Dreissena Impacts on Unionidae: General Trends in North America and Europe and Recent Findings from Lake Erie

The continued invasion of zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) in North America and Europe has threatened the survival of native unionid mussels. We used data from multiple waterbodies in Europe and North America to test if the impact of Dreissena on unionids depends on densities of dreissenids in a waterbody, time since invasion, and Dreissena species. We found an overall trend for increase of attached dreissenid weight with unionid host’s size during the first 10 years after Dreissena invasion, however, this adverse impact reduces beyond 10 years after the recorded invasion. We also found that while impacts of zebra mussels on unionids are well described, there is little comparable data for quagga mussels. Considering that the overall dreissenid density in Lake Erie has declined over the last decade, zebra mussels have been almost completely replaced by quagga mussels, and the fact that quagga mussels have weaker attachment strength, we hypothesized that the adverse impact of dreissenids on unionids is now less than the early stages of the invasion. We conducted extensive surveys of unionids in lakes Erie and St. Clair in 2011, and recorded the number, weight, and species of dreissenids attached to unionids shells. Confirming our hypothesis, most of the unionids found were free of dreissenids, and infested unionids had fewer attached dreissenid mussels than in the early 1990s. Despite the quagga mussels’ lake-wide dominance, zebra mussels were more often found on unionids, and their number and weight per host unionid were higher than those of quagga mussels.