Behavioral modification by trematodes shaping the distribution for <i>Batillaria multiformis</i> along rocky intertidal flats

Behavioral modifications of the snail host caused by trematode infections may result in spatial variations in the snail population along the intertidal elevational gradient. In this study, we investigated the impact of trematode infections on the behavior and distribution of the first intermediate host snail, Batillaria multiformis, on the rocky tidal flat of Kan-onji in the inner Seto Inland Sea, Japan. Seventy sampling points were randomly selected using quadrats across the intertidal gradient. A total of 1,322 collected B. multiformis snails were examined for parasitism. A higher number of infected snails, including those infected by Cercariae batillariae and Cercaria hosoumininae, were observed in the lower elevational gradient. The mark and recapture field experiment was conducted to assess the movements of the snails across the intertidal gradient. About a hundred samples were recovered after four months from the release. Most of the infected snails moved to the lower shore, whereas a large number of uninfected snails migrated to the higher shore. These findings support our hypothesis that C. batillariae and C. hosoumininae alter the behavior of Batillaria multiformis, possibly to pursue favorable habitats for parasite transmission to the second intermediate host such as fishes or crustaceans.