Pyganodon grandis

Geographic Range

The giant floater is a widespread species, found throughout te Mississippi and Missouri river drainages, the St. Lawrence drainage,, Gulf of Mexico through Louisiana and Texas. In Canada it is in the Interior basin from western Ontario to Alberta.

In Michigan, P. grandis is found throughout streams, lakes and rivers in the state. In general, it is more common in lakes, but is widespread throughout river systems. (Burch, 1975)


The giant floater is found in all habitats, but mainly in lakes or slower moving waters, including backwaters of streams, rivers, and impoundments. It can colonize newly impounded streams. Generally, it is found in substrates of mud or sand, but can also be found in gravel. (Cummings and Mayer, 1992; Oesch, 1984; van der Schalie, 1938; Watters, 1995)

  • Aquatic Biomes
  • rivers and streams

Physical Description

The giant floater is up to 25.4 cm (10 inches) long , and is elliptical or elongated in shape. The shape often varies. The shell is usually fairly thin and inflated. The anterior end is broadly rounded, the posterior end bluntly pointed. The dorsal hinge line is slightly curved and the ventral margin is straight or slightly curved.

Umbos are full, raised slightly above the hinge line and are situated slightly towards the anterior part of the shell. The beak sculpture has three to five double-looped ridges.

The periostracum (outer shell layer) is smooth, yellow to yellow-green with rays in younger individuals. Older specimens tend to be more brown.

On the inner shell, the left valve lacks pseudocardinal and lateral teeth. There is a slight thickening sometimes where the lateral tooth would be.

The beak cavity is broad and shallow. The nacre varies from silvery white, yellow, pink or copper.

In Michigan, this species can be confused with Pyganodon lacustris, paper pondshell, creeper, and cylindrical papershell. Pyganodon lacustris is generally more elongate. The paper pondshell has flattened umbos. The creeper may have a slightly waved and thickened hinge. The cyclindrical papershell generally has a beak sculpture of concentric, single loops. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    25.4 (high) mm
    1.00 (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.

In the Huron River in Michigan, Pyganodon grandis was gravid from early August to the following mid-April. It probably spawns from May through July in Michigan. (Lefevre and Curtis, 1912; Watters, 1995)

  • Breeding interval
    The giant floater breeds once in the warmer months of the year.
  • Breeding season
    In Michigan, the breeding season is probably May through 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.

  • 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)

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 giant floater attracts its fish hosts 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. (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.

Glochidial metamorphosis and natural infestations have been observed for bluegill, green sunfish, black crappie, rock bass, largemouth bass, bluntnose minnow, central stoneroller, common shiner, striped shiner, blackchin shiner, blacknose shiner, brook silverside, Iowa darter, Johnny darter, rainbow darter, and yellow perch.

In lab trials, glochidial metamorphosis was observed on banded killifish, golden topminnow, longnose gar, longear sunfish, pumpkinseed, creek chub, golden shiner, redfin shiner, blacknose dace, brook stickleback (Arey, 1932; Howells, 1997; Lefevre and Curtis, 1910; Penn, 1939; Trdan and Hoeh, 1982; Tucker, 1928)

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

Pyganodon grandis is fairly common throughout its range. However, it is considered Threatened in Vermont. (Hove, 2004)

Other Comments

The genus Anodonta is synonomous with Pyganodon. Because of the variation in shell morphology, Pyganodon grandis was once considered to be several species. (Hoeh, 1990)


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.


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

Arey, L. 1932. The formation and structure of the glochidial cyst. Biological Bulletin (Woods Hole), 62: 212-221.

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.

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

Hoeh, W. 1990. Phylogenetic relationships among Eastern North American Anodonta (Bivalvia: Unionidae). Malacological Review, 23(1-2): 63-82.

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

Howells, R. 1997. New fish hosts for nine freshwater mussels (Bivalvia: Unionidae) in Texas. Texas Journal of Science, 49: 255-258.

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.

Penn, G. 1939. A study of the life cycle of the freshwater mussel, Anodonta grandis, in New Orleans. Nautilus, 52: 99-101.

Trdan, R., W. Hoeh. 1982. Eurytopic host use by two congeneric species of freshwater mussel (Pelecypoda: Unionidae: Anodonta). American Midland Naturalist, 108: 381-388.

Tucker, M. 1928. Studies on the life cycles of two species of fresh-water mussels belonging to the genus Anodonta. Biological Bulletin (Woods Hole), 54: 117-127.

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

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.