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 M icrophallus sp. has been demonstrated to alter the behavior of the New Zeal and freshwater snail Potamopyrgus antipodarum in a way that increases the p robability that infected snails will be eaten by the final host (waterfowl) . infected snails are found foraging on top of rocks more often in the earl y morning when waterfowl;are feeding and less often in the afternoon when u nsuitable hosts (fish) are feeding. The mechanism(s) that the parasite util izes to produce this behavioral change is not known. The: present study inv estigated three possible behaviors (phototaxis, geotaxis, and photokinesis) that the parasite could alter that may account for the behavioral change s een 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, un infected snails were found to positively orient towards gravity, while infe cted snails did not. Also, both infected and uninfected snails were found t o be positively photokinetic (they move faster in the light than in the dar k), but Microphallus-infected snails were found to move more slowly than un infected snails. The differences found between infected and uninfected snai ls may be part of the manipulative effort of the parasite, but by themselve s the differences are not sufficient to explain the patterns observed in th e field.