Species Profile

Threehorn Wartyback

Scientific Name: Obliquaria reflexa
Taxonomy Group: Molluscs
Range: Ontario
Last COSEWIC Assessment: May 2013
Last COSEWIC Designation: Threatened
SARA Status: No schedule, No Status

Individuals of this species may be protected under Schedule 1 under another name; for more information see Schedule 1, the A-Z Species List, or if applicable, the Related Species table below.

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Quick Links: | Description | Habitat | Biology | Threats | Protection | Other Protection or Status | National Recovery Program | Documents

Image of Threehorn Wartyback


The Threehorn Wartyback (Obliquaria reflexa) is one of Canada’s 54 freshwater mussel species. It is a rare mussel, only found in North America and the only member of the genus Obliquaria found in Canada. It is characterized by the following features: Medium-sized mussel, with an average length of 4 centimeters; thick shell is circular to triangular in shape; front end of the shell (“anterior”) is rounded, while the back of the shell (“posterior”) is bluntly pointed; a single row of 2–5 large knobs or “horns” across the tops of both shells; raised part of the shell (“beak”) is finely sculptured and raised above the hinge line; shell colour can be green, tan or brown; numerous thin rays, or one wide green ray along its horns, may or may not be present; and inside of the shell (“nacre”) is pearly white and shiny near the posterior.


Distribution and Population

The Threehorn Wartyback is found only in North America, where it is broadly distributed from the Gulf of Mexico to the Great Lakes. In the United States, the Threehorn Wartyback is widely distributed across 21 central states throughout the Great Lakes, Mississippi River and Mobile River drainages. It is generally in decline throughout the Great Lakes drainage and possibly lost from Pennsylvania, imperiled in Ohio and vulnerable in Indiana and Wisconsin. In Canada, this mussel was historically known only in Ontario, in Lake St. Clair, the Detroit River, western Lake Erie and the Sydenham, Thames and Grand rivers. It is now believed to be lost from the Great Lakes, though small populations remain in the Sydenham, Thames and Grand rivers.



The Threehorn Wartyback prefers large river habitats with moderate currents and firm bottoms (substrates) of gravel, sand and mud. It is typically found at depths of up to 6–7 metres.



Considered a short-term brooder, Threehorn Wartyback mussels spawn in the spring and the larvae (glochidia) are released by the end of July. Like most other freshwater mussels, the glochidia are parasitic on fishes (i.e., 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 Threehorn Wartyback are essentially sessile and may move only a few meters along the substrate for the remainder of their lives. This mussel is a moderately long-lived species, with some individuals living up to 18 years. Like all species of freshwater mussels, the Threehorn Wartyback filters its food from the water. Bacteria and algae are its primary food sources.



Remaining Threehorn Wartyback mussels are significantly threatened by pollution related to urban and agricultural activities. Specifically, it is the sediment loading that leads to the clogging of the mussel’s gill structures, while nutrient loading and contaminants degrade water quality and overall habitat. Infestations of aquatic invasive species also remain a threat. Specifically, the Zebra Mussel is largely responsible for the loss of Threehorn Wartyback populations within the Great Lakes and connecting channels. By attaching to the Threehorn Wartyback by the hundreds, Zebra Mussels interfere with the native mussel’s ability to feed, move, breathe and reproduce. The Zebra Mussel continues to similarly threaten the remaining riverine populations of the Threehorn Wartyback. Additionally, Round Gobies are currently impacting native fish communities, including fish hosts that support native mussels. Human recreational activities, such as driving all-terrain vehicles (ATVs) over fragile mussel beds in the Sydenham River, are also another known threat.



Federal Protection

In Canada, this species is currently under consideration for listing as Threatened under the federal Species at Risk Act (SARA). More information about the SARA, including how it protects individual species, is available online at AquaticSpeciesAtRisk.ca or on the SARA Registry.

Provincial and Territorial Protection

To know if this species is protected by provincial or territorial laws, consult the provinces' and territories' websites.


Other Protection or Status

If listed under the SARA, a recovery strategy and action plan will be developed to prevent the loss of the Threehorn Wartyback in Canada, involving research, land and water stewardship, monitoring and awareness activities. Critical habitat for this species will also be identified under the SARA, allowing for greater protection and recovery of its habitat.



PLEASE NOTE: Not all COSEWIC reports are currently available on the SARA Public Registry. Most of the reports not yet available are status reports for species assessed by COSEWIC prior to May 2002. Other COSEWIC reports not yet available may include those species assessed as Extinct, Data Deficient or Not at Risk. In the meantime, they are available on request from the COSEWIC Secretariat.

4 record(s) found.

COSEWIC Status Reports

  • COSEWIC Assessment and Status Report on the Threehorn Wartyback Obliquaria reflexa in Canada (2014)

    Threehorn Wartyback is a medium-sized freshwater mussel generally reaching 40 mm in adult length (maximum length of 55 and 80 mm reported in Canada and the United States, respectively). The shell is thick, circular to triangular in shape, rounded on the anterior end and bluntly pointed on the posterior. The most obvious characteristic of the Threehorn Wartyback is the single row of 2 - 5 large knobs or “horns” that give rise to the common name of this species. The Threehorn Wartyback is the only member of the genus Obliquaria that occurs in Canada.

Response Statements

  • Response Statement - Threehorn Wartyback (2013)

    This rare species historically occurred in the Great Lakes drainages including Lake St. Clair, western Lake Erie, and the Grand, Thames, and Detroit rivers. The species has not been found since 1992 in Lake St. Clair and the Detroit River and may be extirpated there due largely to the impacts of Zebra and Quagga mussels. It was last recorded from the Canadian side of Lake Erie in 1997. Pollution (sediment loading, nutrient loading, contaminants and toxic substances) related to both urban and agricultural activities represents a high and continuing threat at the three remaining riverine locations.

COSEWIC Annual Reports

  • COSEWIC Annual Report – 2012-2013 (2013)

    Under Canada’s Species at Risk Act (SARA), the foremost function of COSEWIC is to “assess the status of each wildlife species considered by COSEWIC to be at risk and, as part of the assessment, identify existing and potential threats to the species”. COSEWIC held two Wildlife Species Assessment Meetings in this reporting year (October, 2012 to September 2013) from November 25 to November 30, 2012 and from April 28 to May 3, 2013. During the current reporting period, COSEWIC assessed the status or reviewed the classification of 73 wildlife species. The wildlife species assessment results for the 2012-2013 reporting period include the following: Extinct: 0 Extirpated: 2 Endangered: 28 Threatened: 19 Special Concern: 19 Data Deficient: 4 Not at Risk: 1 Total: 73 Of the 73 wildlife species examined, COSEWIC reviewed the classification of 50 species that had been previously assessed. The review of classification for 26 of those species resulted in a confirmation of the same status as the previous assessment.

Consultation Documents

  • Threehorn Wartyback - Consultations on listing under the Species at Risk Act (2014)

    The Threehorn Wartyback has recently been assessed as “Threatened” by the Committee on the Status of Endangered Wildlife in Canada. Before deciding whether this species will be protected under the Species at Risk Act, Fisheries and Oceans Canada would like your opinion, comments and suggestions regarding the possible ecological, cultural and economic impacts of listing or not listing it.