No One's Sure What's Causing This 'Environmental Catastrophe'

Freshwater mussels have been dying out in North America, and the people who study them still don't know why. The continent once hosted 300 freshwater mussel species—many living for decades, some more than a century—anchored in riverbeds and helping to filter vast amounts of water, per Scientific American. Now, about 10% are already gone, and roughly a third of the rest are considered threatened or endangered. "No other group of animals on our continent is so gravely imperiled," notes the Bay Journal. Heavy dam building in the 20th century destroyed prime habitats and likely caused around 20 extinctions, but the mystery now is what's driving steep declines in rivers that aren't dammed and are cleaner than they were decades ago, per Scientific American. "There's this environmental catastrophe going on, and we really don't know what's causing it," says the US Forest Service's Wendell Haag.

Haag suspects Asian clams (Corbicula fluminea), a small, invasive creature now found in 47 states and often carpeting streambeds by the thousands per square yard. In a 2018 experiment in Kentucky's Rockcastle River, juvenile mussels grew poorly where Asian clams were densest, even when water quality looked fine. Early results from a 13-state, 90-stream study that Haag is leading point in the same direction: The usual suspects—pesticides, too much sediment, mining pollution—don't consistently line up with mussel crashes, but Asian clams do. One theory is that the clams strip so much food out of the water that young native mussels starve; another is that the clams may introduce unseen pathogens. So far, however, scientists combing through baby mussels from Haag's study have found DNA from thousands of previously unknown viruses—but no clear mussel killer.

While scientists hunt for answers, hatcheries in the Southeast are trying to keep the rarest species alive. Biologists are collecting gravid (i.e., pregnant) females of species like the Coosa moccasinshell and Southern combshell, using host fish to carry their parasitic larvae, then raising the juveniles for a year or more before carefully "planting" them back into chosen stretches of river. Success, when it comes, is slow. Still, in places where booming Asian clam populations have unexpectedly declined, Haag is beginning to see young native mussels again—a hint that, if the underlying threat can be managed, some of America's most overlooked sea creatures might yet stage a comeback. (Check out what Maryland is doing to restore its freshwater mussel population.)