This award will support collaborative research between Dr. Thomas J. Hilbish, University of South Carolina, and Dr. Brian Bayne, Institute for Marine Environmental Research, Plymouth, England. The objective of the research is to investigate the genetic basis of physiological variation in two species of the mussel Mytilus in an area of apparent hybridization in southwest England. The research represents a unique opportunity to analyze the behavior of potential gene complexes in traits known to be important to the survival of an organism in its natural environment. In addition the work should shed light on the dynamics of gene flow in an area of species overlap. Genetic and physiological differentiation occurred between allopatric populations of Mytilus during the Pleistocene. One taxon occurs predominantely in cold-water, while the other is a warm-water form. In southwest England the two taxa hybridize and certain populations provide an array of genotypic combinations that will allow the investigators to analyze the genetics of physiological differentiation of the two taxa. The investigators will utilize physiological traits that may explain the differences in biogeography of the two taxa. This project will benefit from Dr. Bayne's expertise in evolutionary physiology of the Dr. Hilbish's background in physiological-evolutionary genetics.