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 subclassElasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under
this broader definition, the earliest known sharks date from more than 420
million years ago.[1]
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.[2] They
breathe through five to seven gill slits.
Sharks have a covering ofdermal denticles that protects their skin from damage
and parasites in
addition to improving their fluid dynamics. They also
have several sets of replaceable teeth.[3]
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.
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