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Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River Niger at Agenebode in Edo State, Nigeria

Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River... Hindawi International Journal of Zoology Volume 2021, Article ID 3151609, 7 pages https://doi.org/10.1155/2021/3151609 Research Article Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River Niger at Agenebode in Edo State, Nigeria 1 1 2 M. O. Agbugui , F. E. Abhulimen , and H. O. Egbo Department of Biological Sciences, Edo State University Uzairue, Iyamho, Edo, Nigeria Department of Morbid Anatomy and Histopathology, Edo State University Uzairue, Iyamho, Edo, Nigeria Correspondence should be addressed to M. O. Agbugui; agbugui.marian@edouniversity.edu.ng Received 8 April 2021; Revised 28 May 2021; Accepted 11 June 2021; Published 19 June 2021 Academic Editor: Joao Pedro Barreiros Copyright © 2021 M. O. Agbugui et al. &is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. &is study was conducted to record the anatomy and histological features of the gastrointestinal tract of the trunk fish Gymnarchus niloticus captured at Agenebode in the Lower River Niger, Edo State. &e gastrointestinal tract was segmented into the mouth, oesophagus, stomach, pyloric caeca, and intestine. &e slender (taeniform) body that is fuller at the head tappers at its end. &ere are no spines on this fish, and the fins used for locomotion are the dorsal fins and not the typical pectoral fins as in other fish. &e oesophagus, stomach, pyloric caeca, intestine, and cloaca (anterior, intermediate, posterior, and rectum) were analysed for histological examination. &e stomach had significantly higher villi and thicker inner circular muscles compared to the intestine and oesophagus. &e lamina propria was thickest in the stomach which is significantly higher when compared with the oe- sophagus, but not with the intestine. However, the intestine showed significantly thicker outer longitudinal muscle, while gastric glands were observed only in the stomach. &e histological features were closely associated with the functions of the different segments of the gastrointestinal tract. In conclusion, the histological features of the gastrointestinal tract of G. niloticus are consistent with the feeding habit of a carnivorous fish. In conclusion, the GIT is a complex structure composed of organs that reveals that it is a higher vertebrate created for a hardy survival, consistent with the feeding habit of a carnivorous fish. &e histology reveals a GIT formed by four conspicuous layers from the inside to the outside of the mucosa, submucosa, and an inner longitudinal layer of the muscularis and the outer circular of the muscularis typical of higher vertebrates. and imports, G. niloticus has not received any extensive 1. Introduction study of its biology [1]. Gymnarchus niloticus (Cuvier 1829), commonly known as In recent times, G. niloticus has been enlisted as an the Nile knife fish, trunk fish, or aba, is one of the most endangered species in Nigeria, although it is classified as valued fishes along the River Niger mostly by the inhabitants least concerned (LC) to the rest of the continent [2]. Its of Agenebode and Idah. &e trunk fish is highly valued for its decline (due to irregular and unauthorized fishing practices, good taste, rich nutrients, though oily flesh, ability to grow as destruction of habitat, overfishing, and human activities large as 25 kg, highly valued in customary rites for marriage along the river) as a need for conservation has been reported and community celebrations. &e aforementioned makes the by [3, 4]. To avoid this highly and economically valued fish trunk fish a highly priced economic commodity which is from going into extinction, crucial measures need to be put often preferred fresh or smoked. It is called eta by the Afemai in place to enhance the preservation of this fish species, of Bode, Asa in Idah. protecting its habitat, breeding, production, culture, and G. niloticus is endemic to the freshwaters of tropical propagation at a commercial scale. It is, therefore, necessary Africa. It is an electric fish and the only species in the genus that ample knowledge of the biology of G. niloticus be Gymnarchus, family Gymnarchidae. Despite these attributes studied to provide maximal information that should provide 2 International Journal of Zoology baseline information with a view to effectively manage the to its tail and making it blunt, the fins have no spines, the species for sustainable fisheries. Hence, this research is straight head of the trunk fish is devoid of scales, but the aimed at the study of the biology of G. niloticus in the River entire body is covered with small cycloid scales. &ere are Niger at Agenebode to determine the anatomy and physi- no spines on fins or body of this fish. &e tail is the main ology and histology of its mouth and gastrointestinal tract. organ used for locomotion. &e dorsal fin and pectoral fins consist of soft rays. G. niloticus does not exhibit sexual dimorphism. &e small mouth of the trunk fish is terminal, 2. Materials and Methods nonretractable, and opens up to 5% of its total body length; 2.1. Study Area and Description. &e River Niger is the the head is covered with thick flesh making the eyes well longest river in West Africa discharging into the Atlantic proofed. &ere are two pairs of nostrils on the