A group of researchers has identified freshwater mussels in the upper reaches of the river in Wisconsin that are over 100 years old based on the timing of dam construction.
Biologists from the Wisconsin Department of Natural Resources (DNR), the University of Minnesota, and the National Park Service discovered a population of spectaclecase mussels (Cumberlandia monodonta) in the upper St. Croix River at the St. Croix Falls dam in August 2021. The spectaclecase mussel is a native freshwater species in the United States, listed as endangered by the U.S. Fish and Wildlife Service (FWS) and the International Union for Conservation of Nature.
The research team poses with the long-lived mussel population on the St. Croix River. (Photo: NPS)
According to FWS, large mussels can grow up to 23 cm and are distributed across at least 44 streams in the Mississippi, Ohio, and Missouri River basins in 14 states. However, spectaclecase mussels are gradually disappearing in these three states and now exist in only 20 streams. Today, spectaclecase mussels can still be found in Alabama, Arkansas, Illinois, Iowa, Kentucky, Minnesota, Missouri, Tennessee, Virginia, West Virginia, and Wisconsin.
When the biologists found the population of spectaclecase mussels in August on the St. Croix River, they could not determine their age on site by counting growth rings due to erosion on the shells. Instead, they estimated that the mussels are over 100 years old based on the dam’s construction date in 1907. “Native mussels can live for a very long time,” said Lisie Kitchel, a conservation biologist at DNR. “Finding a surviving population of mussels is an exciting discovery because the fish, which are the hosts that mussels need to reproduce, cannot swim upstream due to the St. Croix Falls dam built in 1907.”
As female spectaclecase mussels mature, they release larvae called “glochidia.” The larvae must attach to the gills or fins of fish to continue developing into juvenile mussels. After that, the juvenile mussels detach, and if they fall into a suitable area, they will grow into mature mussels. In this way, mussels use fish to move upstream and settle. The biologists returned the living spectaclecase mussels to their original habitat. The research team also collected the shells of dead mussels for laboratory analysis to accurately determine their age.
Spectaclecase mussels are declining due to various factors related to human activity, including dam construction, increased river sedimentation, pollution, dredging, and canal building. According to FWS, dam construction has contributed more to the decline and extinction of spectaclecase mussels than any other activity. This practice isolates populations upstream and downstream, blocking the migration routes of many fish species that mussels depend on. Dam construction creates small, unstable populations with lower chances of survival.
Wisconsin is home to 50 species of native mussels, 24 of which are endangered, threatened, or in need of conservation. Native mussels play a crucial role in river ecosystems, helping to clean water by filtering pesticides, mercury, and pollutants, while also providing food for various animal species.
