First Go the Shellfish

Ocean acidification continues to alarm scientists, as new evidence emerges about the impacts on shellfish, a vital part of the food chain for both marine animals and people.A broad survey, spanning from the tropics to the poles, found thinning shells among a variety of shellfish including clams, sea snails and sea urchins. Researchers from British Antarctic Survey, the National Oceanography Centre, Australia’s James Cook and Melbourne Universities, and the National University of Singapore collaborated to investigate the impacts of ocean acidification in 12 regions. The results were published August 5 in the journal Global Change Biology.

Four types of marine species were collected by scuba divers working at research stations from Svalbard in the Arctic through tropical sites in Singapore and Australia to Rothera Research Station in the Antarctic.

Carbon dioxide emissions not only concentrate in the earth’s atmosphere, but are also absorbed by oceans, making them more acidic. The ocean acts as a sponge for carbon dioxide (CO2) emissions, helping to slow the impacts of climate change on land. But marine life must contend not only with rising temperatures, but also higher acidity, in what National Oceanic and Atmospheric Administration scientists have called a “double whammy” for ocean creatures.

The impact of acidification on the size and weight of the four species’ shells and skeletons is already apparent, researchers found.

Higher acidity in the oceans makes calcium carbonate, the building block for bones and shells, less available to sea creatures. In turn, their shells become thinner and smaller, the researchers found, a problem some scientists have termed “osteoporosis of the sea.”

“This effect is strongest at low temperatures and the results showed polar species to have the smallest and lightest skeleton, suggesting that they may be more at risk in the coming decades as the oceans change,” Professor Lloyd Peck of British Antarctic Survey said. In the polar regions, shellfish depend on their shells to protect them against sea ice and other difficult conditions.

Prospects seem better in warmer climates, Prof. Peck added. “If there is time for species to evolve in temperate and tropical regions it is one way they may be able to overcome some of the future effects of ocean acidification,” he said.

Temperature and pressure also impact calcium carbonate’s availability, so species in different places have evolved to fit the conditions in their regions.

“In areas of the world’s oceans where it is hardest for marine creatures to make their limestone shell or skeleton, shellfish and other animals have adapted to natural environments where seawater chemistry makes shell-building materials difficult to obtain,” another researcher, Sue-Ann Watson, Ph.D., of James Cook University said. “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.”

But for this to happen, the greenhouse gas emissions that cause acidification may need to slow. And for many decades, emissions have been rising rather than falling.

Another team of scientists recently found that the oceans have been absorbing a sharply increasing amount of carbon dioxide each decade. As emissions have quadrupled over the past 50 years, carbon sinks have steadily pulled roughly half of this CO2 from the atmosphere, according to a study published August 2 in the journal Nature, two days before the shellfish research was released.

“Humanity is getting an assist on climate change from natural systems, otherwise the carbon dioxide in the atmosphere would be twice as high,” Pieter Tans, one of the study’s authors, told U.S. News and World Report. “But CO2 is an acid and the amounts [absorbed] are so massive that I don’t see how we can remedy coming acidification.”

The impacts of a weakened shellfish population may be felt by animals further up the food chain – including people. Shellfish are a fundamentally important source of food for a broad variety of marine predators, including many familiar animals. Creatures ranging from stingrays, which dig out shellfish from ocean floors, sea birds like herons or cranes, and mammals like otters and seals, all depend on shellfish as a key part of their diets.

The shellfish industry is also a major source of revenue in many coastal regions, and seafood is the main source of protein for many people worldwide.

With concerns like these in mind, scientists worldwide are working to coordinate efforts to combat ocean acidification. This summer, the International Atomic Energy Agency launched a new initiative, the Ocean Acidification International Coordination Centre, to help address the growing problem of ocean acidification and to promote coordinated global action. And in July, 2,600 of the world’s top marine scientists signed a statement urging immediate action to limit greenhouse gas emissions. The question remains: to what degree will policymakers and the public will heed these calls?