TURNOVER OF POSTLARVAL BIVALVES IN SEDIMENTS OF TIDAL FLATS IN KONIGSHAFEN (GERMAN WADDEN SEA)

After larval settlement, juvenile bivalves may rapidly re-enter the wa ter column and attain secondary dispersal by byssus-drifting. In order to estimate the quantitative importance of byssus-drifting, the abund ance of drifters in the water column, their re-entry into the sediment , and their density in the ambient sediment were measured simultaneous ly over 3 months on a tidal flat in Konigshafen near the Island of Syl t in the North Sea. Turnover of juvenile clams Macoma balthica and coc kles Cerastoderma edule was more than once per week in summer, showing strong shortterm variability because of semi-lunar rhythms of driftin g activity. While there is currently no evidence for active habitat se lection in settling M. balthica and C. edule larvae, it is suggested t hat habitat selection occurs following postlarval migrations.