Zebra mussel impacts on unionids: A synthesis of trends in North America and Europe

This chapter examines impacts of exotic zebra mussel (Dreissena polymorpha) infestation on unionids from water bodies in North America, Belarus, and Ireland over the past two centuries. A variety of methods were used to assess impacts ranging from short-term studies of multiple water bodies to extensive multi-year studies of a single water body. In general, there was a strong positive linear relationship between numbers of zebra mussels per unionid and zebra mussel density. However, datasets indicated that a high percentage of unionids may be infested with zebra mussels even when zebra mussel density in a water body was low. During the first 10 years subsequent to a zebra mussel invasion, there was an overall trend for increased weight of zebra mussels per unionid with increased unionid size, but this trend decreased 10 years after the initial invasion. We discuss possible mechanisms for coexistence of zebra mussels and unionids and make recommendations for management options and unification of research methods. Finally, we identify research priorities that will provide a better understanding of zebra mussel- unionid coexistence and hence aid in the development of unionid survival and management strategies during the initial stages of invasion by dreissenids or other byssate exotic bivalves.