head for Ocean in Nigeria. It rises up to 240 km and runs 4180 km. efficient intake of air (Figures 2–4). Figure 5 shows the gap &e river is known to harbour 36 families of fresh water fish mouth of G. niloticus with its thick tongue that is and nearly 250 species of which 20 are found nowhere else protrusible. on Earth but Nigeria. From Lokoja, the lower part of the &e mandibular teeth (lower jaw) and maxillary teeth are River Niger, the river runs through Agenebode-Ida, to equipped with one row of nonfused incisor teeth. &e tongue Forcados in delta, then the Nun River in rivers, and further. is long and thick and has no teeth. &ere are no pharyngeal &e river is clean, relatively clear, carrying only a tenth of teeth in the mouth. much sediment. &e River Niger floods yearly beginning Four gills are present on the left and right side of the fish. from September and peaks in November to January. &ese gills are small and bony, and prominent gill arches, gill Agenebode is located at latitude 7.10512 and longitude rackers (11), and gill filaments are fused in pairs (63 pairs). 6.69381 and stretches through an area of 1133 km (Fig- G. niloticus possesses a gastrointestinal tract (GIT) that is ure 1). Agenebode water front is a very busy part of the Niger composed of long and longitudinally organized organ of a located in Edo State, Nigeria, serving portable water for long and tubular oesophagus, long tubular stomach, one domestic and industrial uses. Active fishing activities take lung, two pyloric caeca, a straight intestine, and cloaca that place along the river for subsistence and commercial fishing. were well enveloped by the peritoneal serosa (Figure 6). Figure 7 shows the composite and neat arrangement of the GIT. &e intestine is neatly folded and compacted to ac- 2.2. Collection and Preparation of Specimens. Twenty sam- commodate its length in the stomach of the fish. &e oe- ples of G. niloticus comprising different sizes (length and sophagus leads to a long extensible, tubular-shaped stomach weight) were purchased from fisher folks at landing sites of with a posterior rounded end. fisher folks using nets, traps, and baskets in the River Niger On one side of the stomach arises a pair of long pyloric at Agenebode, Edo State, Nigeria. Immediately after col- caeca. &e left side of the stomach reveals the intestine that is lection, the samples were rinsed with water and wiped dry. 78% of the total body length and ends in the cloaca. &e Samples of fishes were transported to the Laboratory of the stomach is hardly visibly distinguished in with an anterior Department of Biological Sciences, Edo State University large intestine and a posterior small intestine. Uzairue, Edo State. Fishes were rinsed, wiped dry, and A single lung arises from a slit in the right side of the identified to species level using guides, keys, and pictures pharynx seated on the right side of the fish, and these organs provided by [5–7]. &e mouths and intestinal tracts were are held together by sheaths of connective tissue and blood removed. &e oesophagus, stomach, and intestines were vessels. &e liver and pancreas are located on the right side of gently slit open; their contents were rinsed off in gently the stomach. flowing tap water and, thereafter, fixed in 10% &e guts of G. niloticus were formed by four major layers, formaldehyde. which is typical of higher vertebrate animals composed of four conspicuous layers from the inside to the outside of mucosa, submucosa, and inner longitudinal layer of the 2.3. Anatomy and Histology. &e samples were then dehy- muscularis and outer circular of the muscularis drated through a standard ethanol series to 100%, cleared in (Figures 8–10). xylene and embedded in paraffin wax, then sectioned with a &e gastrointestinal tract of the Nile aba revealed rotatory microtome set at 5-6 pm, deparaffinized, and typical layers seen in fish: the serosa, muscle layers (circular stained with haemotoxylin and eosin. Prepared slides of the and longitudinal muscle layers), submucosa, and mucosa gastrointestinal tract will be mounted and examined with the (Figure 8). &e structure of the oesophagus was a long and electronic Olympus microscope (model Bino Cxi IS4381) wide tube with numerous longitudinal folds in the mucosa, (PL120) to view and capture features of biological interest. and the columnar epithelium had numerous goblet cells. Pictures of the mouth, jaws, and pharyngeal teeth are &e stomach of G. niloticus was a long tubular-shaped taken with digital Samsung camera (Model X650B). organ; its mucosa was thick, dense, and filled with nu- merous glandular structures; the columnar epithelium 3. Results revealed numerous gastric pits, gastric glands, and gastric G. niloticus possesses a long, slender (taeniform) body that is cells (Figure 9). &e intestine was a straight tube that was not distinguished into small and large intestine and ends in fuller at the head and tappers at its end. &e body has no caudal fins, anal fins, and pelvic fins. &e dorsal fin stretches the cloaca (Figure 10). International Journal of Zoology 3 ′ ′ ′ 6°41 E 6°42 E 6°43 E Sampling point Main road Upstream Secondary road Midstream Settlemnent Downstream Waterbody 0 500 1,000 1,500 Meters ′ ′ ′ 6°41 E 6°42 E 6°43 E Figure 1: Map of the study area showing sampling stations. Figure 2: Body of Gymnarchus niloticus. oesophagus should naturally prefer small-sized prey. &e 4. Discussion