Effects of Long-Term Ocean Acidification Exposure on the Structural, Mineralogical, and Mechanical Properties of Sea Urchin ( <i>Echinometra</i> spp.) Skeletons at a Natural Volcanic CO ₂ Seep

Two decades of mesocosm studies document generally negative effects of ocean acidification (OA) on adult sea urchin growth, feeding performance, skeletal structure, and strength. Whether experimental observations hold true in natural systems will determine whether they can be extrapolated to predict responses under ecologically relevant contexts. Here, we employ a suite of imaging, chemical, and mechanical techniques to examine the skeletal properties of two closely related sea urchin species (genus <i>Echinometra</i>) living at a natural carbon dioxide (CO₂) seep in Japan. Test plates and spines from urchins living under elevated CO₂ conditions were thinner, more porous, and had less biomineral than those at reference sites; however, tooth structure was resilient to elevated CO₂. The magnesium content of the test and spines did not differ between sites; however, they exhibited reduced nanohardness and became more brittle under elevated CO₂. Together, altered structural and mechanical properties may compromise the protective function of urchin skeletons at the CO₂ seep. These responses have implications for ecosystem structure if urchin function is suppressed at the population level. Future work might explore the repeatability of these findings across successive species and localities to recognize generality, its limits, and the conditions that mediate the influence of OA.