Jordan Richard had barely started his job as a biologist for the U.S. Fish and Wildlife Service’s Southwestern Virginia Field Office, in September 2016, when he got hit with concerning news: freshwater mussels were dying in the Clinch River. Thousands, in multiple parts of the river.
Healthy freshwater mussels aren’t easy to spot: The mollusks spend most of their adult lives half-buried in riverbeds, shells slightly open, siphoning nutrients from the water. They can move using a muscular little “foot” but rarely do. They look like rocks.
“You might have a mussel the size of your hand and you can only see a couple centimeters of it poking up out of the bottom of the river,” Richard told the Bristol Herald Courier.
But when he began visiting the Clinch’s various mussel shoals that fall, he saw huge numbers of the creatures lying fully exposed on the surface of the riverbed.
“Their feet were just kind of hanging out, and their shells were hanging open limply, and there were thousands and thousands ... dying everywhere,” Richard said.
The biologist — who now splits his time between the Fish and Wildlife Service and the University of Wisconsin — said it was “scary” to see so many dying and freshly dead mussels.
He was reluctant to sound an alarm bell without more data, though.
Four Septembers later, after conducting extensive research with a group of colleagues, Richard said they have plenty of evidence that the mussel die-offs have caused a “huge drop” in their populations.
They also have evidence of a culprit. Through genetic testing, the team found a host of viruses in the mollusks — including one that was roughly 11 times more likely to show up in the sick mussels.
The findings, which aren’t conclusive, were published several weeks ago in Scientific Reports, an online open access scientific journal that covers natural sciences.
The group needs more data to confirm them. But Richard said the results are a promising step toward understanding what’s killing the Clinch River’s mussels — and maybe, eventually, figuring out how to save them.
The Clinch River starts in Tazewell County, flowing from cave springs and traveling more than 130 miles across Southwest Virginia and Northeast Tennessee. In conservation-speak, the river is what’s known as a biodiversity hotspot: a region that shelters loads of unique wildlife and plants but also faces a high risk of destruction.
More than 100 fish species, the most of any river in Virginia, swim its waters, including sport fish such as rock bass crappie and freshwater drum. The riverbed beneath them supports 46 species of freshwater mussels. Twenty of those mussel species and five of the fish species are federally listed as threatened or endangered, making the Clinch the area with the highest concentration of rare aquatic creatures in the U.S.
Despite the obscure, monotonous lives they lead, freshwater mussels play an active — and critical — role in the broader life of the river, Richard said.
“We call them ecosystem engineers,” he said.
For one thing, they’re nature’s Brita filters: The mussels feed by sucking algae and other organic matter from the water, a process that also removes toxins and generally cleans the water. Their excretions produce food for bugs, crayfish and other little creatures that, in turn, feed the fish. Their shells stabilize the riverbed, especially during floods. And when they die, the shells become shelter for other small creatures, like the freshwater version of a coral reef.
“[In] places where mussels are doing really well, everything else is doing really well,” said Rose Agbalog, a Fish and Wildlife Services biologist and one of Richard’s fellow mussel researchers.
But for decades, freshwater mussels in the Clinch and across North America have been struggling. Dams have destroyed and fragmented their habitat. Invasive species and pollution have wiped out large numbers of them. (In 1998, a rubber accelerant spill in Tazewell County killed an estimated 18,000 mussels in the Clinch.) And for a long time, people overharvested freshwater mussels for their pearls and shells.
More than 70% of the continent’s known freshwater mussels are now “considered endangered, threatened or vulnerable, with 23 species having gone extinct from the Southeastern United States alone,” Richard, Agbalog and his colleagues wrote in the Scientific Reports study.
“So that’s one of our worries,” said Agbalog, who joined the Southwestern Virginia Field Office not long after Richard. “Mussels are really important, but ... everything else that depends on them — those [species] tend to … head downhill, too, once we start losing mussels.”
The die-offs that started among the Clinch’s mussels in 2016 added a bewildering new threat to the mollusks’ already long list of challenges. And they aren’t just happening in the Clinch: In the past few years, Richard and Agbalog have helped research similarly mysterious die-offs in Washington, Oregon, Michigan, even Sweden and Spain.
“There’s a lot of this stuff,” Richard said of the die-offs.
