Alasmidonta marginata


Jim Rathert
Species of Conservation Concern

Unionidae (freshwater mussels) in the phylum Mollusca


Shell is elongate, triangular, inflated, and relatively thin. Anterior end rounded, posterior end sharply angled, ending in a blunt, squared point. Posterior ridge sharply angled and prominent, posterior slope broad, flat, and covered with fine ridges. Umbo is large, located near the center of the shell, and elevated above the hinge line. Beak sculpture of three or four heavy, double-looped ridges. Shell smooth and dull, yellowish green or bright green with numerous rays and dark green spots present. Posterior slope often lighter than rest of shell.  Sharply angled posterior ridge, poorly developed teeth, and heavy beak sculpture. Pseudocardinal teeth thin and elongate; one in right, occasionally two in the left. Lateral teeth reduced to a thickened swelling along the hinge line. Inner shell beak cavity is moderately deep. Nacre bluish white, occasionally with salmon near the beaks.


Adult length: up to 4 inches.

Habitat and conservation

Medium-sized streams in gravel or mixed sand and gravel.


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 Elktoe Distribution Map
Distribution in Missouri

Sporadic in distribution. Generally in the southern half, in rivers draining the Springfield and Salem plateaus. Also has been found in the Salt River and Cuivre River.


Uncommon; a Species of Conservation Concern. The elktoe is one of many Missouri mussels with a declining population. It is usually not abundant where found. It had been been quite numerous in the Big River before two major intrusions of mud and finely ground rock changed the stream. These incidents were caused by improperly handled wastes from nearby mining activity.

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 the white sucker, northern hog sucker, shorthead redhorse 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.