The shells of California mussels have thinned dramatically in the modern era, probably as a result of ocean acidification, a direct result of increased carbon dioxide in the atmosphere resulting from fossil fuel combustion, say University of Chicago biologists who compared mussel specimens collected in the 1970s with present-day samples.

In the 70s, the shells were on average 32 percent thicker than modern specimens. Going back even farther, the researchers said shells collected by Native Americans 1,000 to 1,300 years ago were also 27 percent thicker than modern shells.

“Archival material provided by past researchers, the Makah Tribal Nation, and the Olympic National Park allowed us to document this intriguing and concerning pattern in shell thickness,” said Cathy Pfister, PhD, professor of ecology and evolution at the University of Chicago and lead author. The study was published June 15, 2016, in the Proceedings of the Royal Society B.

Other recent research has shown that ocean acidification is also putting the economically valuable West Coast Dungeness crab fishery at risk, slowed coral growth, and impairs the feding ability of sharks. And the impacts to oysters have also been well-documented.

As humans burn fossils fuels, the oceans absorb a large portion of the additional carbon released into the atmosphere. This in turn causes pH levels of ocean water to drop, making it more acidic. Mussels, oysters, and certain species of algae have difficulty producing their calcium carbonate shells and skeletons in such an environment, and can provide an early indicator of how increasing ocean acidification affects marine life.

Pfister has documented the acidification of the ocean in previous studies sampling  pH levels in the waters surrounding Tatoosh Island off the coast of Washington state. Other work focused on analyzing carbon and oxygen isotopes taken from modern mussel shells, shells collected by the local Makah tribe between AD 668 and 1008, and shells collected by biologists in the 1970s.

The long-term decline in thickness likely shows a response to ocean acidification, though the researchers also consider other environmental drivers including changes in food supply (e.g. plankton) for mussels.

The researchers also point out that their findings raise concerns about the California mussel’s ability to retain its role as a keystone species in these waters. Decreased shell thickness makes them increasingly vulnerable to predators and environmental disturbances. This in turn could affect interactions with hundreds of other species of organisms that live near mussel beds in tidal waters.

“The California mussel is a common species along the entire west coast of the United States, and their fate will be linked to that of a rich diversity of predators, including sea stars and sea otters, as well as myriad species that are part of the mussel bed habitat,” Pfister said. “It is imperative that we understand more about how these species will change as ocean conditions change.”

Bob Berwyn, Summit County Citizens Voice, 15 June 2016. Article.