Taxonomic insights and evolutionary history in East Asian terrestrial slugs of the genus Meghimatium

[Display omitted] •The phylogeny of fully shell-less slugs Meghimatium is inferred.•The genus Meghimatium shows more phylogenetic diversity than morphology suggests.•The divergence time of the genus is concordant with the formation of the Japanese mainland.•Clades are consistent with the geographic structure.•A DNA sequence-based method is indispensable to delimit Meghimatium species. East Asia, specifically the Japanese Archipelago, is a biodiversity hotspot of both vertebrates and invertebrates. Mollusks represent a burst of species diversity in this region due to the effects of biotic and abiotic factors on their morphological traits, such as shell shape and size. However, the evolutionary history of terrestrial slugs in East Asia remains unknown. In the present study, we investigated the molecular phylogeny of terrestrial slugs of the genus Meghimatium. This genus includes three described and eight undescribed species, and our study used all except for two. Based on phylogeny and the species delimitation tests, the genus Meghimatium was split into many putative species, suggesting higher species diversity than previously thought based on morphological and anatomical studies and that almost undescribed species may be inappropriate. Therefore, morphological traits, such as body size and colour, conventionally considered for classification may easily vary or be similar across geographic region. Moreover, the divergence time of this genus is almost concordant with the geographical time scale of the formation of the Japanese mainland. Our findings suggest that molecular phylogenetics helps classify Japanese Meghimatium slugs, but comprehensive taxonomic revisions using multi-locus analyses are needed.