presence of nonfused incisors avails the fish with the op- &e body of G. niloticus is long, slender (taeniform) with no portunity to tear the prey immediately when it gets into the scales on the straight head, no caudal fin, anal fins, and pelvic mouth. &e long thick tongue which is devoid of teeth fins. &e dorsal fin extends to the end of a blunt tail with no probably holds the prey in place for swallowing. &e gills spines; the entire body is covered with small cycloid scales. possess paired and numerous gill filaments making it pos- With no prominent paired fins, the tail, like most marine sible to filter small food particles and retain many small- fish, is used for movement and gliding from place to place in sized food items. &e long, elongated, and tubular gut of its environment. G. niloticus does not exhibit sexual di- G. niloticus is a typical characteristic of bottom dwelling morphism. Hence, the exposure of the internal cavity is the carnivorous species. most appropriate way to sex the fish is the presence or On one side of the stomach arises a pair of long pyloric absence of ovaries to classify as males or females. &e ossified caeca. &e left side of the stomach reveals the intestine that is nonretractable mouth, which opens to 5% of the species total 78% of the total body length and ends in the cloaca. &e body length, enables the niloticus to swallow a prey 5% of its stomach is hardly visibly distinguished in with an anterior size, bearing in mind that the slim mouth, throat, and large intestine and a posterior small intestine. ′ ′ ′ 7°5 N 7°6 N 7°7 N ′ ′ ′ 7°5 N 7°6 N 7°7 N 4 International Journal of Zoology Figure 3: Head of Gymnarchus niloticus showing two pairs of nostrils, eyes, and a pair of pectoral fins. Figure 6: Alimentary canal of G. niloticus. o: oesophagus; LI: liver; s: stomach; pc: pyloric caeca; L: lungs; I: intestine; R: rectum. vessels. &e liver and pancreas are located on the right side of the stomach. &e guts of G. niloticus were formed by four major layers, which are typical of higher vertebrate animals. &e oe- sophagus is composed of four conspicuous layers from the Figure 4: Gap mouth of Gymnarchus niloticus. inside to the outside of the mucosa, submucosa, and an inner longitudinal layer of the muscularis and the outer circular of the muscularis. &e mucosa showed large amounts of the esophageal length of the lumen of the oesophagus giving oval-shaped appearance in all sections. &e submucosa had numerous rounded folds which were broad, thick with numerous gastric glands. &e longitudinal muscle fibre was distinct. It is worthy of note that the thickness of mucosal projections and the thickness of muscularis of the oe- sophagus greatly decreased from the anterior to the posterior part. &is may be because the anterior oesophagus first receives food and will need greater contraction and movements to move the food bolus caudally during peri- stalsis. It is also reported that the muscularis layer of car- nivorous fish is thick, which is to prevent any damage or engorgement to the mucosa during swallowing of prey Figure 5: Gape mouth of G. niloticus showing long thick tongue. [8–10]. &e stomach of G. niloticus is a long tube that can be stretched longitudinally, suggesting a feeding pattern of A single lung arises from a slit in the right side of the carnivorous fish. &e organ can act as a holding area for pharynx seated on the right side of the fish, and these organs larger bolus such as small fish that they eat. &e histology of are held together by sheaths of connective tissue and blood this stomach showed dense regions of the gastric gland International Journal of Zoology 5 Figure 9: LM of the stomach of G. niloticus. GC: goblet cell; LF: longitudinal fibre; SM: submucosa; M: mucosa; PL: lamina propria; CE: columnar epithelium. Figure 7: &e gut enveloped in peritoneum serosa. &e intestines Figure 10: LM of the intestine of G. niloticus. M: mucosa; GC: are folded in a “U”-shaped form to accommodate length. goblet cell; MF: muscle fibre; GP: gastric pits; V: villi; CE: columnar epithelium. short. &e intestine is a slender elongated tubular organ where food from the stomach passes through to start an alkaline digestion before the absorption of nutrients [12]. In fish, the length of the intestine varies and depends on the diet. &e amount of vegetal material in diet is the main determination factor for intestinal length. Usually, herbiv- orous fish have longer intestines compared to carnivorous fish [13]. In this study, however, the feeding habits show a carnivorous diet. Fish, crustaceans, and arthropods were the main components of the diet found in the alimentary canal. A preference for fleshy food and prey was observed as the fish size increased. &e length of the intestine could probably Figure 8: LM of the oesophagus of G. niloticus. s: serosa; cm: be attributed to the length of the fish and to provide measure circular muscle; mf: muscle fibre; SM: submucosa; GC: goblet cells. for adequate digestion and nutrient absorption of flesh. In this study, the intestinal segments did not show any dif- ference histologically. In the intestine, gastric pits, and revealing gastric cells. &e mucosa of the stomach consists of surface and gastric epithelium. &e surface epithelium is glands, the mucosal folds are numerous, and the presence of large villi is prominent. &e columnar epithelium is distinct. made up of a single layer of