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Gymnarchus

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Gymnarchus niloticus
Gymnarchus swims with its back straight, detecting prey by the way they distort the electric field it generates.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Osteoglossiformes
Family: Gymnarchidae
Bleeker, 1859
Genus: Gymnarchus
Cuvier, 1829
Species:
G. niloticus
Binomial name
Gymnarchus niloticus
Cuvier, 1829
Map
Aba IUCN range
  Extant (resident)
  Presence Uncertain & Introduced

Gymnarchus niloticus – commonly known as the aba, aba aba, frankfish, freshwater rat-tail, poisson-cheval, or African knifefish – is an electric fish, and the only species in the genus Gymnarchus and the family Gymnarchidae within the order Osteoglossiformes. [1][2]

Species Description

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G. niloticus is a part of the ancient taxon of bony-tongue fishes (Osteoglossomorpha).[3] It has an anguilliform (long and slender) body, with brown/grey coloration on the top half of its body and a white underside. This coloration helps it camouflage into the bottom of the lakes and rivers to avoid predation. This body shape and coloration is consistent with bottom-dwelling fish. They have four small and bony gills present on both the left and right sides of their body.[4] They only have one lung which arises via a slit on the right side of their throat on the right side of their body, and their entire body is covered in small cycloid scales.[4]

This species has no caudal, pelvic, or anal fins, and its pectoral fins are small and rounded. The dorsal fin is elongated, running along the back of the fish towards the blunt, finless tail. The dorsal fin is the main source of propulsion, as compared to most other fishes that use their posterior end or caudal fin (tail fin) for thrust generation. Since thrust generation and power are created throughout the body, it has the ability to swim backward more efficiently than other fishes.[5] Small pectoral fins and the absence of pelvic and anal fins may also help with its ability to swim backward since they have more of a streamlined body shape without them.

These fish can grow to be quite large. Juveniles start off small, ranging from 49-72mm (1.9-2.8 inches) and weighing 0.9-0.31 grams.[6] Adults grow up to 1.6 m (5.2 ft) in length and 19 kg (42 lb) in weight.[7] Males are generally slimmer and longer than females.[8] Females are wider than males because they carry eggs.[8] They show signs of negative allometric growth, meaning they get slimmer as they increase in size.[9] With their long, slender bodies, and dark coloration these fish look similar to the American eel (Anguilla rostrata) but should not be confused with them as they live on two different continents.

Electric Field

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Further information: Electroreception

G. niloticus is nocturnal and has poor vision. Instead, it navigates and hunts smaller fish using a weak electric field, as demonstrated by the zoologist Hans Lissmann in 1950, when he noticed that it could swim equally well forwards or backward, clearly relying on a sense other than vision. This opened up research into electroreception and electrogenesis in fish.[10] Like the related elephantfish, which hunts the same way, it possesses an unusually large brain, which is believed to help it interpret the electrical signals.[7] G. niloticus makes its tail negatively charged with respect to its head. This produces a symmetrical electric field around its body, provided it keeps its back straight; it does this by swimming using its fins. This electric field can be incredibly useful in navigation and finding prey as nearby objects distort this field, and it can sense the distortion on its skin.[11] Nearby fish with similar electric discharge frequencies can affect their ability to electrolocate their surroundings.[12] To avoid this, fish shift their discharge frequencies apart from each other to form a larger frequency difference.[12] While the previously mentioned elephantfish is related to G. niloticus, there are other fish that have separately evolved the ability to generate electric fields in nearly identical ways. Eigenmannia, a South American electric fish, processes sensory information extremely similarly to G. niloticus, even though they evolved the trait independently.[12]

Reproduction

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G. niloticus females possess a singular ovary and the males possess a singular testis, this is often referred to as unpaired gonad.[13] The sperm cells lack a flagellum, moving like an amoeba instead.[14] They breed in swamps during high water season in nests that are made out of plant fibers.[8] Females lay their eggs in the floating nests that measure up to 1 m (3.3 ft) across.

