Global diversity of syncarids (Syncarida; Crustacea) in freshwater

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Abstract

Syncarida are malacostracan crustaceans that live exclusively in fresh and brackish waters all over the world. With the exception of a few Anaspidacea that live in free freshwater (streams, ponds, superficial lakes and caves) the great majority inhabit the interstitial groundwater (they are stygobiont that live in groundwater in the wide sense). The Syncarida lack a carapace; have compound eyes (absent in subterranean taxa); the range in body size from 0.55 to 55 mm long and are more or less cylindrical in body shape; they have separate sexes with no free-swimming larval stage. Only the epigean Anaspidaea have coloration. Fossil Syncarida comprises two orders: Palaeocaridacea (five families, 15 genera and 20 species from Europe, USA and Brazil) and Anaspidacea (two monospecific genera from Australia). Anaspidacea also has present-day representatives: five families with 12 genera and 21 living species that live only in the Southern Hemisphere. Bathynellacea, the third order of Syncarida, with no fossil representatives, has two families, with 66 genera and 219 species that are widely distributed throughout all continents, except Antarctica. Since 1950, new species of Bathynellacea have been discovered with regularity, however many countries remain poorly sampled. The accumulation curves for Parabathynellidae, Bathynellidae and the whole of Bathynellacea demostrate that new species descriptions continue to accumulate at a rate that is well beyond the “plateau” level.