By all accounts, freshwater mussels are boring creatures. From the time they fall off their host fish as a larva and settle in a riverbed until they die decades later, they might move a few meters at most. These palm-sized mussels are so mundane that we don't even have population estimates for most species, ecologist Jens Hegg explains.
However, these simple mollusks may hold decades of data detailing the historic conditions of Northwest waterways, such as the Little Spokane or Snake rivers, embedded in their shells.
Earlier this year, Hegg, an assistant biology professor at Gonzaga University, received nearly $100,000 from the M.J. Murdock Charitable Trust to figure out how to read that shell data. His research team will aim to figure out what controls the shell growth of Western ridged mussels, a Northwest native freshwater clam species suffering a population decline.
"In the same way that we pull information from tree rings about climate, you can do the same thing with these guys," Hegg says, pointing at faint striations in concentric ovals along a clam's shell. "As it turns out, a lot of things get recorded in hard parts of animals."
While researching these mussel shells is new to Hegg, he's previously studied waterbody climate conditions using data from the New Zealand snapper's otoliths, or ear bones.
Described in his January 2025 research article in Global Change Biology, Hegg and a group of scientists used a process called "dynamic time warping" to bisect these minuscule bones and uncover the microchemical profile within. By inspecting these chemicals, researchers were able to deduce centuries of information that describes where these fish have been and what the climate was like when they lived.
"That information didn't exist without looking at those records from the animals themselves. These sorts of internal records are also stored in mussel shells, which can stick around for a long time," Hegg explains. "We're still trying to figure out what the chemistry changes in mussels tell us. It's clearly changing through the year. There's peaks and valleys in some of the chemical signatures ... but it's still a little unclear just exactly why they're changing."
In bright red dry suits and snorkeling gear, Hegg and his students will crawl shoulder to shoulder along streambeds to look for mussels in the summers to come. Once the team members locate a bed of mussels, they'll dig a few up in search of any living specimen.
Although researching the chemistry in a mussel shell doesn't require them to be alive, Hegg says he needs at least one living specimen to accurately age these mollusks.
"If you only take dead mussels, it's hard to tell when they died. Those shells could sit around on the stream bottom for a long time, but if you collect a live one, then you've done what we call anchoring the chronology," Hegg explains. "You've anchored it in time, because you know exactly when it died. So then you are able to add mussels to that chronology going back every year by finding mussels that overlap in age."
Once the researchers understand how long these mussels were alive by counting the growth rings on their shells, and after they've figured out which environmental factors caused the shells to grow, they hope to pinpoint what the river was like at different points in time in history. Theoretically, a mussel's rings might grow differently depending on the climate they lived in at different times. Hegg's research into mussel shell growth is foundational, and if he's able to determine this information reliably, climatologists will be able to set specific climate baselines for individual waterways.
Since Western ridged mussels can live upward of 60 years, Hegg says they make the perfect specimen for this type of chronologic research looking back in time. When climate scientists know what a river looked like decades or even centuries ago, they can help set more specific climate goals for that body of water.
"Every stream is unique, so having something like mussel shells, where you could say, 'Oh, this is the temperature history, or this is the flow history,' and we know that from the chemistry and the growth in mussel shells," he says, "that would give you something really specific that you could aim for. And we could potentially learn a lot about really local rivers and streams that we just don't have any records for." ♦
