Oysters, clams, mussels and other bivalves build calcium carbonate shells using mostly raw materials from seawater. A two-day-old oyster larva is already 90 percent calcium carbonate by body weight, ecologist George Waldbusser of Oregon State University in Corvallis and colleagues report May 29 in Geophysical Research Letters.

During their shell-building blitz, larvae rely solely on energy derived from their eggs, the team found in a study of Pacific oysters (Crassostrea gigas) from a commercial hatchery in Oregon. By looking at the forms of carbon present in eggs versus algae provided as oyster food, the researchers found that larvae depend heavily on an egg’s resources for more than a week. The youngsters can’t grab outside food until they construct enough shell to support muscle attachments for feeding appendages, Waldbusser says.

Oyster larvae’s dependence on a fixed energy source could be a problem as atmospheric carbon dioxide rises. Oceans soak up more of the gas, driving reactions that lower the water’s pH and alter the availability of the compounds needed to make shells. Waldbusser and colleagues calculate that the amount of energy that oyster larvae need to build shells grows exponentially as CO₂ dissolved in the water increases.

Previous work has found ocean acidification affects oyster growth and survival, says Annaliese Hettinger, an ecologist at Oregon State who wasn’t involved in the research. “George’s paper is one of the first to point to an actual reason.”

The ocean’s surface waters are slightly alkaline, with an average pH of 8.1 on a scale where anything below 7.0 is acidic. Since the onset of the Industrial Revolution, ocean pH has dropped by 0.1. By 2100, pH could decline another 0.3 units, and some parts of the ocean could become corrosive to shells.

The new findings may help explain why oyster populations could suffer even before that point. Oyster hatcheries in the Pacific Northwest have had disastrous production declines in the last several years, possibly due to seasonal winds that have brought deep, CO₂-rich water to the surface. Although the water hasn’t been corrosive enough to dissolve shells, its decreased alkalinity has made shell-building difficult for larvae, Waldbusser says.

Hatcheries can combat falling pH by buffering water with antacids, Waldbusser says. But globally, he says, the only way to fight dropping pH is to reduce CO₂emissions.

More work needs to explore whether other bivalves are similarly vulnerable. Studies should also examine whether oysters can adapt to higher CO₂, says physiologist Brad Seibel of the University of Rhode Island. It may be that oysters in CO₂-saturated seawater will make eggs with more energy reserves to compensate for larvae’s more laborious shell construction.

Read this article on Science News.