Migratory rhythms of drifting juvenile molluscs in tidal waters of the Wadden Sea

Abstract

After initial spatfall, some species of bivalves and juvenile mud snails may re-enter the water column and thus undergo secondary dispersal. Rhythms of migratory activity were analysed on a tidal flat area near Sylt, E North Sea. All abundant species showed a clear diurnal pattern. The mud snail Hydrobia ulvae was more abundant in the water column during daytime than in the dark, while the reverse was true for the bivalves observed. In addition, cockles Cerastoderma edule and razor clams Ensis directus displayed a semilunar rhythm of drifting activity, and in clams Macoma balthica there was a 10 d periodicity not closely linked to lunar phases. In most species, the quantitative effect of these longer periodicities was stronger than that of the diurnal component. The pattern of migratory activity, resulting from these diurnal and lunar rhythms together with a seasonal component, was distorted by weather conditions in a species-specific way. -from Author