Abstract
This paper discusses the ecology and biogeography of nematodes from freshwater environments, which are extreme in terms of temperature, chemical composition, variability or isolation. The composition of nematode faunas from freshwater pools in bromeliads or tree hollows, hot or mineral springs, pools and bogs in polar regions, seasonal lakes or pools, fresh groundwaters and caves are described and compared. The nematode faunas from these extreme habitats are also compared with those from more typical freshwater environments. Nematodes with evolutionary affinities to freshwaters but found in estuarine sediments are discussed. Conversely, nematodes from freshwaters with evolutionary affinities to otherwise marine taxa are also considered. Common themes in the adaptations of all these nematodes to such unusual environments are synthesized.