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Villosa iris

What do they look like?

Rainbow mussels can be up to 7.6 cm long. They are elongated and oblong with thin shells. Males are flatter compared to females, which are more curved. For both genders, the frontal end is rounded, in females; the back end is rounded as well, while males have a pointed back end. Umbos, or the protruding parts of each shell, are low or slightly raised above the shell’s hinge, with four to six double-looped ridges, the first two or three are concentric. Their outer shell layer is smooth except for growth lines. Their shell is yellow to yellow-green, with heavy broken green rays, especially on the back two-thirds of the shell. On their inner shell, the left valve has two pointed bumps near the hinge, known as pseudo cardinal teeth, which are small, upright and sharp-pointed. Their beak cavity is shallow. Their nacre, or mother of pearl inner shell, is bluish-white, 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 ellipse mussels 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

Where do they live?

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 range is difficult to know because there are uncertainties about 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)

What kind of habitat do they need?

Rainbow mussels are found in cool, clear, upper reaches of small to medium streams. They inhabit 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)

  • 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, known as glochidia. Glochidia are released into the water where they attach to the gills or body surface of host fishes. After attaching, host fishes grow a tissue over the glochidium 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)

How do they reproduce?

The age at which rainbow mussels reach sexual maturity is not known. Rainbow mussels are gonochoristic, which means this species has separate male and female forms. Females are fertilized by many males as sperm drifts in the water. Rainbow mussels are viviparous, meaning they give live birth. Their larvae, are released by the female after they are fully developed. (Haag, 2012; Lefevre and Curtis, 1912; Watters, 1995)

In family Unionidae, breeding generally begins as water temperatures increase. Males release sperm into the water, which is taken in by the females through their respiratory current. Eggs are fertilized internally in the suprabranchial chambers, and then pass into the water tubes of the gills, where they develop into larvae. Rainbow mussels are long-term brooders. In Michigan in the Huron River, females carry eggs from mid-August to the following mid-July and probably spawn from late July to mid-August. (Lefevre and Curtis, 1912; Watters, 1995)

  • How often does reproduction occur?
    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 larvae, known as 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

How long do they live?

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)

How do they behave?

In general, mussels are inactive, although they may move due to changing water levels and conditions. Rainbow mussels specifically are known to make lure displays where they rock back and forth with their lure extended beyond the shell, apparently mimicking crayfish. Although not well 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.

How do they communicate with each other?

In bivalves, most of their sensory organs are found in the middle lobe of the mantle edge. They have fluid filled chambers with a solid pellet (a statolity), known as paired statocysts, which are located in the mussel's foot, and help with 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 fish hosts. Rainbow mussels have a mantle flap resembling an aquatic insect or crayfish. It is unknown whether mussels can recognize specific fish hosts. Glochidia, or larvae, 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)

What do they eat?

In general, members of family Unionidae are filter feeders. Mussels use cilia to pump water into their incurrent siphon (where water is brought in) and food is caught in a mucus lining. Particles are sorted and directed to the mouth. Mussels have been cultured on algae, but may also ingest bacteria, protozoa and other particles. The parasitic glochidial (larval) stage absorbs blood and nutrients from hosts after attaching. (Watters, 1995)

What eats them and how do they avoid being eaten?

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)

What roles do they have in the ecosystem?

Glochidia, rainbow mussels' larvae have successfully metamorphosed by using the following fish hosts: 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. Their mortality and reproduction are impacted by unionicolid mites and trematodes that feed on their gill and mantle tissue, likewise, parasitic chironomid larvae may also destroy up to half their gill. (Cummings and Mayer, 1992; Neves, et al., 1985; Watters and O'Dee, 1997; Watters, 1995; Zale and Neves, 1982)

Species (or larger taxonomic groups) used as hosts by this species
Commensal or parasitic species (or larger taxonomic groups) that use this species as a host

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 usually indicates good water quality. (Farris and Van Hassel, 2007)

Are they endangered?

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).


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.

University of Michigan Museum of ZoologyNational Science Foundation

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Mulcrone, R. 2006. "Villosa iris" (On-line), Animal Diversity Web. Accessed May 30, 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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