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The African knifefish, ''Gymnarchus niloticus'' – also called the ''aba aba'' – is an
electric fish An electric fish is any fish that can Bioelectrogenesis, generate electric fields, whether to sense things around them, for defence, or to stun prey. Most fish able to produce shocks are also electroreceptive, meaning that they can sense electric ...
, living at the bottoms of rivers and lakes. It is the only species in the
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
''Gymnarchus'' and the
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Gymnarchidae, within the order
Osteoglossiformes Osteoglossiformes , meaning "bony tongues" in Ancient Greek, is a relatively primitive order of ray-finned fish that contains two sub-orders, the Osteoglossoidei and the Notopteroidei. All of at least 245 living species inhabit freshwater. They ...
. It is a long slender fish with no
pelvic The pelvis (: pelves or pelvises) is the lower part of an anatomical trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton). ...
or
anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...
s, and a tail fin shaped like a rat's tail. It swims using its elongated
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
, 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 to locate 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 by
electroreception Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such ...
, 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. Globally, its conservation status is 'least concern' but it faces local threats in West Africa from human activities including overfishing and pollution.


Taxonomy

''Gymnarchus niloticus'' was described by
Georges Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...
in 1829, along with the
monospecific In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
genus ''Gymnarchus''. The monogeneric family Gymnarchidae was erected by
Pieter Bleeker Pieter Bleeker (10 July 1819 – 24 January 1878) was a Dutch medical doctor, Ichthyology, ichthyologist, and Herpetology, herpetologist. He was famous for the ''Atlas Ichthyologique des Indes Orientales Néêrlandaises'', his monumental work on ...
in 1859. The
synonym A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
''Gymnarchus electricus'' was accidentally created by
Henri Émile Sauvage Henri Émile Sauvage (22 September 1842 in Boulogne-sur-Mer – 3 January 1917 in Boulogne-sur-Mer) was a French paleontologist, ichthyologist, and Herpetology, herpetologist. He was a leading expert on Mesozoic fish and reptiles.
in 1880, intending to write ''G. niloticus''. The generic name is from Greek , 'naked', and , 'anus'. The specific name is Latin, meaning 'from the
River Nile The Nile (also known as the Nile River or River Nile) is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa. It has historically been considered the longest river i ...
'. The Gymnarchidae is
sister A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to ref ...
to another family of weakly electric fishes, the
Mormyridae The Mormyridae, sometimes called "elephantfish" (more properly freshwater elephantfish), are a superfamily of weakly electric fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order, with around 200 sp ...
; both are within the superfamily Mormyroidea. The ability to generate an electric field is shared and basal to the group. The earliest known fossil remains of ''Gymnarchus'' are from the
Middle Eocene The Eocene ( ) is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''Ēṓs'', ' Dawn') a ...
(
Lutetian The Lutetian is, in the geologic timescale, a stage (stratigraphy), stage or age (geology), age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it ...
) of
Libya Libya, officially the State of Libya, is a country in the Maghreb region of North Africa. It borders the Mediterranean Sea to the north, Egypt to Egypt–Libya border, the east, Sudan to Libya–Sudan border, the southeast, Chad to Chad–L ...
, although its lineage likely diverged from the Mormyridae during the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
.


Biology


Description

''Gymnarchus niloticus'' is a part of the ancient
taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
of bony-tongue fishes (
Osteoglossomorpha Osteoglossomorpha is a group of bony fish in the Teleostei. Notable members A notable member is the arapaima (''Arapaima gigas''), the largest freshwater fish in South America and one of the largest bony fishes alive. Other notable members inclu ...
). It has a long and slender body, with brown/grey coloration on the top half of its body and a white underside. Four small and bony
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
s are present on both the left and right sides of the body, but the species is an obligate air-breather. 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 round cycloid scales. Their
anguilliform Fish locomotion is the various types of animal locomotion used by fish, principally by aquatic locomotion, swimming. This is achieved in different groups of fish by a variety of mechanisms of propulsion, most often by wave-like lateral flexions ...
swimming mode helps them swim effectively in open water as well as more viscous media like thick mud or sand. This species uniquely has no
pelvic The pelvis (: pelves or pelvises) is the lower part of an anatomical trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton). ...
or anal fins, while its
caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...
is shaped like a rat's tail. Its
pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...
s are small and rounded. The
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
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. 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 Allometry (Ancient Greek "other", "measurement") is the study of the relationship of body size to shape, anatomy, physiology and behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in ''On Growth and Form'' and by Juli ...
growth, meaning they get slimmer as they increase in size. The larval fish has an unusual arrangement of nerves in the head. The fish's
karyotype A karyotype is the general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is discerned by de ...
is either 2n=34 or 2n=54
chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s, reported from different locations, suggesting there could be two species in the genus.