columnar epithelial cells, and the gastric epithelium consists of gastric glands. &e longitu- &e intestine has a thick and dense circular muscle, the dinal muscle fibre is thicker than the circular muscle fibre. submucosa has very reduced gastric glands and gastric pits, &e gastric glands are at the base of the submucosa. &e and the columnar epithelium and lamina propria is short. columnar epithelium is visible in cells. In the pyloric caeca, &e authors of [14] reported that, in the intestine, the co- the muscularis consists of the outer longitudinal and an lumnar surface epithelial cells are arranged with long fi- inner thick circular muscle layer and the submucosa has brovascular cores to produce a pattern of villi, increasing the many mucus secreting cells with a visible columnar epi- absorptive surface area. &ere are increasing numbers of thelium which agree with the findings of [11]. &e mucosa is pale, mucin-secreting goblet cells intermixed in the 6 International Journal of Zoology epithelium. &e lamina propria underlies the epithelium Conflicts of Interest and, just beneath, is thin muscularis mucosae. &e brush &e authors declare that there are no conflicts of interest border epithelium of the intestine of G. niloticus is similar to regarding the publication of this paper. that found in the carnivorous species Anguilla bicolor, Anguilla Anguilla, and Salvelinus alpinus. An increase in the number of goblet cells was found in the posterior region of Acknowledgments the intestine. &ese differences are important in the process of expulsion of feces that needs mucus substances for lu- &is research was self-funded. &e authors acknowledge brication to ease the excretion [15]. &is study revealed that Prof. Sunnie Joshua Oniye for research mentorship and the thickness of intestinal villi gradually decreased from the Mr. Fran E. Abhulimen, Department of Biological Science, anterior to the posterior section. &e remnants of food Laboratory Science Unit, Edo State University Uzarue, Edo particles that were not absorbed in the anterior intestine State, Nigeria, for dissection and laboratory assistance. then migrate into the intermediate intestine where the ab- sorption process continues. Since the amounts of food References particles that migrate to the intermediate and posterior intestine were lesser, the number and length of villi were [1] R. Froese and Pauly, “Distribution of Gymnarchus niloticus. significantly reduced. &e fishes demonstrated the highest Gymnarchus niloticus,” 2020, http://www.fishbase.com. [2] A. Azeroual, M. Entsua-Mensah, A. Gethun, P. Laleye, villi measurement when observed in the anterior part of the T. Moelants, and E. Vreven, “Gymnarchus niloticus,” IUCN intestine, compared to the posterior part. Finally, the red list of threaatened species, 2010. remaining unabsorbed food particles and wastes migrate [3] B. A. Falaye, S. O. E. Sadiku, A. N. Okaeme, and A. A. Eyo, into the rectum, waiting to be removed from the body “Preliminary investigation into implication of a single cell through the anus. &e villous folding and microvilli function organism in fish feed bouyancy and flotation,” in Proceedings to increase the intestinal surface areas, which are important of the 19th Annual Conference of the Fisheries Society of for nutrient absorption. &e muscularis of G. niloticus was Nigeria (FISON), pp. 493–499, Fisheries Society of Nigeria, divided into an inner circular layer and an outer longitudinal Ilorin, Nigeria, September 2005. layer, which agreed with the findings of [8]. Carnivorous fish [4] M. O. Agbugui and J. Umaru, “Gymnarchus niloticus (Cuvier, consume various kinds of protein sources and need strong 1830), a threatened fish species in the lower river Niger at muscle contraction at the rectal area to defecate [16]. &e Agenebode, Nigeria; the need for its conservation,” Interna- tional Journal of Fisheries and Aquatic Research, vol. 6, no. 1, propulsive contractions are caused by the muscularis pp. 54–65, 2020. externa. Because of this the thickness of the muscle is more [5] W. Reed, J. Burchard, A. J. Hopson, J. Jenness, and I. Yaro, Fish remarkable at the posterior part of the intestine. &e findings and Fisheries of Northern Nigeria, Vol. 22, Ministry of Agri- agreed with the findings of [16]. culture, Abuja, Nigeria, 1967. [6] K. Pandey and J. P. Shukla, Fish and Fisheries. A Text Book for 5. Conclusion University Students, Rastogic Publications, Meerut, India, 2005. [7] R. Froese and Pauly, “Gymnarchus niloticus,” 2021, http:// &e results obtained from this study show the following: www.fishbase.com. (1) &e anatomical organization and histology of the [8] N. S. Nasruddin, M. N. A. Azmai, A. Ismail, M. A. Saad, H. M. Daud, and S. Z. Zulkifli, “Histological features of the mouth and gastrointestinal tract of Gymnarchus gastrointestinal tract of wild Indonesian shortfin eel, Anguilla niloticus bicolor bicolor (McClelland, 1844), captured in peninsular (2) &e GIT is a complex structure composed of organs Malaysia,” 4e Scientific World Journal, vol. 2014, Article ID that reveals that it is a higher vertebrate created for a 312670, 8 pages, 2014. hardy survival, consistent with the feeding habit of a [9] M. O. Agbugui, S. J. Oniye, J. Auta, and P. T. Bolorundo, carnivorous fish “Gastrointestinal tract of Pomadasys jubelini (curvier, 1860) in the new calabar-bonny river, rivers state, Nigeria,” Interna- (3) &e histology reveals a