Fishes

• Fishes are branchial-breathing, aquatic vertebrates, that is they do not have lungs and obtain the oxygen dissolved in the water through branchial filaments or gills. There is only a small group of freshwater fish that have lungs.
• There are more than 25,000 known species, with the group of the vertebrates being the most numerous today. They are also the oldest, as fossils of animals that lived more than 500 million years ago that have been found.
• They vary considerably in size, from some species of the Gobiidae family, the smallest known vertebrates, which can measure less than 8 mm long, up to the whale shark (Rhicodon typus), which can reach a length of 12 m.
• They live in all kinds of environment: from the surface of the water to depths of 8,500 m; in freshwater (rivers and lakes); in the brackish water of estuaries and coastal lagoons; in seawater; and even in waters with salinity in excess of 60 ppm (more than 60 g of salt per liter of water), known as hyper-saline waters.
• Fish are the most important renewable resource of the planet's waters.
• They are divided into two large groups: the cartilaginous fishes, so called because they have no bones as such, as their skeleton is formed almost exclusively by a relatively soft tissue that goes by the name of cartilage; and the bony fishes, whose skeleton is formed by real bones of osseous tissue.
• The group of the cartilaginous fishes includes sharks, skates, rays, dogfish, etc., and the remaining fish belong to the group of the bony fishes.

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