Ghost knifefish

The ghost knifefishes are a family, Apteronotidae, of ray-finned fishes in the order Gymnotiformes. These fish can be found in the freshwater of Panama and South America.

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. The longest Apteronotid is Apteronotus magdalenensis, reaching 1.3 metres. 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.

Many Apteronotids are aggressive predators of small aquatic insect larvae and fishes, though there are also piscivorous and planktivorous species. Magosternarchus spp. are very unusual, preying on the tails of other electric fishes. Other species, such as Sternarchorhynchus and Sternarchorhamphus, have tubular snouts and forage on the beds of aquatic insect larvae and other small animals which burrow into the river bottom. At least one species (Sternarchogiton nattereri) eats freshwater sponges which grow on submerged trees, stumps, and other woody debris. The genus Apteronotus is artificial and many of the species do not actually belong in it.

The black ghost knifefish (Apteronotus albifrons) and brown ghost knifefish (Apteronotus leptorhynchus) are readily available as aquarium fish. Others are known to appear in the trade but are quite rare.