Parasite-Induced Gigantism in a Snail: A Host Adaptation?

Abstract

I experimentally tested the hypothesis that parasite-induced gigantism in molluscs may be a host adaptation by increasing a host's probability of outliving an infection. I exposed juvenile pulmonate snails from two populations of the hermaphrodite Lymnaea peregra to infections with the trematode Diplostomum phoxini. Only one of the two snail populations studied is naturally in contact with this parasite (P + ) and the other is not (P - ). Diplostomum phoxini induced gigantism only in P + snails. Furthermore, P + snails showed a lower parasite-induced mortality compared to P - snails but infected P + snails did not lay eggs before they died. These results are in agreement with three out of four predictions derived from the hypothesis of gigantism as a host adaptation. The fourth prediction, that some P + snails should survive long enough to outlive the infection and reproduce, was not met. My results do not directly support the idea that parasite-induced gigantism is of adaptive value for L. peregra but suggest that gigantism is unlikely to be purely a parasite adaptation. This study cannot exclude the alternative hypothesis that gigantism is a non-adaptive side-effect of infection.