The mighty mussel
Endangered creatures fill crucial role of environmental sentinels
On a September afternoon that still felt like summer, a small group in boots and waders drove out to a creek near Elgin, sloshed through cold, bubbling water, knelt down and dug their bare hands into the ground. For hours. For days. And all for a mollusk.
Workers from the Shedd Aquarium and Cook County Forest Preserve District spent a few days this week searching Poplar Creek and tagging more than 100 freshwater mussels, a group researchers say is among the most endangered in the world.
But little is known about the unassuming creatures, some of which can live 100 years. So Kentaro Inoue, a Shedd research biologist who has studied mussels for years, is trying to figure out what kind of mussels are in the Chicago area, where they are, how they’re doing and ways they might be protected.
One reason for their obscurity, Inoue said, is a lack of charisma, especially compared with some other aquatic creatures: “A lot of people overlook mussels.”
Chris Anchor, wildlife biologist with the Forest Preserves District, had a similar take: “Without a doubt, they are a much maligned species.”
The Shedd conservation program will help track individuals and survival rates over time at three locations. Inoue will be back next summer at the same spots to do the same thing. He’ll see which mussels make it and how many are new.
Inoue plans to publish the findings and hopes to learn how survival and reproduction are connected to environmental factors. He previously studied mussels in New Mexico and found survival correlated to low water flow. As water levels dropped and sediment entered rivers, populations declined.
Of about 300 species of freshwater mussels in North America, about 10% are already extinct, Inoue said. The majority of what’s left are either threatened or endangered — and state or federally protected. Of the native species found in Illinois, the majority fall into a range from threatened to extinct, according to the state’s Department of Natural Resources.
In Illinois, the shells of freshwater mussels were once used to make buttons, before plastic took over. They were also harvested to produce cultured pearls.
What else is known about freshwater mussels: they’re filter feeders, meaning their food is what’s in the water — bacteria, algae, floating particles. One mussel could filter 5 to 10 gallons per day, Inoue said.
When zebra and quagga mussels invaded Lake Michigan and Lake Huron, they in part helped the lakes surpass Lake Superior in clarity, according to a 2017 study by the Michigan Tech Research Institute.
“People call them livers of the river,” Inoue said. “They are deep-cleaning the water. We don’t really see it, but they’re doing it.”
They’re also a natural counter to erosion, burrowing in the sediment and stabilizing river bottoms. And they’re a part of the larger ecosystem. Like researchers, raccoons and river otters dig mussels up. But unlike researchers, they eat them. Even in death, mussel shells can provide shelter or protection to living creatures.
The smallest mussel can be less than an inch, the larger ones topping 4½ inches. Some, like a hefty plain pocketbook mussel, could weigh as much as a small melon. Their age can be roughly determined by rings on their shells, like trees.
On Wednesday afternoon at Poplar Creek, within the bounds of pink mapping string, clouds of sediment smoked out of the glassy water when a hand or foot hit the creek’s rusty colored bottom.
If you were quick, after plucking a mussel from the creek, you could get a peek at a bloblike entity retreating back into its shell. The work was mostly quiet, aside from a splash, the beep from a scanner detecting a tagged mussel or the recognition of a hard-won catch.
“I found two,” one worker said. “You found two?!” Another find after an ongoing search for a well-hidden mussel garnered an, “Ope! There it is.”
One worker asked Inoue how far mussels can travel. They have a single foot that allows them to move. But, he said, “Some mussels just stay there forever.”
Later in the day, as Inoue tagged the mussels, he showed one with a black tinge on her gill — a sign of pregnancy.
The life cycle of a freshwater mussel is complex, and to proliferate, they rely on fish. Mussel larvae — “little Pac-Man” — must attach to a fish fin or gill to grow, Inoue said. Depending on species, after a few weeks or months, they strike out on their own at the bottom of a river. Some mussel species are particular in their choice of fish, while others are more open to their surrogates.
That dependence contributes to their vulnerability, Inoue said. “If water quality is good for mussels, but not fish, they cannot survive, because they cannot reproduce.”
By the end of the day, 134 mussels had been identified, including 27 new individuals, adding to the more than 600 already marked between locations.
Now, Inoue will head back to the lab and look at the ellipse, the most common of the four species found in Poplar Creek this week and a species of greatest conservation concern in Illinois. He’ll try to understand its genetic structure and examine how populations are related.
Populations of the ellipse are declining across the state, Inoue said. The species need pristine water to thrive, and they are generally found in small to medium-size streams — habitats hard to find in the Chicago area.
Poplar Creek is special, Inoue said, because of its active springs — a rarity in Cook County. He wonders what would happen if the springs, which bring cool water year-round to the creek at a constant temperature, stops.
“I think that’s why the ellipse mussels are pretty abundant in Poplar Creek,” Inoue said — water quality and habitat.
Those two factors may have led to the overwhelming freshwater mussel decline, Inoue said. Mussels are sensitive to environmental change over their sometimes long lifetimes, a tough spot to be in during climate change. It’s difficult to establish new populations once habitat is gone. Pollution and siltation are also detrimental to mussels, as well as nutrient competition from invasive species like Asian clams, found in Poplar Creek as of recent years, and zebra mussels.
In Cook County, there’s also runoff from roads and parking lots to contend with, Anchor said. Few streams are rock- and rubble-bottomed anymore because of human activity.
In the past, 10 to 15 species have been detected in Poplar Creek, Anchor said.
In the ’90s, Anchor studied mussels at Poplar. He wanted to know how far they ranged and more about their host fish. About 100 were marked, and he followed them for three years. One day, he went out to check on them, and a river otter, another endangered species at the time, had consumed about 70 or so of the mussels. He called another biologist for an explanation of what had happened and was met with a comment about being lower on the food chain. “He was not sympathetic for the clam at all,” Anchor said, laughing.
Anchor found that the mussels were “real homebodies.” But a subset did “some rather impressive movement.”
“You don’t think of a clam being able to move around a whole lot,” Anchor said. “They can move several hundred meters if they need to, which I had not expected at all. Most of the clams lived in an area the size of a bedroom and never left it that I could detect. And others, for whatever reason, there wasn’t a sex difference, there didn’t seem to be an age difference, they decided that they wanted to move. And they went both upstream and downstream.”
It might be tricky to identify direct action to save a mussel in the northwest suburbs, but researchers say everyday actions can contribute to healthier habitats, and healthier mussels. The Shedd is asking Congress to protect about a third of land, sea and freshwater habitat by 2030, part of a global effort.
“If something catastrophic were to happen — a horrible drought or a pack of ferocious otters — it can wipe them out,” Anchor said. “They make a fabulous sentinel to tell us what the condition of the habitat is and ultimately what the condition of our environment is.
“To me, the mussels are just as interesting as the otters,” he added. “And most people would say I’m crazy. But they all have a story to tell.”