What was done
The authors employed controlled infusions of pure CO2 to create mean pH values of 8.03 ± 0.03, 7.64 ± 0.12 and 7.31 ± 0.11 (corresponding to atmospheric CO2 concentrations of 400-475, 905-1660 and 2115-3585 ppm, respectively) in filtered seawater that flowed continuously through three sets of multiple tanks into which they had introduced the gametes of two Acropora coral species (A. digitifera and A. tenuis) they had collected during a natural spawning event, after which (seven days later) they determined their percent survival. Then, after ten more days, they documented the size of the developing polyps; and after 14 days they documented the percentage of polyps that had acquired zooxanthellae that the researchers had collected from the giant clam T. crocea and released into the several treatment tanks.

What was learned
Suwa et al. report that "A. digitifera larval survival rate did not differ significantly among pH treatments," and the graphs of their data indicate that survivorship in A. tenuis was actually about 18.5% greater in the lowest pH (highest CO2) treatment than in the ambient seawater treatment. At the end of the subsequent ten-day study, however, polyp size was reduced in the lowest pH treatment, but by only about 14%, which is not too bad for an atmospheric CO2 concentration in the range of 2115-3585 ppm. And in the A. tenuis coral, this reduction in individual size was more than compensated by the even greater percentage increase in survivorship. In addition, after only four days of being exposed to the zooxanthellae derived from giant clams, all polyps in all treatments had acquired a full complement of the symbiotic zooxanthella.

What it means
In discussing their findings, the seven scientists say they indicate that "the survival of coral larvae may not be strongly affected by pH change," or "in other words," as they continue, "coral larvae may be able to tolerate ambient pH decreases of at least 0.7 pH units," which, in fact, is something that will likely never occur, as it implies atmospheric CO2 concentrations in the range of 2115 to 3585 ppm. In addition, if such high concentrations ever were to occur, they would be a long, long time in coming, giving corals far more than sufficient time to acclimate -- and even evolve (Idso and Idso, 2009) -- to adequately cope with the slowly developing situation.

Reference
Idso, C.D. and Idso, S.B. 2009. CO2, Global Warming and Species Extinctions: Prospects for the Future. Vales Lake Publishing, LLC, Pueblo West, Colorado, USA, 132 p.