Ocean acidification: a shrinking problem

N THE clear blue coastal waters of California lives a much cherished mollusc. The Olympia oyster, sought for its rich taste, brings millions of dollars into the local economy. But these oysters are proving useful in other ways too. They have drawn the interest of scientists who want to study how species react to a change that is happening in the oceans.

The change in question is acidification. When carbon dioxide dissolves in water, it makes that water acidic. And as CO2 levels rise in the atmosphere they rise in the oceans too. This could be a problem for creatures like oysters that have shells made of calcium carbonate, a chemical which tends to dissolve in acid.

Recent years have seen reports of the thinning of marine organisms’ shells, but the consequences are unknown. A group of biologists led by Eric Sanford of the University of California, Davis, set out to put that right. As they report in the Proceedings of the Royal Society, they looked in the laboratory at how CO2-enriched seawater affects the growth of Olympia oysters, and how that, in turn, affects the molluscs’ attractiveness to predators—specifically carnivorous snails called Atlantic oyster drills, which, as their name suggests, really belong on America’s east coast but which have invaded the Pacific, too.

Dr Sanford and his colleagues raised the larvae of wild-caught oysters in tanks of filtered seawater for 20 days, and raised oyster drills from eggs in a similar way. Both predator and prey were therefore still juveniles (the oysters’ shells had an area of less than a square millimetre).

Some of the tanks the researchers used were filled with normal seawater while others contained seawater with twice the usual amount of CO2 dissolved in it. The acidity of the world’s oceans is expected to double by 2100, so the second set of tanks contained an approximation of what the sea will be like at the end of the century.

Acidity did indeed affect the oysters’ growth, but not in the way the researchers expected. Both groups’ shells were equally thick, but animals raised in acid water were 30-40% smaller than their confrères.

Even though the acid-raised oysters were smaller, when the snails were given a choice they ate almost 50% more of them than of snails raised in normal water. Why, is not clear. Perhaps their shells, though just as thick, were weaker. Acid-raised snails, though, were just as successfully predatory as the others. Dr Sanford thinks that is because the rasping part of an Atlantic oyster drill’s tongue is made not of calcium carbonate but of chitin—a sugar-based polymer unaffected by mild acids.

Natural ecosystems are, of course, much more complicated than a two-species laboratory experiment. But if Dr Sanford’s results are reflected in the real world as its seas become more acidic, then that is bad news for gourmets—and for all the other animals that like to wait until oysters are a reasonable size before they eat them.

The Economist, 18 January 2014. Article.

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