Monster Energy mornings
Agbalog said she, Richard and their colleagues wound up looking for diseases that could be driving the Clinch River die-offs because just a few mussel species seemed to be affected. One particular species, a large, abundant mussel called the pheasantshell, seemed to be faring the worst.
“The fish and other critters [seemed] to be doing OK — we’d kind of ruled out the chemical or toxicant [causes],” Agbalog said. “That’s what turned us onto thinking, maybe it’s a disease.”
The research team began collecting blood samples from both healthy and sick pheasantshells. Their data would eventually show that the species had declined by 50%-90% at various monitoring sites between 2016 and 2019; one stretch of the river lost an estimated 80,000 pheasantshells during that window.
Freshwater mussels are “supremely easy to catch,” Richard said, once you know how to spot them, it’s just a matter of scooping them from the riverbed. The real challenge of the blood sample collection was everything else, starting with the time frame: Agbalog and Richard typically did their fieldwork in two-day stretches, often working 16 hours each day.
“Usually, we’d start super early in the morning,” Agbalog said. “We’d go to the gas station, get a bunch of Monster Energy drinks and Red Bulls, slam those on the drive, which was ... at least an hour-and-a-half to two hours. And then we would get in the typically super-cold water in our wetsuits.”
The biologists would then haul batches of sick and healthy pheasantshells to the shore, leaving them in the shallows or in buckets while they set up a table for the work. Then it was time to gently pry each shell open, swab it with a little alcohol and draw blood. They transferred some of the blood to petri dishes and the rest to vials stored in dry ice. After that, they’d repeat the process at site after site — as many die-off areas as they could get to.
At the end of each field trip, Richard and Agbalog would rush all of that material, still on dry ice, to a FedEx station to overnight it to the University of Wisconsin lab they were working with. If they didn’t make it by the 7:30 p.m. mailing deadline, they’d either have to find a freezer that could store the samples at roughly minus 109 degrees Fahrenheit — and there aren’t many of those — or let them spoil.
“We never did miss [the 7:30 p.m. deadline],” Agbalog said. “We came close a couple times.”
A possible culprit
The research group used the lab’s genetic sequencing equipment to hone in on viruses present in the mollusks’ blood. Freshwater mussel diseases haven’t been studied much, Richard said, which meant the team wasn’t sure what viruses are normal for the animals to carry. They resorted to identifying every virus they could.
Focusing on the 58 best blood samples from the hundreds they’d gathered, the team identified 17 novel viruses present in the mussels. They measured how much of each virus was present in each sample and ran statistical models to look for correlations.
The results “showed that there’s this one virus that we found over and over and over again in the sick ones and very rarely in the [healthy] control ones,” Richard said.
It was a densovirus, a kind of virus known to trigger deadly outbreaks in a host of other invertebrates such as shrimp, crayfish, moths and cockroaches. “Clinch densovirus 1,” as the group named this one, was significantly more likely to appear in sick mussels than in their healthy counterparts.
It takes a village
Richard said the results were ideal, if not encouraging.
“You never hope that there’s a terrible new virus,” he said. “But I do think we hope it’s a virus [causing the die-offs]. Because if it’s a virus, we might be able to do something about it and it might stay more contained within ... a smaller set of species.”
Richard stressed that the group needs more information before they can definitively link the densovirus to the droves of dying mussels in the Clinch. To that end, they’re setting up a lab where they can study how healthy mussels respond when they’re infected with the virus. Meanwhile, Agbalog has begun making trips to the Clinch for a more targeted study of how healthy mussels respond to the virus in the wild.
Beyond potentially helping the Clinch’s mussels and overall ecosystem, the biologists said, the knowledge they’re gaining through all of this research could also offer hope for other rivers experiencing mussel die-offs.
“The methodology [we used] can be expanded,” Agbalog said. “Now, when Jordan gets a call and somebody says, ‘Ugh, there’s 10,000 dead mussels in ‘X’ river,’ we have the framework for how to start looking at disease.”
Richard said he gets more of those calls every year — some from fellow environmental experts, many from people who simply live along rivers and notice concerning heaps of dead mussels.
“It takes a village to look after a healthy ecosystem,” he said. “If people know that mussels are important and know who to call, when they see something wrong with the mussels, then we’d be a lot better off and able to do something about it.”
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