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Mollusks

Next to arthropods (insects, arachnids, crustaceans, 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 freshwaters, such as rivers, ponds and lakes. These freshwater species include the mussels and snails that live in Winnebago County.

Mussels

Nineteen species of mussels are known to live in the rivers and ponds of Winnebago County: Creek Heel Splitter, Pimple Back, Pistol Grip, Three Ridge, Wabash Pig Toe, Elk Toe, Fluted Shell, Giant Floater, Paper Pondshell, White Heel Splitter, Black Sandshell, Deer Toe, Fat Mucket, Pink Heel Splitter, Plain Pocket Book, Creeper, Ellipse, Spike, and Fawns Foot. The Black Sandshell and Spike are threatened species. Mussels can be found in all four of Winnebago County’s rivers. The Kishwaukee River supports the highest concentration and diversity of mussels in our county.

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.

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.