Scientists find ‘stranded’ 100-year-old mussels surviving in upper St. Croix

Specimens are believed to have persisted above St. Croix Falls dam, cut off from host fish, for more than a century.




4 minute read

DNR, University of Minnesota, and National Park Service biologists display 100+ year old spectacle case mussels found in the St. Croix River. Photo credit: Marian Shaffer, National Park Service

The Wisconsin Department of Natural Resources (DNR) today announced that federal and state endangered native mussels first discovered above the St. Croix Falls dam in the St. Croix River in 1987 are still alive and estimated to be more than 100 years old.

In August 2021, biologists from the DNR, University of Minnesota and the National Park Service searched a stretch of river upstream of the St. Croix Falls Dam where spectaclecase mussels (Cumberlandia monodonta) had been previously discovered 34 years ago, and found a cluster of the native mussels.

Spectaclecase mussels (Cumberlandia monodonta} can grow up to 9 inches long and are named for the shape of their shells, which are elongated and sometimes curved. Spectaclecase are on both the federal and state endangered species lists. (Photo by Marian Shaffer/NPS)

Due to mussel shell erosion, biologists were not able to determine the mussels’ age on-site by counting their growth rings. Instead, biologists estimated the specimens at more than a century old based on when the river was dammed. 

The live spectaclecase mussels were safely returned to their habitat. Dead spectaclecase shells were saved and will be cross-sectioned this winter to verify their age. 

“Native mussels can live a long time, but these mussels were pushing the limits,” said Lisie Kitchel, DNR Conservation Biologist. “Finding some alive was amazing since the host fish species needed for their reproduction have been prevented from getting upstream as a result of the St. Croix Falls dam built in 1907.”

After female spectaclecase mature, they expel their larvae, known as “glochidia,” which must attach to the gills or fins of a specific fish to continue developing into a juvenile mussel before dropping off and growing into an adult mussel. Using fish as a host allows spectaclecase to move upstream and populate habitats it could not otherwise reach.

Recent research has identified mooneye and goldeye fish as host species for spectaclecase. These species are not present above the dam but are found downstream. That means reproduction can occur downstream, and surveys have found younger spectaclecase downstream.

Because of the mussels’ advanced age, biologists are taking action to preserve their genetic stock while they are still alive.

“Now we can implement strategies to propagate and augment the spectaclecase population there or reintroduce mooneye or goldeye above the dam to allow the spectaclecase to reproduce,” said Jesse Weinzinger, DNR Conservation Biologist.

Collaborative partnership seeks to restore spectaclecase mussels

The St. Croix River surveys are part of a collaborative multi-state, multi-agency effort to help recover spectaclecase. The DNR, Minnesota Department of Natural Resources, University of Minnesota, U.S. Geological Survey, U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service are working to provide necessary data for spectaclecase recovery and implementing conservation, restoration and propagation priorities.

Spectaclecase mussels can grow up to nine inches long and are named for the shape of their shells, which are elongated and sometimes curved. Historically, the spectaclecase was found in at least 44 streams of the Mississippi, Ohio and Missouri River basins in 14 states. It has vanished completely in three states and today is rarely found in only 20 streams, according to the U.S. Fish & Wildlife Service.

Their current distribution and status in the Mississippi River and St. Croix River were poorly known until the recent surveys by the DNR and other partners in 2019, 2020 and 2021.

Wisconsin has 50 native mussel species, with 24 endangered, threatened or in need of conservation. Native mussels are important to keeping Wisconsin rivers clean. They filter pesticides, mercury and other pollution from the water and keep it out of fish. They provide food for otters, raccoons, muskrats, ducks, fish and other wading birds. They also help indicate problems that may affect whole lake or river ecosystems.  

Students and volunteers help place hatchery-raised mussels in rivers

The DNR teamed up with partners across Wisconsin to deliver and place more than 1,200 hatchery-raised mussels in several Wisconsin rivers where past water quality problems had taken a toll on mussel populations.

“Propagation and reintroduction are a useful conservation strategy to increase the abundance and distribution of native mussel populations,” said Weinzinger. “We’re thankful for the partnerships that made it possible to augment mussel populations in these streams and provide research going forward to help us shape recovery efforts.”  

Weinzinger said that Genoa National Fish Hatchery raised thousands of fatmuckets (Lampsilis siliquoidea) as part of an experiment to research optimum water quality parameters and feeding rates for best survival.

University of Wisconsin-Stevens Point students worked with the DNR conservation biologists to place over 700 mussels at two streams in Portage County to monitor the growth and survivorship of the animals. Results from this student-led project will help determine the feasibility for future reintroduction efforts.

In southern Wisconsin, DNR partnered with the Upper Sugar River Watershed Association to place an additional 500 fatmuckets into the Sugar River to augment existing populations.

To learn more about how the DNR and our partners work to conserve our native mussels, visit the DNR’s Wisconsin Mussel Monitoring Program webpage. Read the Clam Chronicle newsletter to see how the DNR works with volunteers to build projects, programs, and tools for citizen-based monitoring efforts. 


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17 responses to “Scientists find ‘stranded’ 100-year-old mussels surviving in upper St. Croix”

  1. Brian Mielke Avatar
    Brian Mielke

    They could have been placed there after the dam was built?

