ABSTRACT
This chapter focuses on the group of eurythermal fishes and the changes that occur in tissue ultrastructure during adaptation to altered body temperature. The adaptive import to ectotherms at cold body temperature where rates of cardiovascular adjustment may lag behind alterations in activity and cellular oxygen demand could be considerable. Cytosolic diffusion coefficients for several biologically important molecules, indeed, are highly dependent upon temperature. Both cold-induced responses may contribute to offsetting depressed rates of micromolecular diffusion between cytoplasmic and mitochondrial compartments. One of the most consistently observed ultrastructural changes is that acclimation to cold results in a dramatic increase in the mitochondrial population of skeletal muscle fibers. Any increase detected for the cold-acclimated cellular organization should predict the magnitude of potential compensation to any combination of decreased perfusion and oxygen diffusion coefficients encountered at cold body temperature. A clear relationship between cold body temperature and myoglobin concentration of oxidative muscle has yet to emerge.
