Gymnotiformes

Apteronotidae - They are distinguished from other gymnotiform fishes by the presence of a caudal fin as well as a fleshy dorsal organ represented by a longitudinal strip along the dorsal midline.

Sternopygidae - These fishes inhabit freshwater streams and rivers in Panama and South America.

Hypopomidae - These fish originate from freshwater in Panama and South America.

Gymnotidae - The naked-back knifefishes are a family of knifefishes that are found only in fresh waters of Central America and South America.

Rhamphichthyidae - Sand knifefishes are freshwater fishes of the Rhamphichthyidae family, from South America.