GIT formed by four con- tional Journal of Engineering and Scientific Research, vol. 7, spicuous layers from the inside to the outside of the no. 10, pp. 1086–1105, 2016. mucosa, submucosa, and an inner longitudinal layer [10] M. O. Agbugui and S. J. Oniye, “&e mouth and gastro-in- of the muscularis and the outer circular of the testinal tract of the lung fish Protopterus annectens (owen, muscularis typical of higher vertebrates 1839) in River Niger at Agenebode, Edo state, Nigeria,” Egyptian Journal of Aquatic Biology and Fisheries, vol. 23, Data Availability no. 4, pp. 181–188, 2019. [11] B. L. Silagan, “Histology of the digestive system,” 2011, https:// &e data used to support the findings of this study are www.slideshare.net/IamBrentsls/histology-of-digestive- available within the article. system-stomach-small-intestines. [12] B. W. S. Canan, N. B. Nascimento, D. Silva, and S. Chellappa, Ethical Approval “Morphohistology of the digestive tract of the damsel fish Stegastes fuscus (Osteichthyes: pomacentridae),” 4e Scientific &e care and use of experimental animals in this study were World Journal, vol. 9, 2012. obtained from catch landings of fisher folks. Fishes are for [13] N. E. R. El-Bakary and H. L. El-Gammal, “Comparative sale and as such can be used for research. histological, histochemical and ultrastructural studies on the International Journal of Zoology 7 proximal intestinal of flathead grey mullet (Mugil cephalus) and sea bream (Sparus aurata),” World Applied Science Journal, vol. 8, no. 4, pp. 477–485, 2010. [14] M. L. D. Santos, F. P. Arantes, T. C. Pessali, and J. E. D. Santos, “Morphological, histological and histochemical analysis of the digestive tract of Trachelyopterusstriatulus (Siluriformes: auchenipteridae),” Zoologia (Curitiba), vol. 32, no. 4, pp. 296–305, 2015. [15] H. M. Murray, G. M. Wright, and G. P. Goff, “A comparative histological and histochemical study of the post-gastric ali- mentary canal from three species of pleuronectid, the Atlantic halibut, the yellowtail flounder and the winter flounder,” Journal of Fish Biology, vol. 48, no. 2, pp. 187–206, 1996. [16] X. Dai, M. Shu, and W. Fang, “Histological and ultrastructural study of the digestive tract of rice field eel, Monopterus albus,” Journal of Applied Ichthyology, vol. 23, no. 2, pp. 177–183, http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Zoology Hindawi Publishing Corporation

Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River Niger at Agenebode in Edo State, Nigeria

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Hindawi International Journal of Zoology Volume 2021, Article ID 3151609, 7 pages https://doi.org/10.1155/2021/3151609 Research Article Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River Niger at Agenebode in Edo State, Nigeria 1 1 2 M. O. Agbugui , F. E. Abhulimen , and H. O. Egbo Department of Biological Sciences, Edo State University Uzairue, Iyamho, Edo, Nigeria Department of Morbid Anatomy and Histopathology, Edo State University Uzairue, Iyamho, Edo, Nigeria Correspondence should be addressed to M. O. Agbugui; agbugui.marian@edouniversity.edu.ng Received 8 April 2021; Revised 28 May 2021; Accepted 11 June 2021; Published 19 June 2021 Academic Editor: Joao Pedro Barreiros Copyright © 2021 M. O. Agbugui et al. &is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. &is study was conducted to record the anatomy and histological features of the gastrointestinal tract of the trunk fish Gymnarchus niloticus captured at Agenebode in the Lower River Niger, Edo State. &e gastrointestinal tract was segmented into the mouth, oesophagus, stomach, pyloric caeca, and intestine. &e slender (taeniform) body that is fuller at the head tappers at its end. &ere are no spines on this fish, and the fins used for locomotion are the dorsal fins and not the typical pectoral fins as in other fish. &e oesophagus, stomach, pyloric caeca, intestine, and cloaca (anterior, intermediate, posterior, and rectum) were analysed for histological examination. &e stomach had significantly higher villi and thicker inner circular muscles compared to the intestine and oesophagus. &e lamina propria was thickest in the stomach which is significantly higher when compared with the oe- sophagus, but not with the intestine. However, the intestine showed significantly thicker outer longitudinal muscle, while gastric glands were observed only in the stomach. &e histological features were closely associated with the functions of the different segments of the gastrointestinal tract. In conclusion, the histological features of the gastrointestinal tract of G. niloticus are consistent with the feeding habit of a carnivorous fish. In conclusion, the GIT is a complex structure composed of organs that reveals that it is a higher vertebrate created for a hardy survival, consistent with the feeding habit of a carnivorous fish. &e histology reveals a GIT formed by four conspicuous layers from the inside to the outside of the mucosa, submucosa, and an inner longitudinal layer of the muscularis and the outer circular of the muscularis typical of higher vertebrates. and imports, G. niloticus has not received any extensive 1. Introduction study of its biology [1]. Gymnarchus niloticus (Cuvier 1829), commonly known as In recent times, G. niloticus has been enlisted as an the Nile knife fish, trunk fish, or aba, is one of the most endangered species in Nigeria, although