Alasmidonta marginata
elktoe

The elktoe is found in the upper Mississippi drainage in the Ohio, Cumberland and Tennessee River systems as well as the Susquehanna River. In the Great Lakes region it is in the St. Lawrence drainage from Lake Huron to the Ottawa River. In general its range spans from western New York and Pennsylvania, west to Wisconsin, and south to northern Alabama.

In Michigan this species is found in rivers in both the upper and lower peninsulas, but may be absent from the coldwater rivers and streams in the northwest part of the state. (Burch, 1975)

The elktoe is usually found in larger creeks the upper reaches of rivers. Substrates where it was found are described as mixed sand and gravel. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

The elktoe is up to 10 cm (4 inches) long , and is elongate and quadrate. The shell is usually fairly thin and inflated. The anterior end is rounded, the posterior end is bluntly ended in a squarish point. The dorsal margin is slightly rounded and the ventral margin is nearly straight.

Umbos are broad and elevated above the hinge line. The beak sculpture is three to five heavy, raised, double-looped ridges.

The periostracum (outer shell layer) is smooth, yellow-brown with broad green rays and dark green dots. The posterior end of the shell is usually lighter.

On the inner shell, both left and right valves have a thin, elongated, pseudocardinal tooth. Lateral teeth are absent but there is a swelling on the hinge line.

The beak cavity is moderately deep. Although the nacre is white, occasionally it is has a salmon tint near the beak.

In Michigan, this species can be confused with the snuffbox, slippershell, and deertoe. The snuffbox is generally smaller, more square, and females have teeth ridges at the posterior margin. Slippershells are smaller, therefore growth lines are closer together. The deertoe has a more rounded ventral margin. The elktoe is spotted with dark green dots, which are absent in the other species. The foot of a live elktoe also tends to have an orangish color. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

Fertilized eggs are brooded in the marsupia (water tubes) up to 11 months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults. (Arey, 1921; Lefevre and Curtis, 1910)

The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.

Mussels in general are rather sedentary, although they may move in response to changing water levels and conditions. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn. (Oesch, 1984; Watters, 1995)

The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.

Mussels are heterothermic, and therefore are sensitive and responsive to temperature.

Unionids in general may have some form of chemical reception to recognize fish hosts. Mantle flaps in the lampsilines are modified to attract potential fish hosts. How the elktoe attracts and if it recognizes its fish host is unknown.

Glochidia respond to touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

In general, unionids are filter feeders. The mussels use cilia to pump water into the incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.

The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (Arey, 1921; Meglitsch and Schram, 1991; Watters, 1995)

Unionids in general are preyed upon by muskrats, raccoons, minks, otters, and some birds. Juveniles are probably also fed upon by freshwater drum, sheepshead, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds.

Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue. Parasitic chironomid larvae may destroy up to half the mussel gill. (Cummings and Mayer, 1992; Watters, 1995)

While freshwater mussels require a host fish for metamorphosis, the host for the elktoe is unknown.

There are no significant negative impacts of mussels on humans.

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.

The IUCN has not rated the conservation status of Alasmidonta marginata because not enough is known about its abundance. The elktoe is listed as endangered in Kansas, threatened in Minnesota, and a species of special concern in Virginia and in Michigan. (Hove, 2004)