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A flashlight fish is a type of fish named for the pair of bioluminescent organs located at the front of the fish just below each eye, giving the fish, with its long body and the emission of light at the head, the overall appearance of a flashlight. There are a number of different types of fish that are colloquially referred to as flashlight fish. Though they all typically belong to the family Anomalopidae, a few other types of fish are also sometimes referred to by this name as well. These fish are also sometimes confused with lantern fish, which are similarly bioluminescent but tend to illuminate all along their bodies rather than just at the head.
Flashlight fish are typically dark gray, black or blue in color and are usually either nocturnal or dwell in deep sea regions, though some can come into shallow water during the night. Bioluminescence is the process by which living things such as plants and animals are able to produce light through a chemical reaction. This type of luminescence typically produces very little heat, and it is estimated that as many as 90% of sea creatures may be capable of some form of bioluminescence. The flashlight fish is one particularly spectacular example of this type of luminescence, and small symbiotic bacteria, kept in the organs beneath each eye, produce their notable illumination.
These organs are then used by the flashlight fish to produce light, and even allow the fish to control the direction of the light to an impressive degree. The fish are able to effectively turn the light off and on by changing the direction of the illumination, and so can remain dark to avoid predators. They also flash their lights across long distances to communicate with each other, with various speeds and patterns of flashing used to indicate different messages for warning and mating. Some stories even tell of sailors in certain regions using the fish as guides to navigate through dangerous areas such as reefs by watching the glowing fish beneath the water.
Flashlight fish will also flash their lights quickly, then go dark and speed away from an area to fool potential predators into attacking the wrong location. The lights are commonly used for feeding, as the glow of the light attracts plankton that the flashlight fish then feed upon. These are saltwater fish and can be kept in an aquarium that is specially prepared for nocturnal fish, though they should be kept in small groups as they tend to adapt better and feed more quickly when not alone.