The pattern of planktonic abundance of juvenile Hydrobia ulvae was investigated in the Ria de Aveiro, Portugal, with the use of floating artificial collectors made of ‘hoghair’ filter, which were deployed for 2-day consecutive periods during 3·5 months. Abundance in the collectors followed a remarkably regular semilunar pattern, being higher during spring tides around full and new moons. Cross-correlation analysis between tidal range and captures in the collectors indicated a period of oscillation of the captures of 15 d, with maximum captures tending to occur on the day of the highest amplitude tides. The physical and behavioural mechanisms responsible for the pattern remain largely unknown. However, results from a simple model assuming that migration from the sediment to the water is dependent on current velocity suggest there is a minimum threshold in average current velocity integrated along the tidal cycle (and, therefore, in tidal amplitude) above which juvenile H. ulvae exhibit the floating behaviour. Artificial collectors may provide an effective and relatively easy and inexpensive technique for the study of dispersal of post-metamorphic molluscs.