Ocean acidification and mussels; the role of genetics

In addition to causing global warming, anthropogenic emissions of CO2 are altering the chemistry of Earth’s oceans, turning them more acidic. This change is particularly harmful to calcifying marine invertebrates, such as corals and bivalves, challenging the formation of their shells and skeletons. For many marine invertebrates, and particularly for mollusks, the larval stages are the most sensitive to the changes in seawater carbonate chemistry associated with ocean acidification. These changes induce compounding impacts on the early development of marine mussels. Briefly, exposure to projected declines in seawater pH leads to delayed development and both smaller and abnormalshaped shells in D-veliger larvae (Fig. 1). The abnormal shape is driven by an abnormal development of the underlying soft tissue, the shell field, caused by low pH conditions during the preceding trochophore stage (Fig. 1, 2). To better understand the genetics of pH sensitivity in larval development and the potential for marine mussels to evolve and overcome ocean acidification stress, we used RNA and DNA sequencing, combined with in situ RNA hybridization, to identify genes associated with the abnormal development of the shell field in trochophores of Mytilus galloprovincialis mussels exposed to low pH conditions. We also explored whether those genes harbor genetic variation that could drive rapid adaptation to ocean acidification. We reported, for the first time, a new set of pH-responsive genes associated with the development of the shell field and, secondarily, with the cellular stress response. Remarkably, five genes encompassing both these pathways exhibited large changes in allele frequencies in a genetically diverse larval population exposed to low pH conditions. Thus, while shell field development is highly sensitive to seawater pH, we found that the genes associated with that sensitivity exhibit sufficient genetic variation to support survival of the population under these otherwise stressful pH exposures. These results reinforce the notion that protecting species’ natural genetic diversity is crucial to increasing marine ecosystem resilience in the face of global change.