Insights into the life cycles of trematodes infecting pelagic gastropods in Japan

Pelagic gastropods, including holoplanktonic and neustonic species, have been considered potential sources of trematodes that parasitize marine fishes. However, the taxonomy and life cycles of metacercariae detected in pelagic gastropods remain poorly understood. This study examined metacercaria infections in 10 species of pelagic gastropods ( N = 415) collected from Japanese coastal waters between 2016 and 2025. Metacercariae were detected in four pelagic gastropod species and were identified as seven trematode species based on molecular analyses of the 28S ribosomal RNA and cytochrome c oxidase subunit 1 regions. In holoplanktonic gastropods, six species within the family Lepocreadiidae were molecularly identified as Cephalolepidapedon saba , Prodistomum orientale , Prodistomum sp., Opechona sp., Preptetos sp., and Lepocreadiidae gen. sp., all of which represent new host-parasite records. A metacercaria molecularly identified as Dinurus tornatus (Hemiuridae) was detected in the neustonic gastropod Glaucus atlanticus. These results confirm that pelagic gastropods can act as the second intermediate or paratenic hosts and suggest predator–prey relationships with marine fishes. In addition, rediae obtained from benthic gastropods (cowries) were examined molecularly and identified as Prodistomum sp., thereby clarifying aspects of its life cycle.