Moitessier's pea clam Pisidium moitessierianum (Bivalvia, Sphaeriidae): a cryptogenic mollusc in the Great Lakes

Pisidium moitessierianum Paladilhe, 1866, a small pea clam native to Europe , was identified for the first time from the lower Great Lakes basin based on an examination of historical collections of Pisidium performed by V. Ste rki in 1894 and 1903 and new material collected during 1997 and 1998. Durin g recent surveys, P. moitessierianum individuals were found in the St. Clai r River delta, Lake St. Clair and western Lake Erie, but were not detected in the Detroit River or western Lake Ontario. Pisidium moitessierianum was collected on sand, silty sand and mud substrata from water depths ranging b etween 0.6 and 5.4 m. Populations occurred at an average density of 51 ind. m(-2) and included juveniles and adults. All individuals were less than 2. 0 mm in length. We examined the structure of the umbos and hinge, surface s culpture and shape of the shell, and the anatomy of gills, mantle and nephr idia in populations from the lower Great Lakes and Ukrainian inland basins (Dnieper River and Lake Beloye). The results indicated that the Great Lakes ' pea clams match European specimens of P. moitessierianum in these conchol ogical and anatomical characteristics. As with other nonindigenous sphaerii ds in the Great Lakes, P. moitessierianum was likely introduced through shi pping activities into the Great Lakes, possibly as early as the 1890s.