Amblema plicata
threeridge

The three-ridge mussel occurs in the Mississippi, Gulf of Mexico and St. Lawrence drainages. In the Mississippi it is found from western New York to Minnesota and eastern Kansas. In the Gulf rivers it occurs from central Texas to the Yellow River of Florida. In the St. Lawrence it is absent from Lake Superior and its drainages. Its northern range includes the Red River of the North, Winnipeg River and Nelson River.

In Michigan this species is found mainly in rivers in the lower peninsula from the Saginaw and Grand drainages south. However, there are also records from the Sturgeon River in the upper peninsula. (Burch, 1975)

Amblema plicata is found in small to large rivers and impoundments. It occurs in a variety of substrates, including mud, sand and gravel. (Cummings and Mayer, 1992; Watters, 1995)

The three-ridge is a thick shelled species up to 17.8 cm (7 inches) long, and rounded or quadrate. The anterior end is uniformly rounded, the posterior end truncated, rounded, truncated or bluntly pointed. The dorsal margin is long and straight and the ventral margin is slightly curved.

Umbos are low and slightly raised above the hinge line. Younger specimens will have a beak sculpture of three or four concentric double looped ridges.

The periostracum (outer shell layer) is brown and brownish-black in older individuals. The shell sculpture is distinct, with four to six prominent ridges parallel to the posterior ridge.

On the inner shell, the left valve has two pseudocardinal teeth, which are grooved, large, and triangular. The two lateral teeth are long, slightly curved, and serrated. The right valve has one stout, elevated, triangular and serrated. Anterior to this tooth is a smaller (lamellar) tooth. The one lateral tooth is high, slightly striated and curved.

The beak cavity is moderately deep. The nacre is white, and iridescent at the posterior end.

In Michigan, the three-ridge is fairly distinct and not easily confused with any other species. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

Fertilized eggs are brooded in the marsupia (water tubes) up to two 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. The ridges on the shell of Amblema plicata help it to stay anchored in the substrate. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn. (Oesch, 1984; Watters, 1994; 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. How the three-ridge attracts 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)

Fish hosts are determined by looking at both lab transformations and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.

Lab transformations and natural infections have been recorded for the black crappie, white crappie, largemouth bass, pumpkinseed and yellow perch.

Lab transformations have been oberved for bluegill, green sunfish, rock bass and shortnose gar. (Cummings and Watters, 2004)

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.

Because of its thick and sturdy shell, Amblema plicata is harvested for use by the pearl industry. The shell is sliced and then ground into beads (called "slugs") which are placed in pearl producing oysters. The oysters secrete a nacre over the slugs. The finished pearl has a nucleus of freshwater mussel shell and an outer layer from the saltwater oyster. (Oesch, 1984; Watters, 1995)

Amblema plicata is not on any state or federal lists.

Amblema plicata is synonymous with Amblema costata.