FLOATING OF MUD SNAILS HYDROBIA-ULVAE IN TIDAL WATERS OF THE WADDEN SEA, AND ITS IMPLICATIONS IN DISTRIBUTION PATTERNS

Juvenile mud snails Hydrobia ulvae disperse by floating at the water s urface in summer. The routes of dispersal are determined by the hydrog raphy of the specific area and can be successfully predicted by a hydr ographic model. Along these routes, juveniles may aggregate in tempora ry ''satellite'' sites. Turnover of organisms was high at these sites. On average, an individual only stayed for 2 days in June and July, an d a few hours in August. Consequently, abundance may vary by an order of magnitude within a few days. In August, the satellite sites were de populated again, presumably due to behavioural changes. Because of the high mobility, caution is advised when including juvenile mud snails in descriptive or experimental field studies. High turnover rates of i ndividuals in the sediment have also been recorded for juvenile bivalv es and various meiofaunal taxa. The value of single-site sampling may therefore be questioned with respect to migrating benthic species. Ins tead, a mapping of resident and satellite subpopulations is recommende d.