Invasive terrestrial slugs (Stylommatophora: Limacidae) are an under-characterized component of biodiversity change in northern East Asia. Morphological identification alone often fails to distinguish closely related species, limiting accurate assessment of species diversity and invasion histories. We conducted a mitochondrial cytochrome c oxidase subunit I (COI)–based molecular assessment of terrestrial slugs collected from multiple localities in Hokkaido, Japan, between 2024 and 2025 to clarify taxonomic identities and reassess morphology-based records. COI sequences were analyzed using maximum likelihood phylogenetic inference, revealing three distinct invasive taxa: Ambigolimax valentianus , Deroceras laeve , and Deroceras reticulatum . Specimens from eastern Hokkaido clustered robustly within the D. reticulatum clade, providing the first molecular confirmation of this species in the region. Morphological observations showed substantial overlap among species, highlighting limitations of morphology-only identification. Our findings refine previous records of Deroceras species in Hokkaido, demonstrating that invasive slug diversity has been underestimated. This study establishes a COI-based molecular baseline for invasive terrestrial slugs in Hokkaido, providing essential information for future biodiversity assessment and invasion monitoring in the Asia–Pacific region.
Tomoyasu et al. (2026) studied this question.