The effect of a trematode parasite (microphallus sp.) on the response of the freshwater snail potamopyrgus antipodarum to light and gravity

Parasites often influence the behavior of their hosts in ways that increase the probability of transmission of the parasite. The digenetic trematode Microphallus sp. has been demonstrated to alter the behavior of the New Zealand freshwater snail Potamopyrgus antipodarum in a way that increases the probability that infected snails will be eaten by the final host (waterfowl). Infected snails are found foraging on top of rocks more often in the early morning when waterfowl are feeding and less often in the afternoon when unsuitable hosts (fish) are feeding. The mechanism(s) that the parasite utilizes to produce this behavioral change is not known. The present study investigated three possible behaviors (phototaxis, geotaxis, and photokinesis) that the parasite could alter that may account for the behavioral change seen in the field. Infected and uninfected snails were assessed in terms of their orientation to light (phototaxis), orientation to gravity (geotaxis), and movement in response to light (photokinesis). There was no evidence of phototactic behaviors in either infected or uninfected snails. However, uninfected snails were found to positively orient towards gravity, while infected snails did not. Also, both infected and uninfected snails were found to be positively photokinetic (they move faster in the light than in the dark), but Microphallus-infected snails were found to move more slowly than uninfected snails. The differences found between infected and uninfected snails may be part of the manipulative effort of the parasite, but by themselves the differences are not sufficient to explain the patterns observed in the field.