Education

North America's tremendous richness of pearly freshwater mussels is unique. Biologists have identified nearly 300 species of mussels living in our streams and lakes. Freshwater mussels are bivalve mollusks with two hard outer shells and soft tissues inside. Prehistoric American Indians ate the soft tissues and used the lustrous shells for making tools and ornaments. Historically, the shells have been prized as raw material for manufacturing buttons and are currently used in the production of cultured pearls.

Many mussels are sensitive to changes in their environments. Populations of mussels have declined alarmingly in recent decades because of siltation, pollution, and competition from exotic mollusks like the zebra mussel. Of the eighty mussel species native to Illinois, more than half are currently threatened, endangered, extirpated, or extinct. We can protect mussels and other aquatic wildlife by cleaning up our streams and lakes to create healthy habitats where these animals can thrive.

Gallery of Several Illinois Mussels Species

Anatomy

Freshwater mussels are members of the Phylum Mollusca, Class Bivalvia, and Order Unionoida. They have soft inner bodies and hard outer shells consisting of two valves, one on the left side and one on the right side. The shells are joined by an elastic ligament that stretches across a hinge at the top (dorsal margin). The shells of different species vary in size, shape, thickness, color, and in the presence or absence of sculpturing (ridges or bumps) on the outer surface. The structure of the hinge also differs. Some species have interlocking "teeth" (pseudocardinal and lateral teeth) that ensure proper alignment of the valves. In other species the hinge teeth are reduced or absent.

Mussels' soft tissues include an enveloping mantle that secretes the shell, a single large foot for moving short distances, and adductor muscles for keeping the valves tightly closed. Mussels also have two pairs of multipurpose gills. The gills are used for respiration, for moving microscopic food particles to the animal's mouth, and also for housing and nourishing mussel larvae (glochidia) in females.

Illustrations:

Above: Medial and lateral views of a mapleleaf mussel (Quadrula quadrula) showing the anatomical features of the shell (from Cummings and Mayer 1992: 7)

Right: Soft-tissue anatomy (from Burch 1973: 7).

Life History

Illustration: Generalized life cycle of freshwater mussels (from Cummings and Mayer 1992: 2.)

Freshwater mussels have an elaborate reproductive system. This system not only provides a way for mussels to generate offspring; it also helps each species increase its range. Mussels generally have separate sexes (male and female). However, some mussels are hermaphrodites, in which each individual has both male and female reproductive organs in its body. During spawning, males release sperm into the water. The sperm are drawn inside the female's shell, where they fertilize eggs in her body. The fertilized eggs develop into larvae (glochidia) and are stored for a time in the female's gills. When the glochidia mature, the female generally expels them into the water where they must attach themselves as parasites to the gills or fins of fish. In some species, part of the female's mantle resembles a swimming minnow that lures potential host fish and increases the chance that her larvae will attach to a suitable fish and survive. Larvae remain on the host fish for a period of weeks or months. During this time the larvae metamorphose and develop adult organs and structures. Young mussels then detach from their host and drop to the bottom of the body of water. Thanks to the swimming fish, they may now be far away from their parents.

Mature mussels spend most of their lives, which range from 10 to 100 years, partially or wholly buried in the bottoms of streams and lakes. Mussels often have specialized habitat preferences. Many species cannot live in muddy streams with excess amounts of silt.

Commercial Harvest

Freshwater mussels are commercially important. Beginning in 1891, mussels were harvested to manufacture shell buttons for clothing. Many towns along the Mississippi and Illinois rivers had button factories or buying stations where musselors sold their shells. The shell-button industry thrived until the 1940s, when plastic replaced shell as the preferred raw material for buttons. In the early days, no one attempted to manage or control the mussel harvest, and many of the mussel "beds" were severely depleted or destroyed.

Mussel harvesting resumed in the 1960s to provide the shell implant or "nucleus" for producing cultured pearls. Harvesting continues in the Mississippi River and some of its tributaries. Over seven-million pounds of mussels, valued at over six-million dollars, were harvested in 1990. No one knows if mussels can survive at current harvesting rates. Several states in the Midwest are developing new, uniform harvest regulations to better manage this important natural resource.

