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abyssal zone[ə′bis·əl ′zōn]
the zone of the greatest sea depths (more than 2 km). Areas of the ocean floor with depths over 6 km are ordinarily considered ultra-abyssal. The relief is represented by large ocean basins and by underwater ranges and plateaus. The water is characterized by relatively slight mobility, constantly low temperatures (1–2°C, in the polar regions below 0°C), constant salinity of 34.7–34.9 per hundredth, and constant density. Sunlight is totally absent from the water. These conditions, as well as the enormous pressure characteristic of the abyssal zone (20–110 meganewtons or 200–1,100 kg/cm2), determine the uniqueness of the organic world. The bottoms are various silts of organic or mineral origin. The plant world is represented chiefly by certain bacterial forms. Animals of the abyssal zone are blind or have enormous eyes. Many of the organisms are luminescent. Ultra-abyssal animals are characterized by antiquity and a sharply expressed endemism.
O. K. LEONT’EV