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Abstract. Dreissenid bivalves, Dreissena polymorpha (zebra mussel) and Dreissena bugensis (quagga mussel) are biofouling species that invaded the Great Lakes region of North America from source populations in Europe in the 1980s. Initially, D. polymorpha spread faster and farther; however, D. bugensis have recently displaced D. polymorpha in many areas of the Great Lakes and was the first to be found west of the Continental Divide. Early detection of dreissenids is important in anticipating and preventing potentially high economic impacts. To study population dynamics and to enhance detection methods, we assessed ''spawnability'' using a serotonin bioassay and developed a new, sensitive, multiplex PCR method to identify veligers and verify adult species. Contrasting riverine populations were identified in the Saginaw River (100% D. polymorpha) and the Detroit River at Belle Isle (100% D. bugensis in 2010), and mixed populations of mussels (10% to ;50% D. polymorpha) were found in Saginaw Bay, Lake Huron. In 1994, when the Detroit River population at Belle Isle was virtually all D. polymorpha (Ram et al. 1996), spawning could not be induced by serotonin until late May, and peak spawnability did not occur until early June. In 2010, D. bugensis at the same site could be induced to spawn in the first week of April, and reached near maximal spawning intensity by mid-May. In 2010, Detroit River veligers were first observed in April and, by PCR species-specific detection, were 100% D. bugensis. Veligers changed to a mixed population of both species later in May and rose to a peak, mixed population in early June. These experiments demonstrate a quantitative, species-specific detection of dreissenid veligers, and lay the groundwork for determining the role of early reproduction and other mechanisms in mediating the displacement of one species by a closely related ''cousin.''