Movement and wave dislodgment of mussels on a wave-exposed rocky shore

Postlarval dispersal of mussels has the potential to greatly influence the dynamics of mussel assemblages on rocky shores. We individually tagged mussels (Mytilus trossulus Gould, M. edulis Linnaeus) in situ to compare rates of movement and loss between habitats (tidepools, emergent rock), positions (inside, outside of patches), and seasons. Between 7% and 86% of tagged individuals (5-25 mm shell length) moved ≥1 cm within 2-4 week intervals. Rates of movement were greater in July, when wave forces are lower, than in October, and were greater for mussels outside of patches than for those inside patches. Most tagged mussels moved distances of 1-2 cm, although 9% of movements were >10 cm. Many of the tagged mussels disappeared over the course of the 3-4 week monitoring intervals, most likely due to wave dislodgment. The frequency of mussel disappearance was generally similar inside and outside of patches and between habitats, with the exception of a higher disappearance rate in October than July 1995 for mussels in tidepools but not on emergent rock. This study demonstrates that mussel patches on a wave-exposed shore are dynamic, with movements constantly rearranging individuals within patches, and high rates of loss of individuals, presumably from wave disturbance.