Ocean Acidification Weakens Mussels' Grip

Ocean absorption of CO2 from human activity is loosening shellfish's ability to cling
cooked mussels are sorted and prepped for the Mussel Eating Contest

Flickr/Beth Jusino

The strength of a mussel, the shellfish's ability to grasp tightly to rocks, docks and ships despite crashing waves or prying fingers, is legendary. Scientists have even studied how mussels, using slender fibers called byssal threads that are simultaneously hard and stretchy, are able to cling so tight in a rough, wet environment, in hopes that humans could mimic that technology to create strong, flexible textiles.

Now, climate change is impairing that ability to cling. Researcher Michael O'Donnell, an ecologist at the University of Washington, has shown that ocean acidification, a process in which absorbing large amounts of carbon dioxide lowers the pH of oceans, is weakening mussels' byssal threads.

O'Donnell and his colleagues took bay mussels from San Juan Island, in the Puget Sound, and put them in chambers with seawater at different pH levels. They found that the mussel byssal threads grown in acidified conditions with a pH lower than 7.6 were 40 percent weaker than normal. The research was published Sunday in the journal Nature Climate Change.

Bay mussels play a key ecological role in the natural world, said Brian Gaylord, a professor of evolution and ecology at the University of California, Davis, Bodega Marine Laboratory. As mussels cluster together in beds, they provide a protective home for hundreds of other species.

"Sea urchins, little tiny crabs, marine worms live down in there," along with many other species, Gaylord said.

The shellfish industry, already adjusting to the fact that acidifying oceans hurt the abilities of sea creatures like oysters to make their shells, is also likely to experience losses when mussels lose their ability to cling.

Worldwide, mussel cultivation is a major food source and a $1.5 billion industry, according to the U.N. Food and Agriculture Organization. In Belgium, for example, catching and eating mussels is almost a national pastime.

In the Puget Sound, farmers cultivate mussels by setting out wooden rafts with long ropes that fall into the sea. The mussels settle on the ropes and grow to full size. Then the shellfish growers pull them up and scrape the mussels off.

Harvesting gets harder
If the mussels' byssal attachments are weakened, they are more likely to fall off the rope when it gets pulled up.

Sloughing off, as it is called, is already a problem that the shellfish industry has developed some solutions for, like putting discs on the rope every so often so when a mussel falls off it hits the disc and doesn't take off all the mussels below it.

"Already one of your biggest challenges as a mussel farmer is to prevent your mussels from sloughing off and dropping to the bottom," said Bill Dewey, a spokesman for Taylor Shellfish Farms, a company that has already had to deal with the negative impacts of acidification on the shells of the oysters it grows.

"The changing seawater chemistry means that those threads are going to get weaker. That's not going to be a good thing," Dewey added.

O'Donnell's study is unique because it's one of the few to look at how acidification affects a different part of the shellfish, said UC Davis' Gaylord.

Much of the research on ocean acidification to date has focused on the effect changing seawater chemistry has on the calcium carbonate shells of shellfish.

"This sort of pushes us to expand the spectrum of responses [to acidification] that we might want to consider," Gaylord said.

Do we care?
Gaylord said another step would be to conduct research like this on other mussel species.

"The obvious thing would be to do experiments on the open coast mussel, the California mussel, which routinely gets bashed by waves repeatedly."

Researchers have already published one study showing the number and size of California mussels on one island off the coast of Washington state were reduced as the pH decreased, he noted.

O'Donnell made a point of saying his research does not mean that if the ocean's pH decreases, mussels will go extinct.

"It's not so dramatic that they are just going to disappear and fall off the rock, but it's enough to change the dynamic of a mussel bed," O'Donnell said.

Whether it matters if there are fewer mussels and a less diverse coastline, he said, is up to humans to decide.

"As a scientist, the question I would like to know is, a hundred years from now, what will these shorelines look like? As citizens, people should wonder, do we care what they look like? And if things change drastically because of our activities, do we care about that?"


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