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Rainbow Shell (Villosa iris)



Rainbow shell (Villosa iris), a mussel 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. The 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 RankG5Q
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.

Last revised: Tuesday, February 19, 2019