Sharing the load: a survey of parasitism in the invasive freshwater pulmonate, 'Physa acuta' (Hygrophila: Physidae) and sympatric native snail populations

The highly invasive freshwater snail, 'Physa acuta', was examined for parasites over temporal and spatial scales. All populations studied had a high prevalence of the parasitic morph of the oligochaete worm, 'Chaetogaster limnaei'. This worm was also found to infect two native snails, 'Glyptophysa gibbosa' and 'Austropeplea tomentosa', at lower prevalences. Seasonality and collection sites did not significantly influence the worm's infection dynamics, but strong positive correlations were found between the sizes of 'P. acuta' individuals and both the likelihood of infection and infection intensity. The ectocommensal morph of 'C. limnaei' was found in all three pulmonate species. Physa acuta was infected with two digenean trematode species which were also found in two native snails, 'Posticobia brazieri' and 'G. gibbosa'. One digenean, found within the renal gland of P. acuta, did not significantly influence the presence or infection intensity of the renal gland-dwelling C. limnaei parasitic morph. All parasites infecting P. acuta were also found to be infecting sympatric native freshwater snails. The effects that such host sharing by parasites may have upon the establishment of P. acuta as a successful invader merit further study.