Most deep-sea fishes, for example, are small, usually only a few inches in length, with soft bodies and minimal bone structure, which are adaptations to the tremendous pressures. Bioluminescence, or the generation of light by living organisms, is common among deep-sea fishes. Such light is produced by special cells or by bacteria that live within the fish. Some fishes have lights of different colors and on various parts of the body that flash on and off constantly. Although the exact purposes of the light displays are unknown, they presumably function as a form of communication.
Some predatory species use bioluminescence to lure prey.
Deep-sea angler fish, for example, have long filaments with a light on the end dangling over the top of the head. In the mid-1990s a team of researchers announced that their discovery of a microbe, Methanococcus jannaschii, confirmed the existence of a third major form of life. The scientists said that the microbe possesses a large number of genes previously unknown, leading them to classify it as a member of a branch of life called Archaea. The two previously accepted basic branches of life are Prokarya, single-celled organisms of the kingdom Monera, and Eukarya, usually subdivided into the kingdoms of animals, plants, and protists. Some scientists believe that the members of Archaea may represent the earliest forms of life on Earth.