 170 to 180 PAR, Steridium E-500 photon absorption, was measured using the Diving PAM (DIVING- Sylvania FHO24W/T5/865 and Innova 4230, Sylvania F15W/865) PAM-II, Walz, Germany). PAM measurements of each fragment set at 27–27.5°C (control treatment) and 35.5–36°C (heat were taken twice per week at 10:00 h. treatment). The number of settled larvae (metamorphosed and attached to substrate) were counted for 17, 24 and 48 h after Statistical analysis incubation at both temperatures and the number of settled larvae was Larval heat stress experiment recorded. Settled larvae were defined as those that were attached to After testing data normality and homogeneity of variance with a the well or CCA and deposited a basal plate that was visible after diagnostic plot using the ‘stats’ package in R (https://www.r-project. metamorphosis. This was distinguished from only metamorphosed org/), differences in larval survival were assessed using the non- larvae (metamorphosis but no attachment to substrate), which were parametric ‘wilcox.test’ from the package ‘ggpubr’ (https://rpkgs. excluded from this analysis. datanovia.com/ggpubr/) to determine whether the mean values between each cross at the control and heat treatments (two Adult heat stress experiment independent groups) were statistically different. Colonies were kept in outdoor aquaria under the following conditions before fragmentation: 0.2 μm FSW, 27.5°C, P Settlement experiment CO 400±60 ppm and salinity of 35 psu. Three to five colonies The percentage of settled larvae was first analyzed using the base representing different genotypes for each population were ‘stats’ package in R to test the normality and homogeneity of the fragmented using a ‘diamond-tipped’ bandsaw (Table S1). Each values. If non-normal distributions were present, as shown through colony was divided into a minimum of six fragments, with each diagnostic plots, the random factors cross and plate were tested for placed into one of six experimental tanks (n=3 control tanks, n=3 their contribution to variability in settlement. The ‘ggpubr’ package heat tanks). For the Davies population, five genotypes were selected and Wilcoxon’s test (Wilcoxon, 1945) were used to statistically and cut into 82 fragments; three genotypes were used from the Esk compare the median percentage settlement between heat and control population (51 fragments); and three genotypes from the Keppels treatments at each time point. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Adult heat stress experiment previously been used in corals (Quigley et al., 2018) to determine Several coral traits have been identified as important biometrics for how selection on host–symbiont communities may adapt to restoration, including partial mortality and bleaching, in conjunction different selective environments. Contour plots of R, S and h for with classic response measurements such as survival (Baums et al., larvae grouped by maternal reef of origin were made using the 2019). Here bleaching was assessed by the change in colour from packages ‘plotly’ (https://plotly.com/) and ‘dplyr’ (https://CRAN.R- photographs, where a 6 is indicative of a non-bleached, healthy project.org/package=dplyr). fragment and a 0 is a white, bleached fragment. Differences in the photophysiological responses of the algal symbionts within corals RESULTS (ΔF/F ′), percent necrosis, bleaching and survival were evaluated Differences in larval survival under heat stress with respect to temperature treatment and coral population using The influence of both temperature treatments on larval survival linear models implemented in the ‘lme4’ package (Bates, 2005). varied by population cross, where larval lineages produced from The metric ΔF/F ′ and the percentage of necrosis were treated as Davies, Esk and Keppels corals displayed both high and low continuous variables, and temperature treatment and population were survival under control and heat conditions (Fig. 2A). Both median set as fixed factors in each model. Replicate tank and fragment holder survival and the variance around the median under both control and (‘sticks’) were set as random effects. These factors were not heat conditions varied across the lineages. Overall, the Keppels significant and were dropped from the final model (Table S3). intrapopulation larvae survived better under heat stress compared Once these random effects were dropped, the linear model was re- with interpopulation larvae, whilst the Esk and Davies larvae fitted, and all model assumptions were checked (linearity, normality survived better when crossed with either of the other two reefs as and homogeneity) using diagnostic plots in the ‘stats’ package in R. interpopulation crosses. The overall ‘winners’ under heat stress were Finally, a negative binomial generalized linear model (nbGLM) Davies×Esk and Esk×Keppels (which also had high survival under was used for bleaching, a linear model (LM) for ΔF/F ′ and control conditions), while the overall losers were Keppels×Davies the percentage of necrosis, and a generalized linear model (GLM) for and Keppels×Esk. survival. The function ‘anova’ was used to calculate model P-values, When scaled to the population level, Davies purebred larvae used for interpreting the significant difference among means. Post and larvae produced using Davies eggs were only significantly hoc pairwise comparisons of population and temperature treatment different in their survival between temperatures for one of the three −2 −2 were then run on the model outputs (Tukey, 1977). For survival data, lineages (P=0.71, 2.50×10 and 9.70×10 for Davies×Davies, a binomial distribution was used to determine whether there was a Davies×Esk and Davies×Keppels, respectively; Fig. 2B). Survival significant difference between populations and temperature under heat stress was higher in the interpopulation crosses compared treatments. For each trait, the statistical difference in median values with the purebred larvae (median survival: Davies×Davies= was assessed using the packages ‘ggplot2’ and ‘plyr’ (Wickham, 56.70%, Davies×Esk=75.00%, Davies×Keppels=62.30%). Again, 2011, 2016), with Wilcoxon’s test (Wilcoxon, 1945). The percentage Esk purebred larvae and larvae produced with Esk eggs were only change of adult responses by latitude was calculated and plotted using significantly different in their survival between temperatures for one the ‘ggplot2’ package (Wickham, 2016). All analyses were carried of the three lineages, with the same (reciprocal) cross significantly −2 out using RStudio, version 2.13.2 (https://www.rstudio.com/). different (P=4.10×10 , 0.53 and 0.29 for Esk×Davies, Esk×Esk and Esk×Keppels, respectively). Survival under heat stress was Predicting response gradients to selection higher for interpopulation crosses than for the purebred Esk larvae The density plots of adult and larval survival were made using the (Esk×Davies=61.70%, Esk×Esk=38.30%, Esk×Keppels=81.70%). ‘ggplot2’ package (Wickham, 2016). The breeder’s equation, The variation in median survival was also greater in purebred Esk R=h S (Falconer and Makcay, 1996) was used to calculate larvae compared with other lineages under heat stress. Keppels expected responses to selection (R) and potential constraints to the interpopulation larvae all had lower survival under heat stress −2 −8 evolution of heat tolerance given selective potentials (S) and compared with control conditions (P=3.20×10 and 2.30×10 for narrow-sense heritability for this trait (h ). This approach has Keppels×Davies and Keppels×Esk) whilst purebred larvae had A B DD DE DK ED EE EK KD KE KK * *** 50 100 150 27.5 35.5 Temperature (°C) Cumulative hours Fig. 2. Larval survival under experimental stress. (A) Percentage survival over time of n=85 different Acropora tenuis lineages of larvae exposed to control (27.5°C) and heat stress (35.5°C) conditions. (B) Percentage survival of each cross type over time of larvae exposed to control (27.5°C) and heat stress (35.5°C) conditions. The coloured boxes around the crosses indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Wilcoxon’s test was performed to analyze statistical differences. Survival (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 approximately equal survival between treatments (P=0.61). Median There was a significant effect of temperature