Temporal trends in shell calcification of a foundational bivalve: combining baselines and short-term experiments to understand species’ responses in a changing ocean

Anthropogenic ocean acidification (OA) is potentially a major threat for marine calcifiers. While much work has been done to assess how OA can affect calcification related traits, temporal spans of such studies remain limited. Better information about how calcification traits vary across different temporal scales and across populations is essential for better predicting species responses to OA. Here, we combine paleontological and historical data with recent field surveys and short-term experiments to quantify levels of plasticity in calcification rates and shell mineralogy of the California mussel (Mytilus californianus), a rocky intertidal foundational species. Using Pleistocene (MIS 5e) fossil assemblages in conjunction with historical data (2010) to assess how intraspecific investment in shell carbonate has changed over time, we found that calcification index (CI), defined as thickness per unit size, showed a significant increase from Pleistocene to present (2017-2018). In addition, shell mineralogy, specifically the calcite/aragonite ratio of the shell, has also changed significantly since the 1950’s, with the proportion of calcite in the shell increasing significantly. We also conducted a short-term experiment where we monitored the ability of this species to repair the shell (i.e. precipitate new carbonate) in tanks reflecting current ocean pH and pH predicted for the year 2100. Surprisingly, rate of shell repair was not significantly different between the two groups. Overall, our results show that both CI and shell mineralogy of M. californianus vary substantially over time in response to environmental perturbations, implying considerable plasticity in these traits.