Naturally acidified habitat selects for ocean acidification-tolerant mussels

Ocean acidification severely affects bivalves, especially their larval stages. Consequently, the fate of this ecologically and economically important group depends on the capacity and rate of evolutionary adaptation to altered ocean carbonate chemistry. We document successful settlement of wild mussel larvae (Mytilus edulis) in a periodically CO₂-enriched habitat. The larval fitness of the population originating from the CO₂-enriched habitat was compared to the response of a population from a nonenriched habitat in a common garden experiment. The high CO₂-adapted population showed higher fitness under elevated Pco₂ (partial pressure of CO₂) than the non-adapted cohort, demonstrating, for the first time, an evolutionary response of a natural mussel population to ocean acidification. To assess the rate of adaptation, we performed a selection experiment over three generations. CO₂ tolerance differed substantially between the families within the F₁ generation, and survival was drastically decreased in the highest, yet realistic, Pco₂ treatment. Selection of CO₂-tolerant F₁ animals resulted in higher calcification performance of F₂ larvae during early shell formation but did not improve overall survival. Our results thus reveal significant short-term selective responses of traits directly affected by ocean acidification and long-term adaptation potential in a key bivalve species. Because immediate response to selection did not directly translate into increased fitness, multigenerational studies need to take into consideration the multivariate nature of selection acting in natural habitats. Combinations of short-term selection with long-term adaptation in populations from CO₂-enriched versus nonenriched natural habitats represent promising approaches for estimating adaptive potential of organisms facing global change.