Throughout his professional life, U.S. Forest Service researcher Wendell Haag has studied freshwater mussels in their hotspot of biological diversity, which extends across a vast swath of the southeastern United States. His more than 30-year career has been overshadowed by a single question: Why are the bivalves in this region disappearing?
Freshwater mussels are one of the most endangered groups of animals in the world. North America is currently home to more than 300 native species — many with colorful colloquial names like fuzzy pigtoe, purple warty back, fluted elephant ear, orangefoot pimpleback, pink heelsplitter, and monkeyface — of which 70 percent are either endangered, threatened, or of special concern. Some southeastern watersheds have seen losses of a third to half of their mussel species in the last half century; another 21 southeastern species are believed to be extinct, and dozens more persist only in fragments of their former range.
Threats to mussels come from many quarters — polluted runoff, the damming of waterways, warming water temperatures, pathogenic viruses, and invasive competitors — but an explanation for the extent of the U.S.’s most widespread die-off, which began decades ago in the Southeast, has remained out of reach.
Recent findings have helped biologists develop techniques for raising the most threatened mussel species in labs.
“The region where these die-offs have been happening is overlaid on the area of the highest mussel diversity not only in the U.S. but in the world,” said Haag, who works out of the USDA’s Southern Research Station, in Frankfort, Kentucky. “These are catastrophic events, and we don’t know what’s causing them.”
The die-offs have gotten little attention outside of scientific circles — a fact that does not surprise Art Bogan, research curator of mollusks at the North Carolina State Museum of Natural History. “They are not charismatic like pandas, or gorillas, or even some fish,” he said. “Some people refer to them as pet rocks — having that much sex appeal.”
For a long time, very little was known about freshwater mussels and their unique lifecycle, including the fact that mussels must parasitize certain species of fish to complete their reproductive cycle. This recent finding has helped biologists develop techniques for raising the most threatened mussel species in labs to keep them from blinking out.
“All we can do is keep these animals in captivity, put them in streams, and hope for the best,” said Haag. “It’s kind of a gamble because we don’t know what causes these declines in the first place, and we don’t know, whatever those factors are, if they are still in play or [have] come and gone.”
Researcher Wendell Haag wades into Dunkard Creek in Pennsylvania for a study on freshwater mussels. Michael Swensen
After female mussels take in sperm that males eject in the water, the larvae they produce, called glochidia, hatch on her gills but need further development outside the mother. Female mussels in the eastern U.S. have evolved with a lure-like appendage. When a fish comes to investigate what looks like a minnow, the mussel ejects a cloud of babies, which move through the fish’s mouth and into its gills. (Other mussels release clumps of glochidia that look to fish like morsels of food or webs of larvae into which fish swim.) The young mussels develop in the gills for two to five weeks, until they are mature enough to drop to the stream bed and start life on their own.
Native mussels don’t move much after they establish, perhaps 50 to 75 yards, and most stay in place for their lifetimes. Some species can live more than 100 years, but more commonly, they live for 10 to 50 years. Native mussels vary widely in size — from the size of a pinky fingernail to a foot across — and provide a range of ecosystem services: They are sometimes called the liver — or the kidneys — of a river. “They filter all sorts of things out of the water,” said Christopher Eads, a biologist at North Carolina State University who studies mussels and operates a lab that raises mussels for reintroduction. “Silt, algae, bacteria, fungus, fine organic matter. Each mussel can filter between five and 20 gallons a day. When you multiply that by thousands and tens of thousands of mussels and many, many river miles, they are doing a huge service for us.”
Experts worry the newly arrived golden mussel could spread through North American waterways like other nonnatives.
Mussels not only clean water, they also stabilize river habitat, transfer the particulate matter they take in onto the riverbed, and create biodiversity hotspots by increasing the abundance and diversity of insects around them. Mussels also provide food for birds, fish, and other wildlife. And their presence, experts say, is a good indicator of river health.
Over the last 30 years, scientists in the U.S. have exerted much time and effort studying native mussels, including the construction of laboratories and hatcheries that raise and reintroduce imperiled species. Even so, it may be a losing battle. Last year, Texas listed six species as endangered, and a comprehensive survey published in 2022 found only 17 of the 36 species of mussels once found in South Dakota. The U.S. Fish and Wildlife Service has proposed designating critical habitats across 17 states to protect four endangered mussel species.
The situation is no better globally. Seven of Europe’s 16 mussel species are listed as threatened. (There was a bit of good news during the cleanup of the Seine River for the 2024 Olympics in Paris, when researchers found the DNA of three mussel species thought to be near extinction.) The number of species in Asia rival the U.S., but not as much is known. Pressures on mussels in South America are similar to those in North America; in Argentina, scientists found 35 species in 1990, of which 17 have not been seen since.
