Latin name

Gymnarchus niloticus

Other name

Aba, aba aba.

Identification

The Frankfish has a long and slender body. It possesses four small bony gills on both the left and right sides, yet this species is an obligatory air-breather. On the right side of the body is a single lung, which vents through an opening to the right of the throat. The body is covered in small, round cycloid scales. The electric organ develops from striated muscles; during this process, the filaments thicken, lose their striations, and create positive and negative ends within its constituent electrocytes. The larval fish have an unusual arrangement of nerves in the head.

Features of Fish Fins

A unique feature of this species is the absence of pelvic (ventral) or anal fins. The caudal fin is rudimentary, resembling a rat's tail. The pectoral fins are small and rounded. The dorsal fin is elongated, running along the fish's back to the blunt, finless tail. This dorsal fin is the primary source of locomotion, whereas typical fish use their tail fin, powered by large muscles in the back and tail, to generate thrust. This allows the Frankfish to swim backward as easily as forward.

Fish Coloration

The upper half of the Frankfish's body is brownish-gray, while the lower half is white.

Distribution

They are found in the tropical waters of Africa. They occur in lakes and rivers within the Nile, Turkana, Chad, Niger, Volta, Senegal, and Gambia River basins.

Habitat

This fish species is a bottom-dweller in freshwater.

Size

Adults can reach up to 1.6 m (5.2 ft) in length and weigh up to 19 kg (42 lbs). They exhibit signs of negative allometric growth, meaning they become more slender as they increase in size.

Behavior

An anguilliform (eel-like) swimming mode helps this fish swim efficiently in open water as well as in more viscous environments like thick mud or sand. Frankfish are nocturnal and have poor eyesight.

Food and Feeding Habits

They navigate and hunt for small fish using a weak electric field. They possess an exceptionally large brain that allows them to interpret electrical signals. They charge their tail negatively relative to their head. This creates a symmetrical electric field around the body, provided they hold their back straight while swimming with the fin. This electric field allows the fish to orient themselves and locate prey, as nearby objects distort the field, and they can sense this distortion through their skin.

Frankfish are predators in both juvenile and adult stages. Juveniles primarily catch aquatic insects and decapod crustaceans. Adults catch a variety of small prey, including aquatic insects (28%) and fish (27%), as well as smaller quantities of copepods, shrimp, crabs, frogs, and snails.

Reproduction

Females possess a single ovary, and males a single testis, meaning both sexes have unpaired gonads. The sperm lack a flagellum and move in an amoeboid fashion. They reproduce in swamps during the flood season when their river floodplains are underwater. They build large elliptical nests up to 1.5 m (4.9 ft) in diameter at a depth of about 1–1.5 m (3.3–4.9 ft), choosing densely vegetated swamps as preferred nesting sites. They use the shoreline plant Echinochloa pyramidalis as nesting material, which is only available to the fish during floods. Flooding triggers spawning. The female lays between 620 and 1,378 eggs in the nest. The eggs, measuring about 4.7 or 5.4 mm (in two different populations), are the largest among all species in the superfamily Mormyroidea. A sex ratio skewed toward males may ensure the fertilization of the small number of large eggs. Along with Pollimyrus, this genus is distinguished within the Mormyroidea by providing parental care for their young. The adults continue to guard the fry after they hatch.

Fishing

This species is commercially important for fishing.

Relationship with a person

With its good taste and substantial meat yield, the Frankfish is a valuable food source in several West African countries.