by Dan Kelner
Have you ever heard of a purple wartyback, pink heelsplitter, black sandshell, or lilliput? All are Minnesota mussels.
Imagine these mussels: hickory nut, snuffbox, and spectaclecase. Or these: washboard, pistolgrip, and pimpleback. Butterfly, elephant-ear, monkeyface, elktoe, deertoe, and fawnsfoot live here too. As their names suggest, mussels can be eye-catching, yet they remain unfamiliar to most people because they spend their lives partially buried in river and lake bottoms, where they can be mistaken for rocks.
Freshwater mussels live on every continent except Antarctica. North America has the most--nearly 300 species, including Minnesota's 48 (two of which have disappeared from the state). These animals provide food for fish and wildlife, they clean water by filtering out particles to eat, and they show a river's health by reacting to pollution and other changes in the water.
Freshwater mussels belong to the world's second largest group of animals, called mollusks. Minnesota mollusks also include snails, slugs, and clams.
A mussel is a bivalve mollusk: A hinge holds together its two hard shells, or valves. Inside the valves, a sheet of tissue called the mantle surrounds the body. A mussel has a single large foot and no head or eyes.
Minnesotans often mistakenly use the term clam when referring to a freshwater mussel.
Freshwater mussels and clams are both bivalve mollusks, but they differ in important ways. Mussels make pearls, but clams do not . Freshwater mussels have parasitic young, but clams do not. Minnesota has only a few species of tiny clams--peaclams and fingernail clams.
The mussel opens its shells, sticks out its foot, and slowly pulls itself along, leaving a snakelike track.
In defense the mussel "clams up" or pulls in its foot and siphons and locks shut, using two strong adductor muscles.
Mussels prefer streams, rivers, and lakes with wind-driven currents because the flowing waters help bring in food particles and oxygen and carry away wastes. A mussel can bury itself in the river or lake bottom with only its siphons poking up into the water. Its valves open to extend its incurrent siphon to take in water, oxygen, and microscopic food particles--plankton and detritus. The food sticks to the lining of its gills, and hairlike cilia slowly sweep the tidbits into its mouth. Its excurrent siphon expels filtered water and waste back into the river.
Muskrats, raccoons, and other wildlife eat mussels. Native American Indians ate lots of freshwater mussels, but the tastier mussels and clams people eat today come from the ocean.
Some species can grow older than 100 years. Some only live a few years. Most live 10 to 40 years. Much like counting the rings on a tree, you can estimate a mussel's age by counting the rings, or annuli, that form on its shell each winter when growth slows or stops.
Mussels don't pair up and mate the way many animals do. Much is left to chance. During a breeding season, a female may produce tens of thousands of baby mussels. Yet she'll be lucky if one settles down in the river bottom and matures to adulthood.
Not only did Native American Indians eat mussels, but they also used them to make jewelry, tools, and utensils. They even used mussel shells as currency--items for trade.
Up until the 1960s, almost everybody in this country used a bit of mussel every day: They wore clothes with buttons made of mussel shells. Beginning in the late 1800s, mussels were commercially harvested to make mother-of-pearl buttons. It was a multimillion-dollar industry. Thousands of people collected or sold mussels or worked in button factories. Lake Pepin on the Mississippi River and the Snake River near Pine City were important button producers. Overharvest and pollution reduced the mussel population. Plastic eventually replaced shells for buttons.
Minnesota's mussels sometimes make pearls. When an irritant, such as a grain of sand or a tiny invertebrate, gets inside the mussel's shell, the mantle secretes a substance called nacre, or mother-of-pearl. The nacre coats the sand over and over, thus creating a pearl. The pearl has a beautiful luster and color--white, silvery, pink, salmon, red, copper, brown, lavender, purple, green, blue, cream, or yellow--and comes in all kinds of shapes. The lining of a mussel's shell has the same luster and color because it is also coated with layers of mother-of-pearl.
In the late 1800s and early 1900s, Minnesota pearl hunters popped open hundreds of mussels to find a single pearl. They were most likely to find pearls in deformed shells that had an opening to let in sand to make a pearl. No pearl hunting is allowed in Minnesota today.
Until the mid-1990s, shell companies harvested mussels in Minnesota to make, or culture, pearls in oysters from the ocean. Some shell companies in other parts of the country have stockpiled shells and are still selling them to places in the Far East. Pieces of freshwater mussel shell are ground into small balls, then inserted into live oysters. The oyster coats the ball with mother-of-pearl, forming a perfectly round pearl. A necklace of cultured pearls can cost thousands of dollars.
If you see lots of mussels in a stream, chances are the stream has clean water, good fishing, and plentiful wildlife such as muskrats, otters, herons, and eagles. The mussel at the bottom of the food chain helps keep the stream healthy and livable.
On the other hand, if you find empty shells but no living mussels, you could have cause for alarm.
What caused the mussels to die? Perhaps someone is dumping raw sewage or pesticides upstream. Maybe a new dam built downstream has backed up the water and silt has smothered the mussels. Or maybe a dam has stopped fish from moving upstream, and freshwater mussels cannot reproduce without fish for their hitchhiking glochidia.
Exotic zebra mussels have invaded some Minnesota waters and attached to freshwater mussels. They compete for food and smother freshwater mussels. Commercial harvest of freshwater mussels has put pressure on some mussel species.
Because of such troubles, mussel populations have dropped. More than 70 percent of North American species are extinct, endangered, or declining. In Minnesota more than half of native species are listed as endangered, threatened, or of special concern. At least two species have disappeared.
To better protect mussels and their habitat, we first need to find where they live now and where they lived in the past. In 1999 biologists with the Minnesota Department of Natural Resources began the first statewide survey of lakes and rivers to find freshwater mussels. Their survey will take six years to complete.
No live mussels may be collected in Minnesota. No live or dead mussels may be collected in National Park Service units, including the St. Croix River. If you pick up a live mussel, return it to the water immediately, being careful to place its foot end in the stream bottom so about two-thirds of the shell is buried.
With a fishing license, a person may possess up to 24 whole or 48 half shells of dead mussels. However, it is illegal to collect state listed species. Since many common species look like listed species, and even experts can confuse them, it is best to leave mussels where you find them.
Dan Kelner, former malacologist with DNR Ecological Resources, coordinated the statewide mussel survey, funded by the Legislative Commission on Minnesota Resources.
A complete copy of the article can be found in the July-August 2000 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.