The composition of seawater affects the hardness of sea urchin shells

Experiments on Recent sea urchins grown in seawater with a low Mg/Ca ratio have shown that they produce skeletons with lower magnesium content and nanohardness. These results may suggest that at certain times in the geological past (during the so-called calcite seas with a low Mg/Ca molar ratio) sea urchins were more susceptible to damage by abiotic processes and/or predation.