knifefish

Any of a group of fishes in which the body is deep at the front and drawn to a narrow or pointed tail at the rear, the main fin being the well-developed long ventral (stomach) fin that completes the knifelike shape. The ventral fin is rippled for forward or backward movement. Knifefishes produce electrical fields, which they use for navigation. (Genus Gymnotus and other allied genera, family Gymnotidae.)