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in general, any species of fishes (class Osteichthyes) that are found at extreme ocean depths, usually more than 600 m and even to as much as 2,700 m (2,000 to 9,000 feet). The species, which represent more than a dozen families of marine fishes, are characterized by huge mouths, enlarged eyes, and the presence of luminous organs on some or several parts of the body. The light-producing organs serve to attract either prey or potential mates. These and other peculiar traits of deep-sea fishes represent evolutionary adaptations to the extreme pressure, cold, and particularly the darkness of their environment. The fish life of the deep-sea habitat is among the most specialized of any habitat in the world. Because competition is less intense in the deep sea than in shore or shallow waters, many primitive types and groups still persist there.