|either of two viviparous tropical American freshwater cyprinodont fishes, Anableps anableps or A. microlepis, that swim at the surface of the water and have half of each eye specialized for seeing in air, the other half for seeing in water|
either of two species of tropical American river fishes of the genus Anableps (family Anablepidae, order Atheriniformes). Four-eyed fishes are surface dwellers and have eyes adapted for seeing both above and below the water surface. The eyes are on top of the head, and each is divided into two parts, an upper half for vision in air, and a lower half for vision in water; hence, the common name. The fish grow to a maximum length of about 30 centimetres (one foot)
Learn more about four-eyed fish with a free trial on Britannica.com.