No One’s Sure What’s Causing This ‘Environmental Catastrophe’

Freshwater mussels, once widespread across North America, are disappearing at an alarming rate, and scientists are still struggling to understand why. Historically, the continent supported roughly 300 species of these animals. Many of them lived for decades, and some survived for more than a hundred years. Anchored in riverbeds, they played a major ecological role by filtering huge amounts of water. Today, about 10 percent of those species have already vanished, and nearly one-third of the remaining species are listed as threatened or endangered. According to the Bay Journal, no other group of animals in North America faces such severe danger.

Part of the decline can be traced back to the extensive dam construction of the twentieth century. Dams destroyed large areas of the river habitats mussels depend on and are believed to have contributed to around 20 species extinctions. However, researchers are now observing steep population drops even in rivers that have no dams and are cleaner than they were decades ago. Wendell Haag of the U.S. Forest Service says the situation amounts to a major environmental crisis whose cause remains unclear.

One leading theory focuses on the spread of Asian clams, an invasive species known scientifically as Corbicula fluminea. These small clams have spread across the country and are now found in 47 states, often covering riverbeds in extremely dense numbers. In an experiment conducted in 2018 on Kentucky’s Rockcastle River, Haag found that young mussels struggled to grow in areas where Asian clams were most abundant, even when the water itself appeared healthy.

Haag is also leading a larger study covering 90 streams in 13 states. Early findings point in a similar direction. Common environmental suspects such as pesticides, sediment buildup, or mining pollution do not consistently match up with where mussel populations collapse. The presence of Asian clams, however, often does. Scientists suspect the invaders may be consuming so much microscopic food that young native mussels cannot get enough to survive. Another possibility is that the clams may carry diseases that affect native species. Researchers examining young mussels from the study have discovered genetic traces of thousands of previously unknown viruses, but none has yet been confirmed as the cause of the die-offs.

While researchers search for answers, conservation efforts are underway to keep the rarest species from disappearing. Hatcheries across the southeastern United States are working to breed and raise endangered mussels. Biologists collect gravid, or pregnant, females from species such as the Coosa moccasinshell and the Southern combshell. Their larvae, which naturally attach to certain fish species for development, are placed with host fish and allowed to grow. After the young mussels are raised in captivity for a year or longer, they are carefully returned to selected river habitats.

Progress in restoring these populations is slow. Still, scientists have noticed something encouraging. In some locations where Asian clam numbers have unexpectedly dropped, young native mussels have begun appearing again. That observation suggests that if the underlying threat can be identified and controlled, these often overlooked animals may still have a chance to recover.