Deep-Sea Zone

(bathyal zone), a zone of the world’s oceans occupying an intermediate position between the neritic (shoal) and abyssal (deep-water) zones. Geomorphologi-cally, the deep-sea zone corresponds approximately to the continental slope. At present the term is encountered primarily in hydrobiology as the name of a zone of habitation of organisms preferring ocean depths in the range of 200 to 2,500 m. The deep-sea zone is characterized by the following ecological conditions: considerable pressure (up to 25 meganewtons per sq m or up to c. 250 kilograms-force per sq cm); almost total absence of light; insignificant seasonal variations in temperature and water density; relatively weak water mobility (but significantly greater than that of the abyssal zone); frequent ruggedness of the ocean bed; and a discontinuous sedimentary cover (with many outcrops of bedrock). Various representatives of the zoobenthos and many fish that are intermediate to abyssal forms predominate; plant life is extremely poor owing to the weak illumination or total absence of light.