Water is a "living tissue" influenced by chemical, biological and physical factors that, in turn, are influenced by local and climatic factors. Fish have to adjust physiologically to these alterations in habitat to survive. Physiological adaptation is a dynamic and never-ending process that has resulted in myriad fish groups adapted to the vast environmental diversity which exists on Earth. Moreover, adaptively modified organisms acquire greater ability to exploit the full range of natural environments, by adopting new modes of life in many situations. This book is a journey through fish adaptations.