2.1 IntroductionFish are a wide and heterogeneous group of vertebrates that comprise about 40,000 species, divided into three classes: Agnatha (jawless fish represented by hagfish and lampreys), Chondrichthyes (cartilaginous fish comprising sharks, rays, and skates), and Osteichthyes (bony fish). Fish heterogeneity is based on many aspects of their biology and habitats. There are fish species ad..
