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

Carbon dioxide-induced ocean acidification are 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, the understanding concerning their combined effects is still lacking. To bridge this gap, we conducted a long-term mesocosm experiment using mussel populations with different history of predator exposure: crab-experienced and crab-naïve. Mussels were exposed to either lower pH or crab cue and the combination of both these treatments for four months. 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 experience to crabs showed greater plasticity in response to crab cues than crab-naïve mussels. However, this differential response was eliminated by ocean acidification. Exposure to low pH and crab cue resulted in antagonistic interactions for all traits, except for shell length where the combined effect was additive. However, there was no difference among populations in the interaction type for any of the traits. Our study may provide implications for the management of mussel populations under climate change.