Ocean acidification is leading to changes in the oceanic carbonate system. As a result, calcium carbonate saturation horizon is shallowing, especially at high latitudes. Biogenic high magnesium-calcites could be particularly vulnerable, since their solubility is either similar or greater than that of aragonite. Cidaroid urchins have magnesium-calcite spines covered by a polycrystalline cortex which becomes exposed to seawater when mature (not covered by an epidermis). However, deep species live at low calcium carbonate saturation states, especially at high latitudes. We describe here the morphology and the magnesium content of Ctenocidaris speciosa spines collected at different depths from the Weddell Sea (Antarctica) and relate the features with seawater calcium carbonate saturation. We observed that the spines cortex of C. speciosa presented a thicker inner cortex layer and a lower [Mg2+] below the aragonite saturation horizon. We suggest that the cortex of cidaroid spines is able to resist to low calcium carbonate saturation state.
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