The Great Lakes region has come to be occupied by at least two species of Dreissena, the zebra mussel, D. polymorpha and the quagga mussel, D. bugensis. The species are distinguished on the basis of shell morphology and electrophoretic patterns. Comparisons of the posterior mantle regions reveals differences between the two species in the extent of sensory papillae above the exhalant siphon and the point of union of the middle mantle fold.