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washboard (Megalonaias nervosa)

Features and Behaviors

The washboard mussel has a large, dark, thick, heavy shell with "V"-shaped ridges in the front and large folds on the sides and back. The anterior end of the shell is rounded, and the posterior end of the shell is squared. The inside of the shell is white, sometimes with purple or rust-colored blotches. This mussel may grow to 11 inches in length.

The washboard mussel lives in large rivers that have a good current. Sometimes it is found in medium-sized streams that have a mud, sand or gravel bottom. The washboard mussel often lives for more than 10 years, and some individuals may live for more than 100 years. The sexes are separate. The male releases sperm into the water. The female draws sperm in through the incurrent siphon, along with water and other particles. Eggs are fertilized internally. The fertilized eggs develop into an intermediate larval stage called glochidia. Glochidia are stored in the female's gills, giving them a safe place to develop and providing plenty of oxygen. In the spring or summer, glochidia are released from the female into the water, where they begin their parasitic stage. Glochidia attach to a fish and form a cyst. Within the cyst, the larva transforms into the adult form, a process that may take from one to 25 weeks depending on the host, water temperature and place of attachment. It breaks free of the cyst and drops to the bottom where it begins its adult life, if conditions are favorable. Host fishes for this mussel include 16 species. The washboard mussel is a filter-feeder, bringing in water and the organic matter it contains through the incurrent siphon, filtering the particles out, then sending the rest of the water away from the body through the excurrent siphon. Particles filtered include plankton and detritus.

Illinois Range


​Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Unionoida
Family: Unionidae

Illinois Status: common, native