Fishes inhabit most aquatic environments on our planet, living and reproducing in extremes of temperature, salinity, pH, pressure, oxygen levels, and food availability. Adaptation to many of these situations involves enhancements or suppression of existing metabolic pathways coupled with membraneadaptation to maintain membrane properties via “homeoviscous adaptation.” The study of metabolism involves not only an understanding of the biochemical pathways consuming or producing high-energy compounds but also comprehension of their compartmentation at the subcellular, cellular, tissue, and organ levels. The transport and communication between these compartments involve membranes with their associated protein and lipid components. Many metabolic processes span more than one subcellular compartment and relatively few studies have focused on how these are related. Studies of plasma turnover of metabolites do provide some insights into the overall metabolism of compounds but there are only a few of these available for Œshes.