Fish comprise over 28,000 species, with a remarkable variability in morphology, physiology and environmental adaptation. Due to the huge diversity of their habitats, fish make an excellent model to depict the interplay of morphological, physiological and biochemical aspects and are often used to study adaptive process to new environment. In this class, we will present a window to an array of adaptation of fish inhabitant which enable them to subsist and survive in the diverse environment. The topics cover fish in environmental conditions such as intertidal habitat, caves, Antarctic, tropical coral reef and deep waters .The chapters also discuss mitochondrial functions in the cold circadian rhythm, endocrinology of migratory fish life cycles.