Elucidation of the life cycle of the trematode (Digenea: Himasthlidae), using environmental DNA detection methods.

Detection approaches based on environmental DNA (eDNA) are widely used for free-living species but remain underutilized for parasite species. This study applies eDNA detection methods to elucidate the life cycle of the trematode , which infects the socioeconomically and ecologically important edible cockle () as its second intermediate host along the northeastern Atlantic coast, including Arcachon Bay, France. The first intermediate and definitive hosts remained unknown. To identify these hosts - presumed to be a gastropod and a shorebird - we developed a quantitative PCR (qPCR)-based eDNA approach targeting partial and gene regions of . We tested for eDNA presence in water samples containing separately five dominant gastropod species and fecal samples from known cockle predators, the European oystercatcher () and gulls ( spp.), collected in Arcachon Bay. eDNA was only detected in water containing the needle snail (), with cercarial emergence confirming infection in 1.6% of individual hosts. Morphological analysis of the cercarial and metacercarial stages revealed variability in collar spine visibility. Additionally, was detected by qPCR in 42% of oystercatcher feces and no gull feces, suggesting oystercatchers are the definitive host. This study is the first to elucidate the complete life cycle of , identifying as its first intermediate host and as its definitive host. Our findings highlight the potential of eDNA approaches for resolving parasite life cycles and enabling advances in ecological research on .