This story originally appeared in bioGraphic, an online magazine about nature and sustainability powered by the California Academy of Sciences.

Under cover of darkness, thieves dove into the inky waters of Tennessee’s river sanctuaries and scooped up endangered washboard mussels by the thousands. The thick shells of these animals — some as big as Frisbees — were destined to be cut into cubes, polished round, and implanted in saltwater oysters to grow cultured pearls. “Every one of them was almost a twenty-dollar bill,” says David Sims, then a game warden with the Tennessee Wildlife Resources Agency who was charged with surveying these bivalves. Poachers could wipe out an entire bed of mussels in a single night.

Today, 25 years later, Sims can rattle off the common names of freshwater mussel species like a lineup of Seussian characters: Appalachian monkeyfaces, white wartybacks, Tennessee heelsplitters, fat pocketbooks, shiny pigtoes, pistolgrips, and spectaclecases. The orange-footed pimpleback conceals in its bumpy case a fleshy appendage the color of Orange Crush soda. “They’re hard to find,” Sims says, lifting a glistening mussel from a gurgling tank. “We hold on to them, once we’ve got them.” He cradles the creature like a rare jewel as the mussel pulls the two halves of its shell closed and an elegant arc of water shoots out.

North America hosts the richest variety of freshwater mussels in the world, and the epicenter for this biodiversity is in the southeastern United States. In surveys, biologists have found more species in a few square meters of the Tennessee River than are found in all of Europe. The smallest mussel species can fit on a pinky fingernail when fully grown; the largest have the diameter of a dinner plate. Their shells grow rings over time, like trees, and some can live to be more than a hundred years old.

Mussels are critical, if unsung, players in river systems. They spend their lives half-buried in sediment, stabilizing riverbeds and preventing erosion. There they serve as food for birds, muskrats, and otters; and when they die, their shells are adopted as secondhand homes by crayfish and aquatic insects. Most importantly, mussels filter river water — up to 20 gallons per mussel per day by some estimates. They live off the algae and plankton gleaned from this filter feeding, and in the process remove silt and toxic substances from the water, including some heavy metals, harmful bacteria, pharmaceuticals, and endocrine-disrupting chemicals such as PCBs. “They’re like little miniature treatment plants,” says stream ecologist Jesse DiMartini, of the DuPage County Forest Preserve District in Illinois. But, despite their capacity to take up harmful chemicals, mussels have their limits and can be wiped out in vast numbers when toxin levels are too high.

Considering that more than two-thirds of tap water in the U.S. comes from lakes and rivers, mussels have a direct impact on human health and well-being. And yet our cities, towns, and industries have generated the pollution and habitat threats that are largely responsible for the uncertain future these creatures now face. Of the roughly 300 mussel species native to the U.S. and Canada, well over a hundred are endangered or threatened. Seventy others are of special concern, which means they’re especially vulnerable to environmental change. The United States Geological Survey calls freshwater mussels the most imperiled group of organisms in the country.

On a map of the Cumberland River, Sims can point to mussel colonies that thrived just 20 years ago, but are now submarine graveyards: tens of thousands of empty, algae-covered shells splayed open, a lasting testament to what’s been lost. The causes of these losses are many — poaching, dams, agricultural runoff and invasive species among them — but the effects are always the same: “They didn’t reproduce, and now they’re gone,” Sims says.

To rebuild populations, Sims has spent his career submerged in local waterways. Through dogged trial and error, he’s figured out what big-river mussels require to live and reproduce. The payoff — if he succeeds — will be keeping a few endangered species from the brink. It will take a continent-wide network of scrappy efforts like his, each propagating their own resident bivalves, to reestablish these key members of their ecosystems and undervalued contributors to our public health before they’re lost for good.

