The Round Pigtoe

Description
Habitat
Threats
Further Information
Scientific Information

Description

The Round Pigtoe (Pleurobema sintoxia) is one of Canada’s 54 freshwater mussel species and belongs to the Family Unionidae. Its lifespan is unknown, but other members of its genus can live for more than thirty years. The Round Pigtoe has the following features:

• a medium-to-large freshwater mussel, reaching lengths between 7.5 and 13 cm;
• adult shells are deep reddish-brown in colour, juveniles are tan;
• inside of shell (nacre) is white or shades of pink;
• shell is usually somewhat rectangular in shape, relatively thick and solid with a roughened surface; and
• raised part at the top end of the shell (beak) is compressed, turned forward and slightly elevated, extending beyond the hinge line.

Pleurobema sintoxia

Photo credit: Environment Canada

Habitat

In North America, the Round Pigtoe was once widely distributed from New York and Ontario, west to South Dakota, Kansas and Oklahoma, and south to Arkansas and Alabama. In Ontario, it occurred in lakes Erie and St. Clair and in the Detroit, Grand, Niagara, Sydenham and Thames rivers. Though its current distribution is similar to the historical range, only a few of these mussels remain in the Grand and Thames rivers, and small, but reproducing populations exist in the Sydenham River and the delta area of Lake St. Clair.

The Round Pigtoe can live in a wide range of habitats, from small rivers in areas of moderate flow with gravel, cobble and boulder substrates (bottoms) to larger rivers in mud, sand and gravel at varying depths. Its breeding season lasts from early May to late July and the larvae (glochidia) are released before winter. Like most other freshwater mussels, the glochidia 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 into the substrate below. Adult Round Pigtoe are essentially sessile and may move only a few metres along river or lake bottoms.

The host fishes for this mussel in Canada are the Spotfin Shiner (Cyprinella spiloptera), Northern Redbelly Dace (Phoxinus eos), Bluntnose Minnow (Pimephales notatus), Bluegill (Lepomis macrochirus) and Central Stoneroller (Campostoma anomalum).

Like all species of freshwater mussels, the Round Pigtoe filters its food from the water. Bacteria and algae are its primary food sources.

Map showing the Round Pigtoe distribution in Canada. The map indicates areas of current and historic sites in southern Ontario.

Threats

The introduction and spread of the Zebra Mussel (Dreissena polymorpha) throughout the Great Lakes severely reduced or eliminated the Round Pigtoe in Lake St. Clair, Lake Erie, and the Detroit and Niagara rivers. Loss of habitat and decreased water quality from siltation and pollution continue to threaten this species, as remaining populations are found in heavily populated areas with poor agricultural and urban land-use practices. The Round Pigtoe was also harvested for the historic pearl button industry and is now used in the cultured pearl industry. Commercial harvesting is closed in Canada and most of the U.S., making poaching a possible future threat to this mussel.

Further Information

Under SARA, a recovery strategy and an action plan have been developed to prevent the loss of the Round Pigtoe in Canada and to maintain/return healthy self-sustaining populations of Round Pigtoe in the Sydenham River, Bear Creek and the St. Clair River delta, and to reintroduce healthy self-sustaining populations to the Thames and Grand rivers.
Research, land and water stewardship, monitoring and awareness activities are underway. Provincial legislation regulating agricultural nutrients and protecting clean water in Ontario have been enacted. Critical habitat for the Round Pigtoe has also now been identified under SARA, allowing for greater protection and recovery of its habitat.

For more information, visit the SARA Registry at www.SARAregistry.gc.ca.

Scientific Information:

Scientific Name: Pleurobema sintoxia
SARA Status: Endangered (July 2005)
COSEWIC Status: Endangered (May 2004)