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.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

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.

• Habitat Regions
• freshwater freshwater

• 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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes shaped differently

Development

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.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

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.

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.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• viviparous viviparous

Females brood fertilized eggs in their marsupial pouch. Fertilized eggs develop into glochidia. There is no parental investment after females release the glochidia.

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

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• parasite parasite
• motile motile
• sedentary sedentary

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.

• Communication Channels
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations
• chemical chemical

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.

• Primary Diet
• planktivore planktivore
• detritivore detritivore

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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 .

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.

• Ecosystem Impact
• parasite parasite

Economic Importance for Humans: Positive

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

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.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Renee Sherman Mulcrone (author).

References

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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 http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html .

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 http://www.fw.umn.edu/Personnel/staff/Hove/State.TE.mussels .

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 animaldiversity.ummz.umich.edu/accounts/unionidae .

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.