Summer 2002

Freshwater Mussels

Friendly Filters

A group of volunteers kneel down, shoulder to shoulder, across Otter Creek in St. Charles, Illinois. They look as if they are panning for gold, but these diligent folks are searching for something even more valuable to local conservationists — live freshwater mussels.

	Volunteer Heidi Rush shuffles mussels in Otter Creek. Photo courtesy of St. Charles Park District.

Mussels may be found on every continent except Antarctica, but eastern North America has the most diverse population of freshwater mussels in the world — nearly 300 species. Illinois alone has approximately 105 species of freshwater mussels.

The common names given to mussels (or clams) clearly demonstrate their diversity — elktoe, slippershell, pocketbook, pink heelsplitter, elephant-ear, washboard, white catspaw, sheepnose and monkeyface. These elusive animals are most commonly found in rivers and streams, either partially or totally submerged beneath sand, mud or gravel beds. Some species may also live in ponds and lakes.

All freshwater mussels share a unique method of reproduction. Unlike some other species of mollusks, the sexes are separate. To begin the reproductive process, the males expel sperm into the water, which enter the females through the incurrent siphon, where internal fertilization of the eggs takes place.

The fertilized eggs then develop into larvae, called glochidia, and are stored in the female's gills. During the spring and summer, the female discharges her glochidia into the water where they float about until they find a host, usually a fish, to attach to and form cysts. The glochidia can attach themselves either inside the gills of a fish or to its fins. While some species of mussel are able to use any kind of fish as a host, others are species-specific. A few varieties are actually capable of attracting a potential host fish with their own natural "lure."

Attached to the host fish, the larval glochidia grow to resemble adult mussels. They may remain attached anywhere from one to twenty-five weeks, depending upon the host, location of attachment and water temperature. After this growth takes place, they break away from the cysts and drop to the stream, river, or lake bottom where they will complete their development into adults. Many species of freshwater mussels have long life spans, from ten to one hundred years. The number of growth ridges on a mussel's shell can often be used to determine its age.

Freshwater mussels are filter feeders that constantly siphon water through an intake valve, called the incurrent or branchial siphon, and pump it back out through the excurrent or anal siphon. They feed on detritus, which is organic matter on the bottom of rivers and ponds, and plankton, which consists of microscopic plants and animals suspended in the water. This feeding method helps to keep our rivers, streams, and lakes clean. Chicago-area biologists such as Roger Klocek, director of conservation at the John G. Shedd Aquarium in Chicago, recognize this vital role that freshwater mussels play in our environment. After several mussel surveys and years of research, Klocek has found that "Mussels are nature's primary water treatment plant for suspended substances in streams. When they were abundant, they could do a prodigious filtering job."

According to Klocek, however, "Mussels are disappearing from our streams, lakes, and rivers across the land," causing great concern among environmentalists. Predators such as raccoons, muskrats, and otters already keep mussel populations in check, leaving behind piles of empty shells, called middens, along riverbanks. Moreover, intensive harvesting for human uses, as well as pollution from insecticides, fertilizers, and other chemicals, has decimated mussel populations. Peoria, for instance, was part of a booming button industry early last century (mussels were used to make pearl buttons). In 1950's Japan, mussels were used to culture pearls. Additionally, recent research conducted by Klocek and others reveals that erosion from land development may also be a significant factor in mussel decline. In Illinois alone, there are twenty-seven species of mussels that are endangered or threatened.

But the prospects are not all bleak: local conservationists and volunteers are pitching in to make a difference for the mussels. Embankment restoration projects such as the one in St. Charles help eliminate stream bank erosion caused by construction. To give the mussels in these areas a fighting chance, scientists and volunteers collect them, gather data on them, and then physically carry them upstream where they are "re-planted" in the streambed, safely away from disturbance.

With the help of such people, freshwater mussels will not only survive, but will continue to flourish in our local waters, quietly helping to keep our environment clean and healthy for many generations to come.

— Barbara A. Schreiber