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 back to more than 420 million years ago.

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 (39 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 survive and be found 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 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. 
Many shark populations are threatened by human activities.

------------------------------------
Physiology

Buoyancy
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. 
Instead, sharks rely on a large liver filled with oil that contains squalene, 
and their cartilage, which is about half the normal density of bone. 
Their liver constitutes up to 30% of their total body mass. The liver's effectiveness is limited, 
so sharks employ dynamic lift to maintain depth when not swimming. 
Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. 
Most sharks need to constantly swim in order to breathe and cannot 
sleep very long without sinking (if at all). 
However, certain species, like the nurse shark, 
are capable of pumping water across their gills, allowing them to rest on the ocean bottom.

Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. 
Researchers use this condition to handle sharks safely.

Respiration
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.

The respiration and circulation process begins when deoxygenated blood travels 
to the shark's two-chambered heart. 
Here the shark pumps blood to its gills via the ventral aorta artery where 
it branches into afferent brachial arteries. 
Reoxygenation takes place in the gills and the reoxygenated blood flows 
into the efferent brachial arteries, 
which come together to form the dorsal aorta. 
The blood flows from the dorsal aorta throughout the body. 
The deoxygenated blood from the body then flows through the posterior cardinal veins 
and enters the posterior cardinal sinuses. 
From there blood enters the heart ventricle and the cycle repeats.

Thermoregulation
Most sharks are "cold-blooded" or, more precisely, poikilothermic, 
meaning that their internal body temperature matches that of their ambient environment. 
Members of the family Lamnidae (such as the shortfin mako shark and the great white shark) 
are homeothermic and maintain a higher body temperature than the surrounding water. 
In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, 
which the body retains via a countercurrent exchange mechanism 
by a system of blood vessels called the rete mirabile ("miraculous net"). 
The common thresher shark has a similar mechanism for maintaining an elevated body temperature, 
which is thought to have evolved independently[not in citation given].

Osmoregulation
In contrast to bony fish, with the exception of the coelacanth, 
the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their 
marine environments because of the high concentration of urea (up to 2.5%[38]) 
and trimethylamine N-oxide (TMAO), 
allowing them to be in osmotic balance with the seawater. 
This adaptation prevents most sharks from surviving in freshwater, 
and they are therefore confined to marine environments. A few exceptions exist, 
such as the bull shark, which has developed a way to change its 
kidney function to excrete large amounts of urea. 
When a shark dies, the urea is broken down to ammonia by bacteria, 
causing the dead body to gradually smell strongly of ammonia.

Digestion
Digestion can take a long time. The food moves from the mouth to a J-shaped stomach, 
where it is stored and initial digestion occurs. 
Unwanted items may never get past the stomach, 
and instead the shark either vomits or turns its stomachs inside out 
and ejects unwanted items from its mouth.

One of the biggest differences between the digestive systems of sharks and mammals 
is that sharks have much shorter intestines. 
This short length is achieved by the spiral valve with multiple turns 
within a single short section instead of a long tube-like intestine. 
The valve provides a long surface area, requiring food to circulate 
inside the short gut until fully digested, 
when remaining waste products pass into the cloaca.

------------------------------------
Senses

Smell
Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) 
between the anterior and posterior nasal openings, 
with some species able to detect as little as one part per million of blood in seawater.

Sharks have the ability to determine the direction of a given scent 
based on the timing of scent detection in each nostril. 
This is similar to the method mammals use to determine direction of sound.

They are more attracted to the chemicals found in the intestines of many species, 
and as a result often linger near or in sewage outfalls. 
Some species, such as nurse sharks, 
have external barbels that greatly increase their ability to sense prey.

Sight
Shark eyes are similar to the eyes of other vertebrates, 
including similar lenses, corneas and retinas, 
though their eyesight is well adapted to the marine environment 
with the help of a tissue called tapetum lucidum. 
This tissue is behind the retina and reflects light back to it, 
thereby increasing visibility in the dark waters. 
The effectiveness of the tissue varies, with some sharks having 
stronger nocturnal adaptations. 
Many sharks can contract and dilate their pupils, like humans, 
something no teleost fish can do. 
Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. 
To protect their eyes some species have nictitating membranes. 
This membrane covers the eyes while hunting and when the shark is being attacked. 
However, some species, including the great white shark (Carcharodon carcharias), 
do not have this membrane, but instead roll their eyes 
backwards to protect them when striking prey. 
The importance of sight in shark hunting behavior is debated. 
Some believe that electro- and chemoreception are more significant, 
while others point to the nictating membrane as evidence that sight is important. 
Presumably, the shark would not protect its eyes were they unimportant. 
The use of sight probably varies with species and water conditions. 
The shark's field of vision can swap between monocular and stereoscopic at any time. 
A micro-spectrophotometry study of 17 species of shark found 10 had only 
rod photoreceptors and no cone cells in their retinas 
giving them good night vision while making them colorblind. 
The remaining seven species had in addition to rods a single type of 
cone photoreceptor sensitive to green and, seeing only in shades of grey and green, 
are believed to be effectively colorblind. 
The study indicates that an object's contrast against the background, 
rather than colour, may be more important for object detection.

Hearing
Although it is hard to test the hearing of sharks, 
they may have a sharp sense of hearing and can possibly hear prey from many miles away.
A small opening on each side of their heads (not the spiracle) 
leads directly into the inner ear through a thin channel. 
The lateral line shows a similar arrangement, 
and is open to the environment via a series of openings called lateral line pores. 
This is a reminder of the common origin of these two vibration- and sound-detecting organs 
that are grouped together as the acoustico-lateralis system. 
In bony fish and tetrapods the external opening into the inner ear has been lost.

