For most, the term “endangered species” conjures up charismatic creatures like polar bears, whooping cranes and sea turtles in dire need of conservation. But few would guess that among the most imperiled group of animals in the United States are freshwater mussels. Some 35 species of these mollusks have gone extinct in the past few centuries. And more than 90 other species are on the brink of extinction and currently listed under the Endangered Species Act.
It is not difficult to fathom why this mass disappearance of mussels is off most people’s radar. While mussels come in a variety of vivid shell colors and sport eccentric common names like Fuzzy Pigtoe, Pink Heelsplitter, Purple Catspaw and Orange Pimpleback, these mollusks lack the charming cuteness of a black-footed ferret or the grizzled gravitas of a California condor.But according to John Pfeiffer, a research zoologist at the National Museum of Natural History, mussels’ simple appearance disguises “one of biodiversity’s best kept secrets.” As the museum’s curator of bivalves, Pfeiffer oversees a collection of some 240,000 mollusk records including oysters, scallops and clams. Around 10% of these records represent freshwater mussels, an eclectic group with surprisingly complex behaviors and an outsized impact on ecosystems around the world. “What seem like unassuming rocks full of snot are amazing animals with complex behaviors ” Pfeiffer said.
Like other bivalves, mussels are composed of a soft-bodied animal sandwiched between a pair of shells connected by a ligament, which acts as a door hinge. They spend much of their time wedged into the river bottom, filtering out bits of algae and phytoplankton as they pump water through their siphon and along their gills. As adults, a single mussel can filter between 10 and 20 gallons of water a day.
But these mollusks are not always passive filter feeders. In their earliest life stages, mussels are parasites that latch onto the gills and fins of a fish (only one species of mussel is known to target a non-fish host, opting instead for a large salamander called a mudpuppy to be the surrogate for its larvae). But depositing larvae onto swimming fish can be difficult for adult mussels, which lack sight and a broad range of motion. So many species sport lures that stick out of their shells to entice fish. Over the eons, evolution has fine-tuned these lures to the point where they are convincing mimics of common fish prey items like crayfish, worms and insects. “People often don't think of behavioral adaptations when they think of mussels but these animals are literally fishing,” Pfeiffer said.
When a hungry fish spots the mussel’s dangling lure, it pounces at what looks like its favorite food . “But instead of getting lunch, it gets a mouthful of parasitic mussel larvae,” Pfeiffer said. The larval mussels attach to the fish’s gills and stay put for several weeks as they metamorphosize. Once they reach their juvenile form, they drop off into the streambed and begin filter-feeding.
More than a third of the world’s roughly 950 known species of freshwater mussels are concentrated in North America. Most are found along the bottoms of rivers, streams and lakes throughout the Midwest and Southeastern United States. There were once so many freshwater mussels in spots along the Ohio River that people claimed to be able to cross the entire river on mussel beds.
Washington, D.C. was also prime mussel real estate. The area west of the Washington Monument was once a sprawling wetland called the Potomac Flats that was home to several mussel species including large Yellow Lampmussels and chocolate-brown Triangle Floater mussels. But in 1870, the US Army Corps of Engineers dredged the Potomac River and filled in the flats with more than 12 million cubic yards of sediment. This created the land where the Lincoln, Jefferson and Martin Luther King Jr. monuments now stand. But it also snuffed out the flat’s thriving wetland community. Some of the District’s mussel species have not been seen in decades.
Freshwater mussels around the world are facing a similar plight. In addition to habitat loss, dams pose major risks to mussels by restricting the flow of rivers and halting the movement of the fish that mussels rely on. Another threat is invaders like zebra mussels, a biological expat from Eurasia that outcompetes native mussels for food and space. Increasing levels of pollution, especially toxic pesticides and industrial runoff, and sediment smother these filter-feeding specialists.
Humans have also overharvested freshwater mussels for the past two centuries. Before the advent of plastic buttons, mussels were collected in bulk around the turn of the twentieth century by the pearl button industry. Workers would punch out pieces of the mussel’s shell, which were then crafted and polished into buttons. In 1905, the town of Muscatine, Iowa, produced 1.5 billion mussel buttons alone.
One of the best places to understand the decline of freshwater mussels is in the rows of cases lined up in the museum’s Department of Invertebrate Zoology. Each case houses several drawers brimming with mussel shells that range in size from a pinky nail to a loaf of sourdough bread. Each shell is meticulously labeled, helping researchers like Pfeiffer pinpoint where and when it was collected.
“Without these specimens, we would not be able to know the historical distribution of species or the species richness of a particular river,” Pfeiffer said. “They allow us to verifiably measure how mussel biodiversity has changed across time and space.”
Understanding where mussels were living in the past is key to charting their recovery. Pfeiffer is now working with the Anacostia Watershed Society to raise awareness of local mussel diversity and restoration. Since 2019, the Anacostia Watershed Society has released more than 24,000 mussels into Washington’s Anacostia River, which will filter an equivalent of 132 Olympic-sized swimming pools each year.
According to Pfeiffer, this is great news for the river’s entire ecosystem. “Freshwater mussels are ecosystem engineers, they have an outsized and positive impact on the river’s ecology,” he said. By filtering excess nutrients and pollutants out of the water, improving conditions for many aquatic plants and animals. In turn, mussels are a key cog in the food web, providing nourishment for a range of local birds, fish and mammals. They also serve as vital indicators on water quality to the humans in the area that rely on the river. “Freshwater mussels are doing all those things and so much more,” Pfeiffer said.
When Was the World Our Oyster? We Asked the Anthropologist Investigating Sustainable Oyster Practices Through History
The Story Behind the Seashells By the Seashore
2022 in Review: The Year’s Top Discoveries by Museum Researchers
The True Story Behind How Pearls Are Made