PRIMARY AND SECONDARY SETTLEMENT BY THE GREENSHELL MUSSEL PERNA-CANALICULUS

The settlement and postsettlement dispersal behavior of the mussel Per na canaliculus was investigated in laboratory- and field-based experim ents to determine the role of primary and secondary settlement in its early life history. Field survey data showed size-specific patterns of mussel residency on a variety of rocky shore floral and faunal substr ata. Primary settlement (<0.5 mm), both in the laboratory and in the f ield, was largely on the hydroid Amphisbetia bispinosa and the turfing algae Corallina officinalis, Champia laingii, and Laurencia thyrsifer a, In the field, postlarvae between 0.5 and 5.5 mm occurred mostly on the algae C. officinalis, C. laingii, L. thyrsifera, Melanthalia absci ssa, Pterocladia lucida, Gigartina cranwellae, and Gigartina alveata. Juvenile and adult mussels (>5.5 mm) were resident predominantly on P. lucida, G. alveata, and Pachymenia himantophora. Size-frequency data from established mussel beds indicated low levels of primary settlemen t, with the majority of recruitment coming from secondary settlement o f larger individuals. Recruitment patterns were consistent with Bayne' s primary-secondary settlement model. Substrata deployed onto and near the shore for 21 days recruited both a primary settlement cohort and a secondary settlement cohort of mussels too large to have originated from primary settlement. Differential residency patterns in the field and settlement/recruitment experiments suggested a change in substratu m preference by juveniles as a function of size and age. It is suggest ed that mucus drifting was the likely means of movement for young post larvae among habitats. Mucus-drifting experiments demonstrated that po stlarvae <6 mm in length were able to slow their rate of descent in th e water column to 30% of the passive sinking speed using long mucus th reads. The size at which P. canaliculus were able to use this method o f dispersal greatly exceeds that seen in Mytilus edulis.