Images:
Above: "Mussel fishing on the Illinois River, Pearl, IL." (photo courtesy of Marshall County Historical Society).
Below Left: Drilled threeridge mussel (Amblema plicata) and button blanks (photo by R.E. Warren, Illinois State Museum).
Below Right: Cultured pearl necklace and earrings (photo by R.E. Warren, Illinois State Museum).

Conservation

Historically, Illinois had as many as eighty mussel species in its rich network of streams and lakes. However, recent stream surveys in Illinois and throughout the United States have documented drastic declines in mussel populations. Today mussels may be the most endangered group of animals in North America. Of the eighty species that once lived in Illinois waters, only fifty-nine have been found in the state since 1970. Eleven of the remaining fifty-nine species are now known from only a single river system or population, and they may disappear by the end of the decade.

Many factors are responsible for the decline of freshwater mussels, including siltation, pollution, loss of habitat, channelization of streams, and competition from exotic species like the zebra mussel.

Glossary

Bivalve - mollusk with a shell made of two hinged valves

Channelization - the straightening and/or deepening of a river channel

Endangered - a species faced with the danger of extinction

Extinct - a species that no longer exists

Extirpated - a species that has been eliminated from a particular area, but still exists somewhere else

Exotic - from another part of the world; foreign

Glochidium - the larva of a freshwater mussel (Superfamily Unionoidea) that generally lives as a temporary parasite on a host fish

Hermaphrodite - an animal or plant normally having both male and female reproductive organs

Introduced - brought in and established in a new place or surroundings

Larva - the newly hatched, immature form of an animal that undergoes metamorphosis, differing markedly in form or appearance from the adult

Metamorphosis - a marked change in the form or structure of an animal occurring after birth or hatching (vt. metamorphose)

Mollusk - soft-bodied animal with a muscular head and foot and a mantle, which usually secretes a protective shell

Parasite - an organism that grows, feeds, and is sheltered on or in a different organism (host) while contributing nothing to the survival of the host

Pseudocardinal Teeth - structures resembling teeth used in some species to hold the shell together

Siltation - deposition of fine mineral particles (silt) on the beds of streams or lakes

Spawn - to deposit sperm or eggs into the water

Threatened - a species likely to become endangered in the foreseeable future

Bibliography

Baker, F.C. 1898. The Mollusca of the Chicago Area, Part I: The Pelecypoda. Bulletin of the Chicago Academy of Science 3(1): 1-30 + 27 plates.

Baker, F.C. 1906, A catalogue of the Mollusca of Illinois. Bulletin of the Illinois State Laboratory of Natural History7(6): 53-136 + map.

Cummings, K.S. 1991. The aquatic Mollusca of Illinois. In our living heritage: the biological resources of Illinois, edited by L.M. Page and M.R. Jeffprds, pp. 429-439. Illinois Natural History Survey Bulletin 34(4): 357-477.

Cummings, K.S. 1995. Illinois' most endangered species. The Illinois Steward 4(1): 6-8.

Cummings, K.S., and C.A. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Manual 5. Illinois Natural History Survey, Champaign. 194 pp.

Herkert, J.R. (editor). 1992. Endangered and threatened species of Illinois: status and distribution, Volume 2 - Animals. Illinois Endangered Species Protection Board, Springfield, IL. 142 pp.

Parmalee, P.W. 1967. The fresh-water mussels of Illinois. Popular Science Series Vol. 8. Illinois State Museum, Springfield. 108 pp.

Starrett, W.C. 1971. A survey of the mussels (Unionacea) of the Illinois River: a polluted stream. Illinois Natural History Survey Bulletin 30(5): 267-403.

Thiel, P.A., and A.W. Fritz. 1993. Mussel harvest and regulations in the Upper Mississippi River system. In Conservation and management of freshwater mussels, Proceedings of a UMRCC Symposium, 12-14 October 1992, St. Louis, Missouri, edited by K.S. Cummings, A.C. Buchanan, and L.M. Kock, pp. 11-18. Upper Mississippi River Conservation Committee, Rock Island, Illinois. 189 pp.

Warren, R.E. 1995. Illinois mussels: the silent storytellers. The Living Museum 57(2): 19-22.

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