Contraintes écophysiologiques de la distribution d'une espèce : divergence parmi les populations sympatriques de Dreissena polymorpha (Pallas) et de D. bugensis (Andrusov) dans l'estuaire et du fleuve Saint-Laurent

Authors: Casper, Andrew F
Advisor: Johnson, Ladd Erik
Abstract: Spatial differences in environmental conditions can lead to plastic physiological responses in many organisms. Yet stable but divergent environmental conditions over multiple generations can produce intrinsic local differences in an organism’s physiology. The goal of this research is to determine whether a contiguous population of adult zebra and quagga mussel (Dreissena polymorpha and D. bugensis) in a stable mosaic of environmental constraints has developed divergent traits and if so are they plastic or irreversible adjustments. Comparison among contrasting St. Lawrence River water-masses found population differentiation in condition (tissue glycogen content, RNA/DNA ratio of tissue as well as shell strength and mass. Though the soft tissue responses of zebra and quagga mussels were similar, the lighter shell of the quagga mussels appears to be an advantage where the two species co-occur. Field sampling and laboratory experiments show that the downstream distribution of zebra mussels is constrained by the tidal intrusion of salinity, but counter-intuitively that the animals at this limit were actually in better condition than those upriver. To explore whether the observed population differentiation represents plasticity or alternatively intrinsic local differentiation, reciprocal transplants of adult mussels from the fluvial estuary and the Great Lakes water masses 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 the riverine environment where they coexist, quagga mussels grew faster than zebra mussels, supporting observations that they are displacing zebra mussels throughout the Great Lakes – St. Lawrence system. Yet quagga mussels did not perform as well in the estuarine environment suggesting that their influence will be limited to the strictly freshwater unless they can adjust physiologically as zebra mussels did. Comparisons of shell growth between summer and winter transplants indicated that summer is more productive and that quagga mussels grow faster than zebra mussels. Yet the summer growth rates of the two species measured at the same site in consecutive years reversed, indicating both spatial and temporal components to growth and production.
Document Type: Thèse de doctorat
Issue Date: 2007
Open Access Date: 12 April 2018
Grantor: Université Laval
Collection:Thèses et mémoires

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