Enlarged Stem Glands in the Foot of the Post-Larval Mussel, Mytilus Edulis: Adaptation for Bysso-Pelagic Migration

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Abstract
Following primary settlement and metamorphosis in Myiilus edulu (L.), the original larval stem glands (S1 and S2) increase enormously in size in the plantigrade. These glands, packed with mature secretion, discharge into a multi-partitioned posterior ducted system which is transitional in form between that of the larva and the adult. The gross enlargement of the stem glands appears to be an adaptation for frequent and copious production of byssus material during the bysso-pelagic and attachment phases of early post-larval life. This interpretation is supported by the fact that the volume ratio of the stem glands to the foot declines during subsequent growth of the young mussel and that other glands take over the function of byssus stem formation in mature mussels. Enhanced dispersal of post-larvae by bysso-pelagic drifting would account for the widespread and heavy fouling of offshore platforms by the almost exclusively intertidal mussel, particularly where these structures are far distant from coastal stocks.