Five new mussel species reveal how much biodiversity US rivers still conceal

Researchers have described five new-to-science freshwater mussel species from rivers in the Southeastern U.S.The mussels looked like known species, so the team used DNA to tell them apart.Freshwater mussels filter and clean river water, but are the most imperiled group of animals in North America.Several of the new species are rare, but only one has been proposed for federal protection. For 20 years, researchers snorkeled through rivers across the southern U.S., feeling along muddy bottoms for mussels. Many of the shells they pulled up looked like familiar species, but their DNA told a different story. Scientists from Shedd Aquarium, Arkansas State University, and the U.S. Geological Survey (USGS) have described five new species of freshwater mussels from rivers in the states of Missouri, Arkansas, Mississippi, Alabama, Louisiana, and Texas, in a study published in Zootaxa. Several are rare and confined to single river basins, the authors write. “Our discovery and descriptions of these new freshwater mussel species demonstrate how much we still have to learn about the biodiversity in our rivers,” Nathan Johnson, a study co-author and research biologist at the USGS Wetland and Aquatic Research Center, said in a press release. Mussels are hard to identify from their shells. For example, mussels in headwater streams tend to grow flatter shells, while the same species downstream grow rounder ones. Males and females differ as well. So the researchers turned to genetics. In many cases, they clipped a bit of tissue or took a swab, then returned the mussel alive to the river, according to the study. The team sequenced three genes and compared more than 440 shell outlines. Each major river basin turned out to hold its own genetic lineage. Researchers named the new species for where they live. The Mobile hickorynut (Obovaria sipseyensis) is named for Alabama’s Sipsey River, from a Choctaw word meaning “swamp.” It now survives only in upper tributaries of the Tombigbee River. The White River Hickorynut (O. neeskaensis) takes its name from Nee Ska, the Quapaw name for the White River. It comes in two looks: Rare individuals in the Ozark highlands grow slimmer, tan or greenish shells streaked with thin dark rays, while down on the Mississippi floodplain, the shells turn thick, plump, and nearly black. Louisiana and Texas have their own species. The Big Thicket hickorynut (O. pinetorum) clings to a few scattered spots in the Calcasieu, Sabine, and Neches rivers. Its scientific name means “belonging to the pine forests,” a nod to the Piney Woods region also known as the Big Thicket. The shell is dark brown to black, about the size of a golf ball, with fine rays that sometimes merge into broad bands. You’d have a hard time finding the Tallahatta hickorynut (O. tallahattae). It’s now known only from a roughly 16-kilometer (10-mile) stretch of Mississippi’s Big Black River. It’s named for the Tallahatta Formation, a band of pale rock that underlies its range. Its dark, often rayless shell is longer and more oval than that of its relatives. That shape made it the one new species the shell analysis could reliably pick out, correctly sorting 88% of specimens. Scientists from Shedd Aquarium, Arkansas State University, and the U.S. Geological Survey (USGS) have described five new species of freshwater mussels from rivers in the states of Missouri, Arkansas, Mississippi, Alabama, Louisiana, and Texas. Image courtesy of Shedd Aquarium/USGS The Acolapissa hickorynut (O. lumbo) is the biggest of the five new species, reaching about 6.4 centimeters (2.5 inches). Its species name comes from a Choctaw word for round, and the shell lives up to it. The outside is chestnut brown with a satiny, cloth-like sheen. The inner shell can be white, pale pink, or orange. It lives in the Pearl, Pascagoula, and Lake Pontchartrain drainages in Louisiana. Its common name honors the Acolapissa people, who lived along the Pearl River and Lake Pontchartrain. In Choctaw, Acolapissa means “those who look out for people.” The authors write that the name fits mussels, which serve as sentinels of water quality. Freshwater mussels spend most of their lives half-buried in river bottoms, pumping water through their bodies. As they feed, they strip out algae, sediment, nutrients, harmful bacteria, and metals from the water. Mussels grow slow and live a long life, often 100 years or more. At a conservative rate of 30 liters (8 gallons) per day over a 60-year lifespan, an individual mussel filters about 663,000 liters (175,500 gallons) during its lifetime, about one-fourth of an Olympic-sized swimming pool. But, according to the USGS, “freshwater mussels are the most imperiled group of animals in North America.” Today, 66% of North America’s freshwater mussel species are at risk. In the early 1900s, people harvested them in huge numbers to make pearly buttons from their shells. Today, pollution and new infrastructure continue to drive their populations down, and their numbers have never been lower. A freshwater mussel found in Louisiana (Olumbo sp.) As they feed, mussels filter out algae, sediment, nutrients, harmful bacteria, and metals from the water. Image courtesy of Matthew Duplessis/ Louisiana Department-of-Wildlife-and-Fisheries Another vulnerability is their unique life cycle, which requires the young to hitch a ride on fish. The tiny mussel larvae clamp onto a host fish’s gills or skin for weeks to months before dropping off as juveniles. Scientists don’t yet know which fish the new species depend on, but their relatives use small bottom-dwellers like darters and sculpins, the study notes. Dams block host fish and turn flowing rivers into still pools, while silt, pollution, and invasive zebra mussels add pressure, according to the Minnesota Department of Natural Resources. “This combination of characteristics means mussel populations often cannot recover from large death events,” Alan Lymbery, a professor at Murdoch University in Australia, who wasn’t involved in the study, wrote in The Conversation. “This can have devastating knock-on effects to the freshwater ecosystem, as mussels are considered ‘keystone’ species.” There are proven ways to bring mussels back. At the Alabama Aquatic Biodiversity Center (AABC), a state-run facility in the city of Marion, biologists bring rare mussels into the lab and work out which fish host their larvae. Then they raise the young until they’re big enough to be returned to the rivers. The center has worked with about 40 species, and five now have reproducing wild populations, according to Garden & Gun. Moving wild adults and releasing older lab-raised mussels worked best, according to a study of the effort. Raising mussels takes years, and a new population may need another decade or more to sustain itself. For the five new hickorynuts, that work can’t begin until scientists learn which fish their larvae need. Endangered dwarf wedgemussel (Alasmidonta heterodon) is found in the freshwater rivers of the Northeastern U.S. Photo courtesy of Susi von Oettingen/USFWS (CC By 2.0) Of the new species, only the Acolapissa hickorynut has been proposed for federal protection. In 2024, the U.S. Fish and Wildlife Service proposed listing it as threatened under an earlier label, according to the study. The other new species have no federal assessments. The Mobile hickorynut has declined alongside the Alabama hickorynut (O. unicolor), which was proposed as endangered, so it may warrant equal protection, the authors write. They call for state and federal reviews of all five new species. “Conservation efforts can only protect what we recognize,” said lead author Kentaro Inoue, a research biologist at Shedd Aquarium. “[A]nd without this work, we risk certain species going extinct before they’re even known to science.” Banner image of a researcher from the Shedd Aquarium surveys for mussles in a US river. Photo courtesy of Shedd Aquarium Liz Kimbroughis a staff writer for Mongabay and holds a Ph.D. in ecology and evolutionary biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here. Dams, drains and other artificial habitats could buy time for threatened mussels: Study Citations: Inoue, K., Harris, J. L., & JOHNSON, N. A. (2026). 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