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Utterbackia imbecillis

What do they look like?

The paper pondshell is up to 10 cm (4 inches) long , and is oblong and elongate in shape. The shell is usually fairly thin and compressed in younger individuals, inflated in older individuals. The anterior end is rounded, the posterior end somewhat pointed. Dorsally, posterior margin is straight. The ventral margin is straight to moderately rounded.

Umbos are flattened, are not above the hinge line, and are situated anteriorly. The beak sculpture is fine, with two or three double-looped ridges followed by fine concentric lines.

The periostracum (outer shell layer) is shiny and smooth except for growth lines. The shell is yellow in younger individuals. Older individuals are yellow on the umbo, the rest of the shell being greenish occasionally with fine green rays, and the posterior slope is black.

On the inner shell, both valves lack pseudocardinal and lateral teeth. The beak cavity is shallow to moderately deep. Although the nacre is white, silvery or bluish-white and is iridescent at the posterior end.

In Michigan, this species can be confused with the cylindrical papershell, giant floater and creeper. All these species have umbos slightly above the hinge line. The cylindrical papershell often has a slight pinch in the middle and is slightly more inflated. The giant floater is slightly more rounded. The creeper has a small pseudocardinal tooth or swelling. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    10 (high) cm
    3.94 (high) in

Where do they live?

The paper pondshell is found in the Mississippi River drainage, the middle St. Lawrence river drainage, the Rio Grande River system in Texas and the Ochlockonee River system in Florida. In its eastern range U. imbecillis is also found in the Altamaha River system in Georgia north to the Chowan River system, and the Gunpowder River system in Maryland.

In Michigan this species is found in Lake Erie and the Clinton, Huron, St. Joseph (Maumee River), the Grand and Kalamazoo Rivers, and the Saginaw River system, as well as lakes in these systems. (Burch, 1975)

What kind of habitat do they need?

On the Huron it was found mainly in the upper portion of the river. In general it is usually found in ponds, lakes, or mud-bottomed pools of creeks and rivers. (Cummings and Mayer, 1992; van der Schalie, 1938)

  • These animals are found in the following types of habitat
  • freshwater
  • Aquatic Biomes
  • rivers and streams

How do they grow?

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)

How do they reproduce?

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.

Utterbackia imbecillis is a long-term brooder. In the Huron, it was gravid from late July to early June, and the spawning season is probably June to July.

Individuals of this species have been found to be hermaphroditic. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

  • How often does reproduction occur?
    The paper pondshell breeds once in the warmer months of the year.
  • Breeding season
    In Michigan, the breeding season is probably June to 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

How long do they live?

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.

How do they behave?

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)

How do they communicate with each other?

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 paper pondshell attracts or if it recognizes its fish host is unknown.

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)

What do they eat?

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)

What eats them and how do they avoid being eaten?

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)

What roles do they have in the ecosystem?

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.

Natural infestations of Utterbackia imbecillis have been observed on bluegill, and paper pondshell glochidia metamorphosed on these fish in lab trials as well.

Other sunfish that paper pondshell glochidia metamorphose on include the green sunfish, pumpkinseed, warmouth, rock bass, dollar sunfish, longear sunfish, black crappie, and largemouth bass.

In other lab trials, glochidia metamorphosed on the spotfin shiner, greenthroat darter, yellow perch and the banded killifish. Utterbackia imbecillis has also metamorphosed on non-fish species (the tiger salamander and bullfrog) as well as non-native fish species (not listed here). (Hoeh and Trdan, 1985; Hove, et al., 1995; Howells, 1997; Parker, et al., 1980; Stern and Felder, 1978; Trdan and Hoeh, 1982; Watters and O'Dee, 1997)

Species (or larger taxonomic groups) used as hosts by this species

Do they cause problems?

There are no significant negative impacts of mussels on humans.

How do they interact with us?

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

Are they endangered?

Utterbackia imbecillis is considered Endangered in New Mexico, and Special Concern in South Carolina. (Hove, 2004)

Some more information...

This species was formerly known as Anodonta imbecillis.


Renee Sherman Mulcrone (author).


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.

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

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, E. Long, M. Peteler, E. Peterson. 1995. Anodontoides ferussacianus and Anodonta imbecillis host suitability tests. Triannual Unionid Report, 6: 1.

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.

Parker, R., M. Vidrine, C. Hackney. 1980. A new centrarchid host for the paper pond shell, Anodonta imbecillis Say (Bivalvia: Unionidae). ASB (Association of Southeast Biologists) Bulletin, 27: 54-55.

Stern, E., D. Felder. 1978. Identification of host fishes for four species of freshwater mussels (Bivalvia: Unionidae). American Midland Naturalist, 100: 233-236.

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

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

Watters, G., S. O'Dee. 1997. Surrogate hosts: transformation on exotic and non-piscine hosts. Triannual Unionid Report, 11: 35. Accessed October 04, 2005 at

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.

University of Michigan Museum of ZoologyNational Science Foundation

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Mulcrone, R. 2006. "Utterbackia imbecillis" (On-line), Animal Diversity Web. Accessed February 26, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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