ATTACHMENT AND SETTLEMENT OF POSTLARVAL MUSSELS (MYTILUS-EDULIS L) INTHE SCHLESWIG-HOLSTEIN WADDEN SEA

A number of spat collectors were designed and tested to monitor the sp atfall of mussels (Mytilus edulis) in the Schleswig-Holstein Wadden Se a. Initial trials using vertically suspended sisal ropes placed in the intertidal gave an indication of the importance of the orientation of the collectors to the direction of the tidal stream. Rotatable and cy lindrical collectors, using a polyester petticoat gauze as a settlemen t substrate, were subsequently designed to function independently of t he direction of the tidal current. The location and intensity of larva l attachment to artificial collectors and naturally occurring filament ous substrates varied greatly. Settling intensity was closely correlat ed with the length of the current vector to which the collector was ex posed and/or to its height in the water column. Two main settlement se asons were apparent: the first from late May to early August, and a se cond, shorter and with lower abundance, in late September into October . Settlement on the collectors and subtidal hydroids during winter pro vides field evidence for the ability of larvae to delay metamorphosis, and suggests their presence throughout the year. Length-frequency dis tributions of plantigrades from the collectors were comparable to thos e found attached to natural filamentous substrates over the same perio d. The substrate choices observed during this study confirm that plant igrades preferentially attach to filamentous substrates, and subsequen tly migrate onto pitted and creviced hard surfaces.