Rainbow Mussel

Villosa iris

SARA Status
No Status
NS
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

SARA Status

  • No Status NS
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX
COSEWIC Status
Not at Risk
NR
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

COSEWIC Status

  • Not at Risk NR
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX

Description

The Rainbow (Villosa iris) is one of Canada's 54 freshwater mussel species and belongs to the family Unionidae. It is a small, pretty looking mussel that gets its common name, “Rainbow”, from the shimmering, iridescent inside of its shell. The Rainbow can also be recognized by the following features:

  • Narrow, long and oval-shaped shell;
  • Average length is 5.5 cm;
  • Low and rounded along the back (posterior) ridge;
  • Low and compressed beaks; sculpture contains four to six bars;
  • Medium-sized, well developed hinge teeth;
  • Yellow, yellowish-green or brown shell with many broken, dark green rays of differing widths; and
  • Inside of shell (“nacre”) is shimmering, silvery white.

Habitat

The current distribution of the Rainbow in North America is similar to its historical distribution: from Wisconsin east to Ontario and New York, and south to Oklahoma, Arkansas and Alabama. However, this species has been declining in part of its range—particularly in the Great Lakes, where it has been lost from Lake Erie and the Detroit and Niagara rivers and much of Lake St. Clair.

In Canada, the Rainbow occurs only in Ontario where it is now found in low numbers in the St. Clair River delta and the Saugeen, Maitland, Bayfield, Ausable, Sydenham, Thames, Grand, lower Trent, Salmon and Moira rivers. The Maitland River still supports the largest remaining population of Rainbow; overall however, this mussel has been lost from 30 percent of its historical Canadian range.

The Rainbow is most often found in shallow, well-oxygenated reaches of small- to medium-sized rivers, and sometimes lakes, on substrates (bottoms) of cobble, gravel, sand and occasionally mud.

The Rainbow is a long-term brooder; spawning takes place in the late summer and the larvae (glochidia) are released the following spring. Like most other freshwater mussels, the glochidia are parasitic on fishes. The glochidia attach to the gills of a host fish until they reach their juvenile, free-living stage and drop off to burrow in the substrate below. Adult Rainbow are essentially sessile and may move only a few meters along the substrate.

Likely host fishes for the Rainbow in Ontario include the Largemouth Bass (Micropterus salmoides), Mottled Sculpin (Cottus bairdi) and Rock Bass (Ambloplites rupestris).

Like all species of freshwater mussels, the Rainbow filters its food from the water. Bacteria and algae are its primary food sources. Juveniles feed on the same type of food as adults, but filter it from the water trapped in the surrounding substrate - or directly from the substrate - in which they are buried.

Threats

The introduction and spread of Zebra and Quagga Mussels (Dreissena polymorpha and D. bugensis) continues to be a serious threat to the Rainbow, especially in Lake St. Clair. In rivers, the combined impacts of urbanization and agriculture threaten the Rainbow through sediment and nutrient loading, changes to natural water flow (“flow regimes”) from dams and channel alterations, contaminants and toxic substances, habitat alterations as well as any activity that threatens the species' host fishes. The introduction of invasive fish species, such as the Round Goby (Neogobius melanostomus), could also indirectly affect the Rainbow by disrupting the relationship with its host fish.

Further Information

For further information, visit the Species at Risk Public Registry or Committee on the Status of Endangered Wildlife in Canada.

Rainbow Mussel

Rainbow Mussel

Rainbow Mussel

Scientific name: Villosa iris
SARA Status: Endangered
COSEWIC Status
: Special Concern
Region: Ontario

Map: Distribution of Rainbow in Canada

Distribution of Rainbow Mussel in Canada.

Did You Know?

Female Rainbows use a visual display to attract their host fishes, making water quality and clarity an important factor for successful reproduction. They have a mantle flap that mimics a crayfish in shape and movement. When a fish approaches or strikes at the lure, the female mussel releases her glochidia, allowing them to attach to the gills of the fish.

Rainbow Mussel

Image of a Rainbow mussel, interior and exterior of the shell
(Photo credit: Environment Canada)

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