The deep sea biome occurs in that part of the ocean and seafloor beyond the continental shelves. It covers 65 percent of the planet’s surface and reaches depths of -650 ft to -36,198 ft at the bottom of the Challenger Deep in the Mariana Trench. Much of the seafloor region consists of the flat abyssal plain, located on average 3.5 mi below sea level and covered with muds and oozes. Bottom water temperatures are a steady 28° F. The fine sediments on the seafloor come from stream runoff and from a constant rain of debris and organisms from the upper layers of the sea. They are mostly made up of the shells of plankton: silica-based shells from diatoms, radiolarians and silicoflagellates and calcium carbonate shells form foraminiferans, coccolithophores, and pteropods. Calcareous oozes dominate at mid-oceanic ridges; silicaceous oozes are found on the abyssal plain. The abyssal plain is punctuated by mid-oceanic ridges and seamounts.
Hard substrates are scarce but important for life in the deep sea. Among the main sites are exposed volcanic rocks on the mid-oceanic ridges, the steep slopes of seamounts where sediments are unable to collect, concretions of iron and manganese on the seafloor, tubes and shells of invertebrates, and the skeletons of whales and large fish resting on the seafloor. These all become attachment sites for sessile animals, which in turn attract motile animals seeking prey or sheltering habitat.
Important communities with the deep sea biome include: