Many sea animals, including a lot of fish and molluscs that live in the dark in the ocean depths, are equipped with special organs (photophores) that send out light. This special form of light, known as bioluminescence, is produced by some bacteria of the genus Photobacterium. These are symbiotic associations, in which the bacteria are supplied by their host organism with all the nutrients they need, while the fish or mollusc can use light to communicate with individuals of the same species, to defend themselves by causing their predators to lose their sense of direction or to attract preys. In some islands of the southern seas, the fishermen use the photophores of luminescent fish as a bait, since these organs keep sending out light for some time even after the fish has died.

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