Variation in fecundity among populations of snails is predicted by prevalence of castrating parasites

Life-history theory predicts that high mortality should cause selection for high reproductive effort. Because parasitic castration has an equivalent r ole to mortality, from a fitness perspective, populations with high prevale nce of castrating parasites are predicted to exhibit high reproductive effo rt relative to populations with low prevalence. I examined this prediction by studying populations of the freshwater snail. Elimia livescens, that var y in prevalence of castrating trematodes. Specifically, I determined whethe r there was a positive relationship between reproductive output and prevale nce of castrating trematodes among populations. Consistent with predictions , females from populations with a high prevalence of castrating trematodes produced more eggs than females from populations with a low prevalence. Eit her genetic canalization or phenotypic plasticity may have caused the relat ionship between reproductive output and parasitism. By either mechanism, th e results suggest that castrating parasites shape the life histories of the ir hosts.