In general, higher magnesium levels in calcite imply higher solubilities (Morse and Mackenzie, 1990), but to date little is known about the direct influence of seawater pH decrease on skeletal Mg/Ca ratio, and subsequent solubility, in sea urchins (Calosi et al., 2013): mid-term in situ experiments (2–4 days exposure) on Arbacia lixula showed no significant changes in ions in coelomic fluid or carbonated tissues (no skeleton dissolution), while on P. lividus even a possible increase in net calcification has been hypothesized.