Host and Parasite Counteradaptations: An Example from a Freshwater Snail

Abstract

Host-parasite interactions may lead to a variety of outcomes. Trematode infection of pulmonate snails is often associated with increased growth and/or survivorship of snail hosts. We use the freshwater pulmonate Lymnaea elodes and its trematode parasites to test whether this increase is a parasite adaptation, a host adaptation, or a side effect that serves no adaptive function for either participant. Field experiments indicate that trematode parasitism significantly reduces host fecundity and causes a temporary elevation and subsequent reduction in host growth. A 2-yr field survey of the prevalence of trematode infection in three snail populations revealed a significant positive relationship between shell size and prevalence. Apparently, L. elodes does not outlive its trematode infections. When survey data are compared with theoretical curves generated from a simple model of the system, it appears that trematode parasitism increases the survivorship of infected snails. Overall, the results suggest that increased survivorship in trematode-infected L. elodes is a parasite strategy for providing a stable, longterm resource for the parasite.