| Gymnarchus niloticus | |
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Scientific 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 | |
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| Aba IUCN range Extant (resident) Presence Uncertain & Introduced | |
TheAfrican knifefish,Gymnarchus niloticus – also called theaba aba[a] – is anelectric fish, living at the bottoms of rivers and lakes. It is theonly species in thegenusGymnarchus and thefamilyGymnarchidae, within the orderOsteoglossiformes.[1][2] It is a long slender fish with nopelvic oranal fins, and atail fin shaped like a rat's tail. It swims using its elongateddorsal fin, allowing it to keep its body straight while it moves. This in turn enables it to produce a steady but weak electric field, which it uses tolocate its prey. It is large for a river fish; adults can reach 1.6 m (5.2 ft) in length and 19 kg (42 lb) in weight.
In 1950,Hans Lissmann noticed that the fish could swim equally well forwards or backwards, clearly relying on a sense other than vision. He demonstrated that it could locate prey byelectroreception, making it the first fish known to have this ability.
The fish is considered good to eat in West Africa, where it has a wide but scattered distribution. It is important culturally, as it is given as a gift in community celebrations and marriages. Its conservation status is globally of least concern, but it faces local threats in West Africa from human activities including overfishing and pollution.
Gymnarchus niloticus was described byGeorges Cuvier in 1829, along with themonospecific genusGymnarchus. The monogeneric family Gymnarchidae was erected byPieter Bleeker in 1859. ThesynonymGymnarchus electricus wasaccidentally created byHenri Émile Sauvage in 1880, intending to writeG. niloticus.[3] The generic name is from Greekgymnos, 'naked', andarchos, 'anus'.[3] The specific name is Latin, meaning 'from theRiver Nile'.[4]
The Gymnarchidae issister to the another family of weakly electric fishes, theMormyridae; both are within the superfamilyMormyroidea. The ability to generate an electric field is shared andbasal to the group.[5]
The earliest known fossil remains ofGymnarchus are from theMiddle Eocene (Lutetian) ofLibya, although its lineage likely diverged from the Mormyridae during theLate Cretaceous.[6][7]
Gymnarchus niloticus is a part of the ancienttaxon of bony-tongue fishes (Osteoglossomorpha).[8] It has a long and slender body, with brown/grey coloration on the top half of its body and awhite underside.[2] Four small and bonygills are present on both the left and right sides of the body,[9] but the species is an obligate air-breather.[2] There is a single lung on the right of the body, which arises via a slit on the right of the throat; the body is covered in small roundcycloid scales.[9] Theiranguilliform swimming mode helps them swim effectively in open water as well as more viscous media like thick mud or sand.[10]
This species uniquely[11] has nopelvic oranal fins, while itscaudal fin is shaped like a rat's tail.[11] Itspectoral fins are small and rounded. Thedorsal fin is elongated, running along the back of the fish towards the blunt, finless tail. The dorsal fin is the main source of propulsion, whereas typical fishes use their tail fin, powered by the large muscles of the back and tail, to generate thrust. This enables it to swim backwards as easily as forwards.[12]
Juveniles range from 49–72mm (1.9–2.8 inches) and weighing 0.09–0.31 grams.[citation needed] Adults grow up to 1.6 m (5.2 ft) in length and 19 kg (42 lb) in weight.[13] They show signs of negativeallometric growth, meaning they get slimmer as they increase in size.[14] The larval fish has an unusual arrangement of nerves in the head.[15]
The fish'skaryotype is either 2n=34 or 2n=54chromosomes, reported from different locations, suggesting there could be two species in the genus.[11][16]
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Gymnarchus niloticus is nocturnal and has poor vision. Instead, it navigates and hunts smaller fish using a weak electric field, as demonstrated by the zoologistHans Lissmann in 1950. He noticed that it could swim equally well forwards or backward, clearly relying on a sense other than vision. This opened up research intoelectroreception and electrogenesis in fish. He demonstrated by experiment that it could locate prey in the dark, using only the prey's electrical conductivity.[17] Like the relatedelephantfish, which hunts the same way, it possesses an unusually large brain, which allows it to interpret the electrical signals.[13]G. niloticus makes its tail negatively charged with respect to its head. This produces a symmetricalelectric field around its body, provided it keeps its back straight; it does this by swimming using its fins. This electric field enables the fish to navigate and find prey as nearby objects distort this field, and it can sense the distortion on its skin.[18] Theelectric organ is derived fromstriated muscle in a developmental process which makes the filaments thicker, loses the striations, and creates positive and negative ends of the constituent electroplates.[19]
