Gymnarchus niloticus – commonly known as the aba, aba aba, frankfish, freshwater rat-tail or African knifefish – is an electric fish

Electric fish

An electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive. Electric fish species can be found both in the sea and in freshwater...

, and the only species in the genus Gymnarchus and the family Gymnarchidae within the order Osteoglossiformes

Osteoglossiformes

Osteoglossiformes is a relatively primitive order of ray-finned fish that contains two sub-orders, the Osteoglossoidei and the Notopteroidei. All of the living species inhabit freshwater...

. It is found exclusively in swamps and near vegetated edges in the Nile

Nile

The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...

, Turkana

Lake Turkana

Lake Turkana , formerly known as Lake Rudolf, is a lake in the Great Rift Valley in Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake...

, Chad

Lake Chad

Lake Chad is a historically large, shallow, endorheic lake in Africa, whose size has varied over the centuries. According to the Global Resource Information Database of the United Nations Environment Programme, it shrank as much as 95% from about 1963 to 1998; yet it also states that "the 2007 ...

, Niger

Niger River

The Niger River is the principal river of western Africa, extending about . Its drainage basin is in area. Its source is in the Guinea Highlands in southeastern Guinea...

, Volta

Volta River

The Volta is a river in western Africa that drains into the Gulf of Guinea. It has three main tributaries—the Black Volta, White Volta and Red Volta...

, Senegal

Sénégal River

The Sénégal River is a long river in West Africa that forms the border between Senegal and Mauritania.The Sénégal's headwaters are the Semefé and Bafing rivers which both originate in Guinea; they form a small part of the Guinean-Malian border before coming together at Bafoulabé in Mali...

, and Gambia

Gambia River

The Gambia River is a major river in West Africa, running from the Fouta Djallon plateau in north Guinea westward through Senegal and The Gambia to the Atlantic Ocean at the city of Banjul...

 basins.

Description and biology

G. niloticus has a long and slender body, with no caudal, pelvic, or anal fins. The dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 is elongated, running along the back of the fish towards the blunt, finless, tail, and is the main source of propulsion. It grows up to 1.6 metres (5.2 ft) in length and 19 kilograms (41.9 lb) in weight.

G. niloticus is nocturnal and has a poor vision. Instead, it navigates and hunts smaller fish using a weak electric field similar to that of the related elephantfish

Mormyridae

The family Mormyridae, sometimes called "elephantfish" , are freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 200 species. Members of the family are popular, if challenging, aquarium species...

. Also like the elephantfishes, it possesses an unusually large brain, which is believed to help it interpret the electrical signals. It can make its tail negatively charged with respect to its head. This produces a symmetrical electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

around its body. Nearby objects distort this field, and it can sense the distortion on its skin.

G. niloticus lay their eggs in floating nests up to a metre across. The adults continue to guard the young after hatching.