Family (Animalia): Gymnotidae

The naked-back knifefishes are a family (Gymnotidae) of knifefishes found only in fresh waters of Central America and South America. All have organs adapted to the exploitation of bioelectricity. The family has about 40 valid species in two genera. These fish are nocturnal and mostly occur in quiet waters from deep rivers to swamps. In strongly flowing waters, they may bury themselves.

Physical characteristics

Like the other gymnotiforms, gymnotids have classic knifefish bodies. The body is long and eel-like, the dorsal fin and pelvic fins are absent, and the anal fin is extremely long and used for movement.

The sole member of Electrophorus is the electric eel, which produces both strong (up to 600 volts) and weak (<1 V) electric discharges, for use in predation and communication/navigation, respectively. The electric eel is the largest of the gymnotiform fishes, growing up to more than 2 m (6.6 ft) length. Species of Gymnotus range from about 10–100 cm (0.3–3.3 ft) in total length.

These knife fishes also use electricity to assist in their movement and navigation in the water due to their limited vision.

Reference: Wikipedia