Primary and Secondary Settlement in Mytilus edulis L. (Mollusca)

Abstract

In 1942 Maas Geesteranus postulated that the annual spatfall of mussel larvae in the vicinity of Den Helder, Holland, was scattered over a wide area, that these young mussels subsequently detached themselves from the substrate and that they then entered upon a migratory phase during which further attachment and detachment occurred many times. Eventually the mussels made contact with a substrate suitable for the adult way of life and then final attachment occurred. Verwey (1952) restated this view and stressed the importance of water currents and current-velocity in transporting the young mussels and in determining the site of final attachment. De Blok & Geelen (1958), also working at Den Helder, showed that the larvae of Mytilus edulis attached preferentially to artificial filaments which simulated the naturally occurring filamentous algae on which young mussels were found. They also observed that these young mussels subsequently disappeared from the filaments. Up to the present, however, it has not been demonstrated that the mussel larvae pass successively from the plankton to temporary sites of attachment and thence to their final place of settlement. Population studies therefore have been made to follow the sequence of young stages in the field to confirm that the observed settlement on filamentous algae was the natural prelude to permanent settlement and not a wasteful settlement of larvae on unsuitable substrata. The incidence of various age-groups of M. edulis in the Menai Straits, North Wales, was studied by observing the distribution of (a) the planktonic larvae, (b) the first settlement stages, and (c) the final settlement stages. The first settlement stages will be called early plantigrades and the final settlement stages late plantigrades, after the terminology of Carriker (1961).