Groundwater oligochaetes (Annelida, Clitellata) from the Mercantour National Park (France), with the descriptions of one new genus and two new stygobiont species

Although recognized as an outstanding hotspot of biodiversity for both flora and fauna, the Mercantour massif remains almost totally unexplored in terms of its groundwater fauna. This work presents the first overview of groundwater oligochaete assemblages of the Mercantour National Park after a standardized exploration of both consolidated (fractured massif) and unconsolidated (porous) aquifers. About 40 species of oligochaetes were found at 49 stations representative of the main hydrogeological basins of the Mercantour National Park, from both spring and hyporheic zone habitats. Five stygobiont species are identified, probably all new to science, of which two are formally described: Aberrantidrilus stephaniae Martin n. gen., n. sp. (Naididae Ehrenberg, 1828, Phallodrilinae Brinkhurst, 1971) and Marionina sambugarae Schmelz n. sp., a species belonging to the widespread Marionina argentea (Michaelsen, 1889) species complex (Enchytraeidae Vejdovsky, 1879). The freshwater subterranean species formerly attributed to Abyssidrilus Erseus, 1992 are transferred to Aberrantidrilus Martin n. gen., which means that Abyssidrilus is now restricted to its marine, abyssal species. A dozen of the Mercantour species can be considered as stygophiles. Most stygobiont species are recorded from hyporheic habitats, and stygophiles have a more balanced distribution between both kinds of habitats. The near absence of stygoxene species from the hyporheic zone suggests that this habitat is less affected by the epigean environment than springs. The dominance of enchytraeids among the groundwater oligochaete fauna is here confirmed, and the lumbriculid genus Trichodrilus Claparede, 1862 is also a characteristic faunistic element of the underground freshwater oligochaete communities. The possibility that Aberrantidrilus cuspis n. comb. sensu Sambugar et al. (1999) is a complex of cryptic species is discussed in the framework of recent progress in the knowledge of groundwater biodiversity.