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On “good” bad days, the shells lay open at the bottom of the river, shimmering in the refracted sunlight. Their insides, pearl white and picked clean of flesh, flicker against the dark riverbed like a beacon, alerting the world above to a problem below.

“That’s what we look for in die-offs,” says biologist Jordan Richard, standing knee-deep in the slow-flowing waters of the Clinch River in southwest Virginia. He points at a faint shape submerged about ten feet upstream. “I can tell from here that’s a Pheasantshell, it’s dead and it died recently. The algae development is really light.”

The Pheasantshell is a freshwater mussel; a less-edible version of its saltwater cousin that spends most of its inconspicuous life part-buried in riverbeds, blending in with the rocks and filtering the water around them.

In recent years though, biologists and fisherman noticed something was wrong. On sections of the Clinch and other waterways in the Pacific Northwest and Midwest, dead mussels were turning up on shores and could be seen glinting from the river bottom. Surveys revealed more fresh dead or dying mussels half-buried and rotting in still-clasped shells.

“It would take you 20 to 30 seconds to go from one dead one to another to another,” says Richard, who works with the U.S. Fish and Wildlife Service (USFWS), in Virginia. “And it’s been like that week after week after week [every fall] since September 2016.”

On the Clinch River alone, hundreds of thousands are believed to have perished, a mass mortality event that has baffled scientists and alarmed ecologists.

Freshwater mussels, like pollinators and trees, are critical to their larger ecosystems and the world around them. They create habitat for other species, like freshwater coral reefs, and help maintain the structure and rigidity of the waterways they call home. They scoop up algae and nutrients, processing and concentrating them for others to eat.

Nature’s ‘Brita Filter’ Is Dying and Nobody Knows Why

Photos: Nathan Rott/NPR