treatment −16 survival was lower in the interpopulation larvae compared with (F =887.50, P=2.00×10 ) and heat exposure time 1,1633 −16 purebred larvae (Keppels×Keppels=65.60%, (F =120.70, P=2.00×10 ) on the percentage of settled 3,1631 Keppels×Davies=46.70%, Keppels×Esk=36.70%). larvae, and a significant interaction between temperature treatment −16 and heat exposure time (F =225.42, P=2.00×10 ), 3,1564 Influence of heat stress on larval settlement rates temperature treatment and population-level cross (F =5.30, 8,1564 −6 When exposed to heat, larvae from all crosses significantly P=1.39×10 ). There was no significant interaction between heat decreased in their settlement behaviour (attachment and exposure time and population-level cross (F =1.06, 24,1564 −1 metamorphosis) relative to the control temperature after 17 h P=3.86×10 ), or between temperature treatment, heat exposure −16 −1 (F =370.30, P=2.00×10 ), 24 h (6/9 crosses with 0.00% time and population-level cross (F =1.26, P=1.82×10 ). This 1,433 23,1564 −16 settlement; F =916.50, P=2.00×10 ) and 48 h (7/9 crosses significant decrease in settlement at 24 and 48 h occurred regardless 1,430 −16 −1 with 0.00% settlement; F =722.80, P=2.00×10 ; Fig. 3A–C). of larval cross (F =1.583, P=1.25×10 ) or reef of origin of the 1,334 8,1626 −1 Specifically, larvae from all crosses settled significantly less under maternal coral (F =0.133, P=8.75×10 ). 2,1632 heat stress compared with at control temperatures, regardless of After 17 h of temperature incubation, a median value of 0.00% population cross (P-values in Table S4). After 48 h, Davies×Esk larval settlement was observed for all crosses in the heat treatment and Esk×Keppels showed the greatest percentage settlement and was significantly lower compared with the control temperature compared with the other crosses (median settlement under heat (median 70.00%) across all larval lineages (Wilcoxon’s test, stress=both 0.00%, control=60.00 and 75.00%, respectively), in P<0.001; Fig. 3A). In the control treatment, the median settlement which Davies×Esk was the only population cross that settled under percentage was highest in Davies×Davies and Keppels×Esk, with a heat stress (upper quartile=100%). median of 80.00% settlement. The lowest median percentage Fig. 3. Larval settlement under experimental stress. DD DE DK ED EE EK KD KE KK Percentage of median settlement of n=69 different 100 Acropora tenuis lineages of larvae exposed to control * * * * * * * * * (blue) or heat stress (red) conditions after (A) 17 h, (B) 24 h and (C) 48 h. The coloured boxes around the crosses 75 indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Wilcoxon’s test was performed to analyze statistical differences. DD DE DK ED EE EK KD KE KK * * * * * * * * * DD DE DK ED EE EK KD KE KK * * * * * * * * 27.5 35.5 Temperature (°C) Settlement (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 settlement was observed in the Esk×Esk crosses, with 55.50% Photophysiology settlement and correspondingly high variability. The photophysiological responses, as measured by effective quantum After 24 h (Fig. 3B), median settlement remained at 70.00% in yield (ΔF/F ′), showed that ΔF/F ′ significantly decreased in the m m −15 the control treatment and ∼0.00% in the heat treatment, with all heat compared with the control treatment (LM, P=1.09×10 ; crosses settling significantly less under heat stress compared Fig. 4B) and showed significant differences between population −8 with controls (Wilcoxon’s test, P<0.001). At this time point, origins (LM, P=6.80×10 ). Davies and Keppels fragments reported Keppels×Davies showed the highest percentage settlement the highest yields at control temperatures (mean and median ΔF/F ′ (80.00%) under control conditions, followed by Davies×Keppels >0.60), whereas Esk fragments were extremely low (0.12±0.05, and Keppels×Keppels (75.00%). Finally, after 48 h (Fig. 3C), the median=0.00). Mean ΔF/F ′ recorded from fragments in the median settlement percentage in the control treatment dropped heat treatment was highest in Keppels (0.22±0.07, median=0.00), slightly to 66.67%, where Keppels×Davies crosses again exhibited Davies (0.20±0.05, median=0.00) and finally Esk (0.12±0.05, the highest larval settlement (75.00%). At heat, the only cross to median=0.00), but very variable overall. Relative to control settle included some of the larval replicates within Davies×Esk. temperatures, pairwise comparisons showed that Keppels and Finally, some crosses also experienced mortality of recent recruits Davies fragments showed a significant decrease in ΔF/F ′ −4 from 17 to 48 h (Davies×Davies), whilst others continued to settle (Tukey’stest, P<0.001 for both, median Wilcoxon=2.40×10 for −12 (Davies×Esk). Keppels and P=2.70×10 for Davies), but not Esk (Tukey’stest, −1 P=5.68×10 , median Wilcoxon=0.16). Adult responses to heat stress at the population level Bleaching Necrosis There is a significant effect of temperature treatment (nbGLM, Partial mortality was assessed as the percentage of necrotic tissue −16 P=2.20×10 ; Fig. 4A) and population origin (nbGLM, relative to each fragment (Fig. 4C). Population origin had a −15 −8 P=3.56×10 ) on the median bleaching score of coral fragments significant effect on percentage necrosis (LM, P=3.25×10 ). There after 16 days. At the control temperature, fragments sourced from was no significant difference in percentage necrosis owing −2 Davies recorded a mean colour score of 4.75±0.14 (mean±s.e.m.; to temperature treatment (LM, P=5.50×10 ). At the control median=5.00) whilst fragments sourced from Keppels and Esk temperature, Davies and Keppels corals showed very little to scored 1.85±0.25 (median=2.00) and 1.35±0.39 (median=0.00), no necrosis (median=0.00%), whereas Esk fragments were slightly respectively (Fig. 4A). At heat, fragments from all populations necrotic (median 15.29%). After 16 days under heat stress, bleached heavily (all mean scores <1.00, all median=0.00). Relative fragments sourced from Esk lost, on average, approximately to the control temperature, Davies fragments bleached the most 46.04±9.89% (median=0.00%) of their tissue per fragment, (median percentage change 87.41±5.32% decrease in bleaching compared with 26.67±11.82% (median=0.00%) and 12.20±5.17% category), followed by Keppels (62.96±37.04%) and then Esk (median=0.00%) for Keppels and Davies fragments, respectively. fragments (28.57±28.57%). Pairwise comparisons showed that Pairwise comparisons showed that at the control temperature, Esk bleaching score was significantly different between the control lost, on average, significantly more tissue compared with Davies −3 and heat treatments in Davies (mean Tukey’s test, P<0.001, median (Tukey’s test, P<0.001) and Keppels (Tukey’s test, P=3.39×10 ). −15 Wilcoxon P=1.80×10 ), Esk (Tukey’s test, P<0.001, In the heat treatment, Esk also experienced significantly more −3 median Wilcoxon P=0.11) and Keppels (Tukey’s test, P=0.15, necrosis compared with Davies (Tukey’s test, P=2.76×10 ). −2 median Wilcoxon P=1.50×10 ). All other population comparisons were not significantly different A B E Davies Esk Keppels Davies Esk Keppels 6 * * Bleaching 0.8 ** Necrosis Survival 0.6 ΔF/F ’ 0.4 0.2 C Davies Esk Keppels D Davies Esk Keppels −100 ** 1.0 −19 −23 Latitude 0.8 Esk Davies Keppels 0.6 27.5 31 Temperature (°C) 0.4 0.2 Temperature (°C) Temperature (°C) Fig. 4. Median physiological responses in adult corals exposed to control and heat stress temperatures. (A) Bleaching category core, (B) effective quantum yield, (C) percent necrosis and (D) and percent survival of adult Acropora tenuis fragments. Colonies were collected from three sites on the Great Barrier Reef: Davies reef (tan outlines in A–D), Esk reef (black) and Keppels reef (maroon). (E) Mean percent change in each response by latitude of collection site. Wilcoxon’s test was performed to analyze statistical differences. Necrosis (%) Bleaching category Survival (%) Effective quantum yield Change control to heat (%) Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 −1 (Esk–Keppels, Tukey’s test, P=5.35×10 , Keppels–Davies, had much lower survival at the control temperature, their overall −1 Tukey’s test, P=7.43×10 ). Relative to control temperatures, the mean survival under heat stress was roughly equivalent to Davies median percent necrosis