Living organisms are endowed with ingenious adaptive mechanisms to cope with the adversities of special environments. Due to the vast 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 processes to a new environment. The book covers fish in diverse environmental conditions such as alkaline environments, caves, Antarctic, ice cold lakes, tropical coral reefs, and deep waters. The chapters also discuss mitochondrial functions in the cold, circadian rhythms, endocrinology of migratory fish life cycle and fish muscle function.The topics have been selected in order to present a window to an array of adaptations of aquatic inhabitants which enable them to subsist and survive in the uncommon, and often hostile, external environment.