The abyssal zone is the portion of the ocean deeper than about 2,000 m (6,600 feet) and shallower than about 6,000 m (20,000 feet). The zone is defined mainly by its extremely uniform environmental conditions, as reflected in the distinct life forms inhabiting it. Abyssal fauna, though very sparse and embracing relatively few species, include representatives of all major marine invertebrate phyla and several kinds of fish, all adapted to an environment marked by no diurnal or seasonal changes, high pressures, darkness, calm water, and soft sediment bottoms. These animals tend to be gray or black, delicately structured, and unstreamlined. Mobile forms have long legs; and animals attached to the bottom have stalks, enabling them to rise above the water layer nearest the bottom, where oxygen is scarce. Abyssal crustaceans and fish may be blind. With increasing depth, carnivores and scavengers become less abundant than animals that feed on mud and suspended matter. Abyssal animals are believed to reproduce very slowly.

Source: Britannica