it is classified as valued fishes along the River Niger mostly by the inhabitants least concerned (LC) to the rest of the continent [2]. Its of Agenebode and Idah. &e trunk fish is highly valued for its decline (due to irregular and unauthorized fishing practices, good taste, rich nutrients, though oily flesh, ability to grow as destruction of habitat, overfishing, and human activities large as 25 kg, highly valued in customary rites for marriage along the river) as a need for conservation has been reported and community celebrations. &e aforementioned makes the by [3, 4]. To avoid this highly and economically valued fish trunk fish a highly priced economic commodity which is from going into extinction, crucial measures need to be put often preferred fresh or smoked. It is called eta by the Afemai in place to enhance the preservation of this fish species, of Bode, Asa in Idah. protecting its habitat, breeding, production, culture, and G. niloticus is endemic to the freshwaters of tropical propagation at a commercial scale. It is, therefore, necessary Africa. It is an electric fish and the only species in the genus that ample knowledge of the biology of G. niloticus be Gymnarchus, family Gymnarchidae. Despite these attributes studied to provide maximal information that should provide 2 International Journal of Zoology baseline information with a view to effectively manage the to its tail and making it blunt, the fins have no spines, the species for sustainable fisheries. Hence, this research is straight head of the trunk fish is devoid of scales, but the aimed at the study of the biology of G. niloticus in the River entire body is covered with small cycloid scales. &ere are Niger at Agenebode to determine the anatomy and physi- no spines on fins or body of this fish. &e tail is the main ology and histology of its mouth and gastrointestinal tract. organ used for locomotion. &e dorsal fin and pectoral fins consist of soft rays. G. niloticus does not exhibit sexual dimorphism. &e small mouth of the trunk fish is terminal, 2. Materials and Methods nonretractable, and opens up to 5% of its total body length; 2.1. Study Area and Description. &e River Niger is the the head is covered with thick flesh making the eyes well longest river in West Africa discharging into the Atlantic proofed. &ere are two pairs of nostrils on the head for Ocean in Nigeria. It rises up to 240 km and runs 4180 km. efficient intake of air (Figures 2–4). Figure 5 shows the gap &e river is known to harbour 36 families of fresh water fish mouth of G. niloticus with its thick tongue that is and nearly 250 species of which 20 are found nowhere else protrusible. on Earth but Nigeria. From Lokoja, the lower part of the &e mandibular teeth (lower jaw) and maxillary teeth are River Niger, the river runs through Agenebode-Ida, to equipped with one row of nonfused incisor teeth. &e tongue Forcados in delta, then the Nun River in rivers, and further. is long and thick and has no teeth. &ere are no pharyngeal &e river is clean, relatively clear, carrying only a tenth of teeth in the mouth. much sediment. &e River Niger floods yearly beginning Four gills are present on the left and right side of the fish. from September and peaks in November to January. &ese gills are small and bony, and prominent gill arches, gill Agenebode is located at latitude 7.10512 and longitude rackers (11), and gill filaments are fused in pairs (63 pairs). 6.69381 and stretches through an area of 1133 km (Fig- G. niloticus possesses a gastrointestinal tract (GIT) that is ure 1). Agenebode water front is a very busy part of the Niger composed of long and longitudinally organized organ of a located in Edo State, Nigeria, serving portable water for long and tubular oesophagus, long tubular stomach, one domestic and industrial uses. Active fishing activities take lung, two pyloric caeca, a straight intestine, and cloaca that place along the river for subsistence and commercial fishing. were well enveloped by the peritoneal serosa (Figure 6). Figure 7 shows the composite and neat arrangement of the GIT. &e intestine is neatly folded and compacted to ac- 2.2. Collection and Preparation of Specimens. Twenty sam- commodate its length in the stomach of the fish. &e oe- ples of G. niloticus comprising different sizes (length and sophagus leads to a long extensible, tubular-shaped stomach weight) were purchased from fisher folks at landing sites of with a posterior rounded end. fisher folks using nets, traps, and baskets in the River Niger On one side of the stomach arises a pair of long pyloric at Agenebode, Edo State, Nigeria. Immediately after col- caeca. &e left side of the stomach reveals the intestine that is lection, the samples were rinsed with water and wiped dry. 78% of the total body length and ends in the cloaca. &e Samples of fishes were transported to the Laboratory of the stomach is hardly visibly distinguished in with an anterior Department of Biological Sciences, Edo State University large intestine and a posterior small intestine. Uzairue, Edo State. Fishes were rinsed, wiped dry, and A single lung arises from a slit in the right side of the identified to species level using guides, keys, and pictures pharynx seated on the right side of the fish, and these organs provided by [5–7]. &e mouths and intestinal tracts were are held together by sheaths of connective tissue and blood removed. &e oesophagus, stomach, and