Fish (Class: Fishes) are aquatic vertebrates, covered in scales and endowed with specific adaptations for swimming, such as fins. They breathe primarily through gills located in the recesses of the pharynx. Locomotion is based on a hydrodynamic shape, with lateral movements of the body. Fish are very diverse in their forms, the habitats where they live and in their biology. There is a great variety of species and, for this reason, understanding their evolutionary history and establishment of a proper classification are complex challenges.
Although commonly referred to as a single biological group within the vertebrates, fish are a group so heterogeneous that some of them are evolutionarily more closely related to mammals than other groups of fish. This heterogeneity is due to the fact that they come from different lineages (paraphyletic group) (Nelson, 2006).
Various classifications include five classes: lancelets (Appendicularia), lampreys (Petromyzotida), hagfish (Myxini), sharks and rays (Chondrichthyes) and sardines and perch (Actinopterygii).