An inverse relation between the capacity for growth (i.e. maximum growth potential) and length of the growing season over a latitudinal gradient has recently been discovered in fishes. This intriguing form of intraspecific "countergradient variation" may represent a widespread adaptive strategy for coping with seasonal environments. Using primarily the Atlantic silverside, Menidia menidia, as a model organism, this project will evaluate the selective forces, evolutionary responses, and physiological mechanisms underlying latitudinal adaptation. The premise is that rapid growth in the north is an evolutionary response to an intensely seasonal environment: i.e., one where large size is necessary to survive long winters, but where only a brief growing season is available in which to attain large size. The theoretical prediction that faster growth can occur only at the expense of some other life history function will also be tested. Experiments on laboratory stocks of silversides from different latitudes are designed to: 1) determine the influence of body size, lipid storage and genotype on overwinter survival; and 2) test for genetic variation in energy acquisition and allocation to various metabolic functions such as food processing, growth, swimming and reproductive performance. Results will have implications for our knowledge of life history theory, resource management, and the response of marine organisms to climate change.
