Mar, Fresh. Behav. Physiol, Vol. 30, pp. 251-266 Reprints available directly from the publisher Photocopying permitted by license only © 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint. Printed in India. PREDATOR-INDUCED CHANGES IN BYSSAL ATTACHMENT, AGGREGATION AND MIGRATION IN THE BLUE MUSSEL, MYTILUS EDULIS OLOF REIMER* and MICHAEL TEDENGREN Department of Systems Ecology, Stockholm University, S-10691 Stockholm, Sweden (Received 18 October 1996; In final form 7 May 1997) Blue mussels, Mytilus edulis, were exposed to the water-borne scent of predators, either to the starfish Asterias rubens, or the shore crab Carcinus maenas, in laboratory experiments. Predator- exposed mussels developed a stronger byssal attachment compared to that of unexposed mus- sels, which was significant after about, 24 h, and twice as strong after four days. The byssal attachment response was variable. In the short term (<10h), predator scents could inhibit byssus production in those experimental batches where unexposed mussels had a high rate of byssus production. Predator-exposed mussels also formed larger aggregates, migrated less, and sought structural refuges more often. Experiments showed that these traits gave significant protection against predation from both crabs and starfish. Keywords: Mytilus edulis; predator-induced defence; prey response; byssal attachment; aggregation; migration INTRODUCTION The blue mussel, Mytilus edulis, forms the foundation of many intertidal hard bottom communities in temperate seas (Seed and Suchanek, 1992). Mussels attach to firm substrate, avoiding dislodgement by waves and cur- rents, by means of byssus threads secreted from glands at the base of the foot (e.g. Waite, 1983; Young, 1985). The strength of the byssal attachment * Corresponding author. Tel.: + 46 (0)8 163704. Fax: +46 (0)8 158417. E-mail: reimer@system.ecology.su.se 251