Shark



Fossil range: Silurian–Recent

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Grey reef shark (Carcharhinus amblyrhynchos)

Scientific classification

Kingdom: Animalia



Phylum: Chordata



Class: Chondrichthyes



Subclass: Elasmobranchii



Superorder: Selachimorpha



Orders

Carcharhiniformes

Heterodontiformes

Hexanchiformes

Lamniformes

Orectolobiformes

Pristiophoriformes

Squaliformes

Squatiniformes

† Symmoriida

† Cladoselachiformes

† Xenacanthida (Xenacantiformes)

† Eugeneodontida

† Hybodontiformes





Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs.[1]



Since that time, sharks have diversified into 440 species, ranging 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, which reaches approximately 12 metres (39 ft 4 in) and which feeds only on plankton, squid, and small fish through filter feeding. Sharks are found in all seas and are common down to depths of 2,000 metres (6,562 ft). They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater.[2] They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics so the shark can move faster. They have several sets of replaceable teeth.[3]



Well-known species such as the great white shark, Tiger shark, and the hammerhead are apex predators, at the top of the underwater food chain. Their extraordinary skills as predators fascinate and frighten humans, even as their survival is under serious threat from fishing and other human activities.


Until the 16th century,[4] sharks were known to mariners as "sea dogs".[5] According to the OED the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and used the word to refer to the large sharks of the Caribbean Sea, and later as a general term for all sharks. It has also been suggested to be derived from the Yucatec Maya word for shark, xok, pronounced 'shok'.[6]