Behavioural response of invasiveMytilus galloprovincialisand indigenousPerna pernamussels exposed to risk of predation

We compared the behavioural responses of an indigenous South African mussel (Perna perna) and an invasive mussel (Mytilus galloprovincialis) to the risk of predation. Both species were subjected to the specific risk of predation by the native rock lobsterJasus lalandiiand the general risk of predation simulated by the presence of damaged conspecifics. We hypothesised that, becauseM. galloprovincialisevolved in allopatry from the rock lobster,P. pernawould respond more strongly toJ. lalandii, but that there would be less difference between species in their response to damaged conspecifics. The results confirmed our initial hypotheses.M. galloprovincialiscrawled farther under both predation treatments and generally aggregated more thanP. perna. P. pernahas a larger foot, and our results therefore do not reflect morphological differences between the species but imply greater sensitivity inM. galloprovincialisto chemical cues. Crawling distance was not enhanced under predation threat relative to the control; therefore, increased clumping due to more rapid random movement cannot explain our results, indicating that chemical attraction among individuals is important.P. pernaclumping behaviour was significantly greater when exposed to local lobster effluent or damaged conspecifics, whereasM. galloprovincialisclumped more when exposed to a general threat of predation but exhibited naïve behaviour to local predator effluent. This lack of responsiveness to a native predator could be due not only to the recent arrival of the European mussel, but also to the relatively low selective effects of predation in South Africa.