Missouri Department of Conservation
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Threeridge

Amblema plicata
Family: 
Unionidae (freshwater mussels) in the phylum Mollusca
Description: 

Shell is thick, heavy, rounded to squared, occasionally inflated and covered with three or more posterior folds or ridges. Umbo raised above hinge line. Epidermis is yellowish-green to light brown in juveniles, becomes dark green, brown or black with age. Inside shell beak cavity medium to deep; pseudocardinal teeth large and thick; lateral teeth serrate, straight to slightly curved; nacre (lining) white and frequently stained.

Similar species: Washboard is larger with more complex folding and has bumps on the anterior third of shell.

Size: 
Adult length: 3-7 inches.
Habitat and conservation: 
Small streams to large rivers in mud, sand and gravel.
Foods: 
Algae and fine particles of decaying organic matter; extracts nutrients and oxygen from water drawn into the body cavity through a specialized gill called the incurrent siphon; sediment and undigested waste are expelled through the excurrent siphon.
Distribution in Missouri: 
One of our most widespread mussels; absent from north-central Missouri.
Status: 
Common. Like many mussel species that were formerly common in the northern half of the state, its range has decreased dramatically there, especially in the northwestern quarter. This mussel seems capable of surviving in polluted water that would eliminate many other native mussels, so in the future, this mussel may become the predominant species.
Life cycle: 
Males release sperm directly into water. Females downstream siphon sperm into the gill chamber, where eggs are fertilized. Eggs mature into larvae (called glochidia), which discharge into the water and attach to host fish—in this species, shortnose gar, crappies, yellow perch, rock bass, green sunfish, bluegill and many more. The tiny mussel eventually breaks away and floats to the bottom of the stream, and the cycle repeats.
Human connections: 
Mussels are excellent biological indicators of water quality because they accumulate contaminants in water that can be scientifically analyzed. This species is perhaps the most important shell for the multimillion-dollar cultured pearl industry and was important in the commercial button industry.
Ecosystem connections: 
Mussels act as nature's “vacuum cleaners,” filtering and cleansing polluted waters. They are also an important food source for other species in the aquatic environment.

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