Next to arthropods (insects, spiders, crayfish, etc.) mollusks are the largest group of animals in the world.
Mollusks have soft bodies, which are usually enclosed in thin, hard shells made of calcium. (The word “mollusk” comes from the Latin word mollis, which means “soft.”) Familiar mollusks include mussels, clams, oysters, snails, slugs and squids.
Most live in saltwater, but many inhabit freshwater rivers, ponds and lakes, such as the mussels and snails that live in DuPage County forest preserves.
Along with clams and oysters, mussels are known as “bivalve mollusks.” “Valve” is another name for the hardened calcium carbonate-and-protein casing that protects the creatures’ soft innards. These particular mollusks are “bivalves” because they have two such shells, one on the top and one on the bottom.
Freshwater mussels are filter feeders. Water flows into an “incurrent siphon” inside a mussel’s partially opened shell. From that water, the mussel’s gills take in oxygen and filter nutrients, such as microscopic plants and animals. Water and waste are then discharged through an “excurrent siphon.” A single mussel that weighs a quarter of a pound can process up to a half a gallon of water in a day, which makes these creatures remarkable filtration systems.
Unfortunately, freshwater mussels are the most endangered groups of animals in North America. Of the 300 species known to occur in the United States, over 71 percent are endangered, threatened or of special concern. Historically, Illinois was home to 80 species. Today, six species are extinct, 11 have been extirpated from Illinois, and 23 are on federal or state lists of threatened or endangered species. The remaining species are considered stable for now, and 11 have been found in DuPage.
Several factors have contributed to the demise of freshwater mussels, including pollution; harvesting; the loss of certain “host” fish, which mussels need in order to reproduce; the destruction of habitat; the construction of dams; and the arrival of nonnative species, especially the zebra mussel. Forest Preserve District ecologists hope, though, that river-restoration projects, work to raise mussels at its Urban Stream Research Center, and monitoring will eventually benefit local populations.