Freshwater Mussels

Freshwater mussel underwater
Mussels use siphons to filter water through their body.

In a New Light/Jaden

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.
 
Illustration of mussel expelling glochidia with a catfish in the background. Close up image of glochidia attached to a fin.
Freshwater mussels require a host fish to raise their young.

NPS/Van Tatenhove

Mussels must attract fish close enough for the baby mussels to attach to them. Some mussels produce small packets filled with glochidia that look like tasty insects to fish. Others produce flaps that look like minnows (complete with eye spot, tail, and fins!).

Several species troll “lures” behind them that wave in the current like fish bait. Regardless of how they trick the fish, when the imitation bait is taken, the packets break releasing the glochidia. Some mussel species require specific host fish species for their glochidia.

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.
 
 
Tagged mussels sit on a table
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 about 40 species of freshwater mussels because of the variety of underwater habitats like boulder fields, gravel beds, sand bars, and mucky backwaters. Researchers believe that all the mussel species found in the rivers historically are all still here.

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!
 
US Fish and Wildlife article on the winged mapleleaf mussel’s return

Winged Mapleleaf Mussel’s Return

US Fish and Wildlife's efforts to bolster populations of endangered winged mapleleaf mussels.

10 things about zebra mussels

10 things about zebra mussels

10 things you should know about the invasive zebra mussel

Last updated: September 3, 2018

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