Effects of Trematode Parasites on Habitat Use and Disturbance Response of Freshwater Snails in the Genus Elimia

Abstract

Freshwater snails in the genus Elimia are abundant and important grazers in many southeastern USA streams; in addition, Elimia serves as intermediate hosts to many species of parasitic trematodes and is frequently subject to disturbance from floods. Previous work has shown that parasites can increase Elimia feeding and reduce reproduction; however, the prevalence of trematode infections within Elimia as well as the relationships among parasitism, snail habitat use, and response to disturbance are unknown. I quantified Elimia density, trematode prevalence, and physicochemical habitat conditions, both in wet and dry years, within 7 stream sites in Alabama. All snail populations sampled were parasitized by trematodes to some degree (1–26%). Patterns among streams showed that adult snail density was negatively related to prevalence and percent shade, and was positively related to mean current velocity. Surveys demonstrated that within-streams, parasite prevalence was positively correlated with snail density and stream substrate size; however, it is unknown whether these patterns resulted from parasites altering habitat use by snails, or because infection rates were higher in these habitats for other reasons. I used in situ enclosures with 2 snail density and 2 substrate size treatments to test if substrate size and snail density affected parasite prevalence and growth of Elimia flava. Compared with their contrasting treatment, the higher snail density and larger substrate treatments both showed significantly higher prevalence. In addition, snail growth was significantly lower in the high- (vs. low-) density treatment. There was no effect of parasite prevalence on snail growth. Last, I used artificial streams to investigate how parasitism, snail size, substrate type, and snail behavior influenced the likelihood of snail dislodgment from substrates during high flows. I placed Elimia flava in artificial streams containing tile or gravel substrates, and then exposed snails to progressively increasing flow velocities. Earlier survey results indicated lower snail density and trematode prevalence in high- (vs. low-) flow years. Artificial stream experiments indicated that snails with high parasite loads dislodged faster than snails with low parasite loads or lacking parasites. Given the widespread distribution of trematodes within Elimia populations and the potential for trematodes to influence snail density both within and between stream sites, trematodes appear to have an important influence on population dynamics of their host snails in many southeastern streams. Parasitic trematodes do not appear to alter host behavior as suggested by others, but, because risk of infection is related to substrate type, snails may potentially alter their behavior to minimize risk of parasitism.