  2. David Kesler Avatar
    David Kesler

    Shells need to be sectioned and viewed with microscope. It is impossible to count external rings accurately, especially on old shells. If there are any “young” shells clearly they either are recruiting or were brought in by anglers.

    1. Mark Hove Avatar
      Mark Hove

      We looked long and hard for younger animals, live or dead, and didn’t find any evidence of them at that site.

  3. Eddie Rivard Avatar
    Eddie Rivard

    Restocking Mooneye and Goldeye should be a no-brianer and so should the removal of that dam.

    1. Barry Avatar

      Exactly; foundational to any action plan for their recovery.

    2. Mark Hove Avatar
      Mark Hove

      I’m finding the host fish issue is more complicated than I originally thought. I’ve learned that members of the Mooneye family (Hiodonitidae) are highly migratory (see review in Warren and Burr, 2014). If stocked would they stay in the upper St. Croix? I understand hiodontids are no longer found in the upper river. Did they swim downstream long ago and couldn’t return due to the dam? Did they die from some other cause(s)? Additionally, where would you get a meaningful number of Mooneye for stocking? I’ve used the UMN electrofishing boat to collect Mooneye downstream of the St. Croix Falls dam and only caught a few individuals a day. I’ve found Mooneye die pretty quickly if stressed. Mooneye may be reared in a hatchery but I’ve heard that technology is in its infancy. If you stocked young Mooneye in the upper river would they swim downstream over the dam each year? I think stocking a hiodontid is an interesting idea that would likely require some research and thinking to evaluate its effectiveness as a helpful means of recovery.

      Warren, M. L. and B. M. Burr (eds.). 2014. Freshwater fishes of North America: Petromyzontidae to Catostomidae. Johns Hopkins Press, Baltimore, MD. 644 pp.

      1. Gordon Dietzman Avatar
        Gordon Dietzman


        We occasionally caught mooneyes on the Mississippi River at Hidden Falls during our fishing programs. But it was a fairly rare occurrence, maybe one or two fish per 5-7 programs. I concur with your assessment that they seem easily stressed. They were by far the most fragile fish species we caught and I think they would be very difficult to transport to a new location.

  4. Richard Pollema Avatar
    Richard Pollema

    What a great article and thanks for all the knowledge

  5. Mark Johnson Avatar
    Mark Johnson

    Yes. Get rid of the dam dam.

    1. ROBERT Avatar

      The Dam can control zebra muscles.

      I have seen the devastation of the zebra mussels on the lower st Croix.
      They will cover the native spices and kill it

  6. SigmaH Avatar

    No other energy is in such a way filthy and disgusting as hidropower
    Only fraudsters can call it “clean”

  7. gftiefenbruck Avatar

    I think that there is considerable merit in removing the dam at St. Croix Falls, especially since the St. Croix River has been declared a Wild and Scenic River. However, we really need any source of energy that does not involve fossil fuels to remedy the climate crisis. Perhaps something like a “fish elevator” would be appropriate to restore the continuity of fish populations on the river.

  8. Russ Hanson Avatar
    Russ Hanson

    During the 1930s folks living on the Sterling Barrens (up river from Wolf Creek), harvested clams for food, canning them as did my grandparents. They found a few pearls, but no real size nor quality. Grandpa cut out small necklace crosses from the shells for gifts and to trade at the store. When I was growing up, in the 1950s, we collected a few for food at the Sunrise Ferry Site, where they were easy to find and something new to try for food. No idea of the type we found then, but they were maybe 8 inches long. We were satisfied to eat one meal of them and then leave them alone in the future! Grandma and her family. the Carnes, ran the Sunrise ferry from about 1909 and owned it until it closed in the early 1940s and said that there were many clammers who came to the river, especially in the 1930s with the Great Depression making any free food sought after.

    1. Mark Hove Avatar
      Mark Hove

      That’s fascinating, thanks Russ

  9. Sue Ames-Lillie Avatar
    Sue Ames-Lillie

    In the late 1980’s, I volunteered to work with a group doing clam studies on the St Croix near Interstate Park. I was amazed at the things I learned about the muscles in the St Croix! I remember first hearing about the Winged Mapleleaf, Spectectaclecase, Deer toe, and others and learning about how they are so connected with the rivers and other wildlife. I’m so glad to hear that the research continues, and education is being shared. I hope more of the public gets involved and wants to learn more.

  10. Andrew Glazier Avatar
    Andrew Glazier

    Bivalve molluscs (clams, oysters, and mussels) are newly discovered models of natural aging (1), and the taxonomic class includes species with the longest metazoan life span— exceeding 400 years (2). The class Bivalvia also holds the record within the animal kingdom for the greatest number of species, which attain ages in excess of 150 years (1). For example, the Geoduck clam (Panopea abrupta), the freshwater pearl mussel (Margaritifera margaritifera), and the ocean quahog (Arctica islandica) are all exceptionally long lived attaining maximum ages of 163, 190, and 405 years, respectively (2–4).

  11. Glen Avatar

    Are there plans to reintroduce the fish required for reproduction above the dam?