intestines were vessels. &e liver and pancreas are located on the right side of gently slit open; their contents were rinsed off in gently the stomach. flowing tap water and, thereafter, fixed in 10% &e guts of G. niloticus were formed by four major layers, formaldehyde. which is typical of higher vertebrate animals composed of four conspicuous layers from the inside to the outside of mucosa, submucosa, and inner longitudinal layer of the 2.3. Anatomy and Histology. &e samples were then dehy- muscularis and outer circular of the muscularis drated through a standard ethanol series to 100%, cleared in (Figures 8–10). xylene and embedded in paraffin wax, then sectioned with a &e gastrointestinal tract of the Nile aba revealed rotatory microtome set at 5-6 pm, deparaffinized, and typical layers seen in fish: the serosa, muscle layers (circular stained with haemotoxylin and eosin. Prepared slides of the and longitudinal muscle layers), submucosa, and mucosa gastrointestinal tract will be mounted and examined with the (Figure 8). &e structure of the oesophagus was a long and electronic Olympus microscope (model Bino Cxi IS4381) wide tube with numerous longitudinal folds in the mucosa, (PL120) to view and capture features of biological interest. and the columnar epithelium had numerous goblet cells. Pictures of the mouth, jaws, and pharyngeal teeth are &e stomach of G. niloticus was a long tubular-shaped taken with digital Samsung camera (Model X650B). organ; its mucosa was thick, dense, and filled with nu- merous glandular structures; the columnar epithelium 3. Results revealed numerous gastric pits, gastric glands, and gastric G. niloticus possesses a long, slender (taeniform) body that is cells (Figure 9). &e intestine was a straight tube that was not distinguished into small and large intestine and ends in fuller at the head and tappers at its end. &e body has no caudal fins, anal fins, and pelvic fins. &e dorsal fin stretches the cloaca (Figure 10). International Journal of Zoology 3 ′ ′ ′ 6°41 E 6°42 E 6°43 E Sampling point Main road Upstream Secondary road Midstream Settlemnent Downstream Waterbody 0 500 1,000 1,500 Meters ′ ′ ′ 6°41 E 6°42 E 6°43 E Figure 1: Map of the study area showing sampling stations. Figure 2: Body of Gymnarchus niloticus. oesophagus should naturally prefer small-sized prey. &e 4. Discussion presence of nonfused incisors avails the fish with the op- &e body of G. niloticus is long, slender (taeniform) with no portunity to tear the prey immediately when it gets into the scales on the straight head, no caudal fin, anal fins, and pelvic mouth. &e long thick tongue which is devoid of teeth fins. &e dorsal fin extends to the end of a blunt tail with no probably holds the prey in place for swallowing. &e gills spines; the entire body is covered with small cycloid scales. possess paired and numerous gill filaments making it pos- With no prominent paired fins, the tail, like most marine sible to filter small food particles and retain many small- fish, is used for movement and gliding from place to place in sized food items. &e long, elongated, and tubular gut of its environment. G. niloticus does not exhibit sexual di- G. niloticus is a typical characteristic of bottom dwelling morphism. Hence, the exposure of the internal cavity is the carnivorous species. most appropriate way to sex the fish is the presence or On one side of the stomach arises a pair of long pyloric absence of ovaries to classify as males or females. &e ossified caeca. &e left side of the stomach reveals the intestine that is nonretractable mouth, which opens to 5% of the species total 78% of the total body length and ends in the cloaca. &e body length, enables the niloticus to swallow a prey 5% of its stomach is hardly visibly distinguished in with an anterior size, bearing in mind that the slim mouth, throat, and large intestine and a posterior small intestine. ′ ′ ′ 7°5 N 7°6 N 7°7 N ′ ′ ′ 7°5 N 7°6 N 7°7 N 4 International Journal of Zoology Figure 3: Head of Gymnarchus niloticus showing two pairs of nostrils, eyes, and a pair of pectoral fins. Figure 6: Alimentary canal of G. niloticus. o: oesophagus; LI: liver; s: stomach; pc: pyloric caeca; L: lungs; I: intestine; R: rectum. vessels. &e liver and pancreas are located on the right side of the stomach. &e guts of G. niloticus were formed by four major layers, which are typical of higher vertebrate animals. &e oe- sophagus is composed of four conspicuous layers from the Figure 4: Gap mouth of Gymnarchus niloticus. inside to the outside of the mucosa, submucosa, and an inner longitudinal layer of the muscularis and the outer circular of the muscularis. &e mucosa showed large amounts of the esophageal length of the lumen of the oesophagus giving oval-shaped appearance in all sections. &e submucosa had numerous rounded folds which were broad, thick with numerous gastric glands. &e longitudinal muscle fibre was distinct. It is worthy of note that the thickness of mucosal projections and the thickness of muscularis of the oe- sophagus greatly decreased from the anterior to the posterior part. &is may be because the anterior oesophagus first receives food and will need greater contraction and movements to move the food bolus caudally during peri- stalsis. It is also reported that the muscularis layer of car- nivorous fish is thick, which is to prevent any damage or engorgement to the mucosa during swallowing of