It has long been known that mussel shells contain rings similar to tree rings, but until recently, scientists were unsure just how often these rings were formed, limiting the extent to which shell rings could be interpreted. Haag, along with biological science technician Amy Commens-Carson, recently completed a multiyear study on the formation of rings in mussel shells. This research, soon to be published in the Canadian Journal of Fisheries and Aquatic Sciences, showed that, similar to trees, mussels produce a shell ring each year as the animal grows. The research also showed that mussels are highly sensitive to disturbance.

Mussels are renowned for their sensitivity to environmental conditions. Living most of its life burrowed in a stream bed, a mussel doesn’t want to be disturbed. During warm weather, the mantle, a thin membrane that encloses the mussel’s body, lays down new shell material, increasing the size of the shell and repairing damage. Normally the mussel’s mantle is delicately attached to the edge of the shell. But when a mussel is yanked out of the stream bed—say, by a biologist, a hungry muskrat, or a flood—the mussel clamps its shell shut, retracting the mantle from the edge of the shell. If the mussel survives the encounter and opens again, the mantle doesn’t reestablish connection with the shell edge at exactly the same place as it was before being disturbed. This slight misalignment results in the production of a disturbance ring that is often readily distinguishable from annual growth rings.

Because some mussels can live more than 100 years, their shells provide a long-term record of stream and climatic conditions that can now be interpreted using Haag and Commens- Carsons’ research findings. Annual variations in growth due to changes in temperature, rainfall, and other factors can be examined by looking at the amount of shell produced between annual growth rings. Furthermore, the occurrence of disturbance rings can provide evidence of floods, drought, or other past environmental events.

In collaboration with researchers at the University of Alabama, Haag is now using shell rings to reconstruct past growth histories of mussels and to assess recent changes in hydrologic variables such as flood and drought periodicity. “Mussel shells hold a wealth of information about the environment,” Haag says. “The growth histories recorded in these shells can help us understand past changes in rivers and lakes, as well as predict future effects of climate change on freshwater ecosystems.”