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Contact information
For information on Wisconsin's rare vertebrate animals, contact:
Rich Staffen
Conservation Biologist
For information on Wisconsin's rare invertebrates, contact:
Jay Watson
Conservation Biologist

Rainbow Shell (Villosa iris)



Rainbow Shell (Villosa iris), listed as Endangered in Wisconsin, is found in shallow, flowing, clean small streams with stable gravel substrate in the eastern part of the state. It has a very restricted geographical range in Wisconsin. Known host fish include smallmouth and largemouth basses and rockbass.

State status

Status and Natural Heritage Inventory documented occurrences in Wisconsin

The table below provides information about the protected status - both state and federal - and the rank (S and G Ranks) for Rainbow Shell (Villosa iris). See the Working List Key for more information about abbreviations. Counties shaded blue have documented occurrences for this species in the Wisconsin Natural Heritage Inventory database. The map is provided as a general reference of where occurrences of this species meet NHI data standards and is not meant as a comprehensive map of all observations.

Note: Species recently added to the NHI Working List may temporarily have blank occurrence maps.

Documented locations of Villosa iris in the Natural Heritage Inventory Database as of July 2015.
Summary Information
State StatusEND
Federal Status in Wisconsinnone
State RankS1
Global RankG5
Tracked by NHIY

Species guidance

Note: a species guidance document is not available at this time. Information below was compiled from publication PUB-ER-085-99 (now out-of-print).

Identification: The shell is elongate and elliptical, relatively thin. The male shell is compressed and the female shell inflated. Females are more expanded in the post-dorsal region and are more broadly rounded, while the male is rather sharply pointed. The beak is small and sculptured with distinctive, double-looped bars. The outer surface is colored yellowish-green to brown with dark green rays. The nacre is silvery white and highly iridescent posteriorly. Shell is small, usually less than 3 inches (7.6 cm).

Habitat: Inhabits small streams, living within and below riffles on a sand, gravel or mud bottom in water less than a meter deep.

State Distribution: Occurs in the Mukwonago River. This species may yet be found in rivers of southeastern Wisconsin for which we currently have only historical records. See the species map.

Phenology: Breeding has been observed from May through September. Glochidia have been found on rock bass (Ambloplites rupestris), and smallmouth bass (Micropteris dolomieu) from May through August. Largemouth bass (M. salmoides) are also used as hosts.

Management Guidelines: Because this species inhabits small streams, this mussel is most vulnerable to pollution from runoff and siltation. Habitat protection and water quality improvements would benefit this species. Increased development along waterways in southeastern Wisconsin is of great concern for the continued existence of this species.



Rainbow Shell

Photo © Kevin Cummings.

Wildlife Action Plan

Wisconsin Wildlife Action Plan graphic

Natural community (habitat) associations

The table below lists the natural communities that are associated with Rainbow Shell. Only natural communities for which Rainbow Shell is "high" (score=3) or "moderate" (score=2) associated are shown. See the key to association scores for complete definitions. Please see the Wildlife Action Plan to learn how this information was developed.

Natural community Score
Warmwater streams 3
Coolwater streams 2

Ecological landscape associations

The table below lists the ecological landscape association scores for Rainbow Shell. The scores correspond to the map (3=High, 2=Moderate, 1=Low, 0=None). For more information, please see the Wildlife Action Plan.

This map shows the probability of Rainbow Shell occurring in each of Wisconsin's Ecological Landscapes.  Actual scores can be found in the table to the left.

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Landscape-Community combinations of highest ecological priority*

Ecological priorities are the combinations of natural communities and ecological landscapes that provide Wisconsin's best opportunities to conserve important habitats for a given Species of Greatest Conservation Need. The 10 highest scoring combinations are considered ecological priorities and are listed below. More than 10 combinations are listed if multiple combinations tied for 10th place. For more information, please see the Wildlife Action Plan.

* Ecological priority score is a relative measure that is not meant for comparison between species. This score does not consider socio-economical factors that may dictate protection and/or management priorities differently than those determined solely by ecological analysis. Further, a low ecological priority score does not imply that management or preservation should not occur on a site if there are important reasons for doing so locally.

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Issues/threats and conservation actions

Conservation actions respond to issues or threats, which adversely affect species of greatest conservation need (SGCN) or their habitats. Besides actions such as restoring wetlands or planting resilient tree species in northern communities, research, surveys and monitoring are also among conservation actions described in the WWAP because lack of information can threaten our ability to successfully preserve and care for natural resources.

Threats/issues and conservations actions for rare animals

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Last revised: Thursday, October 08, 2020