Strophitus undulatus
creeper
(Also: squawfoot)

The creeper is widespread throughout most of the eastern half of the United States through the southern edge of Canada, but absent in Florida, Georgia, and South Carolina. Drainages where it is found include the Interior basin from central Texas to Lake Winnipeg, Canada, and the Canadian Interior basin in the Nelson River from western Ontario to Saskatchewan. In the upper Atlantic drainage it is found in the upper Savannah river system of South Carolina. This species is also found in the St. Lawrence river system and Nova Scotia.

In Michigan this species is widespread throughout river systems in both the upper and lower peninsulas. (Burch, 1975)

Strophitus undulatus is found in a wide range of habitats, from headwaters to pools to larger streams. It is probably more scarce in lower reaches of rivers, and is generally absent from land-locked lakes. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

The creeper is up to 10.2 cm (4 inches) long. Headwater specimens are generally smaller than those found in larger creeks. The shell is elliptical or oval and ranges from being thin to fairly thick in older individuals. The anterior end is broadly rounded and the posterior end is bluntly pointed or truncated. The dorsal margin is rounded and the ventral margin is straight to slightly curved.

Umbos are low and raised only slightly above the hinge line. The beak sculpture has three to five v-shaped ridges.

The periostracum (outer shell layer) is green with rays, and brown to black in older individuals.

On the inner shell, the left valve has a pseudocardinal tooth which is mainly a thickening of the hinge line. The tooth is located under the beak. The right valve has a similar thickening of the hinge line, with the tooth anterior to the beak. Lateral teeth are absent.

The beak cavity is shallow. The nacre is cream colored or salmon in the center and bluish-white on the outer margin.

In Michigan, this species can be confused with the cylindrical papershell or giant floater. The cylindrical papershell in general is more cylindrical. The creeper has a beak sculpture that is more coarse. In general, the creeper generally has a more truncated posterior end than the giant floater. The hinge on the creeper is also generally thicker and has a more concentric beak sculpture. (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; Oesch, 1984)

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)

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 creeper recognizes its fish host is not known.

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. (Watters, 1995)

Unionids in general are preyed on 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.

Strophitus undulatus glochidia are unique in that they may metamorphose without the use of a fish host. However, the glochidia have also metamorphosed on several species of fish in lab trials. No natural infestations on fish have been recorded. Species of fish that the S. undulatus glochidia have metamorphosed on include: channel catfish, black bullhead, yellow bullhead, burbot, plains killifish, bluegill, pumpkinseed, black crappie, white crappie, green sunfish, rock bass, smallmouth bass, largemouth bass, common shiner, common stoneroller, river chub, creek chub, blacknose dace, longnose dace, northern redbelly dace, bluntnose minnow, fathead minnow, spotfin shiner, sand shiner, spottail shiner, central mudminnow, banded darter, fantail darter, rainbow darter, Iowa darter, johnny darter, slenderhead darter, logperch, blackside darter, yellow perch, walleye and brook stickleback. (Hillegass and Hove, 1997; Hove et al., 1997; van Snik Gray et al., 1999; Watters, O'Dee, and Chordas, 1998)

Creeper glochidia have been found on: channel catfish, black bullhead, yellow bullhead, burbot, plains killifish, bluegill, pumpkinseed, black crappie, white crappie, green sunfish, rock bass, smallmouth bass, largemouth bass, common shiner, common stoneroller, river chub, creek chub, blacknose dace, longnose dace, northern redbelly dace, bluntnose minnow, fathead minnow, spotfin shiner, sand shiner, spottail shiner, central mudminnow, banded darter, fantail darter, rainbow darter, Iowa darter, johnny darter, slenderhead darter, logperch, blackside darter, yellow perch, walleye and brook stickleback.

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.

Strophitus undulatus is listed as threatened in Iowa and Special Concern in Massachusetts. It is also a Species of Concern in Rhode Island and South Carolina. (Hove, 2004)

Strophitus undulatus is synonomous (same species, but with a former different name) with S. rugosus.