First Record of Arion subfuscus in Japan: A Two-Century, Climate- and Trade-Mediated Invasion Pathway

In 2024, we report the first confirmed occurrence of Arion subfuscus (Arionidae) in Japan—also the first Arion ever documented there. Specimens from Hokkaido (Northern Japan) were identified by morphology and mitochondrial 16S sequencing. Phylogenetic analyses placed the Japanese individuals within the global S1 clade of A. subfuscus , sharing a haplotype with populations in California (USA) and Western Europe. These genetic data suggest a multi-step invasion pathway, originating in Europe, passing through North America, and ultimately reaching Japan. Observed mating pairs and egg-laying in captivity confirm local establishment. Long-term records show substantial warming in Europe, North America, and northern Japan over two centuries, likely reducing thermal barriers to slug survival. Concurrently, expanding global trade and shipping routes have increased propagule pressure between these regions. The combined influence of climate change and globalization appears to have 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.