Edinburgh Napier University

  Hydrobia ulvae were collected from five areas on the Ythan estuary, and the prevalences of digenean parasites in the samples compared. Differences in mean shell height were best explained by differences in parasite prevalence. All snails larger than 6.1 mm were infected, with a strong correlation between size and probability of infection. Comparisons of depth distributions of different size classes of snail in the substratum showed that larger snails were disproportionately represented in the top 5 mm. Snails exposed on the surface of the mud at low tide were found to have a higher chance of infection with metacercarial species compared with those that were buried. Snails infected with metacercarial species were less likely to burrow than those infected with other species, or with uninfected snails, in laboratory experiments. Large snails crawled more slowly and did not react as rapidly to simulated rainfall as small snails.

These observations are interpreted as evidence for parasite-induced changes in snail behaviour.