prey Figure 5: Gape mouth of G. niloticus showing long thick tongue. [8–10]. &e stomach of G. niloticus is a long tube that can be stretched longitudinally, suggesting a feeding pattern of A single lung arises from a slit in the right side of the carnivorous fish. &e organ can act as a holding area for pharynx seated on the right side of the fish, and these organs larger bolus such as small fish that they eat. &e histology of are held together by sheaths of connective tissue and blood this stomach showed dense regions of the gastric gland International Journal of Zoology 5 Figure 9: LM of the stomach of G. niloticus. GC: goblet cell; LF: longitudinal fibre; SM: submucosa; M: mucosa; PL: lamina propria; CE: columnar epithelium. Figure 7: &e gut enveloped in peritoneum serosa. &e intestines Figure 10: LM of the intestine of G. niloticus. M: mucosa; GC: are folded in a “U”-shaped form to accommodate length. goblet cell; MF: muscle fibre; GP: gastric pits; V: villi; CE: columnar epithelium. short. &e intestine is a slender elongated tubular organ where food from the stomach passes through to start an alkaline digestion before the absorption of nutrients [12]. In fish, the length of the intestine varies and depends on the diet. &e amount of vegetal material in diet is the main determination factor for intestinal length. Usually, herbiv- orous fish have longer intestines compared to carnivorous fish [13]. In this study, however, the feeding habits show a carnivorous diet. Fish, crustaceans, and arthropods were the main components of the diet found in the alimentary canal. A preference for fleshy food and prey was observed as the fish size increased. &e length of the intestine could probably Figure 8: LM of the oesophagus of G. niloticus. s: serosa; cm: be attributed to the length of the fish and to provide measure circular muscle; mf: muscle fibre; SM: submucosa; GC: goblet cells. for adequate digestion and nutrient absorption of flesh. In this study, the intestinal segments did not show any dif- ference histologically. In the intestine, gastric pits, and revealing gastric cells. &e mucosa of the stomach consists of surface and gastric epithelium. &e surface epithelium is glands, the mucosal folds are numerous, and the presence of large villi is prominent. &e columnar epithelium is distinct. made up of a single layer of columnar epithelial cells, and the gastric epithelium consists of gastric glands. &e longitu- &e intestine has a thick and dense circular muscle, the dinal muscle fibre is thicker than the circular muscle fibre. submucosa has very reduced gastric glands and gastric pits, &e gastric glands are at the base of the submucosa. &e and the columnar epithelium and lamina propria is short. columnar epithelium is visible in cells. In the pyloric caeca, &e authors of [14] reported that, in the intestine, the co- the muscularis consists of the outer longitudinal and an lumnar surface epithelial cells are arranged with long fi- inner thick circular muscle layer and the submucosa has brovascular cores to produce a pattern of villi, increasing the many mucus secreting cells with a visible columnar epi- absorptive surface area. &ere are increasing numbers of thelium which agree with the findings of [11]. &e mucosa is pale, mucin-secreting goblet cells intermixed in the 6 International Journal of Zoology epithelium. &e lamina propria underlies the epithelium Conflicts of Interest and, just beneath, is thin muscularis mucosae. &e brush &e authors declare that there are no conflicts of interest border epithelium of the intestine of G. niloticus is similar to regarding the publication of this paper. that found in the carnivorous species Anguilla bicolor, Anguilla Anguilla, and Salvelinus alpinus. An increase in the number of goblet cells was found in the posterior region of Acknowledgments the intestine. &ese differences are important in the process of expulsion of feces that needs mucus substances for lu- &is research was self-funded. &e authors acknowledge brication to ease the excretion [15]. &is study revealed that Prof. Sunnie Joshua Oniye for research mentorship and the thickness of intestinal villi gradually decreased from the Mr. Fran E. Abhulimen, Department of Biological Science, anterior to the posterior section. &e remnants of food Laboratory Science Unit, Edo State University Uzarue, Edo particles that were not absorbed in the anterior intestine State, Nigeria, for dissection and laboratory assistance. then migrate into the intermediate intestine where the ab- sorption process continues. Since the amounts of food References particles that migrate to the intermediate and posterior intestine were lesser, the number and length of villi were [1] R. Froese and Pauly, “Distribution of Gymnarchus niloticus. significantly reduced. &e fishes demonstrated the highest Gymnarchus niloticus,” 2020, http://www.fishbase.com. [2] A. Azeroual, M. Entsua-Mensah, A. Gethun, P. Laleye, villi measurement when observed in the anterior part of the T. Moelants, and E. Vreven, “Gymnarchus niloticus,” IUCN intestine, compared to the posterior part. Finally, the red list of threaatened species, 2010. remaining unabsorbed food particles and wastes migrate [3] B. A. Falaye, S. O. E. Sadiku, A. N. Okaeme, and A. A. 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Journal

International Journal of ZoologyHindawi Publishing Corporation

Published: Jun 19, 2021

References