Quadrula pustulosa
pimpleback

The pimpleback is found throughout the Mississippi drainage through mid-Michigan and into Lake Erie. Its range extends from central New York west to southeastern North Dakota, south to eastern Texas and east through northern Mississippi.

In Michigan this species is found in the Grand and Saginaw drainages and in rivers in southern Michigan's lower peninsula. It has also been recorded from Saginaw Bay. (Burch, 1975; Oesch, 1984)

Quadrula pustulosa is found in medium to large rivers and large lakes. Substrates where it is found range from mud, sand, and/or gravel. (Cummings and Mayer, 1992; Watters, 1995)

The pimpleback is up to 10.2 cm (4 inches) long , and rounded. The shell is usually fairly thick, heavy and compressed to moderately inflated. The anterior end is rounded, the posterior end straight and somewhat truncated. The dorsal margin is straight and short and the ventral margin is rounded.

Umbos are raised above the hinge line. The beak sculpture has three to four rounded ridges.

The anterior half of the periostracum (outer shell layer) is smooth and two-thirds of the posterior half are covered with pustules or bumps. The shell is yellow-brown to light brown. Younger specimens have a broken green ray present extending from the umbo to the ventral margin.

On the inner shell, the left valve has two pseudocardinal teeth, which are erect, large, and striated. The two lateral teeth are short to slightly curved and striated. The right valve has one large, grooved, erect triangular pseudocardinal tooth. The area between the pseudocardinal and lateral tooth, the interdentum, is broad.

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

In Michigan, this species can be confused with the purple wartyback and maple leaf. The maple leaf is more quadrate and lacks the green ray. The purple wartyback has a dorsal wing and purple nacre. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

The pimpleback's fertilized eggs are brooded in the marsupia (water tubes) up to 2 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 the 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. How the pimpleback attracts and/or recognizes its fish hosts 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 metamorphosis 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.

Glochidial metamorphosis and natural infestations of Quadrula pustulosa have been observed for the black bullhead, channel catfish, and flathead catfish.

Glochidial metamorphosis has also been observed with the brown bullhead. (Coker et al., 1921; Howard, 1914)

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, Quadrula pustulosa 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.

Quadrula pustulosa is listed as threatened in Virginia. (Hove, 2004)