Successive changes in distribution patterns as an adaptive strategy in the bivalveMacoma balthica (L.) in the Wadden sea

During their first year in the Wadden Sea, high proportions of the tidal-flat populations of the tellinid bivalveMacoma balthica (L.) redistribute twice: immediately after their settlement in late spring, the postlarvae show a net transportation in a shoreward direction — whereas in the subsequent winter, the grown spat move in the reverse direction. As a consequence of these two periods of high mobility, distribution patterns shift twice: though initial settlement takes place mainly in the lower half of the intertidal, most spat grow to a size of ∼0.5 cm in the upper half of it, whereas most adults live in the middle and lower zones. The successive distribution patterns of spat and olderMacoma are described in detail for Balgzand, an extensive tidal-flat area in the westernmost part of the Wadden Sea. Long-term observations in this area and published evidence from other areas are used to evaluate the suitability of different tidal zones as a habitat for successive life stages ofMacoma. For spat, the upper zone is a more favourable habitat than the lower, because predation pressure (mainly from shrimp) and disturbance are less and growth is more rapid. For adults, the upper parts are no longer a favourable habitat, because the few animals that stay suffer from high parasite load, low survival, slow growth and low reproductive output. It is concluded that in their successive life stages the majority ofM. balthica live at the intertidal level most favourable to them. The strategy of timely shifts to areas more suitable to the next life stage contributes to the success of the species: it is the most widespread and common (and one of the most stable) macrozoobenthic species in the Wadden Sea.