- published: 23 Sep 2013
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Sharks are a group of fishes 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 suborder 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 400 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 that 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.
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I went to the marina saw a man in a speedo scratching himself and staring at sharks his shirt was unbuttoned long grey chest hairs and the sharks just kept swimming more bored than annoyed and the man on the tape says that they'd suffocate if the sharks would stop swimming in circles even though there's no place for them to go in this aquarium they just keep swimming in circles they're such noble creatures the marina's main feature and they glide through captivity with oceans more dignity than we'll ever know judging by that speedo and the man on the tape says that they'd suffocate if the sharks would stop swimming in circles even though there's no place for them to go in this aquarium they just keep swimming in circles