The spring rains have come, and the streams are running. Gliding over the somewhat cloudy waters of a creek in west-central Illinois, I took my kayak out to explore the changing landscape.  A few years ago, a friend of mine introduced me to the wild world of freshwater mussels.  So, naturally, while on my kayak I kept an eye out for mussel trails on the streambed. If you have never seen a mussel trail within the gravel or sand, I now challenge you to find one.

Mussel misconceptions

Plenty of mussel misconceptions exist – if freshwater mussels could talk, they would tell you that their marine relatives have more differences than similarities. Yes, both freshwater and marine (saltwater) mussels derive from the same invertebrate phylum, Mollusca, and class, Bivalvia, which means two shells connected by a hinge. They are also both filter feeders. However, they differ in many ways. The most obvious difference is that they live in different habitats — freshwater mussels are all about the “lake (stream) life” while marine mussels have those “salt life” stickers on their make-believe vehicles. The reproductive mechanism for the two groups of mussels also differ — freshwater mussel eggs are fertilized inside the female while marine mussel eggs are fertilized in the water. Freshwater mussels are also on the move, albeit slowly, using a foot they can extend and retract. Marine mussels, conversely, stick to one place, attaching themselves to rocks and other ocean structures using self-created threads called byssus.

Additionally, one may be tempted to use the word “clam” when discussing freshwater mussels. Clams, technically, are classified in a different subclass than marine and freshwater mussels. The zebra mussel, contrary to the use of “mussel” in its common name, is classified within the same subclass as clams.

The specific freshwater mussels living beneath my kayak are part of the Unionidae family of mussels, commonly called river mussels. North America has the highest species diversity of freshwater mussels in the world. 

Mussel life in Illinois streams

If mussels could talk, they would let us know that the water quality of our streams is affecting their survival. Illinois was once home to 80 species of freshwater mussels, however, since 1970 only 59 species have been observed. Interestingly, Alabama has the highest native species diversity of mussels at 181, while states west of the Rocky Mountains may only have 1-3 species. Mussels require a stable substrate that consists of a firm mixture of gravel, sand, and silt.  

University of Illinois researchers have been studying the microhabitats that mussels inhabit within wadable streams in Illinois and how human disturbances may affect populations. They want to understand habitat requirements so managers can develop effective population restoration programs.  Researchers found that mussel species abundance increases when the stream size is larger, when the slope or gradient of the stream is near flat, and when the distance away from a dam increases.

Mussels are workhorses in our streams

Mussels provide a better life for all aquatic organisms. They are filter feeders. Mussels have siphon tubes, both incurrent and excurrent tubes, that bring water into their bodies. They extract nutrients and microscopic life for food and oxygen to breathe — so any pollutants or sediment in streams are being taken in internally by the mussel.  Since the water expelled from the siphon tube is cleaner, a secondary ecosystem phenomenon happens — mussels provide a filtration system for our streams. For a 30-second look at the work that mussels do in cleaning water, check out this video from Minnesota DNR.

A life cycle like no other

If mussels could talk, they’d need a full semester course to explain their life cycle.  Mussels can live a very long time (20-50+ years), however, to make it to adult stage is a miracle. Mussel reproduction occurs anonymously as the male mussel releases sperm into the water. A nearby female uptakes the sperm within her siphon tube where she broods her eggs within pouches on her gills. A female can produce several thousand to 3 million eggs. 

Female mussels are reliant on fish for their glochidia (larval mussels) to continue to develop. And not just any fish — certain species of mussels have specific species of host fish. For example, the Threeridge mussel (Amblema plicata), one of the most prevalent species in Illinois, has been found to utilize largemouth bass, channel catfish, and bluegill. While the Threehorn Wartyback (Obliquaria reflexa) has been found to only use one host fish species, the Striped Shiner. The glochidia have a parasitic relationship with the fish host species, using their developing shell to clamp onto the fish’s gills to uptake nutrients from the fish’s blood. Female mussels are tricky —  they lure fish in close by exposing part of their body tissue (mantle) that looks like a minnow, worm, fly, or other fish food.  Some mussel species release the glochidia in clouds that look like fish food. The glochidia live within the fish’s gills for a few days to weeks, fall off, and settle on the bottom as microscopic juveniles. It takes six years, on average, for a mussel to reach sexual maturity. 

What’s in a name?

Freshwater mussel species have awesome, entertaining common names — you may find a Monkeyface, Spectaclecase, Pistolgrip, or Heelsplitter to name a few.  There are also Butterflies, Maple Leaves, Pocketbooks, and Wartybacks!  The authors of the publication Missouri’s Freshwater Mussels state that most mussel names originated from fishermen of the early 1900s who harvested shells for buttons.

Can I keep mussels if I find them?

Freshwater mussels are the most imperiled aquatic group of organisms in the United States. There are 24 species of mussels on the Illinois Endangered and Threatened Species list.  

In Illinois, mussels are protected under the Illinois Fish and Aquatic Life Code. If you have an Illinois sportfishing license, individuals can possess up to 50 relic, or dead, shells. However, only certain species can be possessed. So, the best way to keepsake your mussel memory is to take a photo and not disturb the mussel in its environment.

What can I do to help freshwater mussels?

Keeping streams clean and healthy is vital to mussels.  You can help!

• Pick up trash. Litter on land typically finds its way to streams.
• Use rain-friendly landscapes that help to keep rainfall as close to where it lands as possible within your yard — and prevents heavy stormwater from gushing to streams picking up sediment and pollution along the way.
• Limit use of chemicals on your lawn and pick up pet waste.
• Implement soil health and conservation practices on your farm to avoid sedimentation and nutrient pollution in our streams.
• Clean off your boat. At the boat ramp when getting off the water, remove any aquatic vegetation and let your boat drain at the site.  This prevents the spread of invasive species like zebra mussels.
• Participate in mussel monitoring through Illinois Riverwatch.

Fun Facts

• Female mussel lures range from glow in the dark to appearing to look like a crawdad.
• Mussels develop their shell in rings like tree growth.

Resources and more information

• Illinois Natural History Survey Mollusk Collection
• Illinois Natural History Survey Mussel Community Data
• Freshwater Mussels of the Upper Mississippi River (field guide)
• Illinois Extension Rainfall Management 
• Illinois Extension Nutrient Loss Reduction Agricultural Practices
• Illinois RiverWatch “Mussel Grubbing: A Citizen Science Treasure Hunt” short documentary
• Cao, Y., Stodola, A., Douglass, S., Shasteen, D., Cummings, K. and Holtrop, A. (2015), Modelling and mapping the distribution, diversity and abundance of freshwater mussels (Family Unionidae) in wadeable streams of Illinois, U.S.A.. Freshw Biol, 60: 1379-1397. https://doi.org/10.1111/fwb.12575