abyssal zone Zone of greatest ocean depth, i.e. below a depth of 2000 m. This zone lies seaward of, and deeper than, the bathyal zone, and covers approximately 75% of the total ocean floor. It is the most extensive Earth environment, cold, dark, with slow-moving currents (less than a few centimetres per second), supporting fauna that typically are black or grey, delicately structured, and not streamlined.

abyssal zone The lower depths of the ocean (below approximately 2000 metres), where there is effectively no light penetration. Abyssal organisms are adapted for living under high pressures in cold dark conditions. See also aphotic zone.