Pimpleback

Quadrula pustulosa

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pimpleback
Jim Rathert
Family

Unionidae (freshwater mussels) in the phylum Mollusca

Description

Shell is thick, rounded to squared, moderately inflated and covered with few to many pustules (bumps) in no discernable pattern; smooth anterior third. Umbo elevated above hinge line and turned forward. Epidermis is yellowish-green to light brown in juveniles, becomes chestnut to brown with age; may have a broad green ray on the umbo. Inside shell beak cavity deep; pseudocardinal teeth well-developed and serrate; lateral teeth moderately short, heavy, serrate and straight to slightly curved; nacre (lining) white, iridescent posteriorly.

Similar species: Wartyback has knobs arranged in two weakly defined rows and the green umbo ray is not present. Purple wartyback is more compressed with a purple nacre.

Size

Adult length: 2-3 inches.

Habitat and conservation

Small streams to large rivers in coarse gravel and sand or mud. The only substrate it cannot tolerate is loose, shifting sand.

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.

image of Pimpleback Distribution Map
Distribution in Missouri

Widespread; statewide.

Status

Common, although degrading water quality and watershed destabilization interfere with the survival of this and all freshwater mussels.

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--this species uses shovelnose sturgeon, black and brown bullheads, channel catfish and others. 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 are long-lived and relatively immobile, accumulating contaminants in water that can be scientifically analyzed. This species is used in the cultured pearl industry. Historically important in the 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.