ⓘ Vision in fishes
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells, and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarized light.
Amongst jawless fish, the lamprey has well developed eyes, although the hagfish has only primitive eyes. The ancestors of modern hagfish, considered protovertebrate was apparently a pretty deep, dark waters where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than flat or concave. Fish vision shows an evolutionary adaptation to their visual environment, for example deep sea fishes eyes adapted to the dark.