Functional consequences of changing shell mineralogy in response to anthropogenic climate change in a foundational marine bivalve

Elizabeth M. Bullard and Kaustuv Roy, Section of Ecology, Behavior and Evolution, University of California San Diego, La Jolla, CA, Maroun Abi Ghanem, Arash Yazdani, Florian Allein, Ivan Torres, Tianqi Ren, Olivia A. Graeve and Nicholas Boechler, Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, CA

Background/Question/Methods
Anthropogenic ocean acidification (OA) is potentially a major threat for marine calcifiers. While a considerable amount of experimental work has been done to assess the effects of OA on calcifiers, analyses of calcification-related traits in wild populations still remain limited. Information about how calcification-related traits vary across natural populations and the functional consequences of these changes is essential for modeling future responses to OA. The California mussel (Mytilus californianus) is a foundational rocky intertidal species. Previous work has shown that the shell mineralogical composition, specifically the ratio of aragonite to calcite, of this species has shifted over the last half a century most likely in response to OA. While this change can potentially help protect the shell from greater dissolution under lower pH conditions, we hypothesized that this switch in mineralogy could also reduce the strength and overall function of the shell. To test the functional consequences of changing shell mineralogy, we sampled 12 different populations of this species along the Eastern Pacific across different temperature and pH regimes. We then tested for the effects of mineralogical compositions as well as other calcification-related traits on shell strength across all of these populations.
Results/Conclusions
Our results show that the morphological traits - shell volume, height, and thickness - were the primary correlates of overall shell strength. Shell volume was positively correlated with both the amount of work it took to fracture a shell as well as max load, while shell thickness and shell height were positively correlated with the max load a shell could carry before fracturing. Surprisingly, we found that shell mineralogical composition, measured as the amount of aragonite or calcite, was not significantly correlated with shell strength in the populations sampled here. Additionally, we found that organic content of the shell was negatively correlated with shell strength and that there was no significant difference in the amount of organic content in calcite and aragonite portions of the mussel shell. Our results suggest that while calcite may be a weaker polymorph of calcium carbonate than aragonite, the overall design of the shell paired with the plasticity in other calcification-related traits could act as a buffer to allow the California mussel to maintain shell strength and function despite significant changes in shell mineralogy.