Sharks are a group of fish characterized by a cartilaginous skeletal system, 5 to seven gill slits behind the head, and also pectoral fins that are not integrated to the head. Modern sharks are categorized within the clade Selachimorpha (or Selachii) and are the sister group to the rays. Nevertheless, the term "shark" has actually also been utilized for extinct members of the subdivision Elasmobranchii outside the Selachimorpha, such as Cladoselache as well as Xenacanthus, along with other Chondrichthyes such as the holocephalid eugenedontidans. Under this wider definition, the earliest well-known sharks go back to more than 420 million years ago. Acanthodians are typically described as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage bring about cartilaginous fish all at once.
Since then, sharks have actually diversified right into over 500 species. They range in dimension from the small dwarf lanternshark (Etmopterus perryi), a deep sea types of only 17 centimetres (6.7 in) in size, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches roughly 12 metres (39 ft) in size. Sharks are found in all seas and are common to midsts of 2,000 metres (6,600 ft). They usually do not live in freshwater although there are a couple of known exemptions, such as the bull shark and also the river shark, which can survive and be found in both salt water and also freshwater. They take a breath through 5 to 7 gill slits. Sharks have a covering of dermal denticles that shields their skin from damage and bloodsuckers in addition to enhancing their fluid dynamics. They have numerous sets of changeable teeth.
Widely known types such as the fantastic white shark, tiger shark, blue shark, mako shark, and also the hammerhead shark are pinnacle predators-- microorganisms on top of their undersea food chain. Several shark populaces are threatened by human activities.
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