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.
For example, light exerts a great influence on the maturation of fish as well as their development and metabolism. For a number of fishes an increase in the amount of illumination produces a significant acceleration in the development of the egg. If fish do not receive the correct amount and intensity of light, they can be severely crippled and may not develop properly. The activity patterns of most fish are also related to light. Vision-orientated predators are most active during the day, often with peaks of feeding in the early morning and evening when invertebrates become more available.