Infection byParorchis acanthus(Trematoda) decreases grazing by the keystone gastropod,Littoraria irrorata

Parasites are well-known to alter the behavior of their hosts, but there is still a paucity of knowledge about how parasites modify the behavior of many ecologically influential host species. I studied the keystone grazer, the salt marsh periwinkle (Littoraria irrorata), to determine the influence of infection by the digenetic trematode, Parorchis acanthus, on its grazing behavior. Comparative laboratory grazing studies of wild-collected and experimentally infected snails revealed that Parorchis decreased grazing on live Spartina by more than 80%. Because of the large ecological influence of Littoraria in southern U.S. marshes, parasite modification of snail grazing may have ramifications for marsh ecosystem stability if parasite prevalence is sufficiently high.