Journal
The structure of gastropod communities was examined from January to June 1999 in four sites of the streams of Mont Saint-Michel Bay along a gradient of salinity, and the occurrence of larval trematodes infecting snails was studied. Abundance and species richness of gastropods increased from polyhaline (95 snails, 1 species) to oligohaline waters (6672 snails, 6 species). Whatever the salinity, the most abundant species was <i>Potamopyrgus antipodarum</i>, an invasive non-indigenous species that represented 80% of the gastropods. Only one male was found in <i>P. antipodarum</i> populations suggesting a predominantly parthenogenetic mode of reproduction. Among 7218 gastropods collected, 1.2% were infected by larval trematodes: 5 species in Lymnaea peregra (4.4%), 4 species in Planorbis planorbis (12.0%), one echinostome in Physa acuta (0.2%), and a new species of Sanguinicola in<i> P. antipodarum </i>(0.5%). This is the first record of infected <i>P. antipodarum</i> in Europe. No parasites were found in polyhaline waters. The prevalence per host population varied from 0 to 100% depending on time of collection, salinity and host species. In the lowest-salinity site, abundance of gastropods and prevalence of trematodes were negatively correlated. The dominance of <i>P. antipodarum</i> in the gastropod communities is discussed in relation with euryhalinity, parthenogenesis and weak rate of parasitism.