was not significantly different for any and Keppels corals, suggesting that the selection differential for −2 population (median Wilcoxon P=0.09, 0.91 and 5.30×10 for survival under heat stress in each population was roughly equivalent Davies, Esk and Keppels). (horizontal dashed black lines), defined as the difference between a selected phenotypic scope (triangle) and the mean percent survival Survival under heat stress (vertical dashed red line). Temperature showed significant effect on survival (binomial LM, −8 P=1.19×10 ). In the control treatment, survivorship was highest Comparisons of purebred and hybrid larval responses to adults in the Keppels fragments (median=1.00), followed by Davies Mean percent survival for larvae in the control treatments were (median=1.00) and Esk (median=0.00) (Fig. 4D). In the heat higher (56.44–78.92%) compared with in the heat treatments treatment, survivorship was highest in the Keppels fragments (41.48–78.40%). As expected, based on similar adult selection (40.00±13.09%, median=0.00), followed by Davies (31.71±7.36%, differentials (Fig. 5A), purebred larvae from Davies×Davies, median=0.00) and Esk (20.00±8.16%, median=0.00). Compared Esk×Esk and Keppels×Keppels responded similarly in the between heat and control temperatures, fragments sourced breadth of larval responses between control and heat treatments from Davies and Keppels significantly decreased in survivorship (specifically, a small difference between treatment responses). In −3 (Tukey’s test, P<0.001 and 4.98×10 , median contrast, interpopulation crosses differed in their responses, in −3 −10 Wilcoxon=5.10×10 and 1.40×10 , respectively), but not Esk which Keppels×Davies and Keppels×Esk had the largest difference fragments, which also survived poorly at the control temperature in mean survival between control and heat treatments (17.14% and −1 treatment (Tukey’s test, P=7.55×10 , median Wilcoxon=0.25). 32.84%) and Esk×Keppels the smallest (0.52%). Given the significant population effect of temperature treatment for The Davies×Davies purebreds exhibited a 56.44±8.77% mean bleaching score, ΔF/F ′ and necrosis (but not survival), the relative survival in the heat treatment, almost 2× greater survival than the differences in responses were also calculated and compared across Davies adults in the heat treatment (31.71±7.36%; Fig. 5B). the latitudinal gradient of adult origin (Fig. 4D). There was no trend Davies×Davies larval crosses demonstrated high variability (e.g. in performance across traits by latitude. flat distribution) in responses in survival in the control treatment, compared with a unimodal response in the heat treatment, Predicting adult and offspring responses using gradients although the mean survival was roughly equal (56.44±8.77% and of selection 55.78±4.71%, respectively, dashed lines). When Davies eggs were Adult responses crossed with sperm from the other central reef, heat tolerance When survival was averaged at the population level, almost all adult increased by 14.46% compared with purebred larvae. Davies×Esk corals collected from Davies exhibited approximately equivalent larvae exhibited unimodal responses for both temperatures, with survival in the control treatment, but when exposed to the heat average survival at 78.38% in the control and 70.24% in the heat treatment, the population responses were generally bimodal, where treatment. Heat tolerance was slightly less for Davies×Keppels some individuals within each population exhibited high survival larvae (55.41%) compared with Davies×Davies, where larval and others low survival (Fig. 5A). Corals collected from Esk and the responses were more variable but still unimodal. Keppels also exhibited this bimodal response for both the heat and The Esk×Esk larval purebreds exhibited 48.29±7.15% survival control temperatures. For both Davies and Keppels corals, the mean in the heat treatment, ∼2.5× greater survival than the Esk adults in survival in the control treatment was high and there was a roughly the heat treatment (20.00±8.16%; Fig. 5C). Esk×Esk larvae in the equal decrease in survival in the heat treatment. Although Esk corals control and heat treatments were relatively flat, demonstrating that AB Davies Esk Keppels DD DE DK 2.5 2.0 1.5 ED EE EK 1.0 0.5 07 25 50 5 100 D KD KE KK Survival (%) 0.04 0.03 0.02 27.5 35.5 Temperature (°C) 0.01 0 25 50 75 100 Survival (%) Fig. 5. Percent survival density plot. Number of (A) adult colonies (y) from each population and (B–D) offspring of each population cross. Arrows indicate the selected phenotypic scope. Horizontal dashed lines represent the selection differential for each population. Vertical dashed lines represent mean percent survival per population. The coloured boxes around the crosses indicate the source reef of the maternal colonies: Davies (tan), Esk (black) and Keppels (maroon). Median survivorship data are presented in Figs 2 and 4D, whereas survival data here are presented as the number of individual colonies per percent survival value. Survival at ambient and hot are shown together to aid in comparison. Density Density Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 both treatments consisted of crosses with high and low survival. produced from selective breeding methods onto cooler reefs to When Esk eggs were crossed with sperm from the other reefs, heat prepare them for warming (Quigley et al., 2018), a method that will tolerance increased by 8.40% and 30.11% in Esk×Davies and also increase the genetic diversity on reefs – fuel for natural Esk×Keppels, respectively, relative to Esk×Esk. Esk×Davis larval selection. Quantifying the feasibility for enhancing corals’ ability to survival responses were weakly unimodal with mean survival at survive further ocean warming is therefore vital for the conservation 70.29±3.47% and 56.68±4.61% for the control and heat treatments. of the world’s coral reefs. Alternatively, Esk×Keppels larvae exhibited strongly unimodal survival responses at 78.92±3.82% in the control and 78.40±2.59% Little variation in adult physiological responses to heat in the heat treatment. stress across three GBR populations The Keppels×Keppels purebreds exhibited 62.84±3.73% Phenotypic variation in organisms drives the capacity for plastic, survival in the heat treatment, ∼1.5× greater survival than adaptive responses to environmental pressure. This variation may be the Keppels adults in the heat treatment (40.00±13.09%; underpinned by genetic variation or by responses mediated by non- Fig. 5D). Keppels×Keppels larval responses formed relatively flat genetic mechanisms, such as changes in the microbiome (e.g. distributions in the control treatment, and unimodal responses in bacteria or Symbiodiniaceae), in which algal symbiont assemblages the heat treatment. When Keppels eggs were crossed with sperm may shape corals’ responses to heat stress (Berkelmans and van from the other reefs, heat tolerance decreased by 17.59% and Oppen, 2006). Understanding the scope for phenotypic variation to 21.35% in Keppels×Davies and Keppels×Esk, respectively. In heat stress at the adult stage is essential to evaluating the scope for Keppels×Davies, the distribution was flat and wide, indicating heritable diversity of heat tolerance at later life stages in corals. variation in survival. The Keppels×Esk density plot for larvae in the Overall, there was no difference in heat tolerance of adult control treatment exhibited a unimodal peak in survival, as well as in southern Keppels corals compared with central Davies corals, with the heat treatment. both populations suffering similar percent necrosis, drops in In the heat stress treatment when larvae were grouped by the photophysiology and lowered survival under heat stress. However, population identity of the maternal colony, larvae produced from the relative change in bleaching was greater for Keppels corals. The Davies and Keppels had selective landscapes that were wider magnitude of bleaching was also more severe in Davies compared compared with Esk larvae (Fig. 6A–C, Table S5), although Davies with Keppels corals. Acropora tenuis in both central and southern and Esk had overall higher selection coefficients compared with reefs generally hosts dominant abundances of Cladocopium Keppels larvae (S=Esk: 60–70, Davies: 54–70, Keppels 50–60; (Rocker