Competitive interactions between two successful molluscan invaders of freshwaters: an experimental study

Abstract

The New Zealand mud snail Potamopyrgus antipodarum (Hydrobiidae) and the pulmonate Physella acuta (Physidae) have invaded freshwaters in many parts of the world and become established. They co-exist in many streams, lakes and ponds in New Zealand, often at high densities. In the present study the effects of intraspecific- and interspecific interactions between the two species on growth and reproductive output were examined in laboratory mesocosms. In 30-day experiments, growth of Potamopyrgus antipodarum was lower in high density treatments than controls providing evidence for competition at higher densities of both snail species. No competitive effect was obtained for Physella acuta when controls were compared with high-density treatments, but growth was reduced at high densities of conspecifics. Numbers of juveniles released by Potamopyrgus antipodarum in 40 day trials declined at high snail densities and were lowest at high densities of conspecifics. Egg production by Physella acuta was also reduced at high snail densities. However, when the two species were kept together at equal densities (total snail density twice that of controls), egg production by Physella acuta was significantly higher than in all other treatments, suggesting facilitation by the congenor. Lastly, in a 10-day experiment, Physella acuta grew faster in water conditioned by Potamopyrgus antipodarum than in Physa-conditioned water, whereas Potamopyrgus antipodarum showed no growth response to Physella-conditioned water. Overall, our results indicate that growth and reproductive output of both snail species are influenced more by the density of conspecifics than the presence and density of the other species. The successful co-existence of the two species in New Zealand freshwaters therefore may be a reflection, at least in part, of few competitive interactions between them.