Giant Floater

Pygandon grandis


giant floater
Cliff White

Unionidae (freshwater mussels) in the phylum Mollusca


Outer shell is large, thick, and elongated to suboval; very inflated. Unbo is swollen and elevated above hinge line. Epidermis is shiny, light tan to light green or brown; becomes black to greenish black with age; young often with faint green rays. Broad beak cavity; teeth absent; nacre (lining) iridescent and variable, often silvery-white, tinged with pink or salmon.

Similar species: Flat floater is more circular and its umbo is almost flush to the hinge line. Paper pondshell has a thinner shell and a flatter umbo.


Adult length: 3-8 inches.

Habitat and conservation

Most common in sluggish sections of ponds, reservoirs, creeks and rivers in mud or silt.


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 Giant Floater Distribution Map
Distribution in Missouri

Found in almost all rivers south of the Missouri River except for a few in the south-central area along the Arkansas border. Also found in five rivers north of the Missouri River.


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--in this species, carp, yellow perch 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.

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.