Spherical lens of fish eye protruding through pupil opening of iris. Flattened camera-like lens of human eye sitting below iris and pupil.The human iris is therefore adjustable according to light intensity,while the fish eye iris is not.
Because fish inhabit water, a dense medium that absorbs, scatters, and bends light to a degree that makes seeing under water different and more complex than seeing in air, they have evolved, our of necessity, specific anatomical characteristics of the eye. In water colors ?behave? differently, and are not so easily differentiated. Therefore, fish eyes, to varying degrees, are equipped with retinal cones, which detect color ranges of color. Remember that there are two types of photo-receptors on the retina of the eye, rods which are sensitive to light in general, and cones which are sensitive to colored light. The ratio of rods to cones varies according to fish habitat. A deep sea fish, for example, which lives in dim light where rods are more useful than cones, may have a rod to cone ratio of several hundred to one. Conversely, a coral reef fish, which inhabits relatively shallow water where color abounds, may have a rod to cone ratio of ten to one. In both cases rods outnumber cones, but cones are clearly more useful on the coral reef. However, there is still much research going on into why coral reef fish are themselves so colorful. One theory involves the flaunting of bright colors as a warning to predators and another involves colors as camouflage. But the salient point here is that most aquarium fish do indeed perceive a complex range of colors, although their perception of color is slightly different than ours. There are different types of cones in a fish retina, and each one is sensitive to a different range of color. Most fish, like humans, have red, green, and blue sensing cones, but the range of each color sensed varies. Additionally, some reef fishes, like damselfish, have been found to perceive UV (ultra violet) light, and to have patterns on their bodies that can only be seen under UV light, presumably for attracting the attention of conspecifics. This is similar to markings that are found on some flowers, and that are only visible to UV sensing insects like bees. There is much going on in the natural world that is undetectable by our human eyes.
References and Further Reading
Castro, Peter and Huber, Michael E. ; Marine Biology; McGraw-Hill College, 3rd Edition, 1999
Gratzek, John B. (Editor); Aquariology: The Science of Fish Health Management; Tetra Press Publication, 1992
Moyle, Peter B. and Cech Jr., Joseph J.; Fishes: An Introduction to Ichthyology; Prentice-Hall Inc., 2000
Stoskopf, Michael K.; Fish Medicine; W. B. Saunders Company, 1993