Rainbow Mussel
Villosa iris
Summary 3
Villosa iris, common name the Rainbow mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.
Distribution 4
Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) This species is found throughout the Tennessee, Cumberland, and Ohio River basins, the upper Mississippi River, and the St. Lawrence River system from Lake Huron to Lake Ontario including their tributaries (Parmalee and Bogan, 1998). Due to much confusion surrounding the use of Unio nebulosus within the Villosa iris complex, Parmalee and Bogan (1998) chose to list all the described taxa from the Ohio, Tennessee, and Cumberland River systems as synonyms of Villosa iris and restrict the use of Villosa nebulosa to the species occurring in the headwaters of the Mobile Bay Basin. In Canada, it was historically known from the Ausable, Bayfield, Detroit, Grant, Maitland, Moira, Niagara, Salmon, Saugeen, Sydenham, Thames and Trent Rivers, as well as Lakes Huron, Ontario, Erie, and St. Clair; but it appears to have been lost from the lower Great Lakes and connecting channels, except for the Lake St. Clair Delta, but it is still extant in most rivers (COSEWIC, 2006). Recently this species has been confirmed to be likely extirpated from the main channel of the Detroit River between Lake St. Clair and Lake Erie, Michigan/Ontario; due to zebra mussel invasion (Schloesser et al., 2006).
Geographic range 5
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, Villosa 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.
Biogeographic Regions: nearctic (Native )
Habitat 6
Habitat Type: Freshwater
Comments: This species lives in riffles along the edges of emerging vegetation, such as Justicia beds, in gravel and sand in moderate to strong current. It becomes most numerous in clean, well-oxygenated stretches at depths of less than three feet (Parmalee and Bogan, 1998). It is most abundant in small to medium-sized rivers but can also be found in inland lakes (COSEWIC, 2006).
Morphology 7
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.
Range length: 7.6 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes shaped differently
Nature serve conservation status 8
Rounded Global Status Rank: G5 - Secure
Reasons: This species is found throughout the Tennessee, Cumberland, and Ohio River basins, the upper Mississippi River, and the St. Lawrence River system from Lake Huron to Lake Ontario including their tributaries and is considered stable in much of its range but is declining significantly in Canada.
Intrinsic Vulnerability: Unknown
Environmental Specificity: Narrow. Specialist or community with key requirements common.
Comments: This species lives in riffles along the edges of emerging vegetation, such as Justicia beds, in gravel and sand in moderate to strong current. It becomes most numerous in clean, well-oxygenated stretches at depths of less than three feet (Parmalee and Bogan, 1998).
Sources and Credits
- (c) Tim Lane, some rights reserved (CC BY-NC-ND), https://www.flickr.com/photos/119210184@N05/12940906273/
- (c) Femorale, some rights reserved (CC BY-NC), http://www.femorale.com/shellphotos/detail.asp?species=Villosa%20iris%20(Lea,%201829)
- Adapted by rkkessler from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Villosa_iris
- (c) NatureServe, some rights reserved (CC BY-NC), http://eol.org/data_objects/28914383
- (c) The Regents of the University of Michigan and its licensors, some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/25067611
- (c) NatureServe, some rights reserved (CC BY-NC), http://eol.org/data_objects/28914390
- (c) The Regents of the University of Michigan and its licensors, some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/31429552
- (c) NatureServe, some rights reserved (CC BY-NC), http://eol.org/data_objects/28914376
