Species- and size-specific infection of snails by Cyclocoelum mutabile (Digenea: Cyclocoelidae).

Infectivity of Cyclocoelum mutabile miracidia to 9 species and up to 4 size classes of pulmonate snails at 14, 16, and 20 C was studied under laboratory conditions. Of the 9 species examined, 6 (Stagnicola elodes, Lymnaea stagnalis, Gyraulus parvus, Gyraulus circumstriatus, Promenetus exacuous, and Armiger crista) were highly susceptible (infection success > or = 25%), 2 (Physa jennessi and Helisoma trivolvis) had low susceptibility (infection success < 25%, > 0), and 1 (Physa gyrina) was not susceptible to infection. Within highly susceptible species, snail size was negatively related to susceptibility and temperature had variable effects. Infection success was not affected by temperature or snail size in species with low susceptibility. Production of cercariae was negatively correlated with susceptibility among snails of different sizes and species, but was not influenced by snail size for a given species. Among species, metacercariae production was typically higher in lymnaeids than in either planorbids or physids. Results of experiments where miracidia were provided with a choice of 2 different snails suggest that they do not discriminate between species with high and low susceptibility.