Light plays a vital role in the life of all oceans. At the simplest level, it provides one of the basic requirements for photosynthesis and promotes development of a food chain. Some species of fish that live in the darker reaches of the oceans also rely on light for survival. Some of these species, such as lantern fish, have even developed their own artificial means of generating light.
Lantern fish are so called on account of the special light-producing organs that are found in their skin. Each light organ, known as a photophore, is connected to the animal's nervous system which, perhaps together with some form of hormonal control, dictates the flashing sequence of these organs. In addition to a series of rows of light-producing organs along their sides (the pattern and number of which varies according to species), some lantern fish, such as those of the genus Diaphus, also have larger organs both in front of and underneath the eyes, rather like a miner's lamp. The former organs give off a twinkling effect as the animals swim, while the latter are far more powerful, effectively lighting up the area immediately ahead of the fish. Some species even have light organs on their tails; the purpose of these is probably to act as false lures to potential predators. The eyes themselves are large with large lenses and pupils and highly sensitive retinas, suggesting that vision is an important sense for these species.
Lantern fish (family Myctophidae) are one of the most important groups of midwater fishes, with some 250-300 species known. Most are small fish, measuring from 0.8-10.4 in (2-15 cm) in length. They are commonly found in large schools. Living at depths between 655-3,280 ft (200-1,000 m), these species undergo nightly migrations to the surface to feed, descending once again to the depths during the day. One explanation for this behavior is that vast quantities of tiny plankton rise to the surface of the ocean at night; species that feed on this rich food source, such as the lantern fish, therefore gain from having a condensed source of food at such times. Lantern fish also take advantage of the fact that they are not alone in harvesting the plankton; other small species such as amphipods and krill are also consumed at such times. By retreating to the gloomy depths during the day, they may also reduce the risk of predation from larger species.
The lantern fish is able to control the intensity and frequency of its flashing lights, and it is likely that the intermittent flashing serves a dual purpose. Many smaller organisms such as krill and copepods are attracted to sources of light and, by responding to the flashes of a lantern fish, unwittingly offer themselves as a meal. Recognition and warning are two other possible functions of the flashing lights. Light-producing organs are commonly found in species inhabiting the darkest regions of the sea, where the dark water can present a problem when trying to find a mate. By detecting and responding to a certain fixed frequency of light flashes, however, a lantern fish may find a mate more easily.