Light exerts a definite influence on a fish's metabolism, maturation, behaviour and colouration. Light conditions in water differ from those on land not only in their intensity but also in the depth of penetration of the various wavelengths. Since a high percentage of red light is filtered out in the first few metres of water, bright red fishes are common. In both situations, red light is virtually absent and a red fish tends to fade into the background rather easily. However, many shallow water fish also have red spots or lines.
Fish have a number of adaptations to varying light levels.
The rods in the retina of the eye are specially adapted for sensing in weak light. The lower part of the retina, in the majority of fish, possesses more cones, and fewer rods, than the upper surface. A rather obvious adaptation to low light levels in a relative increase in the size of eye. This affords a much greater area for light detection. Light also has profound effects upon the internal mechanisms of fish.