| Summary: | Increasing concentrations of atmospheric CO2 are resulting in a decrease in the pH of seawater, so-called ocean acidification (OA). Previous experiments have shown that mussels raised in low pH seawater are typically smaller, have thinner and weaker shells, and exhibit poor physiological condition compared to those living under ambient conditions. In contrast, calcifying predatory crabs have exhibited mixed responses – negative, neutral, or positive – depending upon the species and life stage considered. If crabs get larger and stronger as mussels become smaller and weaker, the predator-prey relationship between the two taxa could be dramatically altered against mussels. I developed a mechanistic model parameterized with data from the field and lab to predict effects of OA and crab predation on mussel population growth and persistence. I evaluated the effects of changes in mussel shell thickness and growth rate, and changes in crab carapace size, claw strength, and feeding and foraging behavior on mussel population fitness quantified as projected reproductive output. Scenarios with the greatest negative impact involved limitations on the ability of mussels to reach a predation-resistant size – specifically, decreases in mussel growth rate and/or increases in crab size. I then tested the response of mussels to the presence of their crab predators under ambient and low pH conditions with manipulative experiments done under controlled conditions in the laboratory. Mussels exhibited increased byssal thread production and clumping in the presence of crabs, indicative of an induced anti-predation defenses. In contrast, pH treatment had little effect on mussel growth, condition, shell mass, or survival. However, mussels from a population routinely exposed to low pH conditions in the field consistently performed better than those from a population naïve to OA conditions, suggesting that acclimation and/or local adaptation may mitigate at least some of the negative effects of OA predicted from simple lab experiments.
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