Shell mineralogy of a foundational marine species, Mytilus californianus, over half a century in a changing ocean

Anthropogenic ocean acidification (OA) is a potential threat for marine calcifying organisms. While much experimental work has been done to assess the impacts of OA on marine calcifiers, studies over long temporal scales and across multiple populations still remain limited. Here, we combine historical data with recent field surveys to quantify the plasticity in shell mineralogy, a key functional trait, of a foundational marine bivalve, Mytilus californianus. Our data suggest that mineralogy in this species is responding more to pH and saturation state changes than warming or decreasing salinity. This study highlights the importance of utilizing long-term data sets and large spatial comparisons to understand and test predictions about species responses to a changing world.