Rivers Paved with Bivalve Gold

At the dawn of the 19th century, early naturalists collected and traded a wide variety of freshwater mussel shells. As recently as a hundred years ago, observers described scenes nearly impossible to envision today: mussels so plentiful they seemed to pave the river bottoms. While people didn’t often eat the mollusks, they were viewed as an abundant source of raw material. Crushed shells filled potholes in roads and served as poultry grit and exfoliants in soap. Harvested from the Mississippi and other big rivers by the millions, their shiny shells were used to make buttons. Indeed, by 1900, half of the entire U.S. supply of buttons was made from mussel shell. Then came the “pearl rushes” of the early 20th century, when freshwater mussels were ravaged for the unlikely chance of finding a marketable pearl inside (most of which were small and misshapen.) Such overharvesting caused local populations to dwindle. And broader threats still loomed.

Beginning in the 1930s, and peaking in the 1960s, rampant dam building nearly eliminated the free flow of rivers throughout the Southeast. There are now 79,000 dams in the U.S. — 1,237 in Tennessee alone. These impoundments block the flow of nutrient-rich water that mussels rely on for food, as well as their access to the fish species the mollusks require to reproduce. Dams also artificially warm or cool water, disrupting the seasonal temperature changes that drive spawning behavior. Plus, since mussels are long-lived, it could take decades before the full effects of dam construction are seen.

The impacts of overharvesting and dams have only been compounded by water pollution. In the mid 20^th century, virtually unchecked dumping of toxic chemicals made rivers across the country uninhabitable for many species. By the 1970s, biologists were describing “mussel deserts” in the Mississippi. The passage of the 1972 Clean Water Act led to dramatic improvements in water quality by reducing industrial pollution from factories, but a variety of chemicals, including agricultural products and pharmaceuticals, still flow in rivers today. Researchers are only beginning to understand the effects of these substances on aquatic ecosystems.

Scientists like Sims still don’t have a firm grasp on which chemicals kill or sterilize which species of mussel, and it’s incredibly complicated to figure out. For example, a 2007 study revealed that the pesticide glyphosate (the active ingredient in Roundup) is toxic to juvenile and larval fatmuckets, but not to adults. Adults are, however, sensitive to one of the inactive ingredients in the pesticide. And that’s just one example. Trying to account for the combinations of substances in a river and how those effects vary with temperature is a daunting task. “It’s embarrassing how little we know about what’s poison in the world,” says malacologist Chris Barnhart, who runs a mussel conservation program at Missouri State University.

Bringing Bivalves Back

Despite the myriad threats mussels face and the important roles they play, for river ecosystems and human health alike, they are hardly a conservation priority. In 2014, the last year for which data are available, the federal government spent less than $10 million on conservation efforts aimed at all endangered mussel species combined, compared to the $190 million dedicated to Chinook salmon — a single species.

In 2006, Sims was recruited to help the Tennessee Wildlife Resources Agency launch a conservation project called the Cumberland River Aquatic Center. Sims’s reputation as a workhorse, together with his successful poaching surveys and advocacy for freshwater habitats, made him the perfect candidate to run the operation. The center was designed to fill a particular void in mussel conservation: propagating endangered big-river mussels, which other labs had struggled to do up to that point. It would also be the first mussel conservation program in Tennessee, the state with the second highest mussel diversity. (Only Alabama has more).

The new facility had a peculiar address, though: It would be housed on the grounds of the second-largest coal-burning power plant in Tennessee. The Tennessee Valley Authority, which operates the plant, originally constructed the building as a catfish hatchery to take advantage of the warm water discharged from the plant. Programs like these satisfy the plant’s environmental stewardship mandate under the National Environmental Policy Act. In 2006, the power company offered the facility to the state wildlife agency for use in mussel conservation — a way of offsetting environmental damage the company had caused to the mollusks’ habitat.

In the shadow of the smokestacks, between the looming coal pile and the four-story boilers, Sims got to work. Bolstered with funds from the Army Corps of Engineers, he patched holes, checked plumbing, and pumped river water into the 10 40-foot-long concrete tanks at a rate of 1,200 gallons per minute. He then gently placed baskets of mussels in these simulated rivers.

