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vision in fishes
Vision is an important sensory system for most species of fish. Fish eyes are similar to terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), 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, while the hagfish has only primitive eyespots.〔N. A. Campbell and J. B. Reece (2005). ''Biology'', Seventh Edition. Benjamin Cummings, San Francisco, California.〕 The ancestors of modern hagfish, thought to be the protovertebrate were evidently pushed to very 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 a flat or concave one. Unlike humans, fish normally adjust focus by moving the lens closer to or further from the retina.〔
Fish vision shows adaptation to their visual environment, for example deep sea fish have eyes suited to the dark environment.
==Water as a visual environment==
Fish and other aquatic animals live in a different light environment than terrestrial species. Water absorbs light so that with increasing depth the amount of light available decreases quickly. The optic properties of water also lead to different wavelengths of light being absorbed to different degrees, for example light of long wavelengths (e.g. red, orange) is absorbed quite quickly compared to light of short wavelengths (blue, violet), though ultraviolet light (even shorter wavelength than blue) is absorbed quite quickly as well.〔Helfman et al, 2009, pp. 84-87.〕 Besides these universal qualities of water, different bodies of water may absorb light of different wavelengths because of salts and other chemicals in the water
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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