Injured conspecifics alter mobility and byssus production in zebra mussels Dreissena polymorpha

In laboratory experiments we studied the effects of different concentrations of chemicals released by crushed conspecifics on zebra mussel mobility and byssogenesis (adhesive thread formation). In response to predation cues, zebra mussels showed reduced numbers of byssal threads, tendency for weaker byssal attachment, shorter distance travelled on sand and slower crawling speed, but duration of crawling did not change. Mussel mobility gradually decreased with increasing concentrations of predation cues. Our findings do not agree with the earlier suggestion that decreased mobility among zebra mussels exposed to predation cues results from acceleration of settlement and increased byssal production dynamics. The results do favour the view that zebra mussels slow their physiological processes in order to lower emissions of metabolites that disclose the location of prey to predators. This study suggests that zebra mussels sense different concentrations of predation cues, make chemosensory assessments of predation risk, and respond accordingly in order to reduce the cost of defences. When faced with predation risk, the zebra mussels seem to trade off resistance to predatory attacks to gain reduced risk of detection, but we argue that profitability of this strategy may depend on the likelihood of immediate attack.