Freshwater Mussels Are Dying—Which Is the Likeliest Culprit?

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In some cases, humans have harnessed the shellfish’s ability to filter water. In 2019, for example, Pennsylvania invested $7.9 million in a mussel hatchery in Philadelphia aimed at improving water quality in the Delaware and Susquehanna watersheds, which together provide drinking water to more than 25 million people. In fact, most water bound for our reservoirs and faucets has probably passed through the interior of a freshwater mussel, according to veterinarian Abbie Gascho Landis’ 2017 book, Immersion. “They belong to both landscape and liquid,” she wrote. “As part of the river’s viscera—tucked among its bones—mussels are vital to the living river, and they cannot survive without it.”

Today, North America’s 300 or so freshwater mussel species go by their common names, which range from the delicate — pheasantshells, finelined pocketbook, downy rainbow, and fragile papershell—to the rude—elephant ear, monkeyface, heelsplitter, orangefoot pimpleback, and fuzzy pigtoe. They can be as small as a quarter or as large as a Frisbee, their shells as thin as a dime or as thick as a finger.

Regardless of size or species, every stage of a mussel’s life cycle reveals the precariousness of the animal’s conditions for survival. To feed, mussels demand water of a certain quality. To reproduce, they need a strong current to carry a flotilla of sperm to a potential mother. Most larval mussels are parasitic and hitch a ride to their future homes on fish, a feat only possible in healthy river ecosystems. Later, young mussels need an intact creek bed where they can mature into adults.

Mussels also don’t have much of a defense system, and “they’re terrible at escaping,” Sasson said. The creatures are vulnerable to whatever gets dumped in waterways, he added, and they “are not really prepared to deal with modern insults.”

Freshwater mussels are even finicky in sickness and death. Monitoring a mussel’s health is nearly impossible, said Tony Goldberg, a veterinary epidemiologist at the University of Wisconsin-Madison and a Strike Force member. Prying the shellfish open can be lethal. And there’s not much in the way of external symptoms to examine. Goldberg can’t easily draw a mussel’s blood or inquire about its diet, like he would do with a dog or cat. “You can’t listen to their heartbeat or take their temperature,” he said. “The best indicator that a mussel is sick is that it’s dead.”

In the United States, humans have driven a lot of mussels to death over the last century. Until the recent mysterious mass die-offs, scientists could explain much of freshwater mussels’ drawn-out decline dating back to the mid-19th century, when people realized they could yield pearls and their shells could be turned into iridescent buttons. By 1900, prospectors had rushed from the Northeast to Arkansas, Wisconsin, Louisiana, and Texas, depleting wild mussel populations along the way. (In a twist of fate, Johan Boepple, father of the US mussel-shell button industry, stepped and cut his foot on a heelsplitter and died from an infection in 1912.)

Overexploitation and the rise of zippers and plastic fasteners brought the pearl button industry to its end in the early 20th century, but a new threat to mussels soon emerged. From the mid-1920s to the mid-1980s, dam construction across the US impacted all but 2 percent of the country’s total stream length, Gascho Landis wrote. To date, most of the continent’s extinction of mussel species are directly attributable to dam-related habitat destruction. In one instance, a single dam on the Coosa River in Alabama drove six species of mussels out of existence.

One-time pollution episodes left further pockmarks in the nation’s mussel beds. In 1998, for example, an overturned tanker dumped 1,350 gallons of a rubber accelerant into an unnamed tributary of Virginia’s portion of the Clinch River. That morning, 7 miles of its water turned milky with chemicals, killing some 18,000 mussels. Virginia’s game and fisheries agency considers the event “the most significant kill of endangered species in the history of the US Endangered Species Act of 1973.”

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