Marine species of the ostracod genus Microceratina Swanson (Cytheruridae, Eucytherurinae) were until now known only from their hard parts, the valves and carapaces, as no living animals have been described. Here we report the first living population, from a tropical anchialine cave. The description of the limbs and hard parts of this new taxon, M. martensi sp. nov., enhances our understanding of the origin and evolution of the cave-dwelling Microceratina – the new species and M. pseudoamfibola (Barbeito-Gonzalez) from an anchialine cave in Southern Italy – and clarifies their affinities with other Eucytherurinae species. Microceratina is known from both Recent and fossil species (Quaternary, Tertiary and Late Cretaceous) from shelf and deep-sea habitats and/or sedimentary facies, located in the Pacific Ocean (along the Australian and New Zealand coasts), the Mediterranean (Greece and Italy), the North Atlantic (British Isles) and the Baltic Sea (Rügen Island). This suggests that the Microceratina group spread through the expanding Tethys Ocean. The morphological traits of the two cave-dwelling species reflect their ecological conditions. Cave-dwelling Microceratina species appear to have originated from epigean shallow water species predisposed to colonize subterranean habitats.

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