Mussels in flow: drag and dislodgement by epizoans

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Abstract
Measurements of attachment strength of the mussels Mytilus californianus and M. edulis were related to measurements of drag on mussels with algal epizoans to understand the ecological mechanics of mussel dislodgement. Attachment strength increased with shell area and was influenced by the location of the mussel within the aggregation, and by the exposure of the habitat. M. californianus had a stronger attachment than M. edulis. In both species, significantly greater force was required to dislodge mussels at the edge of the mussel bed than at the center. The mean attachment strength of M. edulis was 15 times greater in exposed habitats than in protected habitats. In the lab and field, mussels overgrown by kelp encountered flow-induced forces that were 2-6 times greater than flow forces on the mussels alone. Flow-induced forces ranged from 0.12-1.08 N, and increased with velocity from 12 to 62 cm s-1. Surface area of attached kelp was not significantly related to drag. Flow force data and surveys of dislodged mussels at an exposed beach indicate that epizoans increase the risk of mussel dislodgement, which has important implications for intertidal mussel beds impacted by disturbance events.