There are about 600 marine organisms which produce light from their bodies. The main groups of marine luminescent organisms include protozoans, coelenterates, polychactes, molluscs, crustaceans and fishes. Many sea bacteria are also luminescent. Sponges, jellyfish, beetles, flies and earthworms also produce light. This type of luminescence is called bio-luminescence.
Careful studies on fishes and other organisms have revealed that this light is the result of a series of chemical reactions taking place in the organisms. In this contest luminescence is basically of two types: Intracellular and Extracellular. Animals which produce luminescence by intracellular mechanism have luminescent glands. The others which produce luminescence by extracellular mechanism have unicellular or multi-cellular organs in some particular regions of their body. There are two substances (Iuciferin and luciferase) whose interaction produces luminescence. Intracellular light flashes range in duration from a fraction of a second to several minutes. Light producing organs in different sea animals are located in the skin on the ventral musculature and within the abdomen. Light is emitted only from the parts where they have these organs.
Lantern fish and angler fish are two well-known luminescent sea animals. The angler fish has its lighted baits placed in the mouth. Lantern fish produces definite patterns of light.
Luminescence produced by sea animals is helpful in locating fish shoals because the movement of a large number of fishes produces light which can be detected by light sensors. In the last two decades, significant progress has been made in the detection of fish shoals.