Rediscovery and integrative identification of Milax gagates from Japan (Gastropoda: Milacidae)

Milax gagates, native to the western Mediterranean region, has been introduced to many countries including Japan. Despite the need for an anatomical examination for its accurate identification, records of this species in Japan have been based on brief lists in the 20th century and its recent distribution remains unclear. In this study, we rediscovered slugs of M. gagates from Tokyo, Japan, and identified them by examining anatomical traits and a mitochondrial cytochrome c oxidase subunit I (COI) haplotype. This species is believed to be established in the study area based on field observations in 2022 and 2024. The haplotype of the Japanese specimens corresponded to those of non-native populations in the UK, the Netherlands, Portugal, and the Iberian Peninsula, suggesting that it was might not only have been originated from the native range, but also from secondary introduction from non-native areas.