Rainbow Mollusc

(Villosa iris)

Status: Special Concern (ESA 2019, COSEWIC 2015)

Table of Contents:

Species Identification
Diet
Habitat and Range
Biology and Behaviour
Threats/Reasons for Being at Risk
Conservation and Recovery Strategies
How You Can Help

Species Identification:

The Rainbow (Villosa iris) is one of Canada’s 54 freshwater mussel species. It is a small, pretty mussel that gets its common name, “Rainbow”, from the shimmering, iridescent inside of its shell. Narrow, relatively thin, and small (average length of 2”), the shell is oval-shaped, with a rounded posterior in females, and blunt pointed posterior in males.

Diet:

Rainbow larvae, like those of most other freshwater mussels, are parasitic to fish – in The Land Between, their hosts include Smallmouth Bass, Largemouth Bass, Green Sunfish, Rainbow Darter, Yellow Perch, Mottled Sculpin and Rock Bass.
Juvenile Rainbow live completely burrowed in the substrate, where they feed on organic particles present in the substrate or interstitial water; the diet of adult Rainbow is similar, comprising of bacteria, algae and other organic particles filtered from the water column.

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Current distribution (1995-2005) of Villosa iris in Ontario (based on records from the Lower Great Lakes Unionid Database).

Habitat and Range:

This small mussel is widely distributed in southern Ontario. Although it has been lost from Lake Erie and the Detroit and Niagara Rivers, research shows it never common in these waters. Additionally however, two subpopulations (Ausable River and Lake St. Clair) have low abundance and are showing signs of continued decline.

Otherwise, the Rainbow can be found in low numbers in Maitland, Bayfield, Sydenham, Thames, Grand, lower Trent, Salmon and Moira rivers. The Maitland River supports the largest remaining population of Rainbow.

Biology and Behaviour:

Rainbow has separate sexes, but males and females are difficult to tell apart as they differ only slightly in shell shape. Rainbow is a long-term brooder that spawns in the late summer, broods its glochidia (larvae) over the winter and releases them in the early spring. Like most other freshwater mussels, the larvae are parasitic on fishes: they 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.

Sexual maturity occurs between 5 – 9 years of age and individuals can live up to 43 years. Adult Rainbow are essentially sessile and may move only a few meters along the substrate.

Threats/ Reasons for being at Risk:

1. Invasive species: The introduction and spread of Zebra and Quagga Mussels (Dreissena polymorpha and D. bugensis) continues to be a serious threat to the Rainbow, in the Thames, Trent and Moira rivers and Lake St. Clair. This is most notable in the Trent River watershed where dreissenids can form blankets of shells effectively smothering anything below.. 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.

2. Pollution: In rivers, the combined impacts of urbanization and agriculture threaten the Rainbow through sediment and nutrient loading, contaminants and toxic substances, habitat alterations as well as any activity that threatens the species’ host fishes.

3. Anthropogenic infrastructure: Other threats include changes to natural water flow (“flow regimes”) from dams, channel alterations and other system modifications, the severity of which are unknown.

Conservation and recovery strategies:

Rainbow is within the range of several multi-species or ecosystem-based recovery strategies or action plans that have been drafted or completed; because the threats to this species are the same threats that affect habitats as well as other SARs, large scale conservation strategies that seek to address these threats will also support the recovery and conservation of Rainbow.

However, a single-species recovery strategy (and action plan) is best suited for Rainbow. This is because, while its range and distribution overlaps partially with other mussel species at risk in some watersheds, it also occurs in several watersheds where other species at risk mussels do not occur (e.g., Maitland, Saugeen, Salmon and Moira rivers).

How can you help?

Report any sightings of the Rainbow Mollusc to The Land Between, iNaturalist, or the Natural Heritage Information Centre.
Keep the shoreline on your property natural and free of chemicals.
Get in touch with local organizations and government agencies to help protect your local watersheds.
Participate in citizen science shoreline and water rehabilitation projects with local organizations.
Do not release any invasive species into the wild, and make sure you are not relocating any aquatic invasive species when traveling from one area to another.

Additional Resources:

Government of Ontario Rainbow Species at Risk page
COSEWIC 2015 Assessment and Report on the Rainbow
Government of Canada Rainbow Conservation and Recovery Strategy

Sources:

COSEWIC. 2015. COSEWIC assessment and status report on the Rainbow Villosa iris in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xii + 82 pp. (http://www.registrelep-sararegistry.gc.ca/default_e.cfm).

Fisheries and Oceans Canada. 2018. Recovery strategy and action plan for the Rainbow (Villosa iris) in Canada [Proposed]. Species at Risk Act Recovery Strategy Series. Fisheries and Oceans Canada, Ottawa. v + 63 pp.

Government of Ontario. 2016. Rainbow (Villas iris). Retrieved from: https://species-registry.canada.ca/index-en.html#/species/943-644