Gymnarchus

Gymnarchus niloticus
Conservation status
Least Concern  (IUCN 3.1)
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
Aba IUCN range   Extant (resident)   Presence Uncertain & Introduced

Gymnarchus niloticus – commonly known as the aba aba or African knifefish – is an electric fish, and the only species in the genus Gymnarchus and the family Gymnarchidae within the order Osteoglossiformes.

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 by electroreception, making it the first fish known to have this ability.

Description

G. niloticus is a part of the ancient taxon of bony-tongue fishes (Osteoglossomorpha). 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. 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.

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, whereas 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.

G. niloticus 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. Adults grow up to 1.6 m (5.2 ft) in length and 19 kg (42 lb) in weight. They show signs of negative allometric growth, meaning they get slimmer as they increase in size.

Electroreception

Further information: Electroreception

Gymnarchus swims with its back straight, propelling itself with its dorsal fin, not its tail. This allows it to generate a steady electric field to navigate and to detect its prey.

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. He demonstrated by experiment that it could locate prey in the dark, using only the prey's electrical conductivity. Like the related elephantfish, which hunts the same way, it possesses an unusually large brain, which allows it to interpret the electrical signals. 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 enables the fish to navigate and find prey as nearby objects distort this field, and it can sense the distortion on its skin.

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 a jamming avoidance response. 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.

Reproduction

G. niloticus females possess a singular ovary and the males possess a singular testis, this is often referred to as unpaired gonad. The sperm cells lack a flagellum, moving like an amoeba instead. They breed in swamps during high water season in nests that are made out of plant fibers. 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. 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. 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. This may be a reproductive strategy to ensure successful fertilization for the limited number of eggs the females lay.

Feeding habits

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. Adults prefer fish and insects. Due to their small mouths, they tend to eat small-sized prey.

Distribution

Habitat distribution

The Gymnarchus niloticus is a freshwater fish endemic to the tropical freshwaters of Africa. It is found in lakes and rivers in the Nile, Turkana, Chad, Niger, Volta, Senegal, and Gambia basins. 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. 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. Their anguilliform swimming mode helps them swim effectively in open water as well as more viscous media like thick mud or sand.

Conservation status

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'. They also claim that the current population trend for this species is unknown. While they are classified as least concern in most of the countries in Africa, Nigeria has listed them as an endangered species. 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. Ongoing threats for G. niloticus are ecosystem stresses and habitat degradation. 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).

Human use

With good taste and large body size with a lot of meat, G. niloticus is a highly valued food source in several West African countries. It is sold in large numbers during the spring and summer. It is often eaten raw or smoked, and the eggs (which are very large) are edible as well. 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.

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.

It is sometimes found in the aquatic pet trade, where it is known as the aba aba knifefish. Dedicated aquarium enthusiasts can purchase them online, but they are not easy to keep in the average home aquarium, nor are they ethically sourced. Due to the juveniles' dark coloration and ability to swim backward, wild collection has increased.