Subterranean aquatic Oligochaeta

1. Aquatic oligochaetes appear to be pre-adapted to live in the subterranean environment. However, in the absence of genuine troglomorphic characters, the stygobiotic status of groundwater oligochaetes can currently be inferred only from their exclusive presence in the subterranean environment. 2. Aquatic oligochaetes constitute a diverse and mostly endemic fauna. Of the more than 100 species currently considered to be stygobionts, 60% are known only from their type locality. In the Rhône-Alpes region of France, about 93% of samples taken from caves and galleries were found to contain oligochaetes of which 13 species are classified as stygobionts. 3. Oligochaete biodiversity in aquatic subterranean environments probably originated by both preservation of relict forms and speciation in situ, possibly followed by radiations favoured by heterogeneous and fragmented habitats. Close phylogenetic affinities between some tubificid species from continental waters and the deep sea suggest a marine origin of part of the subterranean oligochaetes. 4. Oligochaetes are well known to promote bioturbation in surface habitats. Similar information on subterranean oligochaetes is scarce, but some evidence suggests notable bioturbation activity also of hypogean species, and they may also play other ecological roles (e.g. as prey and consumers) where present in high densities. 5. Valuable studies to enhance knowledge of subterranean aquatic oligochaetes include continued exploration of a variety of groundwater habitats, phylogeographic surveys that take advantage of molecular methods, analyses of species traits of both stygobiotic and epigean oligochaetes in subterranean environments, and laboratory studies to describe life histories and resolve other aspects of oligochaete biology and ecology.