Villosa iris

Geographic Range

Rainbow mussels (Villosa iris) are found in the Ohio, Tennessee and upper Mississippi River systems. In the Great Lakes, they are found in Lake Michigan, Huron, Ontario and Erie drainages. Their southern limit is difficult to define because of taxonomic uncertainties associated with southern species and subspecies. Found in Michigan's lower peninsula, rainbow mussels occur in Lake Michigan tributaries from the Muskegon, south to the St. Joseph River on the west side of the state. On the east side of the state, they are also found in the Saginaw River and Lake Erie drainages. (Burch, 1975)


Rainbow mussels are found in cool, clear, upper reaches of small to medium streams. They inhabit substrates including sandy mud, coarse sand or gravel, in areas with faster currents. In the Huron River they are found on sand and gravel shoals with good currents. They are also found in Lake Erie. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

  • Aquatic Biomes
  • rivers and streams

Physical Description

Rainbow mussels are up to 7.6 cm long. They are elongate and oblong in shape, with a fairly thin shell. Males are compressed and females are inflated. Their anterior end is uniformly rounded; the posterior end is sharply rounded in females and bluntly pointed in males. Their dorsal margin is straight and their ventral margin is straight to gently curved. Umbos are low slightly raised above the hinge line. The beak sculpture has four to six double-looped ridges, the first two or three are concentric. The umbos also have tubercles at the posterior end. Their periostracum (outer shell layer) is smooth except for growth lines. The shell is yellow to yellow-green, with heavy broken green rays. Rays are more numerous on the posterior two-thirds of the shell. On their inner shell, the left valve has two pseudo cardinal teeth, which are small, erect, divergent and sharp-pointed. The two lateral teeth are straight, short and fine. The right valve has one erect, columnar pseudo cardinal tooth. Sometimes there is smaller nacreous swelling anterior to this tooth. The lateral tooth is short and thin. Their beak cavity is shallow. Their nacre is bluish-white, bluer posteriorly, and their beak cavity is cream-colored, the posterior end is iridescent. In Michigan, rainbow mussels can be confused with ellipse mussels and rayed beans. Rays on the ellipse are fine, wavy and generally unbroken. Rainbow mussels also have a longer hinge line and finer teeth. Rayed beans are smaller, generally darker in color and more inflated. (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. Glochidia are released into the water where they must attach to the gill filaments and/or general body surface of a host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. Glochidia metamorphose into juvenile mussels 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)


The age to sexual maturity is unknown for this species. Rainbow mussels are gonochoristic, meaning they have separate sexes. Females are fertilized by many males as their sperm drifts in the water. Rainbow mussels are viviparous, glochidia, their larval stage, are released live from the female after they are fully developed. (Haag, 2012; Lefevre and Curtis, 1912; Watters, 1995)

In family Unionidae, gametogenesis is generally initiated by increasing water temperatures. Their general life cycle includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. Eggs are internally fertilized in the suprabranchial chambers, and then pass into the water tubes of the gills, where they develop into glochidia. Rainbow mussels are long-term brooders. In Michigan in the Huron River, they are gravid from mid-August to the following mid-July and probably spawn from late July to mid-August. (Lefevre and Curtis, 1912; Watters, 1995)

  • Breeding interval
    Rainbow mussels breed once during the warmer months.
  • Breeding season
    In Michigan, their breeding season is probably late July to mid-August.
  • Range gestation period
    11 (high) months

Females brood fertilized eggs in their marsupial pouch. Fertilized eggs develop into glochidia. There is no parental investment after females release the glochidia. (Lefevre and Curtis, 1912; Watters, 1995)

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


Their age can be determined by looking at the annual rings on their shell. There is currently no demographic data available regarding this species, although, members of family Unionidae generally have about a 10 year lifespan or more. (Haag, 2012; Winhold, 2004)


In general, mussels are rather sedentary, although they may move in response to changing water levels and conditions. Rainbow mussels specifically are noted for making an elaborate lure display, in which they rock back and forth with their lure extended beyond the shell, apparently mimicking crayfish. Although not thoroughly documented, mussels may vertically migrate to release glochidia and spawn. They are often found buried underneath substrate. (Haag, 2012; Oesch, 1984)

Home Range

There is currently no information available regarding the home range size of rainbow mussels.

