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Round Pigtoe (Pleurobema Sintoxia)

Summary of Status Report

Pleurobema sintoxia historically occurred in 19 states and the province of Ontario. In the United States, it was found throughout the Mississippi and Ohio River systems. It also occurred in Lake Erie, Lake St. Clair, and their drainages. There are few data available on historical abundance, but Ortmann (1919) described the species as “abundant” in many locations throughout its range at the turn of the 20th century. In contrast, most jurisdictions currently describe the species as uncommon and never abundant, with evidence of recruitment lacking in some streams in Kentucky and Missouri. In Canada, the Round Pigtoe was historically found in the western basin of Lake Erie, Lake St. Clair, and the Niagara, Detroit, Grand, Thames, and Sydenham rivers. It has been lost from Lake Erie, the Niagara and Detroit rivers, and the offshore waters of Lake St. Clair due to impacts of the Zebra Mussel. A significant population was discovered in shallow waters of the St. Clair delta in Lake St. Clair in 1999, but it is not certain that this population will continue to co-exist with the Zebra Mussel. The Round Pigtoe has declined in the Grand and Thames rivers, with only small numbers of large - presumably relic - specimens left. Declines in the Grand River may be reversible, since many other mussel species have recolonized the lower river as a result of significant improvements in water quality over the past 25 years. The healthiest population of P. sintoxia in Canada is in the Sydenham River, where the species is relatively rare (representing about 1% of the mussel community by abundance) but shows signs of continuing recruitment at one or two sites.

The Round Pigtoe is currently 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 (it is not federally listed in the U.S.). Most land along the reaches of the Thames, Grand, and Sydenham rivers where P. sintoxia was found alive in recent years is privately owned and in agricultural use. The population of P. sintoxia in the St. Clair delta is located within the territory of the Walpole Island First Nation. The area is undeveloped and is under the control of the First Nation, which means there is excellent potential for protecting the population from human disturbance. However, it may not be possible to protect it indefinitely from the Zebra Mussel. The most significant threats to the continued existence of the Round Pigtoe in Canada are Zebra Mussels and agricultural impacts.