Selachimorpha is a superordinate predatory cartilage fish, strong jawbone and medium-large, whose members are commonly known as shark or fish.
The etymology of the term is Greek, from σελάχιος seláchios, "cartilage" (zoology applies to those cartilaginous fish with a fusiform or depressed body, a hernia tail and almost semicircular mouth) and μορφή morphḗ, "form".
This grouping includes more than 500 species, for which breathing occurs through the use of a variable number between five and seven of the squirrels. The peculiar characteristic of the shark body is that it is covered with dermal balms that protect the skin from damage due to parasites and improves hydrodynamics.
These fish are also equipped with various sets of spare teeth, which intervene in replacement of lost or damaged ones.
The size of the sharks varies from those of the tiny dwarf shark lantern (Etmopterus perryi), a species that lives deep and measures only 17 cm in length in the male, and 20 in the female, whale shark (Rhincodon typus), The biggest fish ever.
All sharks are carnivores and most of them feed on fish and other marine animals, unlike the larger specimens, such as the aforementioned whale shark, which feed primarily on plankton.
It is generally imagined that sharks live only in salt water, but the Zambesi shark (Carcharhinus leucas) is only the best known of a fairly large number of species of freshwater sharks swimming in both salt water and freshwater , As well as that of river deltas.
As a result of non-injured attacks on human beings, operated by some species in particular, sharks have gained only partially justified reputation for being dangerous.
For this reason, besides the fact that their meat is considered precious in many Asian states, several species of shark are subjected to intensive fishing which puts them at risk of extinction.