Round Pigtoe
Pleurobema sintoxia
Species Description
The Round Pigtoe is a medium to large freshwater mussel that reaches a maximum length of about 13cm in Canada. The Round Pigtoe shell can be highly variable, it typically ranges in shape from triangular or oval. The shell is relatively thick and solid with a rough surface. It may be tan in colour in juveniles but darkens to a characteristic deep reddish brown with age. The beak points anteriorly.
Habitat
The Round Pigtoe prefers medium to large rivers or lakes in areas with moderate flows and mixed substrates (boulder, cobble, gravel, sand, mud).
Distribution in the Sydenham
The Round Pigtoe historically had a wide distribution across the United States and Canada. This mussel inhabited rivers 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, the Round Pigtoe 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, the Round Pigtoe is extant only in the Grand, Thames and Sydenham Rivers and Lake St. Clair.
Round Pigtoe is an imperiled species in parts of its distribution (endangered in Canada). It is limited to a small area of occupancy in the Lake St. Clair and three watersheds in southern Ontario with continuing declines in habitat area, extant and quality.
The population in the Sydenham River is small and there is evidence that recruitment may be declining.
Using DNA to Identify Pigtoes
Round Pigtoe (Pleurobema sintoxia) and the related Wabash Pigtoe (Fusconaia flava) mussels can be hard to tell apart. Wabash Pigtoe is widespread and relatively common, whereas Round Pigtoe is rare and imperiled.
A 2020 study by Willsie, Morris and Zanatta compared physical characteristics to genetics (i.e., DNA barcoding) to test the accuracy of field identifications.
Researchers from Central Michigan University, Fisheries and Oceans Canada, and the Institute for Great Lakes Research collected two hundred and forty-six specimens from rivers in Michigan and Ontario. For each specimen, a preliminary identification was made, shell measurements and foot colour (orange or white) were documented, and photos of the left shell valve were taken. A genetic sample was taken from 133 specimens.
Research found that 82% of field identifications were accurate. Using the shell shape and twenty-one landmarks on the left shell valve, the identification accuracy increased to 99% when compared to the DNA barcoding. Using foot colour alone to assign species identification was only 77% accurate.
From this work, a new DNA-confirmed morphometric database will be available to aid malacologists (scientists who study molluscs which includes mussels, snails, clams, octopus and squid) in the Great Lakes region to differentiate between Round Pigtoe and Wabash Pigtoe.
Why is the Round Pigtoe a Species at Risk?
According to the 2004 COSEWIC assessment report:
Species limited to a small area of occupancy in the Lake St. Clair and three watersheds in southern Ontario with continuing declines in habitat area, extent and quality. Threats include urban, industrial and agricultural development and irreversible impacts from zebra mussels in Lake St. Clair, with potential threats from introduction of zebra mussels in impoundments in the Sydenham River.”