The deep sea

Deep-sea communities are prevalent over a significant proportion of the planet. All deep-sea habitat is in the aphotic zone, well below the distance sunlight can penetrate. Biomass was believed to fall exponentially with depth but novel sampling techniques, principally the epibenthic sled, have disporvied this for some regions. The rate of discovery of new species and the proportion of species currently known from only one sample both suggest that a great number remain to be discovered: estimates range between 500,000 and 10 million.

Ocean trenches are typically close to land masses and tend to have high rates of sedimentation, significant amount of which is of organic origin and an important available food source for trench communities. The water within trenches generally originates from cold surface water at high polar latitudes and is relatively well oxygenated.

Trenches tend to be isolated linear systems that because of their seismic activity form a habitat that is unstable and unpredictable compared with the relative environmental stability of the adjacent abyssal plains. Trench faunas are not rich in species but are often high in numbers of endemic species. There are some 25 genera restricted to the ultra-abyssal (hadal) zone, representing some 10-25% of the total number of genera present, and two known endemic hadal families: the Galatheanthemidae (Cnidaria) and Gigantapseudidae (Crustacea). The latter family contains a single species: Gigantapseudes adactylus. The greatest number of endemic species known from a single trench is a sample of 200 from the Kurile-Kamchatka trench. Ten endemic species are known from the Ryukyu and Marianas trenches.