Parasitism often influences the phenotype of individuals. Many of the resulting changes are due to changes in resource allocation that come with infection. Here we examine the effect of a trematode parasite on the shape and defence morphology of a New Zealand freshwater snail, Potamopyrgus antipodarum. The trematode Microphallus sp. asexually produces hundreds of metacercarial cysts in the snail. The length, width and 2-dimensional area of each snail were measured. Snails were also assessed for their degree of spininess. Snails were dissected to determine gender, brooding condition and parasitism. Snails infected with Microphallus sp. were found to be significantly less spiny than uninfected snails. Microphallus-infected snails were also found to have a significantly greater width to length ratio at larger sizes than their uninfected counterparts. These trends could be explained in at least 3 ways. (1) Infection causes the snails to not produce spines and to become wider. (2) Spiny and narrower snails are more likely to die when they become infected. (3) Spiny and narrower snails are more resistant to infection. The changes in phenotype observed are unlikely to be adaptive for either the host or parasite and probably represent physiological by-products of the host-parasite relationship.

The trematode Microphallus sp. alters the behavior of its snail intermediate host, Potamopyrgus antipodarum, in ways that seem to increase transmission to its final host, e.g., waterfowl, and decrease the probability of being eaten by other predators, e.g., fish. The parasite seems to cause the snail to move from the top to the bottom of rocks at about 0900 hr. Waterfowl feed predominantly before 0900 hr, and fish feed predominantly after 0900 hr. In the present study, we tested the hypothesis that Microphallus sp.-infected snails exhibit a change in behavior at around 0900 hr by examining their response to light and vertical orientation before and after 0900 hr. Results demonstrated that uninfected snails generally move toward light, oriented downward, and move a greater distance in the light compared with the dark at all times of day. Microphallus sp.-infected snails behaved differently from uninfected snails in the early morning but similarly to uninfected snails in the late morning with regard to downward orientation and distance moved in response to light. Snails infected with parasites other than Microphallus sp. behaved similarly to uninfected snails during both time periods. These results suggest that Microphallus sp. manipulates the behavior of Potamopyrgus sp. by altering rates of movement in response to light and vertical orientation in a manner consistent with the hypothesized 0900-hr shift.