nature | shark | ocean | under water | beautiful marine creatures | relaxing scenery | ????????????️

Just a minute :

Shark
Superorder of predatory cartilaginous fish

For other uses, see Shark (disambiguation).
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. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. 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. Notable examples of improper classification include Cladoselache, Xenacanthus, and various other members of the Chondrichthyes class like the holocephalid eugenedontidans.

Quick Facts Scientific classification, Orders ...
Under this broader definition, the earliest known sharks date back to more than 420 million years ago. 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 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. Sharks are found in all seas and are common to depths up to 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 be found in both seawater and freshwater. 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.

Several species are apex predators, which are organisms that are at the top of their food chain. Select examples include the tiger shark, blue shark, great white shark, mako shark, thresher shark, and hammerhead shark.

Sharks are caught by humans for shark meat or shark fin soup. Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from overfishing.

Etymology
Until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in several species termed "dogfish," or the porbeagle.

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. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour.

A now disproven[original research?] theory is that it derives from the Yucatec Maya word xook (pronounced [ʃoːk]), meaning 'shark'. Evidence 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. However, the Middle English Dictionary records an isolated occurrence of the word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology.[original research?]

Note : this video is created with computer

Like | comment | share | subscribe

#Nature #shark #ocean #beautifulmarinelife #relaxing #photography #beautifulscenery #relaxing