The dispersal dynamics of juvenile plantigrade mussels ( Mytilus edulis L.) from eelgrass ( Zostera marina) meadows in Maine, U.S.A.

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Field and modelling studies of the distribution of mussel larvae and juvenile plantigrade mussels in and around eelgrass meadows at Mt. Desert Narrows, Lamoine, Maine have demonstrated the following sequence of events which results in the establishment of mussel beds in shallow subtidal and intertidal waters. Mussel larvae increase on the flood tide in early July and are concentrated in the low current regions above the eelgrass blades. Larger mussel pediveligers decrease in concentration in water masses as they pass over the eelgrass meadow on the flood tide, where they settle on the eelgrass blades, especially the taller reproductive shoots. Mussel plantigrades grow from setting size to about 1 mm shell length by mid-August, when they detach from the eelgrass blades and drift inshore on the flood tide using a byssal sail. A wave of secondary settlement in late summer results in a significant drop in mussel concentration on the eelgrass blades, just prior to the release of eelgrass shoots during their fall die-off. A combined flow and particle tracking model of Mt. Desert Narrows, Maine successfully predicted the patterns of both primary and secondary settlement of mussels in the eelgrass meadows at Old Point, and their eventual dispersal inshore of the meadows in late summer. By using size frequency distributions of mussels collected from plankton tows, eelgrass blades, floating collectors and bottom cultch, we observed the absence of drifting juvenile mussels in the 0.4–0.8 mm size range, and most drifting juveniles in the 1–2 mm size groups. Eelgrass blades provide a predator refuge for mussels during metamorphosis, and allow growth of the stem glands in the foot to form a byssal sail. Successful recruitment to bottom cultch results from the combination of available substrate (eelgrass) for primary settlement, and hydrodynamics.