Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the cladeSelachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache andXenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.[1] Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole.
Since then, sharks have diversified into
over 500 species. They range in size from the small
dwarf lanternshark (
Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the
whale shark(
Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the
bull shark and the
river shark, which can survive and be found in both seawater and freshwater.
[2] Sharks have a covering of
dermal denticles that protects their skin from damage and
parasites in addition to improving their
fluid dynamics. They have numerous sets of replaceable teeth.
[3]
(copied from wikipedia)
The shark is friendly if you don´t hurt or bother him