et al., 2017; Ulstrup and van Oppen, 2003), which could Fig. 6A–C). Combined with narrow-sense heritability estimates, contribute to the similarity in their physiological performance, these differences resulted in overall higher selective responses (R) whereas symbionts from Davies reef or the host corals themselves for Davies and Esk (i.e. more values of R between 40 and 60) may have lower initial tolerances but are able to recover and survive compared with Keppels offspring (Fig. 6D–F). equally well. Taken together, these results suggest that the absolute heat tolerance of both populations was roughly equal. Finally, adult DISCUSSION coral fragments from the Esk population in heat treatment exhibited Mean maximum SSTs are expected to increase by between 2 and the highest bleaching severity, lowest effective quantum yield, 4°C by 2100 globally (IPCC, 2014). Without adaptation, this will highest percentage necrosis and lowest survivorship. It should be likely exceed the thermal thresholds of corals. Understanding the noted that although fragments sourced from the Esk population lost underlying adaptive capacity of wild populations is therefore critical the greatest overall percentage of tissue (necrosis) per population, to forecasting species persistence. Moreover, various conservation fragments in the control treatment were also highly necrotic, strategies are being considered worldwide to help corals withstand suggesting a compromised health state of corals from this increasing ocean temperatures whilst carbon emissions are curbed population, also reflected in the low survival and photosynthesis (National Academies of Science, Engineering and Medicine, 2019). in controls. Both Davies and Esk corals were collected during the This includes the introduction of more heat-tolerant offspring sample trip with the same level of handling. This suggests that Fig. 6. Adaptive landscape of selection for heat 1.0 A D tolerance. (A–C) Modelled selective values (S) and narrow-sense heritability (h ) values of larval survival at 0.5 heat grouped by the population identity of the maternal colony. (D–F) Estimated response to selection (R) for 0.0 h and S. 1.00 B E 0.75 0.50 0.25 0.00 C F Response (R) 60 60 0.5 40 40 20 20 0.0 -20 −20 -0.5 50 55 60 65 70 50 55 60 65 70 Selection (S) Heritability (h ) Keppels Esk Davies Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 transport issues were not the cause of their diminished health state, during transition phases of larvae to polyp (Randall and Szmant, and instead point towards population-level differences between 2009). The reduction in larval settlement could also be a these corals. consequence of energy deficiency. As cellular proteins unfold and The high overall fitness of adult Keppels corals under heat stress aggregate, HSP70, a heat shock protein that refolds degrading was surprising. This population exhibited the lowest bleaching proteins (Daugaard et al., 2007), has been found to be upregulated in severity, highest effective quantum yield and highest survival at Acropora millepora larvae to maintain normal cell function heat. Although enhanced heat tolerance in corals is generally (Rodriguez-Lanetty et al., 2009). This requires vast amounts of attributed to corals from warmer reefs exhibiting higher upper energy, that could potentially otherwise be used for settlement. thermal thresholds (Berkelmans, 2002; Howells et al., 2012; Ulstrup Higher respiration rates at heat also increase metabolic activity et al., 2006), the enhanced performance of Keppels corals may be (Edmunds et al., 2001), which, in turn, increases the amount of food attributed to the greater variability in their local thermal regime. The required to maintain these elevated levels. These factors could all increase in tolerance may also partly be attributed to the coral contribute to the reduced settlement of larvae measured here. symbionts (Howells et al., 2012; Thomas et al., 2018; Ulstrup et al., Overall, although our results showed a significant decrease in 2006) and their interaction with host genetics (Dixon et al., 2015; larval settlement at heat compared with control temperatures, this Smith-Keune and van Oppen, 2006; Thomas et al., 2018), in which did not correspond to reef of origin. Our findings only weakly allude complex holobiont interactions influence the overall heat stress to the potential for selectively bred coral larvae to settle at higher responses via gene regulation, symbiont density control and temperatures. For example, while our results do not demonstrate a assemblage shuffling (Cunning and Baker, 2020; Yuyama et al., significant increase, there was a higher percentage of settled larvae 2018). Our results indicate that the control of heat tolerance is whose maternal colony was from either Esk or Davies (the central, complex and that many factors, including local thermal regime, warmer, inshore reefs in this study) compared with those with a likely play a role. maternal colony from the southern, cooler Keppels reef, in contrast to the adult’s response in which Keppels had higher overall fitness Minimal improvement in larval settlement owing to selection than Davies and Esk. Previous research suggests that mitochondrial for heat tolerance DNA plays a large role in the thermal resistance of corals, alluding Previous breeding experiments have demonstrated the transfer of to a high maternal effect on the heat tolerance of coral offspring increased offspring survival from parents sourced from warm reefs (Dixon et al., 2015; Quigley et al., 2020a,b). There was when reproductively crossed with cooler reefs (Dixon et al., 2015), also some variation in settlement within population crosses under or at least one parent from warmer reefs (Quigley et al., 2020b), heat, demonstrating the potential for plasticity. This aligns with suggesting a genetic contribution to offspring. In this study, larval information that corals found in warmer environments or with high survival was high in the crosses with a maternal colony sourced daily temperature variability have greater genetic plasticity (Kenkel from either Esk or Davies. However, it is currently unknown and Matz, 2017) which can be passed onto offspring. However, the whether an increased propensity for settlement at high temperatures findings presented here are preliminary and it appears that the is also transferable using colonies sourced from warmer reefs to maximum thermal limits of parental corals and the resulting larvae achieve an enhancement in settlement success. Although settlement are not indicative of settlement success. As warming increases in is a heritable trait under control conditions (h =0.49; Meyer et al., severity, these processes may be the first to be disrupted (Radchuk 2009), the overall heritability is low relative to other fitness-related et al., 2019), and assessing the impacts on these and other traits. Moreover, it is well known that settlement in corals is fundamental processes such as recruitment will become negatively impacted by heat. For example, early life-stage A. tenuis increasingly important. settlement decreased by 100% when exposed to >5°C above The lack of a significant effect of parental colony from warmer ambient temperature (Humanes et al., 2016), and by 55% when reefs to enhance settlement at high temperatures may be due to combined with a suspended sediments treatment (Humanes et al., either experimental or biological factors. The high temperatures 2017). Diploria strigosa larvae demonstrated a decrease in chosen here may have surpassed the corals’ settlement ability at settlement behaviour at temperatures exceeding 30°C compared these temperature limits, in which temperatures exceeding 35.5°C with controls (Bassim and Sammarco, 2003), and Acropora were extreme compared with the mean monthly maximums of these palmata settlement decreased by 25% at 31.5°C compared with sites (all 24–27°C), resembling short-term acute heat stress 28°C (Randall and Szmant, 2009). The lack of strong differences in temperature range (Grottoli et al., 2021; McLachlan et al., 2020). settlement success between the crosses here may be reflected in the However, the experimental temperature of many studies does not roughly equal heat tolerance of both Davies and Keppels corals, exceed >5°C above the control temperature (Humanes