Features: The round pigtoe (Pleurobema sintoxia) is a freshwater mussel that lives in sandy substrates in deeper water of large rivers, and the near-shore areas of some of the Great Lakes. Adults have deep-mahogany coloured shells with darker banding, and they can grow to 10 cm long or more.

The lifecycle of the round pigtoe is typical for freshwater mussels: adults are sessile, attached to substrate, and filter food out of the water. Males release sperm into the water column, which fertilizes eggs held by females in a pouch (called a "marsupium") in the gills. When the larvae reach an intermediate stage called the glochidium, they are released in mucus packets. In the case of round pigtoe, these packets are mistaken for food by fish, and when a fish bites into a packet, the glochidia are dispersed and attach to the gills of the fish host. Here the larvae continue their growth until they are juveniles, when they fall to the substrate and are a free-living mussel. Juvenile mussels grow for several years and must attain a size of about 8 cm before they are mature and can breed.

Status: Endangered Provincially and Nationally

Range: The Round Pigtoe is found from New York and Ontario west to South Dakota, Kansas and Oklahoma, and south to Arkansas and Alabama. In Ontario, it is found in the Grand, Thames and Sydenham rivers, and it is found around Walpole Island in Lake St. Clair. Historically, it is known from Lake Erie, and it may persist in a few locations, such as around Long Point. Range Maps

Threats: Round pigtoe has likely always been rare in Ontario, which is the northern limit of its range in North America; surveys in the early 20th century in the Sydenham River reported densities similar to those today. However,its range has almost certainly contracted in the last 50 years, especially with increases in water pollution in the Great Lakes and, more recently, with the invasion of zebra mussels. Zebra mussels are thought to have caused widespread decreases in a number of freshwater mussel species in Lake Erie in the last 20 years. Zebra mussels colonize the same areas as native mussels, but can reproduce much faster in the absence of their natural predators and parasites, and so they quickly exclude native mussels.

The river populations have not yet been subject to competition from zebra mussels, but water pollution is the main threat. Round pigtoe, as with most freshwater mussels, is not tolerant of water pollution or siltation. The rivers where it currently lives flow through some of southern Ontario’s most productive farmland, which are subject to runoff of chemicals, animal wastes and soil.

Protection: The round pigtoe is listed as endangered under Ontario's Endangered Species Act, 2007. The Act protects the species from being killed, collected, possessed, sold or traded. The round pigtoe population in Lake St. Clair is located entirely within the territory of the Walpole Island First Nation, and human disturbance of the habitat is strictly regulated and kept to a minimum. The river populations are in catchment areas that are largely privately-owned and managed as farmland.

Text Sources: Zanatta and Metcalf-Smith 2004

Last Modified Date: October 2008