Threatened means the species lives in the wild in Ontario, is not endangered, but is likely to become endangered if steps are not taken to address factors threatening it.

Date added to the Species at Risk in Ontario List

June 27, 2014

Read the assessment report (PDF)

What it looks like

Threehorn wartyback is a medium-sized freshwater mussel that typically grows to 4 cm long. Its olive-green shell is thick and circular to triangular. Threehorn wartyback gets its name from the row of 3-5 large knobs or horns on its shell.

These mussels live up to 18 years.

Where it lives

This mussel is found in large rivers with moderate current and stable gravel, sand and mud bottoms. It burrows in the riverbed to filter-feed.

Like most mussels, threehorn wartyback females expel their larvae in the gills of host fish, where they live as parasites before forming into free-living mussels. Likely host fish are the common shiner and longnose dace.

Where it’s been found in Ontario

In Ontario, this mussel is found only in the Sydenham, Thames and Grand rivers in southwestern Ontario.

Historically, it was also found in Lake St. Clair, the Detroit River and western Lake Erie.

Recovery strategy

A recovery strategy advises the ministry on ways to ensure healthy numbers of the species return to Ontario.

Read the executive summary and the full document (January 25, 2023).

Government response statement

A government response statement outlines the actions the government intends to take or support to help recover the species.

Read the government response statement (October 25, 2023)

What threatens it

The greatest threats to this mussel are pollution from urban and agricultural sources, increased sediment in its rivers, and invasive zebra and quagga mussels, which have significantly altered the ecosystem of the rivers.

Action we are taking

Threatened species and their general habitat are automatically protected under Ontario’s Endangered Species Act, 2007.

What you can do

Report a sighting

  • Report a sighting of an endangered animal or plant to the Natural Heritage Information Centre. Photographs with specific locations or mapping coordinates are always helpful.


  • Volunteer with your local nature club or provincial park to participate in surveys or stewardship work focused on species at risk.

Be a good steward

  • Invasive species seriously threaten many of Ontario’s species at risk. To learn what you can do to help reduce the threat of invasive species, visit:
  • Private land owners have a very important role to play in species recovery. You may be eligible for stewardship programs that support the protection and recovery of species at risk and their habitats.
  • You can help improve mussel habitat and keep Ontario’s water safe and clean by maintaining natural vegetation next to creeks and rivers. The roots of plants reduce erosion and can stop soil from washing into the river. Fence off streamside areas to keep cattle (and their manure) out of the water. There are many other things that you can do to help reduce soil erosion and you might be eligible for funding assistance. For more information, visit the Ontario Soil and Crop Improvement Association:

Report illegal activity

Quick facts

  • Ontario is the only place in Canada where this species of mussel has been found.
  • The threehorn wartyback has always been rare – the first record in the province of a shell being found was in the Grand River in 1890, but the first live specimen in Ontario was not documented until a century later in 1992.
  • During their immature stage, threehorn wartyback are parasitic and attach themselves to the gills of a fish. They travel with their host until they fall off and carry out a free-living life cycle.