ghost knifefish

The ghost knifefishes are a family, Apteronotidae, of 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 ) where there is little or no light. The Apteronotidae should not be confused with the unrelated Notopteridae, which are also commonly called knifefishes. They are distinguished from other gymnotiform fishes by the presence of a caudal fin (all other families lack a caudal 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 in total length in the smallest species to in the largest. It has been claimed that Apteronotus magdalenensis is up to , but this is not supported by recent studies, which indicate that it does not surpass about . These nocturnal fish have small eyes. Also, sexual dimorphism exists in some genera in snout shape and jaws. Apteronotids use a high frequency tone-type (also called wave-type) electric organ discharge (EOD) to communicate.