Effect of the digenean parasite Proterometra macrostoma on host morphology in the freshwater snail Elimia livescens

Parasitism can affect size in gastropods by altering the host's growth rate, but other morphological effects of parasitism have rarely been examined. In this study, the relationship between variation in host morphology and parasitism was examined in a population of the freshwater snail Elimia livescens. Differences were found in the morphology of snails infected with the digenean Proterometra macrostoma and uninfected snails. In order to differentiate between 2 hypotheses to explain these differences in morphology, snails were experimentally infected in the laboratory and several morphological traits were measured after 180 days. One hypothesis suggests that parasite-induced changes in shell development explain differences in morphology between infected and uninfected snails. The other hypothesis suggests that selective mortality of infected hosts explains the difference. In the experiment, differences were found between infected snails and uninfected snails in overall size but not in any measurements of shape. The short duration of the experiment relative to the duration of most infections may account for why field-infected snails differed in shape but experimentally infected snails did not. Parasite-induced changes in growth rate are the most likely explanation for the larger size of infected snails relative to uninfected snails.