CASPER, AF*; JOHNSON, LE; University of South Florida St. Petersburg: Patterns of microevolution and displacement in a rapidly expanding species: Insights from comparative physiological ecology of zebra and quagga mussels

This research explores whether a contiguous population of adult zebra and quagga mussels (Dreissena polymorpha and D. bugensis) in a stable, divergent mosaic of river environments has developed localized physiological differences and, if so, whether these are plastic or irreversible responses. Previous field and laboratory experiments show that the riverine zebra mussel distributions are constrained by salinity. However zebra mussels from the estuarine transition zone of the St. Lawrence were in better condition (biomass, RNA/DNA ratio, glycogen content) than those from upriver. To explore whether this represents plasticity or alternatively local differentiation, reciprocal transplants of adult mussels from the estuarine transition zone and the river were conducted. Results indicate that shell growth depends on source population, independent of the environment (river source mussels > estuary source mussels). In contrast, RNA/DNA ratio, a short-term measure of tissue condition and growth, was significantly higher for estuary mussels in their source environment. Methodologically this confirms that shell growth is not always an accurately accurate proxy for changes in biomass. Evolutionarily it suggests the differentiation in shell growth and strength in zebra mussels has a clear source population component. Ecologically the second species of dreissenid to arrive, the quagga mussel, appears to have a physiological advantage and is displacing the zebra mussel in river environments. However this is not true in the estuarine transition zone where zebra mussels appear to have made an irreversible physiological adjustment that currently allows them to maintain their dominance.