Strophitus undulatus

Geographic Range

Creepers (Strophitus undulates) are widespread throughout the eastern half of the United States through the southern edge of Canada, although they are absent from Florida, Georgia and South Carolina. They are found in drainages including 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, they are 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, they are widespread throughout river systems in both the upper and lower peninsulas. (Burch, 1975)


Creepers are found in a wide range of habitats, including headwaters, pools and large streams. They are probably scarcer in lower river reaches, and are generally absent from land-locked lakes. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

  • Aquatic Biomes
  • rivers and streams

Physical Description

Creepers are up to 10.2 cm long. Headwater specimens are generally smaller than those found in larger creeks. Their shell is elliptical or oval and may be thin or fairly thick in older individuals. Their anterior end is broadly rounded, their posterior end is bluntly pointed or truncated, their dorsal margin is rounded and their ventral margin is straight to slightly curved. Umbos are low and raised only slightly above the hinge line. Their beak sculpture has three to five v-shaped ridges. Their periostracum (outer shell layer) is green with rays, and brown to black in older individuals. On the inner shell, their left valve has a pseudo cardinal tooth, which is mainly a thickening of the hinge line. The tooth is located under the beak. Their right valve has a similar thickening of the hinge line, with the tooth anterior to the beak. Lateral teeth are absent. Their 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 cylindrical papershells or giant floaters. However, cylindrical papershells are more cylindrical, with a less coarse beak sculpture and giant floaters have a less truncated posterior end, thinner hinges and less concentric beak sculpture. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    10.2 (high) cm
    4.02 (high) in


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)


Increasing water temperatures stimulate male mussels to produce sperm and release it into the water, where it is taken up by nearby females. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

Age to sexual maturity is unknown for creepers. Unionids are gonochoristic (sexes are separate) and viviparous. Glochidia, their larval stage, are released live from the female after they are fully developed. Among unionids, gametogenesis is initiated by increasing water temperatures. The general life cycle of unionids includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. Their eggs are fertilized internally in their suprabranchial chambers and pass into water tubes of the gills, where they develop into glochidia. Creepers are long-term brooder. Gravid females have been observed in the Huron River from late July to the end of May. Glochidia are likely released at the end of May. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

  • Breeding interval
    Creepers breed once in the warmer months of the year.
  • Breeding season
    In Michigan, the breeding season is probably early June to late July.
  • Range gestation period
    10 (high) months

Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female


Their age can be determined by looking at the annual rings on their shell. However, no demographic data has been recorded on this species, although members of family Unionidae generally have about a 10 year lifespan or more. (Haag, 2012; Winhold, 2004)


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

Home Range

The home range size is not known for this species.

Communication and Perception

Most bivalve sensory organs are located in the middle lobe of the mantle edge. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot and help the mussel with georeception or orientation. Mussels are heterothermic, and are therefore sensitive and responsive to temperature. Unionids may have some form of chemical reception to recognize fish hosts, although their method of host recognition is not known. Glochidia respond to touch, light and some chemical cues. In general, when touched or introduced to a fluid, they respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

Food Habits

In general, unionids are filter feeders. 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 directed to the mouth. Mussels have been cultured on algae, but may also ingest bacteria, protozoa and other organic particles. The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed on the host’s tissue through phagocytocis. (Watters, 1995)


In general, unionids are preyed on by muskrats, raccoons, minks, North American river otters and some birds. Juveniles are probably also consumed by freshwater drums, sheepsheads, lake sturgeon, spotted suckers, redhorses and pumpkinseeds. (Cummings and Mayer, 1992; Watters, 1995)

Ecosystem Roles

Creeper glochidia are unique in that they may metamorphose without using a fish host. Early research suggested that creepers did not use fish hosts in the wild, however, more recent evidence has shown they use several fish hosts including channel catfishes, black bullheads, yellow bullheads, burbots, plains killifishes, bluegills, pumpkinseeds, black crappies, white crappies, green sunfishes, rock bass, smallmouth bass, largemouth bass, common shiners, common stonerollers, river chubs, creek chubs, blacknose daces, longnose daces, northern redbelly daces, bluntnose minnows, fathead minnows, spotfin shiners, sand shiners, spottail shiners, central mudminnows, banded darters, fantail darters, rainbow darters, Iowa darters, johnny darters, slenderhead darters, logperches, blackside darters, yellow perches, walleyes and brook sticklebacks. Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on their gill and mantle tissue, likewise parasitic chironomid larvae may destroy up to half of the mussels' gill. (Cummings and Mayer, 1992; Hillegass and Hove, 1997; Hove, et al., 1997; van Snik Gray, et al., 1999; Van Snik Gray, et al., 2002; Watters, 1995; Watters, et al., 1998)

Species Used as Host
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality. (Farris and Van Hassel, 2007)

Economic Importance for Humans: Negative

There are no significant negative impacts of mussels on humans.

Conservation Status

Creepers are listed as a threatened species in Iowa and a species of special concern in Massachusetts. They are also a species of concern in Rhode Island and South Carolina. (Hove, 2004)


Renee Sherman Mulcrone (author).



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


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