Deep-Sea Animals

inhabitants of depths from 500 m to the maximum (approximately 11,000 m). Deep-sea fauna are classified as bathyal, abyssal, and ultra-abyssal or Hadean. As a result of the special conditions of their existence, the fauna of the depths are qualitatively and quantitatively much poorer than those in the higher levels of the sea. Prevalent in the depths are echinoderms, crustaceans, mollusks, and polychaetous worms.

Intensive research on the fauna of the depths was begun in the 1870’s by the English expedition on the Challenger. Fauna of the greatest depths (6-11 km) have been systematically studied only in recent decades by Soviet expeditions on the Vitiaz’ (1949-70) and the Danish expedition on the Galatea (1950-52). In 1958 benthic animals from depths of more than 10 km were obtained by the Vitiaz’ expedition. In 1960 direct observations were made in a bathyscaphe at a depth of 10,900 m by the French scientist J. Piccard and by D. Walsh.

In the depths of the sea there is no sunlight or algae, salinity is constant, temperatures are low, the bottom is semifluid, and there is an abundance of carbon dioxide and enormous hydrostatic pressure (increasing by 1 atmosphere for every 10m). Sources of food for deep-sea animals are bacteria and the “rain of corpses” and organic detritus entering the depths from above; therefore, deep-sea animals are detrivorous and predatory. They are either blind or have highly developed, often telescopic eyes. Many fish and cephalopod mollusks have photophores. In other forms the surface of the body or part of it is luminescent. Hydroacoustic methods are used by deep-sea animals to obtain information. Their coloring is dark (in fishes, velvety black), or there is no pigmentation, and the body is off-white. The low temperature and abundance of carbon dioxide hamper precipitation of lime from solution, a fact which results in decreased calcification of skeletons and sometimes a gelatinous consistency of tissues. Lack of heavy skeletons as well as flattened bodies prevents immersion of deep-sea animals in the ooze. Long extremities (stilts), needles, and stalks hold the body over the bottom. Constancy of environmental conditions has made deep-sea animals highly sensitive to changes in them; however, some species do accomplish large-scale vertical migration. For example, near the shores of Japan cuttlefish (Abraliopsis watasenia) rise to the surface in schools for reproduction.

The meagerness of food reserves is the reason for the small number and sparsity of deep-sea animal populations, the development of predation, and the appearance of trapping and defensive adaptations. Gigantism is quite rare in deep-sea animals. (For example, the polyp Monocaulus reaches a length of 3 m, including its pedicel; Ascidiae are up to 1 m tall; and squids and fishes are 2-5 m.) There are many deep-sea animals with special adaptations—for example, anglers with photophores and baitlike appendages, the large-toothed, snake-like Stomias boa, the eellike, huge-mouthed Saccopharynx and Eurypharynx, phosphorescent anchovies, the colorless, soft-bodied Paraliparis, and the blind Benthosaurus with very long fin rays. The fish Chiasmodon swallows prey two to three times the length of its own body. The shrimp Acanthephyra and the eel Heterotheutis release a phosphorescent fluid like a smoke screen.