Abyssal plains – Abyssal plains are defined by the IHO (2008) as “An extensive, flat, gently sloping or nearly level region at abyssal depths.” Sediment deposited adjacent to the continents forms the continental rise. Seaward of this, the land-derived sediment wedge may extend for hundreds of kilometers onto the ocean basins forming the flat abyssal plains.
Abyssal plains are remarkably flat, having a slope of less than 1:1,000 (or less than 1 m change in height over a distance of 1 km), because of the thick sediment drape that covers and subdues most of the underlying basement topography. The GSFM classifies areas less than 300 m in relief as broadly correlating with abyssal plains. Ocean basins that receive the greatest sediment input have the best developed abyssal plains and are generally located adjacent to passive continental margins (e.g., the Atlantic and Indian oceans and the Gulf of Mexico). Abyssal plains are poorly developed or absent adjacent to active continental margins because the ocean trench systems intercept and subduct land-derived sediments into the earth’s mantle (eg. the North Pacific and the eastern South Pacific Oceans).
The biology of animals that inhabit the deep ocean basins remains one of the least well studied subjects in the biological sciences (Gage and Tyler, 1991). In deep sea benthic environments, assemblages have been described for bathyal, abyssal and hadal depth zones, and there are clear differences in communities inhabiting rocky substrate versus soft substrate habitats. Animals that live in soft-sediments feed on detritus and often live within the sediments (infauna) such that the only sign of their existence in underwater photographs are the burrows in the mud where they live or the trails they leave in the mud from their feeding activities (known as Lebensspuren). Most bivalves live buried in the sediment and extend a siphon into the overlying water column to filter out food particles. Other animals crawl across the sediment, feeding on detritus. Animals that commonly occur in abyssal sediments include molluscs, worms (nematodes, sipunculids, polychaetes, hemichordates and vestimentiferans) and echinoderms (holothuroids, asteroids, ophiuroids, echinoids, and crinoids).