Molecular elucidation of trematode life cycles in Japanese tidal flat and freshwater lake habitats.

Elucidating the life cycles of trematodes is essential for understanding their ecology and evolution. However, it is often difficult due to morphological similarities among closely related species, particularly at larval stages, and the distinct morphologies among different life stages of a single trematode species. Here, we used molecular genetics to differentiate morphologically similar species and to link their different life stages within a single species. We dissected 2152 snails, which serve as the first intermediate hosts of trematodes, and 396 individuals of their potential second intermediate hosts, such as fish, crustaceans, and insects. Using molecular genetics, we identified 25 trematode species in those hosts. We investigated the life cycles of these trematodes using the obtained sequences and identified the first and second intermediate hosts for two species. By integrating our sequences with information from the associated publications, we further inferred first and second intermediate host links for an additional 14 species. Among these, five species also had potential definitive host information at the species level. Nine species were found only in either the first or the second intermediate host. While the current study could not elucidate the life cycles of these 9 species, we consider that the accumulation of trematode DNA sequences, along with host information, in public databases will facilitate future identification of their life cycles. This study highlights the usefulness of molecular approaches to infer trematode life cycles.