![]() Įvidence for this etymology came from the Oxford English Dictionary, which notes shark first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted " sharke" to refer to the large sharks of the Caribbean Sea. Ī now disproven theory is that it derives from the Yucatec Maya word xook ( pronounced ), meaning 'shark'. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour. The etymology of the word shark is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning 'villain, scoundrel' ( cf. This is still evidential in several species termed " dogfish," or the porbeagle. Until the 16th century, sharks were known to mariners as "sea dogs". Since 1970, shark populations have been reduced by 71%, mostly from overfishing. Many shark populations are threatened by human activities. Sharks are caught by humans for shark meat or shark fin soup. Select examples include the tiger shark, blue shark, great white shark, mako shark, thresher shark, and hammerhead shark. Several species are apex predators, which are organisms that are at the top of their food chain. They have numerous sets of replaceable teeth. ![]() Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. 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 be found in both seawater and freshwater. Sharks are found in all seas and are common to depths up to 2,000 metres (6,600 ft). ![]() They range in size from the small dwarf lanternshark ( Etmopterus perryi), a deep sea species that is 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. Since then, sharks have diversified into over 500 species. 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. Under this broader definition, the earliest known sharks date back to more than 420 million years ago. Notable examples of improper classification include Cladoselache, and various other members of the Chondrichthyes class like the holocephalid eugenedontidans. However, the term "shark" has also been (incorrectly ) used to refer to extinct members of the subclass Elasmobranchii, which are technically outside the Selachimorpha clade. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. Sharks are a group of elasmobranch 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. Clockwise from top left: spiny dogfish, Australian angelshark, whale shark, great white shark, horn shark, frilled shark, scalloped hammerhead and Japanese sawshark representing the orders Squaliformes, Squatiniformes, Orectolobiformes, Lamniformes, Heterodontiformes, Hexanchiformes, Carcharhiniformes and Pristiophoriformes respectively.
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