Ghost Knifefishes

The ghost knifefishes consist of the family Apteronotidae, which are ray-finned fishes in the order Gymnotiformes. These fish are native to Panama and South America. They inhabit a wide range of freshwater habitats, but more than half the species in the family are found deep in rivers (typically deeper than 5 m or 16 ft) where there is little or no light. The genus Apteronotus is "artificial" and some of the species do not actually belong in it; it is polyphyletic.

They are distinguished from other gymnotiform fishes by the presence of a caudal fin (all other families lack a caudal or tail fin) as well as a fleshy dorsal organ represented by a longitudinal strip along the dorsal midline. They vary greatly in size, ranging from about 15 cm (6 in) in total length in the smallest species to 60 cm (2 ft) in the largest. It has been claimed that Apteronotus magdalenensis is up to 130 cm (4.3 ft), but this is not supported by recent studies, which indicate that it does not surpass about 50 cm (1.6 ft). These nocturnal fish have small eyes, and some species may exhibit sexual dimorphism in their snout shape and jaws.