Morphology and habitat of Euglesa casertana populations in Yakushima Island, Japan, with implications for its taxonomic status (Mollusca: Sphaeriidae)

Abstract A population of sphaeriid clams with a manuscript name “ Pisidium habei Kuroda” has long been believed to be an undescribed species endemic to the UNESCO World Natural Heritage, Yakushima Island, Southern Japan. Although the known distribution of this taxon was restricted in one narrow high-altitude area of the island, one highland and two lowland populations were discovered in this study. We investigated the morphological and genetic characteristics of previously known and newly discovered populations, and conducted a brief survey of habitat temperature and population density. The examined specimens exhibited variations in external and internal shell morphologies within and among populations, with most falling within the range of the cosmopolitan Euglesa casertana (Poli, 1791). The four populations shared similar 16S haplotypes, which were closest to those of E. casertana from Northern Japan. Based on the phylogenetic position and morphological relationships, the Euglesa populations in Yakushima Island with the unavailable manuscript name “ Pisidium habei ” should be regarded as regional populations of E. casertana . In the lowland habitats of Yakushima Island, water temperatures rose up to 27.1 °C during summer. The significant differences in aquatic environments among the four surveyed localities indicate that Yakushima Island harbors diverse habitats for E. casertana , suggesting that its distribution range in Southern Japan and the surrounding East Asian region potentially includes wider environments than previously assumed. Taxonomic accounts and distributions of many sphaeriids remain uncertain in Japan, and thus, taxonomic and faunal studies employing integrative approaches are required.