Long-term ocean acidification exposure negatively impacts the structural and mechanical properties of sea urchin skeletons at a natural volcanic CO2 seepBiology & Ecophysiology

Callum Hudson¹, Ben Harvey², Sean Connell³, Jeffrey Jolly¹, Timothy Ravasi^{1, 4}

¹Okinawa Institute of Science and Technology, Japan - ²University of Tsukuba, Japan - ³University of Adelaide, Australia - ⁴James Cook University, Australia

Two decades of laboratory studies investigating the direct impacts of elevated CO2 on adult sea urchins document negative effects on growth, feeding performance, skeletal structure and strength. However, laboratory studies often fail to capture the full complexity of natural ecosystems, such as the influence of species interactions or spatiotemporal abiotic variation, factors which may ameliorate or exacerbate the negative effects of ocean acidification. Whether or not experimental observations hold true in natural systems will determine if they can be extrapolated to predict responses in more ecologically relevant contexts. We observed negative effects of long-term exposure to elevated CO2 on the skeletal properties of adult sea urchins (Echinometra spp.) at a natural CO2 seep. Biomineralisation, and in turn skeletal structure, was impaired by elevated CO2; test plates and spines synthesised under acidified conditions were thinner, more porous, and had less biomineral than those at reference sites. Skeletal test plates possessed altered mineralogy (higher Mg/Ca ratio calcite) and showed signs of dissolution, which may explain reductions in skeletal thickness. Together, altered skeleton structure and chemistry compromised the mechanical properties of test plates and spines, which were less hard and more brittle under elevated CO2. These responses may have implications for ecosystem structure if urchin function is suppressed at the population-level. Together, these findings provide evidence that supports general trends from laboratory-based ocean acidification studies and their validity in natural settings, but also identifies responses that may show context-dependency. Future work, therefore, might explore the global repeatability of these findings across successive localities to recognize generality, the limitations of generality and the conditions that mediate the influence of ocean acidification.