Nematodes are probably the most abundant metazoans in the biosphere. They occur in a wide range of habitats, being absent only from the oceanic plankton. In marine sediments, free-living nematodes are the most abundant metazoans, often numbering millions per m² in the sediments. In the deep sea nematodes comprise between 85-95% of the metazoan meiofauna, in poor as well as in rich areas.

Most adult marine nematodes are elongated cylindrical worms, small in size, with only a few tenth of µm in diameter and a few mm in length, and structurally simple, consisting of two concentric tubes. Despite their similar basic morphology nematodes occupy very different ecological roles and trophic levels in sediments. Many species feed on bacteria, on algae or on both; they eat detritus and dissolved organic matter, and a considerable number of nematodes are predators feeding on other nematodes, oligochaetes, polychaetes, etc. This diversity in feeding is also reflected in species diversity. The number of nematode species in most habitats is much higher than that of any other metazoan group. The extremely successful coexistence of many species in a wide range of habitats lies in their physiological, biochemical and behavioural adaptation, not in their structural diversity.

The male of the marine genus Leptolaimus has been collected from Arctic deep-sea sediments at 2500 m water depth. This nematode is a selective deposit-feeder, with a small, unarmed buccal cavity, mainly feeding on bacteria and detritus.

The past decades have seen remarkable changes in key arctic variables, including a significant decrease in both, sea-ice extent and thickness. Enabling the detection of expected changes in abiotic and biotic parameters in a transition zone between the northern North Atlantic and the central Arctic Ocean, the Alfred Wegener Institute for Polar and Marine Research established the deep-sea long-term observatory HAUSGARTEN in the eastern Fram Strait.

Temperatures in the HAUSGARTEN area slightly increased at 2500 m water depth. Changes in the temperature and salinity of Arctic seawater and associated shifts in nutrient distributions will directly affect the marine biota on multiple scales from individuals to populations and communities, consequently altering food web structures and ecosystem functioning. Especially in the deep sea, nematodes contribute significantly to the metabolic balance of food webs and to the degradation and remineralisation of organic matter. A change in nutrient availability and increased water temperatures will have a direct effect on the composition of these communities. Thus, the knowledge of diversity patterns of nematodes, as important component of the sediment-inhabiting biota in the deep sea, will not only enable us to better understand, but also to improve our predictive capabilities concerning the ecological effects of global warming in Arctic deep-sea ecosystems.