As Pacific acidifies, ‘sea butterflies’ are quickly losing their shells

BY Elizabeth Shell  April 30, 2014 at 2:55 PM EST
A healthy pteropod collected during the U.S. West Coast survey cruise (left) and one of a dissolving shell (right). This is the first evidence of marine snails from the natural environment along the U.S. West Coast with signs that shells are dissolving. Photos courtesy of NOAA

A healthy pteropod collected during the U.S. West Coast survey cruise (left) and one of a dissolving shell (right). This is the first evidence of marine snails from the natural environment along the U.S. West Coast with signs that shells are dissolving. Photos courtesy of NOAA

Scientists have known for some time that shells of the tiniest sea life have been dissolving due to an increasingly polluted ocean. Pteropods, ocean-dwelling snails roughly the size of a thumbnail, have been dubbed by some the “canary in the coal mine” for oceanic climate change. Off America’s West Coast, the snails are losing their shells at faster rates than previously thought, according to a new study published Wednesday.

We did not expect to see pteropods being affected to this extent in our coastal region for several decades,” said William Peterson, an oceanographer at NOAA’s Northwest Fisheries Science Center and one of the paper’s co-authors.

The pteropods — meaning “winged foot” and sometimes called sea butterflies — are tiny, transparent marine snails. They use their foot to swim freely in the water, rather than to slug along the ground like their non-swimming cousins. They’re also food for other fish, such as mackerel and herring.

As fossil fuels are burned to create energy, the resulting carbon dioxide is released into the atmosphere, part of which settles into the world’s oceans. As the ocean waters become more C02-enriched, the ocean’s acidity increases, a process known as ocean acidification. At a certain threshold, the acidity can and does dissolve the snail’s shells.

And as the ocean acidifies, the sea butterfly’s habitat becomes more endangered. Sea butterflies, oysters, clams, mussels and corals cannot survive without their shell to protect them. Scientists surveying the U.S.’ west coast found that more than 50 percent of pteropods sampled from central California up through northern Washington had “severely dissolved shells.” Since the pre-industrial era, the percent of pteropods with dissolving shells has doubled. That’s expected to triple by 2050, when coastal waters are estimated to become 70 percent more corrosive than in the pre-industrial era.

Ocean life with skeletons or shells made of calcium carbonate have been found to be especially vulnerable to acidification — mostly small organisms that make up the base of the ocean food chain, as well as corals and mussels.

“We do know that organisms like oyster larvae and pteropods are affected by water enriched with carbon dioxide,” co-author Richard Feely said, noting that more research needs to be done. “The impacts on other species, such as other shellfish and larval or juvenile fish that have economic significance, are not yet fully understood.”


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