They have relatively low fecundity (the total number of eggs a female can lay in a breeding season) with an average of 620 to 1378 eggs in females.[13] This can be considered low compared to the average amount of eggs produced in other fish species since fish typically produce extremely large amounts of eggs due to low chances of offspring survival after they hatch. The adults continue to guard the young after hatching.[7] Guarding the young after hatching is not common practice for most fish, but it can help the young survive especially in species that do not produce many offspring in their lifetime. Males are more common than females, with a ratio of 1:1.4.[13] This may be a reproductive strategy to ensure successful fertilization for the limited number of eggs the females lay.[13]

Feeding Habits

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Feeding habits for fish change throughout their lifetime. G. niloticus is a carnivorous fish that changes its preferred food sources in different stages of its lifecycle. Juveniles of this species specialize in eating aquatic insects and fish parts.[6] Adults prefer fish and insects.[13] Due to their small mouths, they tend to eat small-sized prey.[4]

Distribution

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The Gymnarchus niloticus is a freshwater fish endemic to the tropical freshwaters of Africa.[4] It is found in lakes and rivers in the Nile, Turkana, Chad, Niger, Volta, Senegal, and Gambia basins.[1][2] They are a bottom-dwelling species that live among dense vegetation that they use for hiding during the day and cover for when they hunt at night.[8] They mainly inhabit swamps and the vegetated edges of rivers but are also found living in slow-moving or stagnant water as well as backwater regions.[8] Their anguilliform swimming mode helps them swim effectively in open water as well as more viscous media like thick mud or sand.[15] This helps them efficiently navigate the entirety of their complex habitat.

Conservation Status

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The conservation status of this species is not very clear. The IUCN Red List last assessed G. niloticus in 2019, where they were listed as “least concern”.[16] They also claim that the current population trend for this species is unknown.[16] While they are classified as least concern in most of the countries in Africa, Nigeria has listed them as an endangered species.[4] The decline of this species in Nigeria is thought to be due to the destruction of habitat, unauthorized and irregular fishing practices, overfishing, and human activities near the river.[4] Ongoing threats for G. niloticus are ecosystem stresses and habitat degradation.[16] Causes of these may be pollution (waste water, and agricultural and forestry effluents), natural system modifications (dams and water management/use), biological resource use (logging/wood harvesting, and fishing/harvesting aquatic resources), and climate change/severe weather (droughts).[16] With this fish being economically and culturally important, conservation measures can be taken to prevent the loss of an important species.

Human Use

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With good taste, rich nutrients, and large body size with a lot of meat, G. niloticus is a highly valued food source in several West African countries.[4] While it is commonly sold in markets year-round, it is only sold in large numbers during the spring and summer.[17] It is often eaten raw or smoked, and the eggs (which are very large) are edible as well.[4][17] Due to its rapid growth and demand, it has the potential to be a good fish for aquaculture farming. Although it has the potential for aquaculture, supply greatly relies on wild collection which is insufficient for the amount of demand it has.[6] While it is a common food source, studies have found that it can contain large amounts of pesticides in it.[18] In one study examining pesticide residue levels in smoked Clarias sp., G. niloticus, and Tilapia sp., G. niloticus showed consistently higher residue levels than the others.[18]

G. niloticus is known for its cultural significance in Nigeria and several other West African countries. It is considered to be a high-priced ceremonial fish and has a great amount of sociocultural importance in Nigeria.[10] It is highly valued in customary rites for community celebrations as well as marriages, as they are given as gifts from suitors to the bride’s family.[4][8]

It is also sometimes found in the aquatic pet trade. Known as the Aba Aba Knifefish in the pet trade, dedicated aquarium enthusiasts can purchase them online, but they are not easy fish to keep in the average home aquarium, nor are they ethically sourced. Due to the juveniles’ dark coloration and ability to swim backward, the wild collection of them has increased as they are considered ornamental fish.[6] While they look nice, they are highly aggressive and do not get along well with other fish in tanks.[8] Due to that they would need a separate tank they can be solitary in. As juveniles, when they are caught, they are not at their full size yet and can fit into aquarium tanks, but grow rapidly and can get too large for the average aquarium owners tank after a short amount of time. Their large size requires them to have a very large tank. Due to this, they are not a good fish to keep in the average home community-style fish tank.

