Villosa fabalis
rayed bean

The rayed-bean is found in sporadic in the Ohio River drainage north to Lake Erie drainages in Michigan and Ontario, and in the Duck and the upper Tennessee Rivers.

In Michigan V. fabalis is found in the Detroit River, Lake Erie and its tributaries. (Burch, 1975; Carman, 2001)

The rayed bean is found in lakes and small to large streams. Substrates in inhabits include mud, sand and gravel. This species may also be associated with water willow stands.

In the Clinton River headwaters the rayed bean was found buried in sand among aquatic vegetation roots in about four inches of flowing water. It seems more common in lake plain areas of the Lake Erie basin where substrates are finer. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

The rayed bean is up to 3.8 cm (1.5 inches) long , and is elongate in shape. The shell is usually fairly thick, solid and moderately inflated. The anterior end is rounded, the posterior end bluntly pointed. The dorsal margin is straight and the ventral margin is straight to slightly curved.

Umbos are low, being raised only slightly above the hinge line. The beak sculpture has two or three heavy ridges, knobbed posteriorly.

The periostracum (outer shell layer) is smooth except for growth lines. The shell is green, yellowish-green or brown with numerous dark green wavy rays. Older specimens tend to be more brown.

On the inner shell, the left valve has two pseudocardinal teeth, which are triangular, heavy, large and rough. The two lateral teeth are short, low and heavy. The right valve has one triangular, heavy, large pseudocardinal tooth. Sometimes small tubercular teeth are on either side of this psuedocardinal tooth. The one lateral tooth is also short, low, and heavy.

The beak cavity is shallow. The nacre is white and is iridescent at the posterior end.

In Michigan, this species can be confused with the lilliput and the rainbow. Both the lilliput and rainbow are thinner and have more delicate hinge and teeth. The lilliput is slightly more cylindrical. The rainbow is larger, has more yellowish coloring and more prominent green rays. (Cummings and Mayer, 1992; Parmalee, 1967; 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. Often they are found buried under the substrate. (Oesch, 1984)

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 rayed bean attracts or if it recognizes its fish host is unknown.

Glochidia respond to both 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 fish for V. fabalis has not been determined.

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.

Villosa fabalis is a federal candidate species being considered for listing in the United States. Currently it is listed as endangered in Michigan, Ohio, and New York. Indiana lists the rayed bean as Special Concern.

In Canada, V. fabalis is endangered under the Species At Risk Act. (Environment Canada, 2003; Hove, 2004)