A plain pocketbook mussel displays a lure-like appendage resembling a fish, in the Potomac River in West Virginia. Ryan Hagerty / USFWS
The list of known threats to mussels is long. “Basically it’s humans,” said Bogan. In the U.S., mussel numbers started to decline with the advent of colonial agriculture, which dumped sediment into rivers and buried many mussel species. Mussel populations took a different sort of hit in the mid-1800s, when clothing manufacturers began to use the inside of mussel shells, called nacre, to make mother-of-pearl buttons. Freshwater mussels also produce pearls, and discoveries of the gems across the East, South, and Midwest — one pearl found in New Jersey in 1857 reportedly fetched $1,500, a princely sum at the time — sparked “pearl rushes” that decimated mussel populations.
In the 1980s, invasive mussel species such as quagga and zebra mussels, native to eastern Europe, began threatening mussels in the U.S. Those species are much more aggressive feeders, says Caryn Vaughn, a researcher at the University of Oklahoma. They have a much higher reproductive rate and can attach to the shells of native mussels, killing them outright. In Europe, invasive mussels were found to create conditions unfavorable to natives and push them out of their habitat.
Late last year, biologists discovered that a new invader from Asia, the golden mussel, had arrived at the port of Stockton, in California, likely introduced by ballast water from a ship traveling from an international port. Experts worry it could spread through North American waterways like other nonnatives — on the hulls of recreational boats, in bait buckets, bilge water, and through the movement of waterfowl. “If it invades areas where native mussel populations are already stressed from habitat degradation, pollution, and other stressors,” said Vaughn, “it will likely have impacts similar to zebra mussels, which means the natives would starve and their populations decline.”
By slowing river currents, dams allow suspended sediment to fall onto the river bottom, burying mussel beds.
And in many places climate change is exacting a toll as water temperatures rise. Severe and extended droughts in the southern plains are causing many mussel die-offs, said Vaughn, “not only because of dewatering, but when water levels get low in the summer, the water temperatures get high, and the mussels are basically getting too hot and stressed and dying.”
The widespread construction of dams in the 20th century was likely the most severe blow to freshwater mussels, experts say. By slowing the velocity of river currents, dams allowed suspended sediment to fall onto the river bottom, burying mussel beds. And as barriers to fish migration, dams can effectively block mussel reproduction and migration: without fish to attach to, mussel larvae can’t complete their life cycle. (In streams across the country, dam removal has helped restore some mussel populations.)
As filter feeders, mussels are uniquely susceptible to waterborne pollutants, such as pesticides, heavy metals, pharmaceuticals, and other chemicals. Road salt is implicated in some die-offs; in the Allegheny River, mussels were found with elevated levels of radium, traced to discharge water from oil and gas operations.
Endangered freshwater mussels found in the Allegheny River in Pennsylvania. Ryan Hagerty / USFWS
The fragmentation of river systems is also a serious and growing threat. “Where [mussels] might have had extensive populations throughout river networks, they are getting eliminated from main stems through sedimentation and pollution and getting isolated to some pockets in headwater streams,” said Eads. (Isolated populations can’t interbreed, reducing a species’ genetic diversity and therefore its evolutionary fitness.) “Once that happens, any event can wipe out a mussel bed, whether it’s a beaver dam or a tree falling in the wrong place. It’s pretty dire for some species.”
Despite all that is known about the myriad threats to freshwater mussels, the root cause of the catastrophic die-off in the Southeast, which began in the 1970s and continues to this day in small- to medium-size streams, remains a mystery. Haag, who published a paper on this decline in 2019, is still researching the die-off and studying whether recovery — or even conservation of mussel beds in unaffected areas of the Southeast — is possible. “These streams are still mostly de-faunated,” he said, which causes ecological ripples throughout freshwater ecosystems, from dirtier water to less biodiversity. “They are a keystone animal on which a lot of other ecological functions depend.”
Researchers hope that while freshwater mussels may not be charismatic, they will be deemed deserving of protection.
Haag hypothesizes that mussel decline in the Southeast is related to past colonization of the region’s streams by the invasive Asian clam, which grew super-abundant — perhaps as many as 5,000 per square meter in some places — and knocked many native populations to near-extinction levels. Asian clam populations have since collapsed in the Southeast, but their legacy lives on: “There were so few [native] mussels left they were not able to recover,” said Haag. “Or it’s going to take a long, long time.”
Populations of many mussel species have dropped so low that wildlife agencies now grow them in hatcheries for release in local streams. Some Native American tribes are also operating hatcheries, for environmental as well as cultural reasons. Mussels are a “First Food,” or food of significant traditional importance, to members of the Umatilla tribe, said Alexa Maine, lead biologist for the Confederated Tribes of the Umatilla Indian Reservation’s Freshwater Mussel Research and Restoration Project, in southeastern Washington. They’re also adornment, regalia jewelry, and tools, she said. “All of the tribes in the Columbia Basin have an extensive history with mussels.”
Across the U.S., mussel researchers hope that while freshwater mussels may not be charismatic, they will be deemed deserving of protection by federal and state agencies — which will in turn protect entire river ecosystems. “They are beautiful animals and have value in and of themselves,” said Eads. “If we lose them, the world is losing something special.”
