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).

Fish were the first group of animals with skeletons to appear on Earth and without doubt constitute the largest group of vertebrates (Burnie, 2003). At least 26,800 fish species have been described worldwide (Nelson 2006). Currently, about 200 new species are still being described every year, so the number of valid species could reach 30,000 or 35,000 due to the fact that the areas under study are poorly described and also due to the existence of new and improved methodologies (Eschmeyer, 2004).