Effect of the digenean parasite Proterometra macrostoma on host morphologyin 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 pa rasitism was examined in a population of the freshwater snail Elimia livesc ens. Differences were found in the morphology of snails infected with the d igenean Proterometra macrostoma and uninfected snails. In order to differen tiate between 2 hypotheses to explain these differences in morphology, snai ls 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 bet ween infected and uninfected snails. The other hypothesis suggests that sel ective mortality of infected hosts explains the difference. In the experime nt, differences were found between infected snails and uninfected snails in overall size but not in any measurements of shape. The short duration of t he experiment relative to the duration of most infections may account for w hy field-infected snails differed in shape bur experimentally infected snai ls did not. Parasite-induced changes in growth rate are the most likely exp lanation for the larger size of infected snails relative to uninfected snai ls.