THE EFFECT OF LARVAL TREMATODES ON THE GROWTH AND BURROWING BEHAVIOR OF HYDROBIA-ULVAE (GASTROPODA, PROSOBRANCHIATA) IN THE YTHAN ESTUARY, NORTHEAST SCOTLAND

Hydrobia ulvae were collected from five areas on the Ythan estuary, an d the prevalences of digenean parasites in the samples compared. Diffe rences in mean shell height were best explained by differences in para site prevalence. All snails larger than 6.1 mm were infected, with a s trong correlation between size and probability of infection. Compariso ns of depth distributions of different size classes of snail in the su bstratum 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 sp ecies compared with those that were buried. Snails infected with metac ercarial species were less likely to burrow than those infected with o ther species, or with uninfected snails, in laboratory experiments. La rge snails crawled more slowly and did not react as rapidly to simulat ed rainfall as small snails. These observations are interpreted as evi dence for parasite-induced changes in snail behaviour.