Integrative taxonomy of <i>Semisulcospira kurodai</i> (Mollusca, Semisulcospiridae) with insights into its geographic variation and description of three new species from Japan

<i>Semisulcospira</i> Boettger, is a freshwater snail genus widely distributed in East Asia. The Japanese <i>Semisulcospira</i> consists of widespread riverine species and two species groups that have diversified in Lake Biwa (<i>S. niponica</i> and <i>S. nakasekoae</i> groups). <i>Semisulcospira kurodai</i> Kajiyama &amp; Habe, is a member of the <i>S. niponica</i> group and has been known as the only species that has expanded outside the lake drainage. Although recent studies have suggested the presence of undescribed species in the snails previously identified as <i>S. kurodai</i>, their taxonomic accounts have not yet been examined. In this study, the taxonomy of the species was arranged by a comprehensive taxon sampling and an integrative approach. Analyses based on genome-wide SNPs and teleoconch, protoconch, and radula morphologies clearly discriminated <i>S. kurodai</i> and the three new species. These species are described here as <i>S. masudai</i> Sawada &amp; Miura sp. nov. and <i>S. praecursa</i> Sawada &amp; Miura sp. nov. in the <i>S. niponica</i> group and <i>S. miurai</i> Sawada sp. nov. in the <i>S. nakasekoae</i> group. Morphological analysis revealed sexual size differences in each species. The analysis of geographic variation in <i>S. kurodai</i> detected four genetic subgroups in the species and indicated that morphological variation in the shell and radula of this species was increased by local environmental factors. Genetic relationships within <i>S. kurodai</i> suggest that this species originated not directly around Lake Biwa but in the Chugoku District, where its common ancestor with <i>S. masudai</i> sp. nov. was isolated by the mountainous areas. The similar teleoconch shapes of the <i>S. niponica</i> and <i>S. nakasekoae</i> group species in riverine habitats suggest parallel morphological evolution between groups, as observed in Lake Biwa endemics. The present investigation also clarified that habitat shrinkage of <i>S. kurodai</i> may have caused the current allopatric distribution between <i>S. kurodai</i> and <i>S. praecursa</i> sp. nov. https://zoobank.org/urn:lsid:zoobank.org:pub:8B2A5561-023F-435C-B189-8254BF681630.