Fish are, by far, the most representative group within all vertebrates. With more than 500 million years of evolution (Ordovicic Period), presently there are more than 25 000 species known, divided in two major groups:

  Bony fish (Class Osteichthyes – Ex: Atlantic Sailfish)
  Cartilaginous fish (Class Chondrichthyes – sharks, rays and chimaeras).

Throughout their life history, fish have evolved a series of adaptations that enabled them to occupy almost every aquatic ecosystem on earth. The shape of their bodies and fins, as well as the number of the latter, are a really good example of how they adapted to very distinct environments.
Despite their body shape differences, they all have several characteristics in common: They are cold-blooded (ectothermic), i.e., their body temperature depends directly on the surrounding environment; They breathe through gills - one of the most efficient respiratory organs in the Animal Kingdom; and they have an extra sense – the lateral line which detects movements and vibrations in the surrounding water.