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 clade Selachimorpha (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 and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.
Since that time, sharks have diversified into over 470 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 (39 ft). Despite its size, the whale shark feeds only on plankton, squid, and small fish by filter feeding. Sharks are found in all seas and are common down 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 in both seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They also have several sets of replaceable teeth.
Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Their predatory skill fascinates and frightens humans, even though their survival is threatened by human-related activities.
Other articles related to "shark, sharks":
... In 1991 South Africa was the first country in the world to declare Great White sharks a legally protected species ... Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000 ... Approximately 64,695 Pounds of Shark Fins ...
... The Season 1, Episode 4 sketch, titled "Jaws II," featured the "Land Shark," a cunning urban predator (voiced by Chevy Chase) ... the Land Shark is considered the cleverest of all sharks ... Unlike the Great White shark, which tends to inhabit the waters and harbors of recreational beach areas, the Land Shark may strike at any place, any time ...
... The Land Shark (also land shark, landshark, LandShark) was a recurring character from the sketch comedy television series Saturday Night Live ... the subsequent hysteria over purported shark sightings ...
... Shark Island is an island in Sydney Harbour, Australia ... The name Shark Island is from its shape, which is claimed to resemble a shark ... Just north of the northern tip of the island is Shark Island Light, an active pile lighthouse which was built in 1913 ...
... The Bala shark, Balantiocheilos melanopterus, also known as the tricolor shark, silver shark, or shark minnow, is a fish species of the family ... This species is not a true shark, but is commonly called a "shark" because of its torpedo-shaped body and large fins ...
Famous quotes containing the word shark:
“They will tell you tough stories of sharks all over the Cape, which I do not presume to doubt utterly,how they will sometimes upset a boat, or tear it in pieces, to get at the man in it. I can easily believe in the undertow, but I have no doubt that one shark in a dozen years is enough to keep up the reputation of a beach a hundred miles long.”
—Henry David Thoreau (18171862)
“I have often told you that I am that little fish who swims about under a shark and, I believe, lives indelicately on its offal. Anyway, that is the way I am. Life moves over me in a vast black shadow and I swallow whatever it drops with relish, having learned in a very hard school that one cannot be both a parasite and enjoy self-nourishment without moving in worlds too fantastic for even my disordered imagination to people with meaning.”
—Zelda Fitzgerald (19001948)
“Oh, the shark has pretty teeth, dear
And he shows them pearly white
Just a jackknife has Macheath, dear
And he keeps it out of sight.”
—Bertolt Brecht (18981956)