Marine organisms that live in the water column at great depths (from 500 m).

Inhabitants of depths from 500 m to the maximum depth (about 11,000 m). 

A distinction is made between bathyal, abyssal and ultrabyssal or hadal faunas. Due to the special living conditions, the fauna of the deep is qualitatively and quantitatively many times poorer than in the upper sea horizons; echinoderms, crustaceans, molluscs and polychaetes dominate in the deep. 

The depths were intensively explored by a British expedition in the 1970s. British expedition on the Challenger. The fauna of the deepest depths (6-11 km) has only been systematically studied in recent decades by the Soviet expeditions aboard the Vityaz (1949-70), the Danish expedition aboard the Galatea (1950-52) and others. In 1958, the Vityaz expedition collected benthic animals from a depth of more than 10 km. In 1960, the French scientists J. Piccard and D. Walsh made direct observations on a bathyscaphe at a depth of 10900 m. At depth there is no sunlight, no algae, constant salinity, low temperatures, semi-fluid soils, plenty of carbon dioxide, enormous hydrostatic pressure (increasing by 1 atm for every 10 m). 

Deep-sea animals feed on bacteria as well as on "corpse rain" and organic detritus from above; they are therefore detritivores and predators. 

Deep-sea animals are either blind or have highly developed eyes, often telescopic; many fish and cephalopods have photophores; other forms have a luminescent body surface or parts thereof; hydroacoustic methods are used for information; colouration is dark (velvet black in fish) or unpigmented, and the body is whitish. 

Low temperature and abundance of carbon dioxide make it difficult for lime to precipitate from the solution; this leads to reduced calcification of skeletons and sometimes to jelly-like tissue; absence of heavy skeleton and flattened body prevents immersion in mud: long limbs (stilts), needles, stems keep the body above the bottom. 

The constancy of the environment has led to a high sensitivity to changes in the environment, but some species undertake large-scale vertical migrations; for example, the cuttlefish Abraliopis watasenia off Japan rises to the surface in schools to reproduce. 

The scarcity of food is the reason for the small and sparse settlements of deep-sea animals, the development of predation and the emergence of traps and defences. Giant animals are quite rare (e.g. the polyp Monocaulus can reach 3 m including the leg, ascidians - up to 1 m, octopus and fish - 2-5 m). 

Among the deep-sea fishes there are many with special adaptations, such as the duckfish with photophores and spiny baitfish, the serpentine Stomias boa, eels with huge mouths P Saccopharynx and Eurypharynx, luminous anchovies, colourless soft-bodied Paraliparis, blind with the longest fin rays Benthosaurus, etc. Chiasmodon fish swallow prey 2-3 times their own body length; Acanthephyra shrimp, Heterotheutis cuttlefish release clouds of luminous liquid like a smoke screen.