Freshwater mussels suffer from reputational issues.

To start, they are living rocks, faceless and mostly motionless. When all is well, they hide out of view, tightly shut and burrowed into riverbeds, doing the ecological heavy-lifting of filtering water, storing nutrients and anchoring freshwater food webs. And when they are in trouble, mussels are not particularly charismatic poster children for ecological strife.

After years of searching for a potential explanation for the mysterious die-offs that have suddenly killed thousands of mussels from Washington to Virginia, researchers have identified a potential “mussel-bola” culprit.

In a study published in Scientific Reports, Jordan Richard, a biologist with the U.S. Fish and Wildlife Service and the University of Wisconsin, and his colleagues used genetic testing to identify viruses in mussels. One novel virus, they found, was 11 times more likely to be present in sick mussels.

Federal estimates suggest more than 70% of North America’s freshwater mussels have been driven to endangerment or extinction. While pollution, habitat destruction and other human-caused hazards can explain some of that loss, the sudden die-offs have remained thoroughly unexplained.

“Mussels are disappearing at an alarming rate, but we just don’t understand why,” said Wendell Haag, a research biologist who was not involved in the new paper. The recent paper is the first solid evidence of a pathogen as a possible cause, he said.

The study is among the first to examine mussel die-offs through an epidemiological lens. The researchers focused on blood samples from 58 healthy and diseased pheasantshell mussels. In all, they discovered 17 viruses in the mollusks’ blood.

One of those, a densovirus, belongs to a class of viruses linked with lethal epidemic disease in shrimp, cockroaches, crickets, moths, crayfish and silkworms. It was much more likely to be present, and in higher densities, in sick mussels compared with healthy subjects.

“Mussels are the dark matter of wildlife disease,” said Tony Goldberg, a veterinary epidemiologist. “We know they exist, we know they’re important, but we don’t know what they’re made up of. We’re really starting from scratch.”