Pleurobema sintoxia

Geographic Range

The round pigtoe was historically found in the upper Mississippi drainages and the St. Lawrence drainage. Generally it is found from western New York and Ontario west through Wisconsin and Minnesota to the southeastern portion of South Dakota, south to Oklahoma, Arkansas, Mississippi and Alabama. Its eastern range includes western Virginia, West Virginia, and western Pennsylvania.

In Michigan P. sintoxia is found in mid-Michigan drainages, mainly from the Muskegon and Saginaw River systems on through the southern river systems. Specimens have also been collected in the Sturgeon River in the upper peninsula. (Burch, 1975; COSEWIC, 2004; Cummings and Mayer, 1992)


The round pigtoe is generally widespread from small to large rivers. Substrates it inhabits include mud, sand and gravel with moderate flows.

In Lake St. Clair it is found in nearshore areas less than 1 m with firm, sandy substrates. (COSEWIC, 2004; Watters, 1995; Zanatta, et al., 2002)

  • Aquatic Biomes
  • rivers and streams

Physical Description

The round pigtoe is up to 10.2 cm (4 inches) long , and is round and compressed in headwaters, triangular and inflated in rivers. The shell is usually fairly thick. The anterior end is broadly rounded, the posterior end rounded to bluntly pointed. The dorsal margin is straight to slightly curved and the ventral margin is gently curved.

Umbos are low, being raised only slightly above the hinge line, and can be centered on the shell or more anterior. The beak sculpture with two elevated ridges on the umbo or posterior ridge.

The periostracum (outer shell layer) is green-brown, red-brown or light brown with green rays. Older specimens tend to be more brown and lack rays.

On the inner shell, the left valve has two pseudocardinal teeth, which are rough, large, and grooved. The anterior tooth is about one third the size of the posterior tooth. The two lateral teeth are straight, short, and somewhat striated. The right valve has one large, erect, notched and grooved pseudocardinal tooth. The one lateral tooth has high, broad and striated.

The beak cavity is shallow to moderately deep. Although the nacre is white, occasionally it is has a pink or salmon tint and is iridescent at the posterior end.

In Michigan, this species is most difficult to distinguish from the headwater forms of the Wabash pigtoe, and is often only distinguished by looking at the teeth on the inner part of the valves of the shells. The Wabash pigtoe has a deeper beak cavity. When present in females, glochidial packets from the Wabash pigtoe also tend to be orangish while the round pigtoe has white glochidial packets. Other similar species include the hickorynut and the round hickorynut, which are generally more inflated. The hickorynut is more slightly more oval in shape. The round hickorynut is more rounded and generally lighter at the posterior end. (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 three 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.

Pleurobema sintoxia is a short-term brooder. In southeastern Michigan, it was gravid from mid May to late July, and probably spawns in early May. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

  • Breeding interval
    The round pigtoe breeds once in the warmer months of the year.
  • Breeding season
    In Michigan, the breeding season is probably early to mid-May.
  • Range gestation period
    2.5 (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.

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


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)

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. How the round pigote attracts its fish host is unknown. However, white glochidial packets that are released resemble invertebrates that fish would eat.

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)

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 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.

In lab trials, Pleurobema sintoxia glochidia metamorphosed on the bluntnose minnow, northern redbelly dace, southern redbelly dace and spotfin shiner. (Hove, et al., 1995; Hove, 1995)

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

The round pigtoe is listed as state endangered in Iowa, threatened in Minnesota, and special concern in Michigan. (Hove, 2004)

Other Comments

Pleurobema sintoxia is synonomous with Pleurobema coccineum.


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

external fertilization

fertilization takes place outside the female's body


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.


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.


COSEWIC. COSEWIC assessment and status report on the round pigtoe, Pleurobema sintoxia, in Canada. Ottawa: Committee on the status of endangered wildlife in Canada. 2004. Accessed October 10, 2005 at

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

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.

COSEWIC, 2004. COSEWIC assessment and status report on the round pigtoe, Pleurobema sintoxia, in Canada. Committee on the Status of Endangered Wildlife in Canada, 2004: 1-33. Accessed September 12, 2006 at

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

Hove, M. 1995. Host research on round pigtoe glochidia. Triannual Unionid Report, 8: 8. Accessed October 04, 2005 at

Hove, M. 2004. "Links to each state's listed freshwater mussels, invertebrates, or fauna" (On-line). Accessed September 21, 2005 at

Hove, M., R. Engelking, M. Peteler, E. Peterson, Kapuscinski, A.R., Sovell, L.A. and E.R. Evers. 1995. Suitable fish hosts for glochidia of four freshwater mussels. Conservation and Management of Freshwater Mussels II. Proceedings of a UMRCC Symposium, 16-18 October 1995, St. Louis, Missouri: 21-25.

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.

Zanatta, D., G. Mackie, J. Metcalfe-Smith, D. Woolnough. 2002. A refuge for native freshwater mussels (Bivalvia:Unionidae) from impacts of the exotic zebra mussel (Dreissena polymorpha) in Lake St. Clair. Journal of Great Lakes Research, 28: 479-489.

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.