Nearby fish with similar electric discharge frequencies can affect the ability to electrolocate. To avoid this, fish shift their discharge frequencies apart from each other in ajamming avoidance response.Eigenmannia, a South American electric fish, processes sensory information extremely similar toG. niloticus and likewise employs a jamming avoidance response,evolved convergently.[20]
Gymnarchus niloticus ispredatory both as a juvenile and as an adult. Juveniles mainly catch aquatic insects anddecapod crustaceans. Adults catch a variety of small prey including aquatic insects (28%) and fish (27%), with smaller quantities ofcopepods, shrimps, crabs,[21] frogs, and snails.[1] The speciesis bottom-dwelling andlives in fresh water.[1]
Gymnarchus niloticus females possess a singularovary and the males possess a singulartestis, i.e. both sexes have unpaired gonads.[9][21] Thesperm cells lack aflagellum, moving like anamoeba instead.[22] They breed in swamps during the high water season when their rivers'floodplains are under water. They build large elliptical nests up to 1.5 m (4.9 ft) across at a depth of around 1–1.5 m (3.3–4.9 ft), selecting thickly-vegetated swamps as their preferred nest sites. They use the waterside plantEchinochloa pyramidalis (antelope grass) as nesting material, available to the fish only during floods. Spawning is triggered by flooding.[1][23] The female lays between 620 and 1378 eggs in the nest.[9] The eggs, at around 4.7 or 5.4 mm (in two different populations), are the largest of any species in the Mormyroidea.[24] The sex ratio, biased in favour of males, may help to guarantee that the small number of large eggs are fertilised. Along withPollimyrus, the genus is distinctive among the Mormyroidea in providing parental care to its young.[24]The adults continue to guard the young after hatching.[13] Males are more common than females, with asex ratio of 1:1.4.[9]
Gymnarchus niloticus is a freshwater fishendemic to the tropical freshwaters of Africa.[9] It is found in lakes and rivers in theNile,Turkana,Chad,Niger,Volta,Senegal, andGambia basins.[1][2]
Theconservation status of this species is not very clear. TheIUCN Red List last assessedG. niloticus in 2019, where they were listed as 'least concern'.[25] They also claim that the current population trend for this species is unknown.[25] While they are classified as least concern in most of the countries in Africa, Nigeria has listed them as anendangered species.[9] The decline of this species in Nigeria is thought to be due to thedestruction of habitat, unauthorized and irregular fishing practices,overfishing, and human activities near the river.[9] A specific threat is that the young are often captured along with their parent; since they die in captivity, the population is in danger from this unsustainable fishing approach.[23] Ongoing regional threats forG. niloticus are ecosystem stresses and habitat degradation.[25] Causes of these may be local pollution (waste water, and agricultural and forestryeffluents), natural system modifications (dams and water management/use), biological resource use (logging/wood harvesting, and fishing/harvesting aquatic resources), andclimate change/severe weather (droughts).[25]
With good taste and large body size with a lot of meat,Gymnarchus niloticus is a highly valued food source in several West African countries.[9][26] It is often eaten raw or smoked, and the eggs (which are very large) are edible as well.[9][26] Due to its rapid growth and demand, it has the potential to be a good fish foraquaculture farming.[27]
The fish is highly valued in customary rites for community celebrations as well as marriages in Nigeria, as they are given as gifts from suitors to the bride's family,[9] and to leaders during celebrations.[23]
The fish's unusual mode of swimming has inspired abiomimetic study that has resulted in a prototype undulating robotic fin calledRoboGnilos, enabling detailed examination of the swimming mechanism.[28]
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