Ocean acidification and predation risk, in isolation and in combination, show strong effects on marine mussels

Carbon dioxide-induced ocean acidification is producing a range of new selection pressures on marine calcifying organisms that show phenotypic plasticity in their shell morphology in response to predators. Although there are numerous studies on the effects of ocean acidification and predation risk on marine bivalves in isolation, information concerning their combined effects is still lacking. To bridge this gap, we conducted a long-term mesocosm experiment using mussel populations with different histories of predator exposure: crab-experienced and crab-naïve. Mussels were exposed to either lower pH or crab cues and the combination of both of these treatments for 4 mo. We demonstrate that both crab-experienced and crab-naïve mussels have heavier, thicker, rounder and, thus, stronger shells in response to crab cues, whereas low pH significantly decreased shell mass, thickness and strength. Mussels with previous crab experience showed greater plasticity in response to crab cues than crab-naïve mussels. However, the differences in plasticity between naïve and crab-experienced mussels to crab cues disappeared in the acidification treatment. Exposure to low pH and crab cues resulted in antagonistic interactions for all traits, except for shell length, where the combined effect was additive. However, there was no difference between populations in the interaction type for any of the traits. Our study provides increased understanding of potential implications for mussel populations under climate change.