et al., 2016, suggesting that both populations are roughly equivalent in tolerance 2017; McLachlan et al., 2020; Quigley et al., 2020b). Hence, our and therefore did not produce strong differences in settlement of result could reflect the contribution of higher-than-threshold larvae. Combined, these previous results suggest that selection temperature treatment. Alternatively, environmental factors may should act on this important trait over time if oceans continue to contribute a greater influence in determining settlement compared warm. with host genetics. Specifically, settlement deficiency at high The results from this study show that, when exposed to heat, temperature may be driven by the disruption of the microbial larvae from all crosses significantly decreased in their settlement biofilm needed to induce metamorphosis and settlement, where it is behaviour relative to the control temperature. During periods of well established that settlement is induced by the presence of CCA increased temperature, the cellular processes within larvae become and microbial biofilms (Webster et al., 2004). During periods of compromised, including disruption in the repair of cellular proteins increased temperature, chemical cues released by CCA can be and enzymes (Negri et al., 2007). Consequently, abnormalities weakened and microbial cells present in biofilms (on the CCA) can develop, and the rate of cell cleavage rapidly increases. This cellular become damaged (Randall and Szmant, 2009). Therefore, this impairment could prevent the transition from larvae to recruit by reduction in biochemical cues could be the main contributing factor preventing attachment to substrata or an increase in hypersensitivity to the reduction in larval settlement at higher temperatures. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 In summary, larvae from intrapopulation and interpopulation responses suggests a divergence between adult population mean crosses demonstrated a general inability to settle at high (and responses and interpopulation offspring crosses. The divergence perhaps extreme) temperatures, suggesting that improvement in between thermal responses of different life stages within species has larval settlement responses owing to selection for heat tolerance been demonstrated in other marine organisms such as mollusks may be challenging as a result of the competing influence of (Truebano et al., 2018), in which larval forms are often more or less environmental effects. Combined with information that behavioural vulnerable to heat compared with adults. In this case, mollusks or morphological traits generally respond less to selection compared reported a 1.7–2.1× difference in responses between larvae and with life-history traits (Mousseau and Roff, 1987) and the potential adults, which mirrors reports seen in other invertebrates such as governing importance of environmental factors (e.g. bacterial brine shrimp (2.7–4.9×; Norouzitallab et al., 2014), copepods communities), this suggests that the enhancement of this trait (Tangwancharoen, 2014) and others (Pandori and Sorte, 2019). under heat stress may require the selection through microbial Finally, adaptive response surfaces revealed that when larvae were community contribution more than through processes targeting host grouped by their maternal populations, Davies and Esk offspring genetics. generally had higher selective responses (R) compared with Keppels offspring. Taken together, this suggests that although adult Predicting responses to heat using selection differentials populations may respond similarly to heat, the overall potential of and gradients of selection offspring responses to selection in warmer populations of corals Genetic variation underpins the potential and speed for adaptation from Davies and Esk is greater compared with cooler populations in through natural selection (Falconer and Makcay, 1996). Warming the Keppels. These findings have important implications for influences traits differentially, with morphological traits generally forecasting the impacts of climate change on wild populations of less impacted compared with phenological traits (Radchuk et al., corals and for the development of novel conservation tools such as 2019). Survival was chosen as the trait of interest to examine the assisted evolution of at-risk populations. gradients of selection given the importance of survival and other life- history traits compared with behavioural or morphological traits Conclusions (Mousseau and Roff, 1987). Measures such as narrow-sense As climate change accelerates ecosystem change, critical heritability (h ), selection (S) and responses to selection (R)are information on how important fitness traits will vary in the future useful for quantitatively assessing the ability of organisms to respond is essential to move from understanding impacts to predicting and to their environment, especially future stressors. Narrow-sense forecasting those impacts. This comparative physiological dataset heritability ranges from 0 to 1, where 0 is indicative of no genetic across different life-history stages in one important coral species contribution to trait variance and 1 is indicative of complete provides key mechanistic and adaptive insights into how corals may dominance of genetics in determining trait variance (Falconer and function under heat stress caused by warming oceans, and in Makcay, 1996). Measurements derived from corals suggest that they particular, the heritability of heat tolerance. do have a strong underlying capacity to respond adaptively to heat Acknowledgements either through host genetics (h =mean: 0.86, range: 0.48–0.93) The authors would like to thank the Traditional Owners from whose Sea Country (Dixon et al., 2015; Dziedzic et al., 2019; Kirk et al., 2018; Quigley these corals were collected. Specifically, we would like to thank the Woppaburra, et al., 2020b) or through changes to their symbiont communities Manbarra, Bindal and Wulgurukaba Traditional Owners. We would like to thank (Quigley et al., 2018). It might be expected that heat stress would Andrea Severati, Line Bay, Christine Giuliano and the spawning collections field crews in the central and southern Great Barrier Reef for assisting in the coral colony elicit directional selection through differential mortality of adults, collections. resulting in the survival of a subset of phenotypes at one end of the phenotypic distribution. However, the bimodal responses in survival Competing interests curves of adult corals suggest that heat stress manifests as disruptive The authors declare no competing or financial interests. selection, which may explain the high variability of heat responses across the numerous offspring lineages. Although the underlying Author contributions Conceptualization: K.M.Q.; Methodology: K.M.Q.; Formal analysis: P.W., K.M.Q.; mechanisms are unknown here, the drivers of bimodality may be Investigation: P.W., R.B.C., A.M., H.K., J.L.R., K.M.Q.; Resources: K.M.Q.; Data linked to biochemical complexity (Rezende and Bozinovic, 2019). curation: P.W., K.M.Q.; Writing - original draft: R.B.C., J.L.R., K.M.Q.; Writing - review Selection differentials depend on the heritability of the trait, & editing: P.W., R.B.C., A.M., K.M.Q.; Supervision: K.M.Q.; Funding acquisition: where heritability is generally equal to the slope of the response over K.M.Q. selection, as determined by the breeder’s equation. Interestingly, the Funding selection differentials at heat (i.e. the intensity of adaptive This study was supported by funding from the Australian Institute of Marine Science. responses) were similar across the three populations of adult P.W. was supported by a travel and study grant provided by Ghent University, corals. This mirrors the similar physiological responses of the adult Erasmus, Master of Science in Marine Biological Resources (IMBRSea). corals to heat stress. The selection differential between survival at high temperatures can be described as the difference between the Data availability Physiological data are available as datasets 1 and 2 in the supplementary material mean value measured (Davies and Keppels=∼40–30%, and Esk and analysis code from: https://github.com/LaserKate/KeppelsAGF19 ∼25%) and the desired mean value (e.g. ∼90%). Hence, the selection differential was approximately 55% (90 minus 35%) References across these three populations, which represents a desired 63% AIMS (2020). Initial recovery of the Great Barrier Reef threatened by the third mass increase in potential trait enhancement. When translated to bleaching event in five years. Australian Institute of Marine Science. Aitken, S. N. and Whitlock, M. C. (2013). Assisted gene flow to facilitate local estimated adaptive larval responses, these varied by population adaptation to climate change. Annu. Rev. Ecol. Evol. Syst. 44, 367-388. doi:10. cross. In comparison, responses to selection in well-studied systems 1146/annurev-ecolsys-110512-135747 such as aquaculture species, including fish and shellfish, averaged Bassim, K. and Sammarco, P. (2003). Effects of temperature and ammonium on ∼13% for growth and 4.9% for survival (Gjedrem and Rye, 2018). larval development and survivorship in a scleractinian coral (Diploria strigosa). Furthermore, our comparative analysis between adult and offspring Marine Biol. 142, 241-52. Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Bassim, K. M., Sammarco, A. P. W. and Snell, A. T. L. (2002). Effects of Jones, G. P., Mccormick, M. I., Srinivasan, M. and Eagle, J. V. (2004). Coral temperature on success of (self and non-self) fertilization and embryogenesis in decline threatens fish biodiversity in marine reserves. Popul. Biol. 101, Diploria strigosa (Cnidaria, Scleractinia). Mar. Biol. 140, 479-488. doi:10.1007/ 8251-8253. s00227-001-0722-4 Keith, S. A., Maynard, J. A., Edwards, A. J., Guest, J. R., Bauman, A. G., van Bates, D. (2005). Fitting linear mixed models in R. Obes. Facts 3, 27-30. Hooidonk, R., Heron, S. F., Berumen, M. L., Bouwmeester, J., Baums, I. B., Baker, A. C., Davies, S. W., Grottoli, A. G., Kenkel, C. D., Piromvaragorn, S. et al. (2016). Coral mass spawning predicted by rapid seasonal rise in ocean temperature. Proc. R. Soc. B Biol. Sci. 283, 20160011. Kitchen, S. A., Kuffner, I. B., LaJeunesse, T. C., Matz, M. V., Miller, M. W. et al. (2019). Considerations for maximizing the adaptive potential of restored coral Kenkel, C. D. and Matz, M. V. (2017). Gene expression plasticity as a mechanism of coral adaptation to a variable environment. Nat. Ecol. Evol. 1, 1-6. doi:10.1038/ populations in the western Atlantic. Ecol. Appl. 29, 1-23. doi:10.1002/eap.1978 s41559-016-0014 Berkelmans, R. (2002). Time-integrated thermal bleaching thresholds of reefs and Kirk, N. L., Howells, E. J., Abrego, D., Burt, J. A. and Meyer, E. (2018). Genomic their variation on the Great Barrier Reef. Mar. Ecol. Prog. Ser. 237, 309-310. and transcriptomic signals of thermal tolerance in heat-tolerant corals (Platygyra Berkelmans, R. and van Oppen, M. J. H. (2006). The role of zooxanthellae in the daedalea) of the Arabian/Persian Gulf. Mol. Ecol. 27, 5180-5194. doi:10.1111/ thermal tolerance of corals: a “nugget of hope” for coral reefs in an era of climate mec.14934 change. Proc. R. Soc. B Biol. Sci. 273, 2305-2312. doi:10.1098/rspb.2006.3567 Komyakova, V., Munday, P. L. and Jones, G. P. (2013). Relative importance of Cunning, R. and Baker, A. C. (2020). Thermotolerant coral symbionts modulate coral cover, habitat complexity and diversity in determining the structure of reef heat stress-responsive genes in their hosts. Mol. Ecol. 29, 2940-2950. doi:10. fish communities. PLoS One 8, e83178. doi:10.1371/journal.pone.0083178 1111/mec.15526 Matz, M. V., Treml, E. A., Aglyamova, G. V. and Bay, L. K. (2017). Potential and Daugaard, M., Rohde, M. and Jaattela,M. (2007). The heat shock protein 70 ̈ ̈ ̈ limits for rapid genetic adaptation to warming in a Great Barrier Reef coral. PLoS family: highly homologous proteins with overlapping and distinct functions. FEBS Genet. 14, e1007220. doi:10.1371/journal.pgen.1007220 Lett. 581, 3702-3710. doi:10.1016/j.febslet.2007.05.039 Matz, M. V., Treml, E. A., Aglyamova, G. V., van Oppen, M. J. H. and Bay, L. K. De’Ath, G., Fabricius, K. E., Sweatman, H. and Puotinen, M. (2012). The 27-year (2018). Potential for rapid genetic adaptation to warming in a Great Barrier Reef decline of coral cover on the Great Barrier Reef and its causes. Proc. Natl. Acad. coral. bioRxiv. doi:10.1101/2021.10.06.463349 Sci. USA 109, 17995-17999. doi:10.1073/pnas.1208909109 McLachlan, R. H., Price, J. T., Solomon, S. L. and Grottoli, A. G. (2020). Thirty Dixon, G., Davies, S., Aglyamova, G., Meyer, E., Bay, L. and Matz, M. (2015). years of coral heat-stress experiments: a review of methods. Coral Reefs 39, Genomic determinants of coral heat tolerance across latitudes. Science 348, 885-902. doi:10.1007/s00338-020-01931-9 1460-1462. doi:10.1126/science.1261224 Meyer, E., Davies, S., Wang, S., Willis, B. L., Abrego, D., Juenger, T. E. and Doropoulos, C., Gómez-Lemos, L. A. and Babcock, R. C. (2018). Exploring Matz, M. V. (2009). Genetic variation in responses to a settlement cue and variable patterns of density-dependent larval settlement among corals with elevated temperature in the reef-building coral Acropora millepora. Mar. Ecol. distinct and shared functional traits. Coral Reefs 37, 25-29. doi:10.1007/s00338- Prog. Ser. 392, 81-92. doi:10.3354/meps08208 017-1629-y Moghaddam, S., Shokri, M. R. and Tohidfar, M. (2021). The enhanced expression Dziedzic, K. E., Elder, H., Tavalire, H. and Meyer, E. (2019). Heritable variation in of heat stress-related genes in scleractinian coral ‘Porites harrisoni’ during warm bleaching responses and its functional genomic basis in reef-building corals episodes as an intrinsic mechanism for adaptation in ‘the Persian Gulf’. Coral (Orbicella faveolata). Mol. Ecol. 28, 2238-2253. doi:10.1111/mec.15081 Reefs 40, 1013-1028. Edmunds, P., Gates, R. and Gleason, D. (2001). The biology of larvae from the reef Mousseau, T. A. and Roff, D. A. (1987). Natural selection and the heritability of coral Porites astreoides, and their response to temperature disturbances. Mar. fitness components. Heredity 59, 181-197. doi:10.1038/hdy.1987.113 Biol. 139, 981-989. doi:10.1007/s002270100634 Munday, P. L., Leis, J. M., Lough, J. M., Paris, C. B., Kingsford, M. J., Evans, L. M., Tahmasbi, R., Jones, M., Vrieze, S. I., Abecasis, G. R., Das, S., Berumen, M. L. and Lambrechts, J. (2009). Climate change and coral reef Bjelland, D. W., de Candia, T. R., Yang, J., Goddard, M. E. et al. (2018). Narrow- connectivity. Coral Reefs 28, 379-395. doi:10.1007/s00338-008-0461-9 sense heritability estimation of complex traits using identity-by-descent National Academies of Sciences, Engineering, and Medicine (2019). A research information. Heredity 121, 616-630. doi:10.1038/s41437-018-0067-0 review of interventions to increase the persistence and resilience of coral reefs. Falconer, D. S. and Makcay, T. F. C. (1996). Introduction to Quantitative Genetics, Washington, DC: The National Academies Press. 4th edn. London: Longman Group Ltd. Negri, A. P., Marshall, P. A. and Heyward, A. J. (2007). Differing effects of thermal Ghaffari, H., Wang, W., Li, A., Zhang, G. and Li, L. (2019). Thermotolerance stress on coral fertilization and early embryogenesis in four Indo Pacific species. divergence revealed by the physiological and molecular responses in two oyster Coral Reefs 26, 759-763. doi:10.1007/s00338-007-0258-2 subspecies of Crassostrea gigas in China. Front. Physiol. 10, 1137. doi:10.3389/ Nelson, H. R., Kuempel, C. D. and Altieri, A. H. (2016). The resilience of reef fphys.2019.01137 invertebrate biodiversity to coral mortality. Ecosphere 7, e01399. Gjedrem, T. and Rye, M. (2018). Selection response in fish and shellfish: a review. Norouzitallab, P., Baruah, K., Vandegehuchte, M., Van Stappen, G., Catania, F., Rev. Aquac. 