Scientists are scrambling to understand why thousands of dead mussels are turning up in several rivers across the United States, including one of the world's biodiversity hotspots.
The rapid decline of the pheasant shell mussel in Appalachia's Clinch River may be part of a mass die-off, with consequences for entire ecosystems.
Like White-nose syndrome, which has devastated North American bats, or chytrid disease, which has ravaged amphibian populations around the world, experts worry that the mussel deaths could be the beginning of a widespread species collapse.
As beloved woodland creatures go, freshwater mussels are near the bottom of the list.
But they perform a critical service. One of these unheralded bivalves can filter roughly 40 liters of water per day, removing algae, bacteria, chemicals, silt and other undesirables from waterways.
They clean the water for everything else that lives in or depends on unpolluted rivers and streams, from fish and plants to city water supplies.
"Mussels are the Rodney Dangerfield of wildlife," said Tony Goldberg, a veterinary epidemiologist at the University of Wisconsin. "They don't get no respect," he added, referencing a quote by the U.S. comedian. "They do so much and we don't even know they're there," Goldberg added.
The Clinch River, flowing through the Appalachian Mountains of Virginia and Tennessee, is home to 46 species of freshwater mussels, along with 133 fish species.
Over the last three years, populations of the pheasant shell mussel, one of the larger and more abundant species, have tumbled.
"We go out in the river and see hundreds if not thousands of dead and dying mussels scattered all over the mussel shoals," said Jordan Richard, endangered species biologist with the U.S. Fish and Wildlife Service.
Richard says one stretch of river his group studies has lost 85% of its pheasant shells since 2016.
Researchers in Ohio, North Carolina, Oregon and elsewhere are reporting mass mussel die-offs, too. Some European scientists say they may be seeing the same thing.
It's not clear why it's happening. While pollution, dam building and invasive species all have taken a toll on mussels around the world, Richard says the current die-offs look different.
"There's this specific subset of mystery cases where we have a lot of different kinds of mussels living in the same place together, and then one species suddenly just drops out and you see them all dying very rapidly," he said.
That sounds like a disease. That's why Goldberg is involved. As a wildlife disease hunter, he has studied mysterious deaths of largemouth bass, West Nile virus in American robins, chimps catching colds from people, and more.
But unlike White-nose syndrome or chytrid disease, in which a single pathogen has wiped out millions of creatures across vast distances, Goldberg does not think a single germ is responsible for all the mussel die-offs. Instead, he thinks something else may be weakening the mussels and making them susceptible to infection.
"There's probably some underlying ultimate cause, like climate change or disturbance of habitat near rivers. Something is linking these that we can't figure out yet," he said.
Mussels are having a tough time even without any possible new disease, Richard said, with increasing weather extremes from climate change.
"They've adapted to periods of drought and periods of flood, and they can recover from that," he said. "But when you suddenly have these crazy events back to back to back, mix that with water quality changes, pollution, river impoundments, all that -- that just takes a toll on mussels in general.
"And then now, if we're looking at a potential emerging disease issue," that could be the last straw, he said.