Life is becoming harder and harder for the planet's shellfish, a new study has found, thanks to acidifying seawater that gives them softer and softer shells. And since a wide range of ocean predators rely on shellfish for food — from seals and seabirds to whales and humans — this could be the start of an ecological sea change.

Ocean acidification occurs as seawater absorbs carbon dioxide from the air, which is increasingly rife with CO2 these days due to emissions from burning fossil fuels. The acidity of seawater has already risen about 30 percent since the Industrial Revolution began, and some models predict it will rise another 150 percent by this century's end. Previous studies have shown how acidification can hinder marine animals' ability to grow, especially early in life, by reducing availability of the calcium carbonate they need to build their shells and skeletons (see graphic below).

And according to the new research published in Global Change Biology, a who's-who of shellfish worldwide are already suffering from these problems, and may be passing their misery up the food chain, possibly all the way to top-level predators like people.

The researchers tested four types of shell-building marine animals — clams, sea snails, lampshells and sea urchins — living in 12 different environments from the tropics to the poles. Across the board, they found that as the availability of calcium carbonate decreases, skeletons become lighter and account for a smaller percentage of an animal's overall weight. Seeing this result in all four types suggests the effect is "widespread across marine species," the study's authors said in a statement Sunday, although the problem is more pronounced in certain parts of the world.

"This effect is strongest at low temperatures," says Lloyd Peck of the British Antarctic Survey, one of several reserarch organizations involved in the study. "[T]he results showed polar species to have the smallest and lightest skeleton, suggesting they may be more at risk in the coming decades as the oceans change."

Dire effects in the Arctic and Antarctic may foreshadow similar issues elsewhere, the researchers say, but these regions might also tell us whether some species can adapt in time to survive. For example, polar shellfish have already evolved to withstand conditions generally unfriendly to their kind, so maybe they and their warm-water relatives could similarly adjust to acidifying oceans.

"Interestingly, where ecology requires animals to have strong skeletons — for instance, to protect them from impacts of floating ice in Antarctica — skeletons are made thicker and stronger," Peck says. "However, they still form a smaller part of the animal's body mass, because the shape of the species changes to enclose much more body for a given amount of skeleton. Thus life finds a way, but still follows the overall trends of decreasing skeleton size in areas where the ocean chemistry makes it more difficult to obtain the necessary building blocks."

And if life can find a way in the Arctic, adds co-author Sue Watson of James Cook University, it could potentially find a way almost anywhere — assuming it's not too rushed: "Evolution has allowed shellfish to exist in these areas and, given enough time and a slow enough rate of change, evolution may again help these animals survive in our acidifying oceans."

Image: U.S. National Oceanic and Atmospheric Administration

Related ocean acidification stories on MNN:

MNN tease photo of clams: Shutterstock

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Study: Ocean acidification disrupts food web
As acidic seawater forces shellfish to grow thinner shells, scientists warn of a ripple effect that could alter marine ecosystems around the planet.