Round pigtoe (Pleurobema sintoxia) COSEWIC status report: chapter 2

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Species Information

The Round Pigtoe, Pleurobema sintoxia (Rafinesque, 1820) is a medium to large freshwater mussel (maximum length in Canada ~130 mm) that is usually somewhat rectangular in shape. The shell is relatively thick and solid with a roughened surface. It may be tan in colour in juveniles but darkens to a characteristic deep reddish brown with age.

Distribution

The Round Pigtoe was historically distributed from New York and Ontario in the east to South Dakota, Kansas and Oklahoma in the west and south to Arkansas and Alabama. In Canada, it was known from the Niagara, Detroit, Grand, Thames and Sydenham rivers as well as Lake Erie and Lake St. Clair. Large river populations have declined in the upper Midwest, but many populations survive in tributaries of the Mississippi and Ohio rivers. In Canada, it is extant in the Grand, Thames and Sydenham rivers and Lake St. Clair.

Habitat

The Round Pigtoe appears to be a habitat generalist. It may be found in small, medium-sized and large rivers with moderate flows on mixed substrates of gravel, cobble, boulder, sand and mud. In Lake Erie and Lake St. Clair, it occurs in shallow (<1 m) nearshore areas with firm sandy substrates. In large rivers it is often found at depths greater than 3 m.

Biology

The Round Pigtoe has separate sexes, but males and females look alike. The lifespan is unknown, but other members of the Subfamily Ambleminae tend to be long-lived (30 years or more). Like other freshwater mussels, the Round Pigtoe is parasitic on fish during its larval stage. The breeding season lasts from early May to July, and the larvae are released by the female before winter. Once released, the larvae must attach to the gills of an appropriate fish host and form a cyst. After a period of time, the larvae transform into juveniles that drop off the fish and fall to the substrate to begin life as free-living mussels. Several fishes known to be hosts for the Round Pigtoe in the U.S. also occur in the mussel’s range in Canada (Bluegill, Spotfin Shiner, Bluntnose Minnow, Northern Redbelly Dace). Round Pigtoes, like all freshwater mussels, feed on bacteria and algae that they filter from the water with their gills.

Population Sizes and Trends

The Round Pigtoe is a widely distributed but uncommon species throughout its range. There is evidence that the species was once more abundant in many systems, especially large rivers, than it is now. It has been lost from the Niagara and Detroit rivers and the offshore waters of Lake Erie and Lake St. Clair due to impacts of the Zebra Mussel, Dreissena polymorpha. Small remnant populations still occur in a few nearshore areas in Lake Erie and Lake St. Clair where Zebra Mussel densities are lower. The Round Pigtoe is represented by only a few relic specimens in the Grand and Thames rivers, but appears to be reproducing in the Sydenham River and the delta area of Lake St. Clair.

Limiting Factors and Threats

The Round Pigtoe has been lost from most of its former range in the Great Lakes due to impacts of the Zebra Mussel, and the remaining Canadian population in the St. Clair delta may be at risk. Populations in the Grand and Thames rivers have been nearly extirpated, probably due to the combined effects of municipal and industrial pollution and agricultural impacts in these heavily populated watersheds. The population in the Sydenham River is small and there is evidence that recruitment may be declining. This population is at risk from intensive agriculture and associated heavy loadings of silt and nutrients.

Special Significance of the Species

There are 31 species in the genus Pleurobema, but only P. sintoxia has a range that extends into Canada. The Round Pigtoe is also the only member of the genus that is considered to be stable throughout most of its North American range. The genera Pleurobema and Epioblasma are the most critically imperiled unionid taxa. Thus, even the most common members of these taxa must be protected in order to prevent the extinction of the genera.

Existing Protection or Other Status Designations

The Round Pigtoe is listed as endangered in Iowa and Pennsylvania, threatened in Minnesota, special concern in Michigan and Wisconsin, and a species of special interest in Ohio, and is therefore afforded some protection in these states. There is no specific protection for the Round Pigtoe in Ontario or Canada at the present time.