Electroreception

''Gymnarchus 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. 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 Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such ...
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 An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
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. The
electric organ An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develop ...
is derived from
striated muscle Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. Under the microscope, sarcomeres are visible along muscle fibers, giving a striated appearance to the tissue. The two types of striated muscle a ...
in a developmental process which makes the filaments thicker, loses the striations, and creates positive and negative ends of the constituent electroplates. 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 The jamming avoidance response is a behavior of some species of electric fish#Strongly and weakly electric fish, weakly electric fish. It occurs when two electric fish with wave discharges meet – if their discharge frequency, frequencies are v ...
. '' Eigenmannia'', a South American electric fish, processes sensory information extremely similar to ''G. niloticus'' and likewise employs a jamming avoidance response, evolved convergently.


Ecology

''Gymnarchus niloticus'' is
predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
y both as a juvenile and as an adult. Juveniles mainly catch aquatic insects and
decapod The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and p ...
crustaceans. Adults catch a variety of small prey including aquatic insects (28%) and fish (27%), with smaller quantities of
copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...
s, shrimps, crabs, frogs, and snails. The species is bottom-dwelling and lives in fresh water.


Reproduction

''Gymnarchus niloticus'' females possess a singular
ovary The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...
and the males possess a singular
testis A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is Homology (biology), homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of Androgen, androgens, p ...
, i.e. both sexes have unpaired gonads. The
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
cells lack a
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
, moving like an
amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...
instead. They breed in swamps during the high water season when their rivers'
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...
s are under water. They build large elliptical nests up to across at a depth of around , selecting thickly-vegetated swamps as their preferred nest sites. They use the waterside plant '' Echinochloa pyramidalis'' (antelope grass) as nesting material, available to the fish only during floods. Spawning is triggered by flooding. The female lays between 620 and 1378 eggs in the nest. The eggs, at around 4.7 or 5.4 mm (in two different populations), are the largest of any species in the Mormyroidea. The sex ratio, biased in favour of males, may help to guarantee that the small number of large eggs are fertilised. Along with '' Pollimyrus'', the genus is distinctive among the Mormyroidea in providing parental care to its young. The adults continue to guard the young after hatching. Males are more common than females, with a
sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...
of 1:1.4.


Distribution

''Gymnarchus niloticus'' is a freshwater fish
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
to the tropical freshwaters of Africa. It is found in lakes and rivers in the
Nile The Nile (also known as the Nile River or River Nile) is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa. It has historically been considered the List of river sy ...
, Turkana,
Chad Chad, officially the Republic of Chad, is a landlocked country at the crossroads of North Africa, North and Central Africa. It is bordered by Libya to Chad–Libya border, the north, Sudan to Chad–Sudan border, the east, the Central Afric ...
,
Niger Niger, officially the Republic of the Niger, is a landlocked country in West Africa. It is a unitary state Geography of Niger#Political geography, bordered by Libya to the Libya–Niger border, north-east, Chad to the Chad–Niger border, east ...
, Volta,
Senegal Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...
, and
Gambia The Gambia, officially the Republic of The Gambia, is a country in West Africa. Geographically, The Gambia is the List of African countries by area, smallest country in continental Africa; it is surrounded by Senegal on all sides except for ...
basins.


Conservation status

The
conservation status The conservation status of a group of organisms (for instance, a species) indicates whether the group still exists and how likely the group is to become extinct in the near future. Many factors are taken into account when assessing conservation ...
of this species is not very clear. The
IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological ...
last assessed ''G. niloticus'' in 2019, where they were listed as '
least concern A least-concern species is a species that has been evaluated and categorized by the International Union for Conservation of Nature (IUCN) as not being a focus of wildlife conservation because the specific species is still plentiful in the wil ...
'. 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 An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...
. The decline of this species in Nigeria is thought to be due to the destruction of habitat, unauthorized and irregular fishing practices,
overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing Fish stocks, fish stock), resu ...
, and human activities near the river. 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. Ongoing regional threats for ''G. niloticus'' are ecosystem stresses and habitat degradation. Causes of these may be local pollution (waste water, and agricultural and forestry
effluent Effluent is wastewater from sewers or industrial outfalls that flows directly into surface waters, either untreated or after being treated at a facility. The term has slightly different meanings in certain contexts, and may contain various pol ...
s), natural system modifications (dams and water management/use), biological resource use (logging/wood harvesting, and fishing/harvesting aquatic resources), and
climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
/severe weather (droughts).


Human use


Food

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. 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 Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...
farming.Falaye, A. E., I. O. Opadokun, and E. K. Ajani.
Seasonal variation in the length-weight relationships and condition factor of Gymnarchus niloticus Cuvier, 1829 in Lekki lagoon, Lagos state, Nigeria.
International Journal of Fisheries and Aquatic Studies 2.6 (2015): 159-162.
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, and to leaders during celebrations.


Biomimetic inspiration

The fish's unusual mode of swimming has inspired a biomimetic study that has resulted in a prototype undulating robotic fin called ''RoboGnilos'', enabling detailed examination of the swimming mechanism.


References


External links

{{Authority control Osteoglossiformes Fish of Lake Turkana Monotypic ray-finned fish genera Taxa named by Georges Cuvier Extant Lutetian first appearances