The muscle of mussels: a powerful indicator species!
Marla Mertz, Marion County Conservation
It was quite a challenge to come up with a catchy title about such an unknown and secretive little creature that work tirelessly in our rivers and streams. These are creatures we call “mussels” and they bear names, such as the giant floater, mapleleaf, mucket, pocketbook, and heelsplitter.
After living and working around Lake Red Rock and the Des Moines River, I have become more aware of the river’s moods and sometimes, what seem to be her unfriendly qualities. But, how often do you get the opportunity to learn what’s under the water in our rivers and streams that make an impact on our lives? What? Mussels? Impact our lives?
These amazing little creatures are immensely important to stream ecology and biodiversity. Mussels live on every continent except Antarctica. According to the USGS (U.S. Geological Survey) North America is home to more than one-third of the world’s mussel species, but more than 70 percent of the continent’s 302 species are extinct or endangered. They quietly sit in the water and never utter a sound. They were an important food source for Native Americans, especially in the winter months, and still are for animals like fish, turtles, mink, otters, and raccoons.
Mussels filter algae and other microscopic organisms from the water providing much needed cleaning services. What they don’t digest is spit back out as mucous plugs – a tasty meal for nearby fish. They also make a positive impact on the ecosystem by filtering large volumes of water and reducing suspended particles and contaminate loads.
Freshwater mussels are important to river structure. Mussel beds provide a firm, natural structure where the river bottom would otherwise be a shifting mixture of sand, silt, and clay. With only one muscular foot, their mobility is somewhat confined. It is not easy for mussels to escape disturbances, like droughts, floods, dredging, or contaminants. They become a stable micro-habitat and home to many different species, all of which contribute to a river ecosystem. Algae that grow on mussels are food for small fish and invertebrates which are in turn eaten by larger fish. Crayfish often convert mussel shells into a suitable home and mussel beds provide spawning areas for many of our game fish.
Siltation, pollution, the damming of rivers and streams, over-harvest, and exotic species all have contributed to the decline of freshwater mussels. When river bottoms become clogged with silt mussels disappear. They are extremely susceptible to pollutants like herbicides, pesticides, heavy metals, and other toxic chemicals. All of these chemicals become concentrated in their foot and soft tissue. Mussels are considered an “indicator species” (a specie that defines a trait or environmental condition of its environment) and can help us to identify toxins that may affect the animals and plants in that river, stream, or section that are being exposed. Mussels can sometimes live longer than most humans and given their chances of encountering large disturbances are pretty high that is amazing!
Research on mussels can tell us a great deal about waterway management and disturbances. In September 2010, the Iowa DNR and U.S. Army Corps of Engineers at Lake Red Rock conducted research on mussel movement in relationship to a drawdown. The lake level was going to be dropped 10 feet from the normal conservation pool for maintenance and mussels were going to become exposed along the shorelines. Information and recommendations to assist native mussels of Iowa's rivers and streams gathered from this project included: shortening the drawdown period length, adjusting timing, and reducing the rate at which the water level is dropped. Mussels counted included mapleleaf, giant floater, and fragile papershell. These three are common and are known to exploit impoundment inlets.
Here are photos from mussel research done in Marion County (in Google Photos).
Prior to the 20th Century, mussel beds carpeted miles of river bottom, sometimes from bank to bank. Searching for pearls imbedded in mussels was a common hobby in the mid to late-1800s. By 1899, 41 factories in Iowa used freshwater mussel shells to make buttons. The introduction of plastic buttons brought the pearl button industry to a halt in the 1940s.
Mussels have a unique symbiotic and parasitic relationship with fish. They may have only one foot, but they are able to spread through river systems utilizing one or more fish species to help disperse tiny mussel larvae called glochidia. The spawning male mussels release sperm into the water where it is filtered by the female mussels. Within her gills are specialized brood chambers where the eggs become fertilized and are able to grow and develop. When the female releases her glochidia, they must attach to a suitable host fish for several weeks to complete metamorphosis. Only when completed metamorphosis occurs can the young mussels drop from the host fish.
How do the female mussels know when to release the glochidia onto a fish host? Hint: they can be creative! Some may release their glochidia to resemble macro-invertebrates (underwater insects). As the fish eats what it believes is a meal, it gets a mouthful of glochidia. Some mussels have elaborate lures that mimic aquatic animals, small fish, and crayfish. It’s fun to look into different kinds of mussels and learn their bizarre ways of unloading their glochidia to a fish host! Some of our mussels are pretty picky about what species of fish is the best host.
- Freshwater Mussel Conservation Society - Learn about mussel reproduction
Freshwater mussels are one of the most endangered groups of animals in North America.
That is a pretty strong statement! Think of all the different ecosystems and animals that you have heard about that are either endangered or threatened. Did they include a secretive organism under our rivers and streams? Water is a common thread of life, we need more muscle helping the mussel! Conservation measures to protect Higgins eye pearlymussel in Iowa include the protection and reintroduction in the Iowa River below Iowa City. Higgins eye pearlymussel is found only in the Upper Mississippi River and a few large tributaries from Minnesota and Wisconsin south to Iowa and Illinois. Historically, Higgins eye occurred in the Cedar, Wapsipinicon, and Iowa rivers in Iowa, but is no longer found in these rivers. Never a common species, the Higgins eye pearlymussel was heavily harvested around the turn of the century for use of its shell in mother-of-pearl buttons and pins. More recent threats, including pollution from agricultural and industrial runoff and sedimentation, prompted its listing as an endangered species in 1976 under the Endangered Species Act. The invasive zebra mussel is also bringing this specie to its demise.
How can you help mussels? Let’s see…
- Improve our water quality. Become involved with issues locally and globally. Remember the saying, “All rivers lead to one Great Ocean.” Reducing the sediment build-up on rivers and lake bottoms is a biggie. Buffer strips of grasses and/or trees between water and land help keep chemicals and other pollutants from reaching the water. Terraces and contours can greatly reduce the amount of sediment that runs off the land into our river and lake bottoms. Lawn chemicals don’t just stay on your lawn – they all end up in our rivers.
- Be aware of aquatic invasive species. Have you seen signs around Iowa’s recreational lakes and rivers about the spread of zebra mussels? Zebra mussels tolerate a wide range of conditions and can significantly alter the ecosystem of waterbodies where they become established. Areas with large densities can have up to 6,000 zebra mussels per square foot. Zebra mussels have severely reduced native mussel populations in some areas. Be sure to drain all the water from your boat, motor and trailer, and thoroughly wash after boating or fishing. Be sure to check your hunting decoys, as well. Please report any mussels that may have attached to the vessel.
Learn more about the invasive zebra mussel:
Awesome links to learn more about freshwater mollusks in the Midwest!
- Freshwater mussels and the renewal of the Dubuque Ice Harbor
- Iowa DNR Fishing Regulations (opens as PDF)
- Freshwater Mussels of Iowa
- Mussel Conservation - Click on your state for links to specific local conservation sites
- Freshwater Mollusk Conservation Society
- Missouri's Freshwater Mussels (Missouri Conservationist article August 1999)