Background
The authors say that certain studies that have focused on the early life stages of mollusks "have shown decreased growth rates and increased proportions of abnormal development under low pH conditions." However, they indicate that these studies did not allow unambiguous attribution of the observed changes in physiology to the low pH values.

What was done
To further explore this important subject, Gazeau et al. strove to "assess the impact of several carbonate-system perturbations on the growth of Pacific oyster (Crassostrea gigas) larvae during the first three days of development (until shelled D-veliger larvae)." This work was done using filtered seawater obtained from the Oosterschelde (a nearby tidal inlet) with five different chemistries that were obtained "by separately manipulating pH, total alkalinity and aragonite saturation state."

What was learned
The seven scientists say their results showed that "developmental success and growth rates were not directly affected by changes in pH or aragonite saturation state but were highly correlated with the availability of carbonate ions ... as long as carbonate ion concentrations were above aragonite saturation levels." But when carbonate ion concentrations dropped below aragonite saturation levels, they found that growth and development "strongly decreased."

What it means
Gazeau et al. conclude that according to their results, "the effects of ocean acidification on larvae of Crassostrea gigas from the Oosterschelde estuary during the first three days of development are not significant as long as CO3^2- concentrations remain above aragonite saturated conditions." And they add that "due to relatively high levels of total alkalinity in this area, it is not expected that seawater will become corrosive for aragonite following a decrease of 0.3 to 0.4 pH unit," which is to be compared with the 0.1 decrease in pH that is believed to have occurred since before the beginning of the Industrial Revolution and the present point in time.

In further discussing the subject, the French, English and Dutch researchers write that "most calcifying species, including mollusks, are able to concentrate Ca²⁺ and CO3^2- ions at the site of calcification (McConnaughey and Gillikin, 2008)," and they say that the bivalves they studied "should therefore be able to regulate calcification rates under suboptimal concentrations of Ca²⁺ and CO3^2-." In fact, they note that "Thomsen et al. (2010) have shown that blue mussels are actively growing in a bay of the Western Baltic Sea naturally enriched with high CO2 water," and that "juvenile recruitment occurs in summer time coinciding with low pH levels and aragonite under-saturated conditions." Thus, it would appear that the ocean acidification threat is not something that will seriously affect mollusks any time soon ... if ever. See also, in this regard Ocean Acidification (Effects on Marine Animals - Shellfish) in our Subject Index.

References
McConnaughey, T.A. and Gillikin, D.P. 2008. Carbon isotopes in mollusk shell carbonates. Geo-Marine Letters 28: 287-299.

Thomsen, J., Gutowska, M.A., Saphorster, J., Heinemann, A., Trubenbach, K., Fietzke, J., Hiebenthal, C., Eisenhauer, A., Körtzinger, A., Wahl, M. and Melzner, F. 2010. Calcifying invertebrates succeed in a naturally CO2-rich coastal habitat but are threatened by high levels of future acidification. Biogeosciences 7: 3879-3891.