Revision of the genus Pontohoratia Vinarski, Palatov & Glöer, 2014 (Mollusca: Hydrobiidae) from South Caucasus, with description of one genus and three species

Stygobiotic hydrobiid snails of the western South Caucasus are taxonomically poorly known, and the diagnostic morphological and molecular boundaries within the genus Pontohoratia have not previously been tested using an integrative approach. We provide an integrative taxonomic revision of Pontohoratia based on shell and soft-part morphology and a multilocus dataset of five markers. Phylogenetic analyses recover Pontohoratia s. l. as comprising two deeply divergent, well-supported lineages: Pontohoratia s. str. (including the type species P. birsteini) and the lineage previously identified as 'P. smyri'. The latter lineage is described as Dimonia gen. nov., with Dimonia smyri comb. nov. redescribed, and two additional molecularly distinct lineages reported as Dimonia spp. pending the discovery of diagnostic morphological characters. Within Pontohoratia s. str. we recognise four species confined to subterranean waters of the southern Bzyb Range: P. birsteini (amended redescription) and three new species, P. styx sp. nov., P. acheron sp. nov. and P. cocytus sp. nov. Both genera show narrow ranges largely restricted to individual karst massifs, with syntopic occurrence at two sites, pointing to prolonged historical isolation among karst massifs as a driver of diversification in this subterranean fauna.