Light and Vision in the Aquatic Environment

Abstract

The conditions for vision underwater are more exacting than on land. As the depth increases, the daylight gets progressively less bright, and that which remains comes mostly from above and is restricted to a narrow and variable band of the spectrum. Deeper than about 1,000 m in even the clearest water, there is not enough daylight for vision, and animals must produce their own light. At all depths, light is more strongly scattered than is usual on land, and it is principally for this reason that it would be unusual indeed to be able to see farther than 100 m through the water. Both air and water scatter, refract, and absorb light; but the effects are quantitively much greater in water, and it is often possible to recognize visual adaptations to the particular conditions for vision underwater, and several of these will be considered in this chapter. However, an animal is concerned with the real problems of finding food, finding mates, and avoiding getting eaten, and several optical problems may be involved at any one time. If we are to understand how an animal is adapted to the real world around it. an attempt must be made to take a more integrated view of the problems it encounters in its visual environment.