10, 168-179. doi:10.1111/raq.12154 Bussche, J. V., Vanhaecke, L., Sorgeloos, P. and Bossier, P. (2014). Grottoli, A. G., Toonen, R. J., van Woesik, R., Vega Thurber, R., Warner, M. E., Environmental heat stress induces epigenetic transgenerational inheritance of McLachlan, R. H., Price, J. T., Bahr, K. D., Baums, I. B., Castillo, K. D. et al. robustness in parthenogenetic Artemia model. FASEB J. 28, 3552-3563. doi:10. (2021). Increasing comparability among coral bleaching experiments. Ecol. Appl. 1096/fj.14-252049 31, e02262. doi:10.1002/eap.2262 O’Mahony, J., Simes, R., Redhill, D., Heaton, K., Atkinson, C., Hayward, E. and Heron, S. F., Maynard, J. A., van Hooidonk, R. and Eakin, C. M. (2016). Warming Nguyen, M. (2017). At what price? The economic, social and icon value of the trends and bleaching stress of the world’s coral reefs 1985-2012. Sci. Rep. 6, 1-14. Great Barrier Reef. Deloitte Access Econ. 92, 1-92. doi:10.1038/s41598-016-0001-8 Pandori, L. L. M. and Sorte, C. J. B. (2019). The weakest link: sensitivity to climate Heyward, A. J. and Negri, A. P. (2010). Plasticity of larval pre-competency in extremes across life stages of marine invertebrates. Oikos 128, 621-629. doi:10. response to temperature: observations on multiple broadcast spawning coral 1111/oik.05886 species. Coral Reefs 29, 631-636. doi:10.1007/s00338-009-0578-5 Payne, N. L., Morley, S. A., Halsey, L. G., Smith, J. A., Stuart-Smith, R., Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the Waldock, C. and Bates, A. E. (2021). Fish heating tolerance scales similarly world’s coral reefs. Mar. Freshw. Res. 50, 839-866. across individual physiology and populations. Commun. Biol. 4, 1-5. doi:10.1038/ Howells, E. J., Beltran, V. H., Larsen, N. W., Bay, L. K., Willis, B. L. and van s42003-021-01773-3 Oppen, M. J. H. (2012). Coral thermal tolerance shaped by local adaptation of Quigley, K. M., Bay, L. K. and Willis, B. L. (2018). Leveraging new knowledge of photosymbionts. Nat. Clim. Chang. 2, 116-120. doi:10.1038/nclimate1330 Symbiodinium community regulation in corals for conservation and reef Hughes, T. P., Barnes, M. L., Bellwood, D. R., Cinner, J. E., Cumming, G. S., restoration. Mar. Ecol. Prog. Ser. 600, 245-253. doi:10.3354/meps12652 Jackson, J. B. C., Kleypas, J., van de Leemput, I. A., Lough, J. M., Morrison, Quigley, K. M., Bay, L. K. and van Oppen, M. J. H. (2019a). The active spread of T. H. et al. (2017). Coral reefs in the Anthropocene. Nature 546, 82-90. doi:10. adaptive variation for reef resilience. Ecol. Evol. 9, 1-14. doi:10.1002/ece3.5616 1038/nature22901 Quigley, K. M., Willis, B. L. and Kenkel, C. D. (2019b). Transgenerational Humanes, A., Noonan, S. H. C., Willis, B. L., Fabricius, K. E. and Negri, A. P. inheritance of shuffled symbiont communities in the coral Montipora digitata. Sci. (2016). Cumulative effects of nutrient enrichment and elevated temperature Rep. 9, 1-11. doi:10.1038/s41598-019-50045-y compromise the early life history stages of the coral Acropora tenuis. PLoS One Quigley, K. M., Bay, L. K. and van Oppen, M. J. H. (2020a). Genome-wide SNP 11, 1-23. doi:10.1371/journal.pone.0161616 analysis reveals an increase in adaptive genetic variation through selective Humanes, A., Ricardo, G. F., Willis, B. L., Fabricius, K. E. and Negri, A. P. (2017). breeding of coral. Mol. Ecol. 29, 2176-2188. doi:10.1111/mec.15482 Cumulative effects of suspended sediments, organic nutrients and temperature Quigley, K. M., Randall, C. J., van Oppen, M. J. H. and Bay, L. K. (2020b). stress on early life history stages of the coral Acropora tenuis. Sci. Rep. 7, 44101. Assessing the role of historical temperature regime and algal symbionts on the doi:10.1038/srep44101 heat tolerance of coral juveniles. Biol. Open 9, 1-11. IPCC (2014). Climate Change 2014: Synthesis Report. Contribution of Working Radchuk, V., Reed, T., Teplitsky, C., van de Pol, M., Charmantier, A., Hassall, C., Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel Adamık,́ P., Adriaensen, F., Ahola, M. P., Arcese, P. et al. (2019). Adaptive on Climate Change (ed. Core Writing Team, R. K. Pachauri and L.A. Meyer). responses of animals to climate change are most likely insufficient. Nat. Commun. Geneva: IPCC. 10, 1-14. doi:10.1038/s41467-019-10924-4 Journal of Experimental Biology RESEARCH ARTICLE Journal of Experimental Biology (2022) 225, jeb243344. doi:10.1242/jeb.243344 Randall, C. J. and Szmant, A. M. (2009). Elevated temperature reduces Thomas, L., Rose, N. H., Bay, R. A., López, E. H., Morikawa, M. K., Ruiz- survivorship and settlement of the larvae of the Caribbean scleractinian coral, Jones, L. and Palumbi, S. R. (2018). Mechanisms of thermal tolerance in reef- Favia fragum (Esper). Coral Reefs 28, 537-545. doi:10.1007/s00338-009-0482-z building corals across a fine-grained environmental mosaic: lessons from Ofu, Rezende, E. L. and Bozinovic, F. (2019). Thermal performance across levels of American Samoa. Front. Mar. Sci. 4, 434. biological organization. Philos. Trans. R. Soc. B Biol. Sci. 374, 20180549. do:10. Truebano, M., Fenner, P., Tills, O., Rundle, S. D. and Rezende, E. L. 1098/rstb.2018.0549 (2018). Thermal strategies vary with life history stage. J. Exp. Biol. 221, Rocker, M. M., Francis, D. S., Fabricius, K. E., Willis, B. L. and Bay, L. K. (2017). jeb171629. Variation in the health and biochemical condition of the coral Acropora tenuis Tukey, J. W. (1977). Exploratory Data Analysis. Reading, MA: Pearson. along two water quality gradients on the Great Barrier Reef, Australia. Mar. Pollut. Ulstrup, K. E. and van Oppen, M. J. H. (2003). Geographic and habitat partitioning Bull. 119, 106-119. doi:10.1016/j.marpolbul.2017.03.066 of genetically distinct zooxanthellae (Symbiodinium)in Acropora corals on the Rodriguez-Lanetty, M., Harii, S. and Hoegh-Guldberg, O. (2009). Early molecular Great Barrier Reef. Mol. Ecol. 12, 3477-3484. doi:10.1046/j.1365-294X.2003. responses of coral larvae to hyperthermal stress. Mol. Ecol. 18, 5101-5114. 01988.x doi:10.1111/j.1365-294X.2009.04419.x Ulstrup, K. E., Berkelmans, R., Ralph, P. J. and van Oppen, M. J. H. (2006). Savary, R., Barshis, D. J., Voolstra, C. R., Cardenas, A., Evensen, N. R., Variation in bleaching sensitivity of two coral species across a latitudinal gradient Banc-Prandi, G., Fine, M. and Meibom, A. (2021). Fast and pervasive on the Great Barrier Reef: the role of zooxanthellae. Mar. Ecol. Prog. Ser. 314, transcriptomic resilience and acclimation of extremely heat-tolerant coral 135-148. doi:10.3354/meps314135 holobionts from the northern Red Sea. Proc. Natl. Acad. Sci. 118, 1-9. doi:10. van Oppen, M. J. H., Oliver, J. K., Putnam, H. M. and Gates, R. D. (2015). Building 1073/pnas.2023298118 coral reef resilience through assisted evolution. Proc. Natl. Acad. Sci. USA 112, Schneider, C. A., Rasband, W. S. and Eliceiri, K. W. (2012). NIH Image to ImageJ: 2307-2313. doi:10.1073/pnas.1422301112 25 years of image analysis. Nat. Methods 9, 671-675. doi:10.1038/nmeth.2089 van Tienderen, P. H. and de Jong, G. (1994). A general model of the relation Schoepf, V., Carrion, S. A., Pfeifer, S. M., Naugle, M., Dugal, L., Bruyn, J. and between phenotypic selection and genetic response. J. Evol. Biol. 7, 1-12. doi:10. McCulloch, M. T. (2019). Stress-resistant corals may not acclimatize to ocean 1046/j.1420-9101.1994.7010001.x warming but maintain heat tolerance under cooler temperatures. Nat. Commun. Webster, N. S., Smith, L. D., Heyward, A. J., Watts, J. E. M., Webb, R. I., Blackall, 10, 4031. L. L. and Negri, A. P. (2004). Metamorphosis of a scleractinian coral in response Siebeck, U. E., Logan, D. and Marshall, N. J. (2008). CoralWatch - a flexible coral to microbial biofilms. Appl. Environ. Microbiol. 70, 1213-1221. doi:10.1128/AEM. bleaching monitoring tool for you and your group. Proc. 11th Int. Coral Reef Symp., 70.2.1213-1221.2004 p. 16. Wickham, H. (2011). The split-apply-combine strategy for data analysis. J. Stat. Smith-Keune, C. and van Oppen, M. (2006). Genetic structure of a reef-building Softw. 40, 1-29. coral from thermally distinct environments on the Great Barrier Reef. Coral Reefs Wickham, H. (2016). ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag, 25, 493-502. New York. Sully, S., Burkepile, D. E., Donovan, M. K., Hodgson, G. and van Woesik, R. Wilcoxon, F. (1945). Individual comparisons by ranking methods. Biometrics Bull. 1, (2019). A global analysis of coral bleaching over the past two decades. Nat. 80-83. doi:10.2307/3001968 Commun. 10, 1-5. doi:10.1038/s41467-019-09238-2 Yuyama, I., Ishikawa, M., Nozawa, M., Yoshida, M. and Ikeo, K. (2018). Tangwancharoen, S. (2014). Early life stages are not always the most sensitive: Transcriptomic changes with increasing algal symbiont reveal the detailed heat stress responses in the copepod Tigriopus californicus. Mar. Ecol. Prog. Ser. process underlying establishment of coral-algal symbiosis. Sci. Rep. 8, 16802. 517, 75-83. doi:10.3354/meps11013 doi:10.1038/s41598-018-34575-5 Journal of Experimental Biology

