The discovery of reduced deep-sea environments and their remarkable communities of bacteria and metazoan organisms became one of the most important events in marine biology of last decades. The variable reduced environments share common features such as high content of reduced compounds (sulphides and hydrocarbons), often oxygen deficiency, high abundance and metabolic activity of bacterial populations, and production of autochthonous organic matter by bacteria using energy of chemical bindings (chemosynthesis). Metazoan macro- and megafaunal life in reduced environments is generally characterised by low diversity (which however may increase to the periphery of the biotope), often high degree of endemism and peculiar biological traits. Round worms or nematodes constitute the most important group of the meiofauna with respect to densities and biomass, followed by harpacticoid copepods, nauplii, polychaetes, tardigrades and other groups (Giere 1993). Because marine nematodes have a worldwide distribution, are the most prominent members of the smaller-sized animals and have a direct link with the sediment and with the processes that occur immediately above the sediment, this group can function as an interesting tool to describe habitat heterogeneity in the marine environment. In this thesis, nematodes are used as key group to explore the meiobenthic communities inhabiting different cold seeps associated with pockmarks or mud volcanoes, and located in the North and South Atlantic Ocean. The following questions were assessed: (1) What factors have an important influence on the seep meiobenthos concerning structure, diversity and distribution at multiple scales? (2) To what extent does the meiobenthos take advantage from the typical seep chemosynthetic food sources? (3) Are the meiofaunal species morphologically or physiologically adapted to resist the often toxic geochemical conditions in seeps? (4) What is the specificity of the successful meiofaunal seep species? In order to obtain this overview, the nematofauna was characterised in terms of taxonomic composition, distribution patterns on different spatial scales, habitat preferences, vertical distribution within sediments, phylogeny, presence of symbionts, and nutritional sources
