Epioblasma torulosa

Geographic Range

Historically, the northern riffleshell was found in the Ohio river drainage in Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and west Virginia, and into southeastern Michigan and southwestern Ontario. Today its range is restricted to southwestern Ontario, the Green River in Kentucky, Big Darby Creek in Ohio and French Creek, LeBoeuf Creek and Allegheny River in Pennsylvania.

In Michigan this species was historically found in Lake Erie drainages. Specimens were collected from the Clinton River and lower Huron River, however no individuals have been found since the 1930s. Recent populations were found in the Black River up to 1998, the lower Raisin River up to 1977, and the Detroit River in the 1990s. However, with the invasion of the zebra mussel into the Detroit River, the population is likely extirpated from Michigan. (Burch, 1975; Carman and Goforth, 2000; Hoeh and Trdan, 1985)


In Michigan, this species was historically found in the Black River and in old lake plain areas. In general the riffleshell is found in well-oxygenated waters in large streams and rivers mainly in riffles and runs with fine to coarse gravel. (Carman and Goforth, 2000; Watters, 1995)

  • Aquatic Biomes
  • rivers and streams

Physical Description

The northern riffleshell is up to 7.6 cm (3 inches) long . Males are oblong in shape with a broad sulcus along the posterior ridge. Females are oblong but have a broad, fragile, posterior end. The shell is thick anteriorly and compressed. The anterior end is rounded, the posterior end broadly rounded. Females are slightly truncated posteriorly and and males have an indent. The dorsal margin is curved and the ventral margin is gently curved in the males and may be slightly arched in the females .

Umbos are low and somewhat turned forward. The beak sculpture is fine, with double-looped ridges.

Males may have small tubercles from the beaks to the posterior-ventral margin on the lateral surface. A broad sulcus or depression runs on the posterior ridge of the males, but the sulcus is less prominent in females. The periostracum is greenish, yellowish or tan with numerous fine green rays.

On the inner shell, the left valve has two pseudocardinal teeth, which are triangular, divergent and rough. The two lateral teeth are short, straight and rough. The right valve has one triangular, divergent pseudocardinal tooth, with two smaller teeth on both sides. The one right lateral tooth has a much smaller tooth below.

The beak cavity is moderate to shallow. The nacre is white, and iridescent posteriorly.

In Michigan, the males of this species can be confused with the three-horned wartyback. However, the three-horned wartyback has distinct large knobs alternating from side to side. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range length
    7.6 (high) cm
    2.99 (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)


Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.

In general, gametogenesis in unionids is initiated by increasing water temperatures. The general life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.

Female northern riffleshells will display a spongy, pure white mantle lining presumably to attract its fish host. Other Epioblasma species have recently been observed trapping fish hosts within their valves, and E. torulosa may do the same. (Barnhart and Roston, 2005; Environment Canada, 2004a; Lefevre and Curtis, 1912; Watters, 1995)

  • Breeding interval
    The northern riffleshell breeds once in the warmer months of the year.
  • Average gestation period
    10 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.

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • provisioning
      • female


Growth rings of specimens suggest this species lives up to 15 years. (Carman and Goforth, 2000)


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)

Communication and Perception

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 spike attracts its fish host is unknown.

Glochidia respond to 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)

Food Habits

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)

Ecosystem Roles

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.

In lab trials, Epioblasma torulosa metamorphosed on the banded darter, the bluebreast darter, brown trout and the mottled sculpin. Other hosts are likely for Lake Erie populations. (Watters, 1996)

Species Used as Host

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.

Economic Importance for Humans: Negative

There are no significant negative impacts of mussels on humans.

Conservation Status

Epioblasma torulosa is a federally Endangered species in both the United States and Canada. It is Critically Endangered on the IUCN Red List.

Epioblasma torulosa is likely extirpated from Michigan due to dredging in the Black River and the introduction of the zebra mussel in the Detroit River. However, extant populations are found across Lake Erie in Ontario, Canada drainages. (Environment Canada, 2004b)

Other Comments

The genus Epioblasma is synonomous with Dysnomia and Plagiola.

The northern riffleshell is also considered as a subspecies, Epioblasma torulosa rangiana and is sometimes listed as Epioblasma rangiana. Another subspecies, the tubercled blossom, Epioblasma torulosa torulosa is not in Michigan, but is also considered federally endangered. (Cummings and Mayer, 1992; Watters, 1995)


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


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

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


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.


Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. J. Exp. Zool., 33: 463-499.

Barnhart, C., B. Roston. 2005. Host infection strategy of the snuffbox mussel, Epioblasma triquetra. Fourth biennial symposium, Freshwater Mollusk Conservation Society, May 15-18, 2005, St. Paul, Minnesota: 43. Accessed October 04, 2005 at http://ellipse.inhs.uiuc.edu/FMCS/Symposium/FMCS2005ProgramandAbstracts04-26-2005.pdf.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Burch, J. 1975. Freshwater unionacean clams (Mollusca: Pelecypoda) of North America. Hamburg, Michigan: Malacological Publications.

Carman, S., R. Goforth. 2000. Special animal abstract for Epioblasma torulosa rangiana (northern riffleshell mussel). Lansing, Michigan: Michigan Natural Features Inventory. Accessed October 06, 2005 at http://web4.msue.msu.edu/mnfi/abstracts/aquatics/Epioblasma_torulosa_rangiana.pdf.

Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Champaign, Illinois: Illinois Natural History Survey Manual 5. Accessed August 25, 2005 at http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html.

Environment Canada, 2004. "Northern Riffleshell" (On-line). Species at risk. Accessed October 06, 2005 at http://www.speciesatrisk.gc.ca/search/speciesDetails_e.cfm?SpeciesID=582.

Environment Canada, 2004. "Species At Risk" (On-line). Accessed September 21, 2005 at http://www.speciesatrisk.gc.ca/default_e.cfm.

Hoeh, W., R. Trdan. 1985. Freshwater mussels (Pelecypoda: Unionidae) of the major tributaries of the St. Clair River, Michigan. Malacological Review, 18: 115-116.

Lefevre, G., W. Curtis. 1912. Experiments in the artificial propagation of fresh-water mussels. Proc. Internat. Fishery Congress, Washington. Bull. Bur. Fisheries, 28: 617-626.

Lefevre, G., W. Curtis. 1910. Reproduction and parasitism in the Unionidae. J. Expt. Biol., 9: 79-115.

Meglitsch, P., F. Schram. 1991. Invertebrate Zoology, Third Edition. New York, NY: Oxford University Press, Inc.

Oesch, R. 1984. Missouri naiades, a guide to the mussels of Missouri. Jefferson City, Missouri: Missouri Department of Conservation.

Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus, Ohio: Ohio Department of Natural Resources.

Watters, G. 1996. Hosts for the Norther Riffle Shell (Epioblasma torulosa rangiana). Triannual Unionid Rep., 10: 14. Accessed October 04, 2005 at http://ellipse.inhs.uiuc.edu/FMCS/TUR/TUR10.html#p14a.

van der Schalie, H. 1938. The naiad fauna of the Huron River, in southeastern Michigan. Miscellaneous Publications of the Museum of Zoology, University of Michigan, 40: 1-83.