A team of scientists led by Florida State University have discovered that the shells of California mussels (Mytilus californianus) are being altered at its most basic structural level.

For centuries, the shells have a relatively uniform mineralogical make-up: long, cylindrical calcite crystals arranged in neat vertical rows with crisp, geometric regularity. This, however, appears to be no longer the case for shell specimens collected within the last 15 years.

"When the mussels are ready to build their shells, they first lay down an amorphous soup of calcium carbonate, which they later order and organise," said Assistant Professor of Biological Science Sophie McCoy said. "More recent shells have just started heaping that calcium carbonate soup where it needs to go and then leaving it there disordered."

In addition, the recent shells had elevated levels of magnesium, which indicates that the shell formation process had been disrupted. Healthy shells are comprised mainly of calcium carbonate; any magnesium that is incorporated is a product of trace amounts of ambient magnesium in the environment.

McCoy explained, "When more magnesium is found in the skeleton, it signals that the organism has less control over what it's making."

An increase in the skeletal magnesium alters the strength of important magnesium-oxygen bonds, which is an instructive proxy for a shell's level of organisation.

"When there's not a clear geometric pattern in the skeleton, the bond strengths become more variable, and that's what we're seeing in modern shells," McCoy said.

This variability may just be a positive sign for the species' future.

"An important theme of climate change science is that increased variability might be the new rule," said McCoy. "We know that climate change right now is happening faster than what the Earth has experienced before, but we also see that over these long timescales, things tend to plateau and stabilise. 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."

The findings of the study has been published in the recent issue of Global Change Biology journal.