References

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  1. ^ a b c Lalèyè, P.; Azeroual, A.; Entsua-Mensah, M.; Getahun, A.; Moelants, T.; Vreven, E. (2020). "Gymnarchus niloticus". IUCN Red List of Threatened Species. 2020: e.T181688A134949091. doi:10.2305/IUCN.UK.2020-2.RLTS.T181688A134949091.en.
  2. ^ a b Froese, Rainer; Pauly, Daniel, eds. (2014). "Gymnarchus niloticus". FishBase.
  3. ^ Dymek, Anna M.; Dymek, Jakub; Pol, Przemysław (2022-10-29). "Sciendo". Annals of Animal Science. 22 (4): 1193–1200. doi:10.2478/aoas-2022-0043.
  4. ^ a b c d e f g h i Agbugui, M. O.; Abhulimen, F. E.; Egbo, H. O. (2021-06-18). Barreiros, Joao Pedro (ed.). "Gross Anatomy and Histological Features of Gymnarchus niloticus (Cuvier, 1829) from the River Niger at Agenebode in Edo State, Nigeria". International Journal of Zoology. 2021: 1–7. doi:10.1155/2021/3151609. ISSN 1687-8485.
  5. ^ Li, Fei; Hu, Tian-jiang; Wang, Guang-ming; Shen, Lin-cheng (2005-09-01). "Locomotion of Gymnarchus niloticus : Experiment and kinematics". Journal of Bionic Engineering. 2 (3): 115–121. doi:10.1007/BF03399488. ISSN 2543-2141.
  6. ^ a b c d Oluwale, FV; Ugwumba, AAA; Ugwumba, OA (13-04-2019). "Aspects of the biology of juvenile Aba, Gymnarchus niloticus (Curvier 1829) from Epe Lagoon, Lagos, Nigeria" (PDF). International Journal of Fisheries and Aquatic Studies. 7 (3). {{cite journal}}: Check date values in: |date= (help)
  7. ^ a b c Greenwood, P. H.; Wilson, M. V. (1998). Paxton, J. R.; Eschmeyer, W. N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 84. ISBN 0-12-547665-5.
  8. ^ a b c d e f g Ayoola, S.O.; Abotti, C.E. (2010). "Morphology of Aba Knife Fish (Gymnarchus niloticus) (Cuvier, 1829)" (PDF). World Journal of Fish and Marine Science. 2 (5).
  9. ^ Nwabueze, Agatha Arimiche; Nwabueze, Emmanuel Obiajulu (July 7, 2021). "Impact of environmental variables on abundance, growth and condition factor of Gymnarchus Niloticus (Curvier, 1829) from Umueze-Ossissa Lake System, Southern Nigeria" (PDF). Asian Journal of Agriculture and Biology. 2021 (3).
  10. ^ Lissmann, Hans. "Continuous Electrical Signals from the Tail of a Fish, Gymnarchus Niloticus Cuv", in: Nature, 167, 4240 (1951), pp. 201–202.
  11. ^ Alexander, R. McNeill (2006). "A new sense for muddy water". Journal of Experimental Biology. 2006 209 (2): 200–201. doi:10.1242/jeb.10.1242/jeb.02012. PMID 16391343.
  12. ^ a b c Kawasaki, M. (1993-07-01). "Independently evolved jamming avoidance responses employ identical computational algorithms: a behavioral study of the African electric fish, Gymnarchus niloticus". Journal of Comparative Physiology A. 173 (1): 9–22. doi:10.1007/BF00209614. ISSN 1432-1351. PMID 8366474.
  13. ^ a b c d e Agbugui, M. O.; Abhulimen, F. E.; Adeniyi, A. O. (2021-11-26). "Abundance, Distribution, Morphometric, Feeding Evaluation and the Reproductive Strategies of Gymnarchus niloticus in the Lower River Niger at Agenebode, Edo State Nigeria". Journal of Applied Sciences and Environmental Management. 25 (8): 1371–1377. doi:10.4314/jasem.v25i8.5. ISSN 2659-1499.
  14. ^ Kunz, Yvette W. (2004). Developmental Biology of Teleost Fishes. Fish & Fisheries. Springer. p. 143. ISBN 978-1-4020-2996-7.
  15. ^ Stin, Vincent; Godoy-Diana, Ramiro; Bonnet, Xavier; Herrel, Anthony (December 2024). "Form and function of anguilliform swimming". Biological Reviews. 99 (6): 2190–2210. doi:10.1111/brv.13116. ISSN 1464-7931. PMID 39004428.
  16. ^ a b c d "Gymnarchus niloticus: Lalèyè, P., Azeroual, A., Entsua-Mensah, M., Getahun, A., Moelants, T. & Vreven, E." IUCN Red List of Threatened Species. 2019-05-12. doi:10.2305/iucn.uk.2020-2.rlts.t181688a134949091.en. Retrieved 2024-11-08.
  17. ^ a b Pekkola, Waino (1918). "Seasonal Occurrence and Edibility of Fish at Khartoum". Sudan Notes and Records. 1.
  18. ^ a b Musa, U.; Hati, S.S.; Adamu, Y.I.; Mustapha, A. (2010). "Pesticides Residues in Smoked Fish Samples from North-Eastern Nigeria". Journal of Applied Sciences. 10 (11): 975–980. Bibcode:2010JApSc..10..975M. doi:10.3923/jas.2010.975.980. Retrieved 2024-11-08.
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