Decalcification of a Clam Shell Caused by Trematodes: Side Effect or Manipulation of the Host Phenotype?

Changes in the total weight of live clams Pisidium amnicum and the weight of their shells have been studied by comparing uninfected clams with clams infected by trematods Allocreadium crassum, Phyllodistomum folium, and Bunodera spp. The mean total and shell weights of clams infected with A. crassum are significantly lower than those of uninfected clams. The characteristics of clams infected with P. folium and Bunodera spp. are not different from the control, with the exception of the weight of clams infected with Bunodera spp. Clams infected with mature metacercariae of A. crassum possess light and fragile shells that weigh on average three times less than those of uninfected clams (25.6 vs. 74.1 g). The weight of shells of clams with immature metacercariae is not different from that of uninfected clams. The results are analyzed from the viewpoint of the hypothesis of adaptive manipulation of the host phenotype.