The Role of Current and Light in Explaining the Habitat Distribution of the Lotic Snail Elimia semicarinata (Say)

Abstract

We sampled Doe Run, Kentucky to search for habitat-mediated differences in the abundance and size distribution of a pleurocerid grazer, Elimia semicarinata, predicted by the literature. Adult density was 3× greater in a slow-flow, sunny habitat, than in faster flowing or shaded habitats where densities were similar. Adult size was greatest in slow-flow habitats, and smallest in habitats with faster current velocities. In comparison, juvenile densities were greater in habitats with faster current velocities. Linear regressions indicated that both adult density and size were negatively related to current velocity, but that juvenile abundance was positively related to current velocity; none of the variables were significantly affected by light levels. Periphyton biomass (both chlorophyll a concentration and ash-free dry mass) was generally greater in slow-flow, sunny habitats, and lower in habitats with faster current velocities, suggesting current velocity may indirectly affect snail size distributions by altering food resources. Differences in gastropod size and density were maintained among habitats even though a field experiment suggested rapid colonization rates in all habitats. An experiment with in situ flow baffles revealed that the number and size of snails colonizing flow refugia increased only in habitats with faster current velocities. Taken together, our results suggest that adults may select flow refugia, while juveniles avoid them, and that pleurocerid population dynamics are influenced by current velocity directly by shear stress, or indirectly by effects of current velocity on periphyton biomass. Future studies of grazer-periphyton interactions should take such complexities into account.