Round pigtoe (Pleurobema sintoxia) COSEWIC status report: chapter 6

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Distribution

• Global Range
• Canadian Range

Global Range

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 (Figure 3). It occurred in Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, New York, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, West Virginia, Wisconsin, and Ontario. The current distribution of the Round Pigtoe is similar to the historical range. Although large river populations have for the most part disappeared from the upper Midwest, many populations still survive in tributaries of the Mississippi and Ohio rivers. 

Figure 3: North American Distribution of Pleurobema sintoxia (based on information provided by jurisdictions)

Canadian Range

In Canada P. sintoxia is only known from southern Ontario. The National Water Research Institute’s Lower Great Lakes Unionid Database was used to identify occurrence records for P. sintoxia in Ontario. At the time of writing, the database consisted of approximately 7600 records for 40 species collected from nearly 2400 sites in the lower Great Lakes drainage basin since 1860 (see Metcalfe-Smith et al. 1998a for a detailed description of the database and its data sources). The Round Pigtoe was historically collected from the Niagara, Detroit, Grand, Thames, and Sydenham rivers, as well as Lake Erie and Lake St. Clair. The earliest record of the species in Canada is one fresh whole shell collected in 1885 from the Grand River at Caledonia by J. Townsend (specimen held by the Canadian Museum of Nature; cat. no. 002417). Figure 4 shows the historical distribution of the Round Pigtoe in Ontario, based on 84 records collected between 1885 and 1995. The current distribution of the species, based on 57 records (live animals and shells) collected between 1997 and 2002, is shown in Figure 5. Live specimens were most recently collected from the Sydenham River in the summer of 2002. 

Figure 4: Historical (1885-1995) Distribution of Pleurobema sintoxia in Ontario (based on records from the Lower Great Lakes Unionid Database)

Figure 5: Current (1997-2002) Distribution of Pleurobema sintoxia in Ontario (based on records from the Lower Great Lakes Unionid Database)

Populations of P. sintoxia in the Niagara and Detroit rivers, Lake Erie and Lake St. Clair have been mostly lost due to impacts of the Zebra Mussel (Dreissena polymorpha). Small, isolated pockets of surviving animals may still be found in some nearshore areas that offer refuge from infestation by Zebra Mussels. For example, P. sintoxia was recently found alive in Metzger Marsh on the Pennsylvania shore of western Lake Erie (Nichols and Amberg 1999) and in the St. Clair delta area of Lake St. Clair (Zanatta et al. 2002). Results from a 2001 survey of the Niagara River showed that no live unionids of any species were found. The Long Point sites have not been re-surveyed in recent years, so we assume that the species still occurs there. A reproducing population still persists in the Sydenham River and small, possibly senescent, populations occur in the Middle Thames and Grand rivers (Metcalfe-Smith et al. 1998b, 1999 and unpublished data). Overall, the Round Pigtoe has been lost from about 54% of its former range (in terms of extent of occurrence) in Canada; the historical extent of occurrence was 26 592 km² and the current EO is 12 360 km², with an estimated area of occupancy of about 15 km².