The effects of parasites and predators on the life history and morphology of freshwater snails

Both evolutionary history and present processes affect the life history and morphology of organisms. Among potential evolutionary factors, biological enemies are important agents of selection on the organisms with which they interact. In this dissertation, I describe the effects of parasites and predators on the life-history strategy and morphology of two freshwater snails (Elimia livescens and Potamopyrgus antipodarum). These snails are the first intermediate hosts to several species of digenetic trematodes. E. livescens is also an important prey species for the crayfish Orconectes propinquus.;I examined the effects of parasites on life history in two studies. First, I compared reproductive investment of uninfected snails among populations with variable levels of parasite prevalence. Consistent with life-history theory, female fecundity was positively related to parasite prevalence. In the second study, experimental infection of P. antipodarum revealed that juveniles were susceptible to infection and that infection stunted growth. Contrary to predictions, however, the snails exposed to parasites did not mature earlier in response to parasite exposure. These studies suggest that parasites affect life-history traits and that they contribute to variation in life-history strategies among host populations.;Two experimental studies revealed that both parasites and predators affect shell morphology. In a study of the effects of parasitism on morphology, I found that parasitism altered shell size and aperture shape in E. livescens. I also examined the effects of predators on morphology; I found that juvenile E. livescens exposed to effluent from tanks with crayfish differed in morphology from control snails. An experiment in which crayfish foraged on snails with varying morphologies revealed that the induced morphology protected snails from predation. Additional studies were conducted to determine whether the induced morphology is costly; without costs induction should not be maintained. I did not find costs associated with the protective morphology under laboratory conditions.;The results of this dissertation provide insight into the complexity of interactions involved in the evolution of life histories and morphology and illustrate the importance of studying species interactions when examining the selective environment of organisms.