Attachment and settlement of post-larval mussels (Mytilus edulis L.) in the Schleswig-Holstein Wadden Sea

A number of spat collectors were designed and tested to monitor the spatfall of mussels (Mytilus edulis) in the Schleswig-Holstein Wadden Sea. 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 cylindrical collectors, using a polyester petticoat gauze as a settlement substrate, were subsequently designed to function independently of the direction of the tidal current. The location and intensity of larval attachment to artificial collectors and naturally occurring filamentous substrates varied greatly. Settling intensity was closely correlated with the length of the current vector to which the collector was exposed and/or to its height in the water column. Two main settlement seasons were apparent: the first from late May to early August, and a second, shorter and with lower abundance, in late September into October. Settlement on the collectors and subtidal hydroids during winter provides field evidence for the ability of larvae to delay metamorphosis, and suggests their presence throughout the year. Length-frequency distributions of plantigrades from the collectors were comparable to those found attached to natural filamentous substrates over the same period. The substrate choices observed during this study confirm that plantigrades preferentially attach to filamentous substrates, and subsequently migrate onto pitted and creviced hard surfaces.