Shell Game

Some species build heavier shells in acidic seawater

It is widely feared that ocean acidification—caused by rising atmospheric CO2—will weaken the shells of marine organisms. But a new experiment shows that certain species actually increase their shell production under these seemingly harsh conditions.

To reach this conclusion, researchers raised 18 species of marine organisms in seawater acidified by different levels of carbon dioxide. Ten species—including corals, clams, oysters, urchins, and scallops—produced less shell as CO2 levels went up. But seven species, including lobsters and crabs, built heavier shells at intermediate or high carbon-dioxide levels.

The latter group may be able to convert bicarbonate, which forms after CO2 dissolves in the ocean, into the carbonate needed for shell building, the authors suggest. And carbon dioxide may boost some organisms’ photosynthesis, giving them more energy to carry out the necessary chemical transformations.

But it’s too early to conclude that these hardier species will do better overall as the world’s carbon dioxide levels go up, the authors say. For instance, spending more energy on shell building could take away from other functions such as reproduction. And a predator that does well in acidified waters could still suffer if its thin-shelled prey goes under. ❧
—Roberta Kwok

Ries, J., A. Cohen, and D. McCorkle. 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37(12):1131-1134.

Photo courtesy of Justin B. Ries

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