Rainbow Mussel

(Villosa iris)

Status: Special Concern (COSEWIC 2015, ESA 2016)

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 average length of a Rainbow Mussel is 8 cm. They have a laterally-compressed, elongate-elliptical shape, and they are yellow, yellow-green, or brown in colour. The inside of their shell is iridescent which gives them a rainbow colour. The shell surface is covered with numerous narrow and wide broken dark green lines. While males and females look very similar, male shells are more bluntly pointed and females are more broadly rounded. Rainbow Mussels are also the only Mussel species that has a lure that looks like a crayfish. This lure is used by females to attract host fish to attach their young to.

Diet:

Rainbow Mussels- like all Mussels, are filter feeders which means they eat bacteria and algae by filtering it out of the water. Mussel larvae need to attach to a host fish in order to consume nutrients off their body. This occurs until they are able to grow into juvenile mussels and drop off the fish host.

Map taken from Government of Canada: https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/cosewic-assessments-status-reports/rainbow-2015.html

Habitat and Range:

Rainbow Mussels were once found in many lakes and rivers from New York and Ontario to Wisconsin and Oklahoma, Arkansas and Alabama. The Rainbow Mussel is now lost from the southern Great Lakes range, with the exception of Lake St. Clair. However, there are isolated populations throughout Ontario in the Ausable, Bayfield, Detroit, Grand, Maitland, Moira, Niagara, Salmon, Saugeen, Sydenham, Thames and Trent rivers. They are extirpated from the St. Clair River, Detroit River, Niagara River, and Lake Erie. Their current range in the United States is similar to their historical range, but populations are declining in the West.

They are most often found in shallow (<1m), well-oxygenated, small to medium rivers. They are most often found along the shore of emergent vegetation in moderate to strong currents. Rainbow Mussels can also be found in inland lakes, and they prefer to bury themselves in cobble, sand, gravel, and occasionally mud, or boulder.

Biology and Behaviour:

Rainbow Mussels are sedentary, they bury themselves in the bottom of lakes and rivers, only moving up to a couple of metres in their lifetime. During mating season, males will release sperm into the water where downstream females will filter the sperm out of the water much like they do for food. Once young have formed into larvae, females must then find a suitable host fish to attach their young to. To attract fish for its larvae to attach to, the female Rainbow Mussel produces a special lure that looks like a crayfish to attract host fish. The closer the host fish comes to the Mussel, the easier it is for her to attach her young. Host fish are generally Striped Shiner (Luxilus chrysocephalus), Streamline Chub (Erimystax dissimilis), Smallmouth Bass (Micropterus dolomieu), Largemouth Bass (Micropterus salmoides), Green Sunfish (Lepomis cyanellus), Bluebreast Darter (Etheostoma camurum), Greenside Darter (Etheostoma blennioides), Rainbow Darter (Etheostoma caeruleum) and Yellow Perch (Perca flavescens). Rainbow Mussels are preyed upon by river otters, mink, and muskrat.

Photo by Tim Lane: https://flic.kr/p/kHxwzF

Threats/ Reasons for being at Risk:

1. Invasive species: Zebra Mussels have decimated populations of freshwater mussels in the Lower Great Lakes by virtually eliminating historical habitat. The Zebra Mussel has been shown to directly reduce available food sources in the water column due to its siphoning ability. Zebra Mussels will directly attach to a mussel and prevent them from feeding and reproducing by covering their siphons. The weight of a Zebra Mussel will also deform other Mussel species. Almost 50% of waterways with Rainbow Mussels also have infestations of Zebra Mussels.

2. Pollution: Mussels are sensitive to increased levels of sediment contamination and water pollution. They are primarily filter feeders, while juveniles remain buried in the sediment feeding on particles associated with the sediment. In both cases, filter feeding increases exposure to water and sediment-born contaminants like road salt, wastewater contaminants, arsenic and copper, mercury, and pesticides and fertilizers from farms. Nutrient loadings for freshwater mussels relate to eutrophication effects (algal blooms) that can result in oxygen depletion and algal toxins.

3. Habitat loss and degradation: Destruction of habitat occurs through dredging, ditching and other forms of channelization. Modifications can result in the direct destruction of mussel habitat and lead to siltation and sand accumulation of local and downstream mussel beds. Damming of the stream channel has been shown to detrimentally affect mussels as reservoirs alter downstream flow patterns and distrust the natural thermal profiles of the watercourse which potentially separates mussels from their host fish.

Photo by Tim Lane: https://flic.kr/p/kDGxVV

Conservation and recovery strategies:

Rainbow Mussels are protected under the Ontario Endangered Species Act, Provincial Policy Statement of Planning Act and Aggregate Resources Act, Ontario Lakes and Rivers Improvement Act, and the Federal Fisheries Act (Ontario Fishery Regulations). The Federal Fisheries Act is important for mussel protection, since permits are needed to collect mussels in Ontario which reduces the amount of harvesting that can impact small local populations. OMAFRA also has a Voluntary Land-stewardship Program that works with landowners to reduce erosion, especially for shorelines near agricultural fields.

How can you help?

Stop the spread of invasive Zebra Mussels by keeping your boating and fishing equipment clean when travelling between water bodies.
Help improve mussel habitat and keep Ontario’s water safe and clean by maintaining natural vegetation next to creeks and rivers on your property- the roots of plants reduce erosion and can stop from washing into the river.
Farmers with lakes or rivers that border their fields should fence off shoreline areas to keep cattle (and their manure) out of the water.
Avoid using harmful chemicals on your property.

Additional Resources:

Government of Ontario Rainbow page
COSEWIC 2015 Assessment and Report on the Rainbow Mussel
Trout Unlimited’s page on how to stop the spread of Zebra Mussels
Sydenham River Watershed has an information page with a diagram of the lifecycle of a Rainbow Mussel

Photo by Tim Lane: https://flic.kr/p/kHEDqa

Sources:

COSEWIC 2006. COSEWIC assessment and status report on the Rainbow mussel Villosa iris in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 38 pp. https://www.sararegistry.gc.ca/virtual_sara/files/cosewic/sr_rainbow_mussel_e.pdf

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. https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/cosewic-assessments-status-reports/rainbow-2015.html

Government of Ontario. 2014. Rainbow. Retrieved from: https://www.ontario.ca/page/rainbow-mussel