Ocean Acidification Changing Mussel Shell Structure

California mussels.

California mussels. Sophie McCoy/FSU

For thousands of years, California mussel shells have been made up of long, cylindrical calcite crystals organized in neat, vertical rows. But scientists have found that as ocean acidification has accelerated over the past 15 years, these shells have undergone dramatic structural changes, being built out of unorganized, uneven minerals instead.

“What we’ve seen in more recent shells is that the crystals are small and disoriented,” Sophie McCoy, a biologist at Florida State University who led the study, said in a statement. “These are significant changes in how these animals produce their shells that can be tied to a shifting ocean chemistry.” The new findings were published last week in the journal Global Change Biology.

McCoy, collaborating with scientists at the University of Chicago and the University of Glasgow in Scotland, compared modern California mussel shells collected from Tatoosh Island off the northwestern tip of Washington to shells from the same region dating back thousands of years.

Historically, California mussels have laid down a heap of calcium carbonate fragments, which they later organized into orderly rows to create their geometric shells. Now, it appears the organisms leave the material disordered. The scientists found that in addition to recent shells losing their structural symmetry, the mussels also had elevated levels of magnesium — a sign that the process of shell formation has been disrupted. “When more magnesium is found in the skeleton, it signals that the organism has less control over what it’s making,” McCoy said.

The scientists say the new shell composition doesn’t necessarily spell the end of the species in a climate-changing world. Rather, it could indicate that the mussels are trying to evolve to deal with new ocean conditions.

“Variability is the basis of natural selection, and the fact that we now see so much variability in the mussels’ individual traits means there is potential for natural selection to act,” McCoy said. “It’s true that we might not have as many mussel species, or that their populations might be smaller and have a more restricted range, but I don’t think that we’ll have an ocean with no mussels.”