Fishes of the coral reef show a wide variety of visual adaptations to cope with marked changes in level and spectral characteristic of underwater light. Most fishes found in a reef environment rely primarily upon visual inputs for information about their surroundings. Diurnal fishes are those that are primarily active during daylight hours; nocturnal fishes are active at night; and crepuscular fishes are most active at periods of twilight (around dusk and dawn).

The great barracuda is adapted for diurnal visual hunting, though it will remain active at other times when conditions are appropriate. Spectral shifts of light in the sea are responsible for many of the visual adaptations in coral reef fishes. Light is scattered and absorbed by particles in the water and even by water molecules themselves and, with increasing depth, shorter wavelengths (such as red light) disappear and the end result is a monochromatic blue because of attenuation of all but long wavelengths of light.

Dr William McFarland notes that the light-capturing pigments of many coral reef fishes concentrate around the wavelengths of light dominant at twilight that, significantly, is the a time of great predation risk for both diurnal and nocturnal fishes. Even though such a narrow range of high visual sensitivity may at first seem detrimental to normal performance during the day or night, there has been a tremendous selective force operating that gives fishes their best chance at surviving twilight, a time of difficult lighting that changes very rapidly (rendering most visual creatures at least somewhat vulnerable). Twilight conditions for nocturnal and diurnal fishes are similar in nature to our own experience at the same times — driving at twilight, when light levels are neither dark nor light, is a notoriously dangerous activity. Diving on a coral reef during the twilight 'quiet period' can sometimes be eerie because of the lack of activity and our own visual limitations in half-light conditions. Nocturnal fishes increase their efficiency at light capture through anatomical and physiological adaptations that include presence of a tapetum, a retinal layer that reflects light and that explains the eye shine of many fishes at night when hit with the beam of a diver's light.

Dr McFarland, Dr Craig Hawryshyn (of the University of Victoria), and others have determined that at least some fishes can detect patterns of polarized light through ability to see in the ultraviolet wavelengths. This ability may increase visual contrast (allowing better discrimination of prey, predators, and potential mates), assist in orientation and migration of the fish with reference to the sun's position, and perhaps even allow recognition of individuals or species through different patterns of polarized light reflected from their scales.