the killer shark
Sharks are a group of branchlike 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, as well as other Chondrichthyes such as the holocephalid eugenedontidans.
Under this broader definition, the earliest known
sharks date back to more than 420 million years ago. Acanthodians are 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 over
500 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 (40 ft) in length. Sharks are found in all seas and are
common 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 be found in both seawater and freshwater.Sharks
have a covering of dermal denticles that protects their skin from damage and
parasites in addition to improving their fluid dynamics. They have numerous
sets of replaceable teeth.
Well-known species such as the great white shark,
tiger shark, blue shark, mako shark, thresher shark, and hammerhead shark are
apex predators—organisms at the top of their underwater food chain. Many shark
populations are threatened by human activities.
Unlike bony fish, sharks have a complex dermal
corset made of flexible collagenous fibers and arranged as a helical network
surrounding their body. This works as an outer skeleton, providing attachment
for their swimming muscles and thus saving energy. Their dermal teeth give them
hydrodynamic advantages as they reduce turbulence when swimming.
Tails
Tails provide thrust, making speed and
acceleration dependent on tail shape. Caudal fin shapes vary considerably
between shark species, due to their evolution in separate environments. Sharks
possess a heterocercal caudal fin in which the dorsal portion is usually
noticeably larger than the ventral portion. This is because the shark's
vertebral column extends into that dorsal portion, providing a greater surface
area for muscle attachment. This allows more efficient locomotion among these
negatively buoyant cartilaginous fish. By contrast, most bony fish possess a
homocercal caudal fin.
Tiger sharks have a large upper lobe, which
allows for slow cruising and sudden bursts of speed. The tiger shark must be
able to twist and turn in the water easily when hunting to support its varied
diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel
and herring, has a large lower lobe to help it keep pace with its fast-swimming
prey. Other tail adaptations help sharks catch prey more directly, such as the
thresher shark's usage of its powerful, elongated upper lobe to stun fish and
squid.
Like other fish, sharks extract oxygen from
seawater as it passes over their gills. Unlike other fish, shark gill slits are
not covered, but lie in a row behind the head. A modified slit called a
spiracle lies just behind the eye, which assists the shark with taking in water
during respiration and plays a major role in bottom–dwelling sharks. Spiracles
are reduced or missing in active pelagic sharks. While the shark is moving,
water passes through the mouth and over the gills in a process known as
"ram ventilation". While at rest, most sharks pump water over their
gills to ensure a constant supply of oxygenated water. A small number of
species have lost the ability to pump water through their gills and must swim
without rest. These species are obligate ram ventilators and would presumably
asphyxiate if unable to move. Obligate ram ventilation is also true of some
pelagic bony fish species
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