Predator-induced clumping behaviour in mussels ( Mytilusedulis Linnaeus)

Abstract

This study examined the aggregation behaviour of blue mussels, Mytilus edulis, under threat of predation by European lobster, Homarus gammarus Linnaeus. Risk of predation was simulated using water in which a hungry lobster had been for 24 h, and mussels were initially set in experimental containers either near (0.5 body length apart) or far (1.5 body length apart) from each other. We found that mussels exposed to lobster effluent formed more clumps, more rapidly than mussels in control conditions. The initial distance separating the mussels had no effect on their aggregation tendencies. Overall, a greater proportion of mussels were aggregated in the lobster treatment at the end of the 22-h experiment. This was not simply the result of increased locomotion. Although mussels in lobster effluent did exhibit greater crawling speed in the first hour of the experiment, mussels initially set far apart also showed enhanced locomotion in both lobster and control treatments. Yet, of the mussels initially far from each other, those in lobster effluent formed clumps on average 5 h sooner than mussels in control water. This suggests that chemotaxis may be involved. Although mussels do aggregate under risk of predation in the laboratory, it is not yet known whether predation plays a significant role in the formation of natural mussel beds.