Freshwater snails in competition: alien Physa acuta (Physidae) and native Glyptophysa gibbosa (Planorbidae) in the River Murray, South Australia

Physa acuta (Draparnaud), an invasive species from Europe, is the most abundant freshwater snail in the Lower River Murray. Its ascendancy follows a general decline of native species, including the morphologically and ecologically similar Glyptophysa gibbosa (Gould). We began with two hypotheses. The first required comparisons of the salinity and temperature tolerances and other attributes of eggs, juveniles and adults to reveal differences that might account for the relative abundances of the species. As anticipated, P. acuta proved to have higher fecundity, shorter hatching time and higher salinity and temperature tolerances than G. gibbosa. The second hypothesis concerned a possible competitive interaction between the species. This was confirmed, as when juveniles were kept together, under laboratory conditions, the growth rate of P. acuta increased and that of G. gibbosa decreased (with some mortality). We suggest that the apparent decline of G. gibbosa in the Lower Murray may reflect its sensitivity to salinity and its vulnerability in interactions with P. acuta. This is a global concern as P. acuta has spread also to North America, Japan and South Africa, and there have been reports of impacts on local species.