Predation by Dugesia polychroa (Platyhelminthes: Tricladida) prevents the establishment of Physa acuta (Gastropoda: Pulmonata) in hard-substratum habitats

Despite half a century of research on planarian-gastropod interactions, the population-level impact of planarian predation on littoral pulmonates is not well known. We have quantified the predation of the common lacustrine planaria Dugesia polychroa Schmidt on the ubiquitous snail Physa acuta Drap., a potential keystone grazer in benthic littoral communities, in a high-replicate laboratory experiment. Planarian predation pressure was maintained constant, and experimental populations of P. acuta were size-structured simulating young-rich small groups (re)colonising the habitat (e.g. after release from predation). D. polychroa caused a quick (week 2) reduction in the density of hatchlings and juveniles, followed by significant reduction in the numbers of young (at the onset of reproductive age) and larger, reproductive-age adults (week 3). Planarias did not have any effect on the size of reproductive-age large adults, but oviposition decreased significantly in week 2, and all P. acuta populations in planarian-present aquaria went extinct within six weeks. Dugesiidinduced extinction mainly occurred by precluding population turnover. The results strongly suggest that active predation by D. polychroa on P. acuta can be high under some environmental conditions, effectively preventing the (re)establishment of small, colonising populations of P. acuta in hard-substratum habitats.