Juvenile mud snails Hydrobia ulvae disperse by floating at the water surface in summer. The routes of dispersal are determined by the hydrography of the specific area and can be successfully predicted by a hydrographic model. Along these routes, juveniles may aggregate in temporary “satellite” sites. Turnover of organisms was high at these sites. On average, an individual only stayed for 2 days in June and July, and a few hours in August. Consequently, abundance may vary by an order of magnitude within a few days. In August, the satellite sites were depopulated 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 individuals in the sediment have also been recorded for juvenile bivalves and various meiofaunal taxa. The value of single-site sampling may therefore be questioned with respect to migrating benthic species. Instead, a mapping of resident and satellite subpopulations is recommended.