Background
Citing Hunt et al. (2008), the authors write that "pteropods are pelagic mollusks that play an important role in the food web of extensive oceanic regions, particularly at high latitudes, where they are a major dietary component for zooplankton and higher predators, such as herring, salmon, whales and birds."

What was done
In the words of Comeau et al., "the effect of ocean acidification was investigated using juveniles of the Arctic pteropod Limacina helicina from the Canada Basin of the Arctic Ocean," where they caught and extracted overwintering individuals from depths of 100 to 200 meters through a hole in the ice, which captives they then maintained at three pH levels (8.05, 7.90 and 7.75) for eight days, after which they assessed them for mortality and shell growth.

What was learned
The three researchers found that pH did not impact the mortality of the pteropods; but they note that the degree of linear extension of their shells decreased as pH declined. Nevertheless, they discovered that the pteropods were able to extend their shells at an aragonite saturation state as low as 0.6, which led them to speculate that "the presence of a thin periostracal layer covering the calcareous surface, as shown on the Antarctic pteropod Limacina helicina antarctica (Sato-Okoshi et al., 2010), might, among other mechanisms, protect the shell from a corrosive environment."

What it means
Although much remains to be known about pteropod responses to a potential decline in seawater pH, it would appear that they already possess a certain degree of adaptability to low pH levels; and the results of many similar studies of other calcifying sea creatures suggest that, like them, pteropods may be able to evolve in their ability to successfully cope with declining seawater pH at a rate commensurate with realistic rates of ocean acidification, especially within the context of the acidification analysis of Tans (2009).

References
Hunt, B., Pakhomov, E., Hosie, G., Siegel, V., Ward, P. and Bernard, K. 2008. Pteropods in Southern Ocean ecosystems. Progress in Oceanography 78: 193-221.

Sato-Okoshi, W., Okoshi, K., Sasaki, H. and Akiha, F. 2010. Shell structure characteristics of pelagic and benthic molluscs from Antarctic waters. Polar Science 4: 257-261.

Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future. Oceanography 22: 26-35.