Sims knew that mussels live and die at the mercy of the water quality, and that individual species require very particular parameters. So for six years, he adjusted water flow and squinted at thermometers. Sims tested for ammonia, aluminum, and other contaminants, and filtered out parasites invisible to the human eye. He measured shell growth with calipers and counted growth rings on bumpy shells. Eventually, he learned enough through all this toil and observation to shepherd colonies of endangered pink muckets from larvae to full-grown adults.

After dams had doomed huge numbers of mussels, Sims’s work represented a step toward keeping them alive long enough to find hospitable riverine homes. This was no small feat.

A Complex Courtship

Mussel reproduction is a marvel of evolution. At every step of the process, the likelihood of failure far exceeds the likelihood of success. The process begins when a male mussel discharges a white cloud of sperm into the surrounding water. Most of this sperm will be carried away by the current and go to waste, but some may find its way into the gills of a female, where her eggs lie in wait. Once fertilized, the eggs develop into larvae, called glochidia, which females hold in their gills for a month or more. Some species release their larvae in the cool waters of late summer or early fall; others wait for spring.

In order to mature, the larvae must feed on fish blood. That means when the female releases her glochidia, they must find and latch onto the gills of a fish. But not just any fish will do. Each mussel species requires particular host species; if the larvae latch onto the wrong fish, the fish’s immune system recognizes them as a parasitic invasion to be fought and killed. If the larvae succeed, they will hold on for a couple of weeks until they’re mature enough to drop off and drift to their lifelong home on the river bottom.

To attract a suitable host, mussels have evolved lures that mimic fish food. The lure of the snuffbox mussel, for example, looks like a grub nestled inside its open shells. When a fish swims in for a bite, the mussel snaps shut onto the fish’s head before releasing the larvae into the open mouth of its suffocating prey. A log perch is the preferred fish for a snuffbox; its immune system tolerates the mussel larvae infestation, and it can survive this aquatic chokehold. If a smaller fish, like a darter, goes in for the bait, it may lose its head.

Spectaclecase mussels take a gentler approach. They unreel a six-foot line of mucus with glochidia on the end. When a fish goes in for a bite of the lure, it ends up with a mouthful of larvae instead. That’s at least what biologists suspect. Even though the mechanism is known, Sims says, “no one’s been able to figure out the host fish.” That’s one of the reasons propagating mussels is so finicky. Experiments aren’t technically difficult, but they are time-consuming. They require infesting different types of fish with glochidia and hoping something sticks. If the larvae fail to develop, researchers have to wait another year, until the female mussels have glochidia again, to try with a new host species. The programs simply don’t have sufficient resources, either in time or researchers, to infest every native host fish up front, so they must take a systematic approach.

Successful Conservation Efforts

Of course, infesting fish is just the first step — propagation also involves keeping juveniles alive once they drop off the host, until they’re large enough to avoid being eaten by flatworms or other predators. Because these trial-and-error experiments have multiple steps and a high risk of failure, and because so many questions about host fish and toxicity remain unanswered, the captive breeding of mussels has been slow to succeed. “It’s only in the last five or six years that research facilities have been able to establish new populations,” says biologist Paul Johnson, who runs the Alabama Aquatic Biodiversity Center in Montgomery. The first propagation efforts began in the 1990s, with the first lab-reared populations restored to rivers a few years later. Now researchers want to establish sustainable populations in the river, a goal that’s becoming increasingly attainable. In the past 6 years, Johnson’s lab has released more than 110,000 mussels into rivers throughout the state.

Johnson’s is one of the dozen or so operations that represent hope for the future of freshwater mussels in North America. Although the labs are small, they often collaborate, sharing species and techniques to help move the whole field forward. For example, when a researcher in Virginia found two rare golden riffleshells with glochidia in their gills, he called the Kentucky lab that had pioneered in vitro propagation techniques to raise mussels without host fish. The labs were 350 miles apart, so the two researchers got in their cars and started driving to make the handoff at the midway point. A year later, the Kentucky lab had produced 1,200 juveniles from those two females.

