Aspects of ecology and conservation of sympatric, prosobranch snails in a large river

Abstract

We examined factors influencing distribution of prosobranch snails with field surveys, lab substrate-choice and movement experiments, and outdoor artificial stream experiments manipulating current, substrate and temperature. Restriction of Lithasia obovata to shallow, isolated rock patches and the more continuous distribution of Pleurocera canaliculatum on diverse substrates (shallow to deep water) seem related to food availability, predator vulnerability, wave resistance and dispersal abilities. Shallow, rocky habitats favored by the relatively fragile-shelled Lithasia maximize the more nutritious, epilithic microalgal food while minimizing wave displacement (firmer substrate) and exposure to molluscivorous fish inhabiting deeper waters. The broader distribution of the heavily-armored Pleurocera is attributed mostly to greater dispersal abilities (e.g. via deep water, gravel pathways), prevalence in more substrate types, and resistance to fish predators, but their relatively small foot may account for low densities in very shallow, wave-swept habitats. Lithasia is vulnerable because of its restriction to isolated, shallow rocky habitats, inability to reach deeper gravel dispersal pathways, and greater susceptibility to cold temperatures (slowing colonization of other habitats). Impoundments increase this threat by blocking inter-reach dispersal and hindering intra-reach dispersal through siltation. Pleurocera's survival is threatened by biofouling from zebra mussels which are more prevalent in depths inhabited by this species.