Bioluminescence means light produced by living organisms. Although many groups of organisms (fish, insects, molluscs etc) possess bioluminescent organs they are all normally dependent on bacteria, maintained in special cells, for the production of the light. However some fish do produce a chemical that reacts with water to produce light.
The bacterial light is usually produced as a result of an enzyme (often luciferase) mediated oxidation reaction in which a molecule (often luciferan) changes its shape and emits a single photon of light in the process. Luciferan is a large complex molecule that can later be returned to its original shape through a reduction reaction during which it gains an amount of energy equivalent to the single photon of light it emitted earlier. Because the essential reaction is an oxidation reaction the host organism, in this case the fish, can control the amount of light emitted by controlling the blood flow, and hence the oxygen supply, to the cells containing the bacteria.
The possession of light producing organs is quite common in fish particularly, but not only, in those species that live in deeper waters. Forty-five percent of fish that live at depths below 300 metres, and 75% of all Bathypelagic fish have some light emitting organs.
The light glands of different fish species show quite a wide variety in complexity. The simplest are just a few cells embedded in the skin. More complex systems involve a reflecting layer behind the light source (black or silver), The light producing gland, a color filter and a lens. I some species the lens is derived from a modified scale. The positioning of the light organs is also highly variable between species so that scientists can use the pattern of the light organs to recognize an individual species. Most of the species that possess light organs are pelagic, some exceptions to this can be found in the benthic Macrouridae (Rat-tail fish)
Many species of fish have lights on the lower parts of their bodies. Scientists believe they use these lights to prevent their bodies becoming silhouetted against the surface. A fish looking up from below will see the body of the fish above it against the haze of light that is the surface, by having lights on their underside fish can break up the pattern of their body and effectively hide in the haze.
Lights along the sides of the body are more likely to be for communicating with other members of their own species, either whilst shoaling, or to attract potential mates.
In other fish, such as the well known Angler Fish the light organs seem designed to attract prey. Nearly all angler Fish possess luminous baits. The same applies to other types of fish with lights near their jaws or at the ends of fins or barbels.
Malacocephalus laevis has light producing bacteria in a gland near its anus, when it is disturbed it can eject some of these bacteria into the water through its anus, the sudden appearance of this luminescent mass would easily disorientate a predator. It is possible that fish with lights distant from the centre of their body, such as on or near their tail, may use these lights to disorientate predators. Of course it is highly likely that fish use their light organs for more than one purpose. It is often difficult to unwind time and discern whether or not an adaptation had a single primary cause. While it remains true that an adaptation with more than one positive function is more likely to be selected for than one with only a single positive function it is also true that Nature is prudent of existing adaptations and often works to add functions to an existing adaptation. .
Some fish can actually produce a beam of light, and such a beam would obviously help them find prey. Many bathypelagic fish with light producing organs also have addition light organs that shine into their own eyes, scientists believe that this may work to accustom their eyes to a certain level of light so that when they turn on their main lights they do not blind themselves.
Most luminous fish are teleosts but there are some luminous sharks as well such as Euprotomicrus bispinatus, which is also the smallest shark in the world. Some examples of bioluminescent fish are:- Isistius brasiliensis which has thousands of small luminescent organs on its belly; Anamalops koptroton and Photoblepharon palpebratus which are coastal; Giganticus vanhoffeni and Dolopichthys niger which are both deep-sea angler fish.