Mollusks

Five species of freshwater mussels, including this winged mapleleaf mussel, are listed as federally endangered on the Riverway.

NPS/Rupp

The St. Croix National Scenic Riverway is home to more than 40 species of freshwater mussels--one of the greatest assemblages of these fascinating and sensitive aquatic organisms in the United States. A rare example of an intact river-dependent group of animals, all of the mussel species that existed in the corridor before European settlement are present here today.

The sheer abundance of mussels in the Riverway plays a critical role in maintaining the excellent water quality of the St. Croix-Namekagon river system by filtering water and cycling aquatic nutrients.

The exceptional diversity of mussels in the Riverway provides invaluable opportunities to learn more about these important and declining organisms.

Visit the NPSpecies database for a species list. Select "Other Non-vertebrates" in the 'Category' drop-down box and enter Search. Select "Full list with details" radio button for more information.

 

Duration:

6 minutes, 8 seconds

Join Ranger Liz on an exhibit tour of the Riverway's freshwater mussels.

 

Freshwater Mussels Are Protected

It is illegal to disturb or take any live mussel or empty mussel shell from the rivers.

 

What is a freshwater mussel?

Although looking at mussels might seem about as exciting as looking at rocks, they are actually amazing animals with many secrets yet to be learned!

Mussels are bivalve mollusks (invertebrate animals with soft bodies inside a hard two-part shell) and are sometimes commonly called clams. Like all invertebrates they lack a spine and they produce a hard external shell instead of an internal skeleton. Mussels are filter feeders. This means they take in many gallons of water each day and remove algae and bacteria for food by filtering the water through their gills. This helps clean the water and improves water quality for other animals in the river.

Mussel Life Cycle

Most species of freshwater mussel spend the first 1-3 months of their lives living on the gills or fins of a fish. The tiny juvenile mussels, called glochidia, use nutrients in the fish’s blood to develop their internal organs. When the juveniles fall off, the mussels will grow into adults if they land on a suitable area of the river bottom. Some species live for 80 years or more.

 

Freshwater mussels require a host fish to raise their young.

NPS/Van Tatenhove

 

Female mussels need to find a fish to carry their glochidia, (baby mussels) for a few weeks. Then the young can survive on their own and will drop to the river bottom. Different mussel species use different methods to attract the fish and some are very picky as to what kind of fish they want to attract. Watch some videos of different mussels trying to attract a fish.

 

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  Mussel Displaying

  Female black sand shell mussel tries to attract a fish to host her glochidia (baby mussels) until they can live on their own.

  Duration:

  14 seconds

 

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  Plain Pocketbook Displaying

  A plain pocketbook mussel tries to attract a host fish (likely a bass or perch) to carry the young larvae until they are old enough to drop off and live on their own. The plain pocketbook is common in the river.

  Duration:

  9 seconds

 

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  Pimpleback mussel

  The pimpleback mussel shown here is excreting her young, thinking that the finger is a host fish come to carry her young.

  Duration:

  33 seconds

 

Researchers tag mussels during surveys to study and maintain counts of endangered species like the winged mapleleaf mussel.

NPS/Rupp

Riverway Refuge

The St. Croix and Namekagon rivers are home to these freshwater organisms because of the variety of underwater habitats like boulder fields, gravel beds, sand bars, and mucky backwaters. The requirement of specific host fish, competition from exotic species and their need for free flowing, unpolluted rivers has resulted in the endangerment and extinction of many mussel species. Today, almost two-thirds of all mussel species in the United States are threatened, endangered, or already extinct.In order to protect the federally endangered species and the numerous state threatened species, it is illegal to take any live mussel or empty mussel shell from the St. Croix and Namekagon rivers. Even moving a mussel is prohibited because they can suffocate if you place them back into the sand upside down!

 

Additional Resources

America's Mussels: Silent Sentinels


Invasive Zebra Mussels