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The rainbow is up to 7.6 cm (3 inches) long , and is elongate and oblong in shape. The shell is usually fairly thin. Males are compressed and females are inflated. The anterior end is uniformly rounded, the posterior end sharply rounded in females to bluntly pointed in males. The dorsal margin is straight and the ventral margin is straight to gently curved.
Umbos are low, even, or raised slightly above the hinge line. The beak sculpture has four to six double-looped ridges, the first two or three concentric. The umbos also have tubercles at the posterior end.
The periostracum (outer shell layer) is smooth except for growth lines. The shell is yellow to yellow-green with heavy broken green rays. The rays are more numerous on the posterior two-thirds of the shell.
On the inner shell, the left valve has two pseudocardinal teeth, which are small, erect, and divergent and sharp-pointed. The two lateral teeth are straight, short and fine. The right valve has one erect, columnar pseudocardinal tooth. Anterior to this tooth sometimes is a smaller nacreous swelling. The one lateral tooth has short and thin.
The beak cavity is shallow. The nacre is bluish-white, more blue posteriorly, and beak cavity is cream-colored. The posterior end is iridescent.
In Michigan, the rainbow can be confused with the ellipse and the rayed bean. The rays on the ellipse are fine, wavy and generally unbroken. The rainbow also has a longer hinge line and has finer teeth. The rayed bean is smaller and generally darker in color and more inflated.
The rainbow mussel is found in the Ohio, Tennessee and upper Mississippi river systems. In the Great Lakes it is found in drainages of Lakes Michigan, Huron, Ontario and Erie. The southern limit of this species is difficult to define because of taxonomic uncertainties of species and subspecies in the south.
Found in Michigan's lower peninsula, V. iris occurs in the 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 it is also found in the Saginaw River drainages and Lake Erie drainages.
The rainbow is found in cool, clear, upper reaches of small to medium streams. Substrates it inhabits include sandy mud, coarse sand, or gravel, in areas near faster currents.
In the Huron River it was found on sand and gravel shoals that had a good current. This species is also found in Lake Erie.
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.
The rainbow mussel breeds once in the warmer months of the year.
In Michigan, the breeding season is probably late July to mid-August.
11 months (high)
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.
Villosa iris is a long-term brooder. In the Huron River in Michigan, it was gravid from mid-August to the following mid-July. It probably spawns from late July to mid-August in Michigan.
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.
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.
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. Often they are found buried under the substrate.
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. Mantle flaps in the lampsilines are modified to attract potential fish hosts. The rainbow mussel has a mantle flap resembling an aquatic insect. If the mussel recognizes a specific fish host is unknown.
Glochidia respond to touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut.
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.
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.
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.
In lab trials, Villosa iris glochidia metamorphosed on the green sunfish, smallmouth bass, largemouth bass, Suwanee bass, spotted bass, striped shiner, streamline chub, mosquitofish, greenside darter, rainbow darter, bluebreast darter, blackside darter, and yellow perch.
There are no significant negative impacts of mussels on humans.
Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.
Villosa iris is listed as Endangered in Illinois and Wisconsin. In Michigan and North Carolina it is considered Special Concern.
Renee Sherman Mulcrone
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