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Published: Mar 8, 2022

References

Initial recovery of the Great Barrier Reef threatened by the third mass bleaching event in five years

AIMS
Assisted gene flow to facilitate local adaptation to climate change

Whitlock M. C.
Effects of temperature and ammonium on larval development and survivorship in a scleractinian coral (Diploria strigosa)

Sammarco P.
Effects of temperature on success of (self and non-self) fertilization and embryogenesis in Diploria strigosa (Cnidaria, Scleractinia)

Snell A. T. L.
Fitting linear mixed models in R

Bates D.
Considerations for maximizing the adaptive potential of restored coral populations in the western Atlantic

Miller M. W.
Time-integrated thermal bleaching thresholds of reefs and their variation on the Great Barrier Reef

Berkelmans R.
The role of zooxanthellae in the thermal tolerance of corals: a “nugget of hope” for coral reefs in an era of climate change

van Oppen M. J. H.
Thermotolerant coral symbionts modulate heat stress-responsive genes in their hosts

Baker A. C.
The heat shock protein 70 family: highly homologous proteins with overlapping and distinct functions

Jäättelä M.
The 27-year decline of coral cover on the Great Barrier Reef and its causes

Puotinen M.
Genomic determinants of coral heat tolerance across latitudes

Matz M.
Exploring variable patterns of density-dependent larval settlement among corals with distinct and shared functional traits

Babcock R. C.
Heritable variation in bleaching responses and its functional genomic basis in reef-building corals (Orbicella faveolata)

Meyer E.
The biology of larvae from the reef coral Porites astreoides, and their response to temperature disturbances

Gleason D.
Narrow-sense heritability estimation of complex traits using identity-by-descent information

Goddard M. E.
Introduction to Quantitative Genetics

Makcay T. F. C.
Thermotolerance divergence revealed by the physiological and molecular responses in two oyster subspecies of Crassostrea gigas in China

Li L.
Selection response in fish and shellfish: a review

Rye M.
Increasing comparability among coral bleaching experiments

Castillo K. D.
Warming trends and bleaching stress of the world's coral reefs 1985-2012

Eakin C. M.
Plasticity of larval pre-competency in response to temperature: observations on multiple broadcast spawning coral species

Negri A. P.
Climate change, coral bleaching and the future of the world's coral reefs

Hoegh-Guldberg O.
Coral thermal tolerance shaped by local adaptation of photosymbionts

van Oppen M. J. H.
Coral reefs in the Anthropocene

Morrison T. H.
Cumulative effects of nutrient enrichment and elevated temperature compromise the early life history stages of the coral Acropora tenuis

Negri A. P.
Cumulative effects of suspended sediments, organic nutrients and temperature stress on early life history stages of the coral Acropora tenuis

Negri A. P.
Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (ed. Core Writing Team, R. K. Pachauri and L.A. Meyer). Geneva: IPCC

IPCC
Coral decline threatens fish biodiversity in marine reserves

Eagle J. V.
Coral mass spawning predicted by rapid seasonal rise in ocean temperature

Piromvaragorn S.
Gene expression plasticity as a mechanism of coral adaptation to a variable environment

Matz M. V.
Genomic and transcriptomic signals of thermal tolerance in heat-tolerant corals (Platygyra daedalea) of the Arabian/Persian Gulf

Meyer E.
Relative importance of coral cover, habitat complexity and diversity in determining the structure of reef fish communities

Jones G. P.
Potential and limits for rapid genetic adaptation to warming in a Great Barrier Reef coral

Bay L. K.
Potential for rapid genetic adaptation to warming in a Great Barrier Reef coral

Bay L. K.
Thirty years of coral heat-stress experiments: a review of methods

Grottoli A. G.
Genetic variation in responses to a settlement cue and elevated temperature in the reef-building coral Acropora millepora

Matz M. V.
The enhanced expression of heat stress-related genes in scleractinian coral ‘Porites harrisoni’ during warm episodes as an intrinsic mechanism for adaptation in ‘the Persian Gulf

Tohidfar M.
Natural selection and the heritability of fitness components

Roff D. A.
Climate change and coral reef connectivity

Lambrechts J.
A research review of interventions to increase the persistence and resilience of coral reefs

National Academies of Sciences, Engineering, and Medicine
Differing effects of thermal stress on coral fertilization and early embryogenesis in four Indo Pacific species

Heyward A. J.
The resilience of reef invertebrate biodiversity to coral mortality

Altieri A. H.
Environmental heat stress induces epigenetic transgenerational inheritance of robustness in parthenogenetic Artemia model

Bossier P.
At what price? The economic, social and icon value of the Great Barrier Reef

Nguyen M.
The weakest link: sensitivity to climate extremes across life stages of marine invertebrates

Sorte C. J. B.
Fish heating tolerance scales similarly across individual physiology and populations

Bates A. E.
Leveraging new knowledge of Symbiodinium community regulation in corals for conservation and reef restoration

Willis B. L.
The active spread of adaptive variation for reef resilience

van Oppen M. J. H.
Transgenerational inheritance of shuffled symbiont communities in the coral Montipora digitata

Kenkel C. D.
Genome-wide SNP analysis reveals an increase in adaptive genetic variation through selective breeding of coral

van Oppen M. J. H.
Assessing the role of historical temperature regime and algal symbionts on the heat tolerance of coral juveniles

Bay L. K.
Adaptive responses of animals to climate change are most likely insufficient

Arcese P.
Elevated temperature reduces survivorship and settlement of the larvae of the Caribbean scleractinian coral, Favia fragum (Esper)

Szmant A. M.
Thermal performance across levels of biological organization

Bozinovic F.
Variation in the health and biochemical condition of the coral Acropora tenuis along two water quality gradients on the Great Barrier Reef, Australia

Bay L. K.
Early molecular responses of coral larvae to hyperthermal stress

Hoegh-Guldberg O.
Fast and pervasive transcriptomic resilience and acclimation of extremely heat-tolerant coral holobionts from the northern Red Sea

Meibom A.
NIH Image to ImageJ: 25 years of image analysis

Eliceiri K. W.
Stress-resistant corals may not acclimatize to ocean warming but maintain heat tolerance under cooler temperatures

McCulloch M. T.
CoralWatch - a flexible coral bleaching monitoring tool for you and your group

Marshall N. J.
Genetic structure of a reef-building coral from thermally distinct environments on the Great Barrier Reef

van Oppen M.
A global analysis of coral bleaching over the past two decades

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Early life stages are not always the most sensitive: heat stress responses in the copepod Tigriopus californicus

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Exploratory Data Analysis

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Transcriptomic changes with increasing algal symbiont reveal the detailed process underlying establishment of coral-algal symbiosis

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Predicting selection–response gradients of heat tolerance in a widespread reef-building coral

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Predicting selection–response gradients of heat tolerance in a widespread reef-building coral

Predicting selection–response gradients of heat tolerance in a widespread reef-building coral

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