Threehorn Wartyback

Obliquaria reflexa

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 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 cm;
  • 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.

Habitat

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. This mussel is a moderately long-lived species, with some individuals living up to 18 years.

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.

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.

Further Information

If listed under the Species at Risk Act (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 the Threehorn Wartyback will also be identified under SARA, allowing for greater protection and recovery of its habitat.

For more information, visit the SARA Registry.

Threehorn Wartyback

Threehorn Wartyback

Threehorn Wartyback

Scientific Name: Obliquaria reflexa
SARA Status: No Status
COSEWIC Status: Threatened
Region: Ontario

Image of inside and outside of the Threehorn Wartyback mussel shell

Image of inside and outside of the Threehorn Wartyback mussel shell.
Photo credit: Environment Canada

Did You Know?

The rare Threehorn Wartyback doesn’t resemble any other freshwater mussel species in North America.

And like most other freshwater mussels, the larvae (glochidia) of the Threehorn Wartyback 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.

The most likely host fishes for this mussel species in Canada are the Common Shiner (Luxilus cornutus) and Longnose Dace (Rhinichthys cataractae).

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