Quadrula nodulata

Geographic Range

Quadrula nodulata, also known as the wartyback, is a species of freshwater mussel that is endemic to the United States of America. It is distributed throughout the entire Ohio, Cumberland, and Tennessee systems, and the Mississippi River drainage from Wisconsin and Minnesota, south through Kansas and Oklahoma to Texas, and east to Louisiana and Mississippi through Ohio. (NatureServe, 2013; Williams, 2008)


The wartyback mussel occurs in medium to large rivers and reservoirs with a mud, sand, or gravel bottom. (Cummings and Mayer, 1992)

  • Aquatic Biomes
  • benthic
  • rivers and streams
  • Range depth
    5.5 (high) m
    18.04 (high) ft

Physical Description

The shell of the wartyback is thick. It is rounded at the anterior end, while its posterior end is squared or truncated. The shell's dorsal margin is straight, and the ventral margin is curved in the anterior half, and may be either straight or arched posteriorly. The wartyback's umbos are small, and are only slightly elevated above the hinge line. The beak sculpture of the two rows of raised bumps, or nodules that continue downward on the surface of the wartyback's shell, are separated by a furrow or sulcus. Rows of ridges or pustules occur on either side of the sulcus and on the posterior slope, while the rest of the surface is usually smooth. The wartyback's periostracum is variable, and can be a yellowish green to a light brown with faint rays in small shells, becoming greenish brown, chestnut, or dark brown in larger individuals. Mean shell length in adult wartyback specimens is usually from 3 to 4 inches (7.6 to 10.2 cm).

The wartyback is similar to the purple wartyback (Cyclonaias tuberculata), the winged mapleleaf (Quadrula fragosa), the pimpleback (Q. pustulosa), and the mapleleaf (Q. quadrula), but a lack of green rays, its 2 rows of large pustules, and its straw color form a distinction from these other species. (Cummings and Mayer, 1992; Illinois Natural History Survey, 2013; Minnesota Department of Natural Resources, 2013)

The shell of the wartyback is thick. It is rounded at the front end, while its back end is squared. The color of the shell varies from yellowish green to light brown in small mussels, to greenish brown or dark brown in larger mussels. The average wartyback mussel is usually 3 to 4 inches (7.6 to 10.2 cm) in length. (Cummings and Mayer, 1992; Illinois Natural History Survey, 2013; Minnesota Department of Natural Resources, 2013)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    7.6 to 10.2 cm
    2.99 to 4.02 in


Male mussels release sperm into the water, which are then drawn in by female mussels by an incurrent siphon, and subsequently fertilize the female's eggs. Fertilized eggs are brooded in the gills of the female, until they develop into tiny larvae called glochidia. The wartyback is tachytictic, and will brood their young for a short time before they are released as glochidia. The glochidia live as parasites on the host fish until they develop into juvenile mussels. Fish hosts for the glochidia of the wartyback include black crappie (Pomoxis nigromaculatus), white crappie (P. annularis), bluegill (Lepomis macrochirus), channel catfish (Ictalurus punctatus), and largemouth bass (Micropterus salmoides). Upon their development into juveniles (usually over the course of a couple of weeks),they proceed to detach from the fish and fall to the streambed as free-living mussels. (Parmalee and Bogan, 1998; Watters, 1994)


Reproduction in Quadrula nodulata is limited to males released their sperm into the water. The water current carries the sperm to females downstream, where it is taken up by the incurrent siphon. Spawning in freshwater mussels is typically initiated by changes in water temperature. (Parmalee and Bogan, 1998; Watters, 1994)

After fertilization, females retain the eggs and later the hatched glochidia in their gills. The wartyback is tachytictic, with the offspring remaining with her for only a short period of time. Female wartyback mussels usually brood their eggs between June and July. When the glochidia are ejected from the female's gills, they attach on to fish gills or fins by clasping onto them with their valves. Quadrula nodulata reproduces once a year. (Parmalee and Bogan, 1998; Watters, 1994)

  • Breeding interval
    Wartyback mussels breed once yearly.
  • Breeding season
    Reproduction takes place during June and July.
  • Average number of offspring
    several thousand

Females brood the eggs and glochidia for about a month before releasing the glochidia. Once the glochidia are released, they are independent and do not receive any further parental care. (Parmalee and Bogan, 1998; Watters, 1994)

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


The specific lifespan for Quadrula nodulata is unknown. Freshwater mussels are long-lived animals, and many species may live for several decades, with some individuals living up to a century or more. (Sietman, 2003)


The wartyback mussel spends most of its lifetime buried in the bottom sediments of permanent water bodies, and will often live in multi-species communities called mussel beds. Mussels are primarily sedentary, but are capable of moving around with the use of their foot. The "foot" is a hatchet shaped muscle that can be extended out between the valves (shells). A mussel will burrow its foot into the sediment, and then contract it to pull itself slowly along the bottom of its aquatic habitat. (Sietman, 2003)

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. However, exactly how the wartyback attracts and/or recognizes its fish hosts is unknown. Glochidia respond to touch, light, and some chemical cues. In general, when touched, or when a fluid is introduced, the glochidia will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

Food Habits

Mussel species eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They are able to draw water into their body through their incurrent siphon, remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hairlike cilia that are located on the gills. After digestion, the mussel's waste matter is expelled through the excurrent siphon.

