Background
The authors write that "echinoderms produce their skeletal components from magnesium-rich calcite, a form of calcite that is even more vulnerable to ocean acidification than aragonite (Andersson et al., 2008; Gayathri et al., 2007)," and, therefore, they say that "an understanding of the prospective impacts of ocean acidification on internal skeletal structures of echinoderms is important, as the presence of an internal skeletal structure is unique in comparison to the majority of invertebrates studied to date in ocean acidification research."

What was done
In the words of Schram et al., "two groups of sea stars, each with two arms excised, were maintained on a formulated diet in seawater bubbled with air alone (pH 8.2, approximating a pCO2 of 380 ppm) or with a controlled mixture of air/CO2 (pH 7.8, approximating a pCO2 of 780 ppm)," while "arm length, total body wet weight, and righting responses were measured weekly." Then, after 97 days, which they describe as "a period of time sufficient for 80% arm regeneration," they say that "protein, carbohydrate, lipid and ash levels were determined for body wall and pyloric caecal tissues of intact and regenerating arms of individuals held in both seawater pH treatments."

What was learned
Very briefly, the four U.S. researchers report that "adults of the common soft bottom predatory sea star Luidia clathrata exposed to end-of-century conditions of ocean acidification (pH 7.8) are relatively unimpaired in their regenerative capacity," which "encompasses not only their ability to re-grow their arms, but their ability to allocate materials and energy to regenerated somatic body components." In addition, they say "there is no discernable pattern arising from exposure to a reduced seawater pH of 7.8 for 97 days on righting behavior," which they say is "an integrative measure of stress."

What it means
Schram et al. conclude the report of their work by stating that "the demonstration of an organism's ability to sustain normal functions under these conditions is as equally important to document as those that are negatively impacted," since "this information will be critical to future assessments of prospective impacts of ocean acidification at the community level." We agree; and going one step further, we suggest it is possible that the non-effects of ocean acidification may well prove to be even more important than whatever small negative changes might ultimately be produced by the acidification phenomenon, since under such conditions we may decide there is no need to drastically restructure the way the world obtains its energy.

References
Anderson, A.J., Mackenzie, F.T. and Bates, N.R. 2008. Life on the margin: implications of ocean acidification on Mg-calcite, high latitude and cold-water marine calcifiers. Marine Ecology Progress Series 373: 265-273.

Gayathri, S., Lakshminarayanan, R., Weaver, J.C., Morse, D.E., Kini, R.M. and Valiyaveettil, S. 2007. In vitro study of magnesium-calcite biomineralization in the skeletal materials of the seastar Pisaster giganteus. Chemistry - A European Journal 13: 3262-3268.