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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.

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