The small, hard-shelled sentinels of Minnesota’s river health are in crisis.
Twenty-eight of Minnesota’s roughly 50 native mussel species are considered endangered, threatened or of special concern. Because of mussels’ key role in the aquatic ecosystem, their decline puts the state’s rivers and streams at risk of poorer water quality.
Minnesota is part of a larger trend: Freshwater mussels are considered the most endangered group of organisms in the United States. Their numbers have declined due to habitat loss, pollution, dams and invasive species. Some native mussel species have disappeared entirely from the rivers of the Upper Midwest.
But there are signs of hope: Thanks to conservation and reintroduction efforts, some native mussels — including some federally endangered species — are making a comeback in Minnesota rivers like the Mississippi and the Cedar. And one city along the Mississippi is putting their unique skills as harbingers of the river’s health to work.
‘Clamming up’ to signal danger
Since 2008, the city of Minneapolis has been using mussels as a sort of emergency warning system at its water treatment plant in Fridley, which draws water from the Mississippi River and distributes water to half a million customers a day.
Attached to one wall of the treatment plant is a nondescript tank, fed by a pipe that brings water directly from the river.
Inside the tank are nine mussels, about the size of a child’s fist, with brown-striped shells. Known as fat muckets, they’re native to Minnesota and the Mississippi River.
Attached to each mussel is a little sensor that measures the gap between the animal’s two shells.
If everything's normal, the mussels are relaxed and happy, with their shells open to eat, said George Kraynick, the city of Minneapolis’ water quality manager and laboratory supervisor.
But pollutants such as heavy metals, pesticides or hazardous chemicals from an oil spill would trigger them to stop feeding.
“If there’s any type of contamination, they’re sensitive to that type of toxicity,” Kraynick said. “So they will actually close up.”
Plant staff monitor the mussel activity on a computer graph. If they were to suddenly close, workers would test the water to find out what's happening — the mussels don’t indicate the precise source of the contamination, so human sleuthing comes next — and could even shut down the plant while they investigate the problem.
The mussels in the Minneapolis water treatment plant are decidedly simple when compared to the plant’s complex system of chemical reactions, filters and lab tests that treat the region’s drinking water. But Kraynick said they are a great way to measure what’s going on in the river.
“It's 24/7. It's using nature,” he said. “It's very low tech and it's really almost bulletproof, because these guys are going to know when something's in the river."
Minneapolis is the only city in the country currently using mussels as water quality monitors, but others have expressed interest.
Kraynick plans to upgrade the tank later this year because the mussels have outgrown their sensors. The new system will be able to send text alerts to staff if the mussels close up. There might even be a "clam cam," so people can see inside the tank, he said.
‘Canary in the coal mine’
North America has the most diverse population of freshwater mussels in the world — more than 300 species, all unique.
Their shells come in different sizes and shapes, earning them unusual nicknames like monkeyface, fawnsfoot, pistolgrip and heelsplitter.
They can live as long as 100 years, and have unique life cycles. To reproduce, they need a host fish to carry their tiny larvae, known as glochidia.
Mussels’ deceptive methods of attracting those hosts can seem almost otherworldly, said Bernard Sietman, a malacologist — a scientist who specializes in studying mollusks — with the Minnesota Department of Natural Resources.
Some have fleshy appendages that mimic food to entice a hungry fish. When the fish attacks, the mussel releases its glochidia into the fish’s face and mouth, so they can attach to its gills.
Mussels are such good indicators of what's happening in a river or stream because they’re voracious filter feeders, straining out tiny particles and pollutants in the water and leaving it cleaner. Some mussels can filter as much as 10 gallons of water a day.
“We like to say they’re the canary in the coal mine,” said Teresa Newton, a biologist and expert on native mussels who works with the U.S. Geological Survey. “When you find healthy and diverse populations of mussels in streams, it’s a good sign that the sediment and water quality is pretty good.”
The reasons for the dramatic and widespread decline in mussels in the first half of the last century were probably the result of habitat destruction caused by dams built along major rivers, Newton said. In the early 1900s, mussel hunters harvested them for buttons and pearls.
But the reasons for mussels’ more recent declines is less clear, and could be the result of multiple factors, including increased contaminants, sedimentation and climate change.
“Some mussel species can live 30, 40, 50, 60, 70 years,” Newton said. “It may take a long time to see the effects of some of these actions on native mussel populations.”
Efforts to conserve and restore native mussel populations are underway on rivers around the U.S., with some signs of hope.
“There’s definitely areas in Minnesota and other places in the country, especially below large metropolitan areas, that are recovering,” said Sietman. But he added, “There’s a lot of work left to do.”
Correction(Nov. 25, 2019): A previous version of this story incorrectly stated the year the city of Minneapolis started using mussels and the amount of water the city’s plant distributes to its residents.