Redescription of Semisulcospira reticulata (Mollusca, Semisulcospiridae) with description of a new species from Lake Biwa, Japan

Abstract

Semisulcospira Boettger, 1886 is a freshwater snail genus divergent in Lake Biwa, Japan. Although recent taxonomic studies of the genus have arranged taxonomic accounts of many species in the lake, our knowledge of morphological traits has still been insufficient in S. reticulata Kajiyama & Habe, 1961, which belongs to the S. niponica-group in the genus. Among the various habitats of the lacustrine species, in addition, the species richness of this group in coastal sandy mud areas of the lake has not been completely clarified. In this study, we revisited the taxonomic account of S. reticulata and described a new S. niponica-group species, S. nishimurai sp. nov., from Lake Biwa by evaluating the genetic and morphological relationships within the group. The reconstructed maximum likelihood and singular value decomposition quartets trees, and ADMIXTURE analyses using genome-wide SNP data strongly supported the genetic distinctiveness of most S. niponica-group species. The Random Forest classification showed that S. reticulata possesses a unique, large protoconch without distinct keels, and that S. nishimurai sp. nov. is characterized by a small, less elongated teleoconch and tiny protoconch. The large protoconch of S. reticulata and the narrow radulae of S. reticulata and S. nishimurai sp. nov. suggest their specialization in life history and feeding habits, respectively. The discovery of the new species from sandy mud substrates highlights niche differentiation among Semisulcospira species, and provides new insights into the diversification of the genus in ancient lakes.