The edible cockle (Cerastoderma edule), a common marine bivalve in semi-sheltered sandflats, is a natural host for many parasite species of the genus Himasthla (Echinostomatidae: Trematoda). In a previous paper, Wegeberg et al. (1999) showed segregation of cockle infection by three Himasthla species (H. interrupta, H. continua, H. elongata) in relation to host tissue (foot, mantle, siphon) and host size (1·5 to 14 mm shell length). Following the same experimental procedure, a fourth species, H. quissetensis, an introduced dominant parasite of cockles in Arcachon Bay (south-west France) was investigated. The infection pattern was very similar to the patterns shown by H. elongata and H. continua. Cercariae (the free-living stage shed from prosobranch snails and encysting as metacercariae in bivalves) were most successful in the shell-length range of 6–14 mm, where 74% of the added cercariae were recovered as metacercariae. The comparison with the other Himasthla species supports the previous conclusion that the efficiency of cercariae to infect cockles depends on host size.
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