When mussels are in their parasitic glochidial stage, they absorb blood and nutrients from their hosts. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (Arey, 1921; Meglitsch and Schram, 1991; Sietman, 2003; Watters, 1995)


Unionids in general are preyed upon by muskrats, raccoons, American minks, North American river otters, and some birds. Juveniles are probably also fed upon by a variety of freshwater fish, such as freshwater drum, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds. The wartyback's shell can protect it from some predators, and burrowing in the sediment also allows it to hide from predators. (Cummings and Mayer, 1992; Watters, 1995)

Ecosystem Roles

Mussels act as nature's “vacuum cleaners,” filtering and cleansing polluted waters. They are also an important food source for other species in the aquatic environment. Unionid mortality and reproduction is affected by unionicolid mites, and monogenic trematodes feeding on gill and mantle tissue. In addition, parasitic chironomid larvae may destroy up to half the mussel gill. Glochidia are parasitic and attach to a large variety of freshwater fish to complete development. (Cummings and Mayer, 1992; Missouri Department of Conservation, 2013; Watters, 1995)

Species Used as Host
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Mussels are excellent biological indicators of water quality, because they are long-lived and relatively immobile. As a result of their longevity, and their sessile nature, they absorb many of the contaminants in water that can be scientifically analyzed. The wartyback, along with other freshwater mussel species, has been used in the cultured pearl and button-making industries in the past. (Missouri Department of Conservation, 2013)

  • Positive Impacts
  • body parts are source of valuable material
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Quadrula nodulata on humans.

Conservation Status

Degradation of mussel habitat in streams throughout the wartyback's known range is a continuing threat to this species. Wartyback populations are vulnerable to further decline, because of hydrologic alteration of streams and their watersheds, non-point and point source water and sediment pollution, and the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation. Further survey work in rivers where the wartyback was formerly documented is needed to verify its status in that former range. The wartyback is critically imperiled in Wisconsin, Minnesota, Iowa, Ohio, and Oklahoma. It is vulnerable in Illinois, Missouri, Indiana, Tennessee, and Mississippi, and imperiled in Texas, and Kansas. Federally, it does not have any special conservation status, and is listed as a species of "least concern" by the IUCN. (Minnesota Department of Natural Resources, 2013; NatureServe, 2013)

Other Comments

The wartyback was previously recorded as Quadrula nodulata, but is now known as Amphinaias nodulata. (Graf and Cummings, 2007)


Eric Krumm (author), Minnesota State University, Mankato, Robert Sorensen (editor), Minnesota State University, Mankato, Angela Miner (editor), Animal Diversity Web Staff.



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


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

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


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.


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


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


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


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. Journal of Experimental Zoology, 33: 463-499.

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

Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest.. Champaign, Illinois: Illinois Natural History Survey.

Graf, D., K. Cummings. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida).. Journal of Molluscan Studies, 73: 291-314.

Illinois Natural History Survey, 2013. "Quadrula nodulata" (On-line). Accessed February 22, 2013 at http://wwx.inhs.illinois.edu/collections/mollusk/publications/guide/index/36.

Meglitsch, P., F. Schram. 1991. Invertebrate Zoology. Oxford, England: Oxford University Press.

Minnesota Department of Natural Resources, 2013. "Species Profile of Quadrula nodulata (Wartyback)" (On-line). Accessed March 23, 2013 at http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=IMBIV39090.

Missouri Department of Conservation, 2013. "Wartyback species profile" (On-line). Accessed March 25, 2013 at http://mdc.mo.gov/discover-nature/field-guide/wartyback.

NatureServe, 2013. "NatureServe Explorer" (On-line). Accessed March 20, 2013 at http://www.natureserve.org/explorer/servlet/NatureServe?sourceTemplate=tabular_report.wmt&loadTemplate=species_RptComprehensive.wmt&selectedReport=RptComprehensive.wmt&summaryView=tabular_report.wmt&elKey=108359&paging=home&save=true&startIndex=1&nextStartIndex=1&reset=false&offPageSelectedElKey=108359&offPageSelectedElType=species&offPageYesNo=true&post_processes=&radiobutton=radiobutton&selectedIndexes=108359.

Parmalee, P., A. Bogan. 1998. The freshwater mussels of Tennessee.. Knoxville, Tennessee: The University of Tennessee Press.

Sietman, B. 2003. Field guide to the freshwater mussels of Minnesota.. St. Paul, Minnesota: Minnesota Department of Natural Resources.

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

Watters, G. 1994. An annotated bibliography of the reproduction and propagation of the Unionoidea (Primarily of North America).. Ohio Biological Survey Miscellaneous Contributions, No. 1, Columbus, Ohio: 158 pp.

Williams, J. 2008. Freshwater Mussels of Alabama & the Mobile Basin in Georgia, Mississippi & Tennessee.. Tuscaloosa, Alabama: University of Alabama Press.