By 2012, Sims’s project, too, was flourishing. He and his team had resettled more than 18,000 adult mussels into river habitats. Encouraged by this success, other centers began sending him breedstock — or starter mussels — so Sims could try to emulate those results with other endangered species from nearby states. Those were heady days in the world of freshwater mussel conservation, but then Sims received some sobering news.

A Turn for the Worst

In the fall of 2012, the 50-year-old coal-burning power plant announced plans to make a long overdue upgrade: installing new scrubbers to remove sulfur dioxide from its emissions. This was good news for air quality, but the new construction meant the hatchery that had been Sims’s headquarters for the past six years would be demolished. Then, to make matters worse, flatworm parasites snuck into his tanks and devoured thousands of juveniles.

Sims loaded what was left of his mussels into coolers, put the coolers in the back of his pickup, and drove away. The best location he could come up with was a wildlife agency storage shed on a stagnant Cumberland backwater about 20 miles away. In the shed, Sims kept a few brown shells, dry, empty, and open, lined up on his desk in the shed like a malacological memento mori. The living mussels in his care at that time numbered about 300. They sat in tanks, filled with water from the near-motionless cove outside. The shed also lacked the space for Sims to put a fish and a mussel in the same tank, which meant he couldn’t work on propagation at all. The goal was pure survival.

Fortunately for Sims and his mussels, environmental groups rallied shortly after the plant’s announcement. The Center for Biological Diversity, the Tennessee Environmental Council, and the Sierra Club threatened to sue the Tennessee Valley Authority for violating the Endangered Species Act if the mussels weren’t given a new home. In response to the criticism and the threat of a lawsuit, the Tennessee Valley Authority changed course. In February 2013, they offered to build Sims’s operation a new $1.5 million facility.

A Room with A View to the Future

In early 2016, Sims loaded the last of the mussels from the shed back into coolers, put the coolers into his pickup, and drove them to their new home. The new center is cavernous — big enough to hold two-dozen hot tub-sized tanks fed by filtered Cumberland water. It includes raceways, boat storage, classroom space, and a room six times the size of the shed dedicated entirely to propagation. There’s also another malacologist on staff, senior scientist Dan Hua, who started out in China’s freshwater pearl cultivation industry. While at Virginia Tech, she earned a reputation for nurturing endangered juvenile mussels until they grew large enough to tag — a bottleneck that the program had been struggling to overcome for years. The secret to Hua’s success was feeding the mollusks with the nourishment found in natural water sources, like ponds, rather than commercial food. Sims sees Hua’s novel techniques as the biggest boon yet for the center — and for the future of big-river mussels in Tennessee.

By last December, things were once again looking up, Sims says. His population of pink muckets had gone from dozens to thousands. The juveniles looked like pebbles strewn across the gravelly bottoms of the simulated rivers. He hopes to eventually turn the colonies of several species into sustainable mussel populations in the Cumberland. “Maybe in 10 or 20 years we’ll see them spread,” he says. “If we can do that up here, there’s a real good chance we can downlist some, like the pink muckets.”

There are no guarantees when it comes to this work, though. Sims knows this firsthand. What works for one species or population may not work for another. New construction projects and invasive species encroach on native habitats. Human populations are growing, spreading, and increasing their environmental impact. And that’s not to mention the effects of a changing climate. There is a lot of work to do to keep lowly mussels alive, but there are steely labs around the country with workhorse biologists like Sims, who have devoted their careers to the challenge.

At the end of last year, Sims returned to a spot a few miles downstream of Carthage, Tennessee where he had placed 100 pink muckets along the Cumberland River bottom two years earlier. From the riverbank, he could see all the females clearly displaying their lures. “They were just beautiful,” he says. The mussels had successfully fertilized eggs and were trying to attract host fish. “Nobody else saw it, but I could see all their lures in that one little spot,” he says. “They were doing their thing.”

Read more about endangered freshwater mussels.

Previously in The Revelator:

Climate Change Puts Sea Turtles in the Hot Seat