It takes time before a defensive phenotype can be effectively developed, which handicaps sessile bivalves exposed to acute predation risks. In a laboratory experiment, we examined whether predation threats induce zebra mussels to limit metabolic rates, serving as a fast-response defence that reduces the chances of chemical detection by predators. We measured the respiration rate of mussels exposed to predation treatments (fish fed zebra mussels, fish fed chironomids, crushed zebra mussels) and to predation-free conditions, and we tested the effect of these treatments on attachment strength and aggregation. Compared with the predation-free controls, all mussels in the predation treatments tended to initially have suppressed metabolic rates. The rate of metabolism increased over time in all treatments, but only in the presence of fish fed chironomids was the increase significantly greater than in the control. Attachment strengths and aggregation rates were similar for all treatments after 7 days. Our results provide the first evidence of predation-induced changes in zebra mussel metabolic rates. We suggest that mussels respond differently to different types of predation threats. Immediately after receiving predation cues, they react promptly by lowering their production of metabolites, but over time, they re-adjust their response to the actual predation threat present.
