Sharks
Sharks are a group of elasmobranch fishcharacterized by a cartilaginousskeleton, five to seven-gill slit son the sides of the head, and pectoralfins that are not fused to the head. Modern sharks are classified within the thecladeSelachimorpha(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 asCladoselacheandXenacanthus, as well as other Chondrichthyes such as the holocephalideugenedontidans.
Under this broader definition, the earliest known sharks date back to more than 420 million years ago.[3]Acanthodiansare 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 over500 species. They range in size from the small dwarflantern shark(Etmopterus perryi), a deep-sea species of only 17 centimetres (6.7 in) in length, to the whaleshark(Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40ft) in length.[4]Sharks are found in all seas and are common to depths of 2,000 metres (6,600ft). They generally do not live in freshwater although there are a few known exceptions, such as the bullshark and the rivershark, which can be found in both seawater and freshwater.[5]Sharks have a covering of dermaldenticles that protects their skin from damage and parasites inaddition to improving their fluiddynamics. They have numerous sets of replaceable teeth.[6]
Well-known species such as the tigershark, blue shark, great white shark, mako shark, thresher shark, and hammerheadshark are apexpredators—organisms at the top of their underwater foodchain. Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from overfishing
By
SAI SHIV SHANKER R
Grade 6C
Orchids The International School Vivekananda
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