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Ocean Acidification and the Velvet Swimming Crab
Small, D., Calosi, P., White, D., Spicer, J.I. and Widdicombe, S. 2010. Impact of medium-term exposure to CO2 enriched seawater on the physiological functions of the velvet swimming crab Necora puber. Aquatic Biology 10: 11-21.

The authors write that ocean acidification "is predicted to play a major role in shaping species biogeography and marine biodiversity over the next century." In fact, they note that "surface water pH has already dropped by 0.1 unit," citing Orr et al. (2005), and they state that "current predictions forecast reductions of 0.5 to 0.7 units by the years 2100 and 2300, respectively," citing Caldeira and Wickett (2003). Hence, they decided to see what effects this phenomenon might have on the velvet swimming or "devil" crab, as Necora puber is sometimes called.

What was done
Working with adult individuals collected from the lower intertidal zone of Mount Batten Beach, Plymouth, UK, Small and his colleagues tested the effect of 30 days' exposure of the crabs to seawater maintained in 4-L aquaria at pH values of 8.0 (control), 7.3 (a value representative of that predicted for 2300 by Caldeira and Wicket, 2003), and 6.7 (a level "potentially resulting from CO2 storage or pipeline leaks," as per Blackford et al., 2008), while they simultaneously measured various types of crab responses.

What was learned
The five UK researchers determined, as they describe it, that "Necora puber was able to buffer changes to extra-cellular pH over 30 days exposure," and to do it "with no evidence of net shell dissolution." In addition, they say that "tolerance to heat, carapace mineralization, and aspects of immune response were not affected by hypercapnic conditions," i.e., conditions that lead to more than the normal level of carbon dioxide in an organism's blood. About the only negative finding was a decline in whole-animal oxygen consumption, which they described as being "marginal" between the control and medium hypercapnic conditions, but as "significant" at the ungodly pH value of 6.7.

What it means
Small et al. conclude their report by saying that their results "confirm that most physiological functions in N. puber are resistant to low pH/hypercapnia over a longer period than previously investigated," and, we would add, over a far greater range of pH reduction than what could ever be produced, even by the burning of all fossil fuels in the crust of the earth, as per the analysis of Tans (2009).

Blackford, J.C., Jones, N., Proctor, R. and Holt, J. 2008. Regional scale impacts of distinct CO2 additions in the North Sea. Marine Pollution Bulletin 56: 1461-1468.

Caldeira, K. and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.

Orr, J.C., Fabry, V.J., Aumont, O., Bopp, L., Doney, S.C., Feely, R.A., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R.M., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R.G., Plattner, G.-K., Rodgers, K.B., Sabine, C.L., Sarmiento, J.L., Schlitzer, R., Slater, R.D., Totterdell, I.J., Weirig, M.-F., Yamanaka, Y. and Yool, A. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437: 681-686.

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

Reviewed 4 May 2011