Background
The authors write that "ocean acidification is predicted to have significant effects on benthic calcifying invertebrates, in particular on their early developmental states," and they note that "echinoderm larvae could be particularly vulnerable to decreased pH, with major consequences for adult populations."

What was done
Martin et al. explored the effect of a gradient of decreasing pH from 8.1 to 7.0 -- corresponding to atmospheric CO2 concentrations of ~400 ppm to ~6630 ppm -- on the larvae of the sea urchin Paracentrotus lividus, a common but economically and ecologically important species that is widely distributed throughout the Mediterranean Sea and the northeast Atlantic from Ireland to southern Morocco. This they did, as they describe it, by using "multiple methods to identify the response of P. lividus to CO2-driven ocean acidification at both physiological (fertilization, growth, survival and calcification) and molecular (expression of genes involved in calcification and development) levels."

What was learned
The eleven researchers found that "Paracentrotus lividus appears to be extremely resistant to low pH, with no effect on fertilization success or larval survival." They did, however, discover that "larval growth was slowed when exposed to low pH," but they report that there was "no direct impact on relative larval morphology or calcification down to pH 7.25," which equates to an atmospheric CO2 concentration of ~3560 ppm. In addition, they found that "genes involved in development and biomineralization were upregulated by factors of up to 26 at low pH."

What it means
Martin et al. conclude that their results reveal "plasticity at the gene expression level" in P. lividus that "allows a normal, but delayed, development under low pH conditions."