Naturally acidified habitat selects for ocean acidification–tolerant mussels

Ocean acidification severely affects bivalves, especially their larval stages. Consequently, the fate of this ecologically\udand economically important group depends on the capacity and rate of evolutionary adaptation to altered ocean carbonate\udchemistry.Wedocument successful settlement ofwild mussel larvae (Mytilus edulis) in a periodicallyCO2-enriched\udhabitat. The larval fitness of the population originating fromthe CO2-enriched habitat was compared to the response of a\udpopulation from a nonenriched habitat in a common garden experiment. The high CO2–adapted population showed\udhigher fitness under elevated PCO2 (partial pressure of CO2) than the non-adapted cohort, demonstrating, for the first\udtime, an evolutionary response of a natural mussel population to ocean acidification. To assess the rate of adaptation,\udwe performed a selection experiment over three generations. CO2 tolerance differed substantially between the families\udwithin the F1 generation, and survival was drastically decreased in the highest, yet realistic, PCO2 treatment. Selection of\udCO2-tolerant F1 animals resulted in higher calcification performance of F2 larvae during early shell formation but did not\udimprove overall survival. Our results thus reveal significant short-term selective responses of traits directly affected by\udocean acidification and long-term adaptation potential in a key bivalve species. Because immediate response to selection\uddid not directly translate into increased fitness,multigenerational studies need to take into consideration themultivariate\udnature of selection acting in natural habitats. Combinations of short-termselectionwith long-term adaptation in populations\udfrom CO2-enriched versus nonenriched natural habitats represent promising approaches for estimating adaptive\udpotential of organisms facing global change