Freshwater mussels are the silent superstars of rivers and streams across the world. The little mollusks, which range in size from a small coin to a pack of playing cards, filter out algae, silt and pollutants, making waterways habitable for other aquatic life. But as Travis Loller at Associated Press reports, many different species of mussels experienced mass die-offs in recent years—and researchers are struggling to understand why.

Biologists at a regional branch of the United States Fish and Wildlife Service in southewestern Virginia first learned about losses in fall of 2016 when locals in reported that large numbers of pheasantshell mussels (Actinonaias pectorosa) were dying in the Clinch River along the Virginia and Tennessee border.

Jordan Richard and Rose Agbalog, both USFWS biologists, and their team confirmed the die off, finding dead mussels strewn across the river bottom, reports Carrie Arnold at National Geographic. They were unable to find a cause of death that year, and by the next fall, they encountered the same thing. In 2018, the pheasantshells once again died off, this time along with many other mussel species. In total, the pheasantshells have decreased by about 90 percent in parts of the Clinch River, along with about 50 percent of another 30 species of mussels in the ecosystem.

The Clinch River isn’t alone. Biologists have also recorded recent mass freshwater mussel die-offs in the Pacific Northwest, Oklahoma, Wisconsin, Michigan and even one in Spain.

What exactly is killing off the mollusks is not clear, and testing has not identified a single culprit. At first, researchers suspected chemical spill or some pollutant was responsible. But the fact that only one species—the pheasantshell—was affected at first suggests a disease is responsible. “It is weird to keep finding dozens or hundreds of dead pectorosa and other species look like they're doing just fine,” Agbalog tells Nathan Rott at NPR.

But eventually, other species also took a hit, which makes the issue even more complex. “From an epidemiologist perspective that is a red flag for infectious disease,” Tony Goldberg, a University of Wisconsin epidemiologist specializing in wildlife diseases who is investigating the die-off tells Emily Holden at The Guardian.

But researchers have not yet identified a common pathogen affecting mussels across species. Instead, it’s likely a variety of diseases are hitting mussels in various river systems. “There’s not some mussel Ebola sweeping across the world to take out every mussel everywhere,” Goldberg tells Loller.

Even before the die-offs, mussels in North America weren’t in great shape. For centuries, they were overharvested to make buttons, and more recently they’ve been impacted by pollution, dam development and altered river habitat. Of 300 mussel species native to the North America, over 70 percent are endangered and dozens have already gone extinct, reports NPR’s Rott. In the Clinch River alone, there used to be 56 species of mussels. Ten have gone extinct and another 20 are considered endangered, including the fluted kidneyshell, snuffbox, birdwing pearlymussel, and shiny pigtoe.

While most people won’t notice the mussels disappearing, they will eventually notice the effects on river ecosystems, which can be dramatic. National Geographic’s Arnold reports that mussels filter almost every drop of water that moves through a river, which keeps the water clean. When the mussels die en masse, the decomposition leads to brief burst of productivity. But after that, the loss of the natural filter leads to darker, dirtier water—and, ultimately, a crash in biodiversity.

“If we have one of these die-offs, the river will never look the same, even to a casual observer,” Goldberg tells The Guardian’s Holden. “The bottom will be different. The fish and other wildlife will be different. It’ll smell and look different. It’ll be cloudier. It’s just not going to be the same river.”

The hope is that the pathogen can be identified. Goldberg tells Holden that researchers are developing new methods for stopping viruses in wildlife including vaccines that can be administered to eggs or via probiotics.

NPR’s Rott reports that if things continue to decline in Clinch River, the Virginia Department of Game and Inland Fisheries has a backup. The agency runs a nursery for freshwater mussels, where they breed healthy Clinch River pheasantshells collected from the die-off reached them.

“If this thing continues, at least we’ve got this other basket of eggs upstream so it’s not all a loss,” says Tim Lane, Virginia's southwest region mussel recovery coordinator.