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.