Communication and Perception

Most bivalve sensory organs are found in the middle lobe of the mantle edge. Fluid filled chambers with a solid granule or pellet (a statolity), known as paired statocysts, are located in the mussel's foot, and assist in georeception and orientation. Mussels are heterothermic, and therefore sensitive and responsive to temperature. In general, members of family Unionidae may have some form of chemical reception to recognize fish hosts. Closely related bivalves have mantle flaps modified for attracting potential fish hosts. Rainbow mussels have a mantle flap resembling an aquatic insect or crayfish. It is unknown whether mussels can recognize a specific fish host. Glochidia respond to touch, light and some chemical cues. In general, when touched or when a fluid is introduced, they respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Haag, 2012; Watters, 1995)

Food Habits

In general, members of family Unionidae are filter feeders. Mussels use cilia to pump water into their incurrent siphon where food is caught in a mucus lining in the demibranches. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but may also ingest bacteria, protozoa and other organic particles. The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within glochidia feed off the host’s tissue through phagocytocis. (Watters, 1995)


In general, members of family Unionidae are preyed upon by muskrats, raccoons, minks, otters and some birds. Juveniles may also be preyed upon by the following fish species: freshwater drums, lake sturgeon, spotted suckers, redhorses and pumpkinseeds. (Cummings and Mayer, 1992; Watters, 1995)

Ecosystem Roles

In lab trials, rainbow mussels’ glochidia metamorphosed on green sunfishes, smallmouth bass, largemouth bass, Suwanee bass, spotted bass, striped shiners, streamline chubs, mosquitofishes, greenside darters, rainbow darters, bluebreast darters, blackside darters and yellow perch. The mortality and reproduction of members of family Unionidae are affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue, likewise, parasitic chironomid larvae may also destroy up to half of their gill. (Cummings and Mayer, 1992; Neves, et al., 1985; Watters and O'Dee, 1997; Watters, 1995; Zale and Neves, 1982)

Species Used as Host
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence usually indicates good water quality. (Farris and Van Hassel, 2007)

Economic Importance for Humans: Negative

There are no significant negative impacts of mussels on humans.

Conservation Status

Overall, rainbow mussels are considered to have a G5Q conservation status, which means they are common and widespread. However, in Illinois and Wisconsin, they are considered endangered, and in Michigan and North Carolina they are considered a species of special concern. (Asher and Christian, 2012; Hove, 2004)


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. Journal of Experimental Zoology, 33: 463-499.

Asher, A., A. Christian. 2012. Population characteristics of the mussel Villosa iris (Lea) (rainbow shell) in the Spring River Watershed, Arkansas. Southeastern Naturalist, 11:2: 219-230.

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

Farris, J., J. Van Hassel. 2007. Freshwater Bivalve Ecotoxicology. Pensacola: Society of Environmental Toxicology and Chemistry.

Haag, W. 2012. North American Freshwater Mussels: Natural History, Ecology, and Conservation. New York: Cambridge University Press.

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

Lefevre, G., W. Curtis. 1912. Experiments in the artificial propagation of fresh-water mussels. Proceeding of the International Fishery Congress, Washington, 28: 617-626.

Lefevre, G., W. Curtis. 1910. Reproduction and parasitism in the Unionidae . Journal of Experimental Biology, 9: 79-115.

Neves, R., L. Weaver, A. Zale. 1985. An evaluation of host fish suitability for glochidia of Villosa vanuxemi and V. nebulosa (Pelecypoda: Unionidae). America Midland Naturalist, 113: 13-19.

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., S. O'Dee. 1997. Potential hosts for Villosa iris (Lea, 1829). Triannual unionid report, 12: 7.

Winhold, L. 2004. "Family Unionidae " (On-line). Animal Diversity Web. Accessed November 25, 2013 at

Zale, A., R. Neves. 1982. Fish hosts of four species of lampsiline mussels (Mollusca: Unionidae) in Big Moccasin Creek, Virginia. Canadian Journal of Zoology, 60: 2535-2542.

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.