INFLUENCE OF PARASITES AND SUBSTRATE SIZE ON HYDROLOGIC DISTURBANCE RESPONSE OF THE STREAM SNAIL ELIMIA

Stream dwelling snails in the genus Elimia are frequently subject to hydrologic disturbance, and also serve as intermediate hosts to parasitic trematodes. In order to study how parasitism influences snail response to disturbance we conducted field surveys over 7 sites during 2002 (drought year, few storms) and 2003 (normal to wet year, several storms) to measure changes in snail and parasite abundance. In addition we used artificial streams to further investigate how parasitism, size, substrate, and snail behavior influenced snail dislodgement. We placed Elimia flava in artificial streams containing tile or gravel substrates, and then exposed them to progressively increasing flow velocities (~10, ~40, ~90 cm/s) for 5 minutes each. We recorded snail behavior and time to dislodgement, and then preserved snails to quantify their size and parasite load. Survey results indicated decreases in snail abundance and trematode prevalence in high flow years. Artificial stream experiments showed snails with high parasite loads dislodged faster than snails without parasites. There was no effect of substrate size on time to dislodgment but there was a significant interaction between parasite load and substrate size. Additionally, parasitism also appeared to affect movement patterns: snails showing predominantly downstream movement had higher parasite loads than those that did not. Behavior was also related to dislodgement probability, as snails moving upstream or to the waterline remained on the substrate longer than snails not showing those behaviors. Parasitism, substrate composition, and snail movement are useful predictors of the likelihood of dislodgement, and parasitism and substrate may both influence how snails respond to high flow disturbance.