Modern mussel shells much thinner than 50 years ago

Shells from the Pacific Northwest are nearly a third thinner now than specimens collected in the 1970s

Date: June 14, 2016

Source: University of Chicago Medical Center

Shells of California mussels collected from the Pacific Ocean off the coast of Washington in the 1970s are on average 32 percent thicker than modern specimens, according to a new study published by University of Chicago biologists.

Shells collected by Native Americans 1,000 to 1,300 years ago were also 27 percent thicker than modern shells, on average. The decreasing thickness over time, in particular the last few decades, is likely due to ocean acidification as a result of increased carbon in the atmosphere.

"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.

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.

In previous studies, Pfister and her colleagues documented declining pH levels in the waters surrounding Tatoosh Island off the coast of Washington state. In 2011 they further analyzed 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.

For the new study, the researchers compared the thicknesses of the same sets of shells. On average, the shells provided by the Makah Cultural and Research Center were 27.6 percent thicker than modern counterparts. Shells from the 1970s were 32.2 percent thicker. Shells collected from a different Native American site in Sand Point, WA, dating between 2150 and 2420 years old were almost 94 percent thicker than modern shells.

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