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threeridge (Amblema plicata)

Features and Behaviors

The threeridge mussel has an elongate or rounded, thick shell. Three or more nearly parallel ridges or folds are present on the posterior half of the shell. Both ends of the shell are rounded. The inside of the shell is pearly white, sometimes with a purple tint on one end. The outside of the adult's shell is brown or black. This mollusk may grow to seven inches in length.

The threeridge mussel lives in rivers and impoundments with a mud, sand or gravel bottom. This mussel often lives for more than ten years with some individuals living for more than 100 years. The sexes are separate. The male releases sperm into the water. The female draws sperm into her shell, along with water and other particles, through her incurrent siphon. Eggs are fertilized internally. 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 larvae transform into the adult form, a process that may take from one to 25 weeks depending on the host, water temperature and place of attachment. The larva breaks free of the cyst and drops to the bottom where it begins its adult life, if conditions are favorable. There are 15 species of host fishes for the threeridge mussel: shortnose gar, northern pike, highfin carpsucker, channel catfish, flathead catfish, white bass, rock bass, green sunfish, pumpkinseed, warmouth, bluegill, largemouth bass, white crappie, black crappie and sauger. The threeridge mussel is a filter-feeder, bringing in water and the organic matter it contains through the incurrent siphon, filtering particles out, then sending the rest of the water away from the body through the excurrent siphon. The particles filtered include plankton and detritus.

Illinois Range


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

Illinois Status: common, native