First record of Arion subfuscus in Japan: a two-century, climate- and trade-mediated invasion pathway

We report the first confirmed occurrence of Arion subfuscus (Arionidae) in Japan—also the first Arion ever documented there. Specimens collected in 2024 in Hokkaido (northern Japan) were identified by morphology and mitochondrial 16S sequencing. Phylogenetic analyses placed the Japanese individuals within the S1 clade of A. subfuscus , sharing a haplotype previously reported from multiple sites in North America (and also recorded in Western Europe). These genetic data suggest a multi-step invasion pathway, originating in Europe, passing via North America, and ultimately reaching Japan. Observed mating pairs in the field and egg-laying in captivity confirm local establishment. According to long-term data from the Japan Meteorological Agency (JMA), annual precipitation in northern Japan has increased over the past decades. Concurrently, expanding global trade and shipping routes have increased propagule pressure between these regions. The combined influence of climate change and globalization likely facilitated this transcontinental invasion. Given the ecological risks posed by invasive Arion (crop damage, pathogen transmission, and native gastropod displacement), our study underscores the need for vigilant biosecurity monitoring and inspection at major trade gateways to prevent further spread.