Change in Thermal Metabolic Rate Reaction Norms of Daphnia in Response to Rearing Temperature

Abstract

Temperature is an important environmental factor that affects the distribution of organisms. It affects the biochemical and physiological processes which are the basis of life. The metabolic cold adaptation (MCA) hypothesis predicts an increase in the metabolic rate of ectotherms from cold environments compared with their more temperate counterparts. This hypothesis is one of the most controversial in ecophysiology. MCA is an example of counter gradient variation, a geographical pattern of genotypes where genetic influences on a trait oppose environmental influences. Environmental gradients are common in nature and are considered to have major effects on intraspecific variation patterns. In this study, I tested the MCA hypothesis at an intraspecific level. As a study model I utilized the water flea, Daphnia magna. These are planktonic crustaceans, common in lakes and ponds, and have a wide geographical and thermal distribution. I tested the MCA hypothesis in a laboratory experiment in which oxygen consumption of water fleas reared at three different temperatures for one year (12 ? 35 generations), were measured at three different experimental temperatures. My results show that the animals from the coldest rearing temperature (10 °C) had the highest metabolism at all three experimental temperatures (10, 17 and 25 °C), compared with animals reared at higher temperatures (17 and 25 °C). Elevated metabolism in animals from cold environment is consistent with the metabolic cold adaptation hypothesis. The present study does however not provide conclusive evidence that clone-specific variation in thermal performance has a genetic basis. Organisms may adjust their thermal reaction norms as a response to the thermal regime in which they live through three mechanisms: acclimation, epigenetic effects and evolution, and these could not be distinguished in experimental design used here.