| Summary: | Ocean acidification and ocean warming compromise the capacity of calcifying marine organisms to generate and maintain their skeletons. While many marine calcifying organisms precipitate low-Mg calcite or aragonite, the skeleton of echinoderms consists of more soluble Mg-calcite. To assess the impact of exposure to elevated temperature and increased <i>p</i>CO<sub>2</sub> on the skeleton of echinoderms, in particular the mineralogy and microstructure, the starfish <i>Aquilonastra yairi</i> (Echinodermata: Asteroidea) was exposed for 90 days to simulated ocean warming (27 °C and 32 °C) and ocean acidification (455 µatm, 1052 µatm, 2066 µatm) conditions. The results indicate that temperature is the major factor controlling the skeletal Mg (Mg/Ca ratio and Mg<sub>norm</sub> ratio), but not for skeletal Sr (Sr/Ca ratio and Sr<sub>norm</sub> ratio) and skeletal Ca (Ca<sub>norm</sub> ratio) in <i>A. yairi</i>. Nevertheless, inter-individual variability in skeletal Sr and Ca ratios increased with higher temperature. Elevated <i>p</i>CO<sub>2</sub> did not induce any statistically significant element alterations of the skeleton in all treatments over the incubation time, but increased <i>p</i>CO<sub>2</sub> concentrations might possess an indirect effect on skeletal mineral ratio alteration. The influence of increased <i>p</i>CO<sub>2</sub> was more relevant than that of increased temperature on skeletal microstructures. <i>p</i>CO<sub>2</sub> as a sole stressor caused alterations on stereom structure and degradation on the skeletal structure of <i>A. yairi</i>, whereas temperature did not; however, skeletons exposed to elevated <i>p</i>CO<sub>2</sub> and high temperature show a strongly altered skeleton structure compared to ambient temperature. These results indicate that ocean warming might exacerbate the skeletal maintaining mechanisms of the starfish in a high <i>p</i>CO<sub>2</sub> environment and could potentially modify the morphology and functions of the starfish skeleton.
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