Electroreception
The ampullae of Lorenzini are the electroreceptor organs. 
They number in the hundreds to thousands. 
Sharks use the ampullae of Lorenzini to detect the electromagnetic 
fields that all living things produce. 
This helps sharks (particularly the hammerhead shark) find prey. 
The shark has the greatest electrical sensitivity of any animal. 
Sharks find prey hidden in sand by detecting the electric fields they produce. 
Ocean currents moving in the magnetic field of the 
Earth also generate electric fields that sharks can use for orientation and possibly navigation.

Lateral line
This system is found in most fish, including sharks. It detects motion or vibrations in water. 
The shark can sense frequencies in the range of 25 to 50 Hz.

------------------------------------
Life history

Shark lifespans vary by species. Most live 20 to 30 years. 
The spiny dogfish has the longest lifespan at more than 100 years. 
Whale sharks (Rhincodon typus) may also live over 100 years.

Reproduction
Unlike most bony fish, sharks are K-selected reproducers, 
meaning that they produce a small number of well-developed young as 
opposed to a large number of poorly developed young. 
Fecundity in sharks ranges from 2 to over 100 young per reproductive cycle. 
Sharks mature slowly relative to many other fish. 
For example, lemon sharks reach sexual maturity at around age 13–15.

Sexual
Sharks practice internal fertilization. 
The posterior part of a male shark's pelvic fins are 
modified into a pair of intromittent organs called claspers, 
analogous to a mammalian penis, of which one is used to deliver sperm into the female.

Mating has rarely been observed in sharks. 
The smaller catsharks often mate with the male curling around the female. 
In less flexible species the two sharks swim parallel to each other 
while the male inserts a clasper into the female's oviduct. 
Females in many of the larger species have bite marks 
that appear to be a result of a male grasping them 
to maintain position during mating. 
The bite marks may also come from courtship behavior: 
the male may bite the female to show his interest. 
In some species, females have evolved thicker skin to withstand these bites.

Ovoviviparity
Most sharks are ovoviviparous, meaning that the eggs hatch in the oviduct 
within the mother's body and that the egg's yolk and fluids 
secreted by glands in the walls of the oviduct nourishes the embryos. 
The young continue to be nourished by the remnants of 
the yolk and the oviduct's fluids. 
As in viviparity, the young are born alive and fully functional. 
Lamniforme sharks practice oophagy, where the first 
embryos to hatch eat the remaining eggs. 
Taking this a step further, sand tiger shark pups 
cannibalistically consume neighboring embryos. 
The survival strategy for ovoviviparous species is to 
brood the young to a comparatively large size before birth. 
The whale shark is now classified as ovoviviparous rather than oviparous, 
because extrauterine eggs are now thought to have been aborted. 
Most ovoviviparous sharks give birth in sheltered areas, 
including bays, river mouths and shallow reefs. 
They choose such areas for protection from predators 
(mainly other sharks) and the abundance of food. 
Dogfish have the longest known gestation period of any shark, at 18 to 24 months. 
Basking sharks and frilled sharks appear to have even longer gestation periods, 
but accurate data are lacking.

------------------------------------
Behavior
The classic view describes a solitary hunter, ranging the oceans in search of food. 
However, this applies to only a few species. Most live far more social, sedentary, 
benthic lives, and appear likely to have their own distinct personalities. 
Even solitary sharks meet for breeding or at rich hunting grounds, 
which may lead them to cover thousands of miles in a year. 
Shark migration patterns may be even more complex than in birds, 
with many sharks covering entire ocean basins.

Sharks can be highly social, remaining in large schools. 
Sometimes more than 100 scalloped hammerheads congregate 
around seamounts and islands, e.g., in the Gulf of California. 
Cross-species social hierarchies exist. 
For example, oceanic whitetip sharks dominate silky 
sharks of comparable size during feeding.

When approached too closely some sharks perform a threat display. 
This usually consists of exaggerated swimming movements, 
and can vary in intensity according to the threat level.

Speed
In general, sharks swim ("cruise") at an average speed of 8 kilometres per hour (5.0 mph), 
but when feeding or attacking, the average shark can reach 
speeds upwards of 19 kilometres per hour (12 mph). 
The shortfin mako shark, the fastest shark and one of the fastest fish, 
can burst at speeds up to 50 kilometres per hour (31 mph). 
The great white shark is also capable of speed bursts. 
These exceptions may be due to the warm-blooded, 
or homeothermic, nature of these sharks' physiology. 
Sharks can travel 70 to 80 km in a day.

Intelligence
Sharks possess brain-to-body mass ratios that are similar to mammals and birds, 
and have exhibited apparent curiosity and behavior resembling play in the wild.

Sleep
All sharks need to keep water flowing over their gills in order for them to breathe, 
however not all species need to be moving to do this. 
Those that are able to breathe while not swimming breathe 
by using their spiracles to force water over their gills, 
thereby allowing them to extract oxygen from the water. 
It has been recorded that their eyes remain open while 
in this state and actively follow the movements of divers swimming 
around them and as such they are not truly asleep.

Species that do need to swim continuously to breathe go 
through a process known as sleep swimming, 
in which the shark is essentially unconscious. 
It is known from experiments conducted on the spiny dogfish that its spinal cord, 
rather than its brain, coordinates swimming,
so spiny dogfish can continue to swim while sleeping, 
and this also may be the case in larger shark species.

Feeding
Most sharks are carnivorous. 
Basking sharks, whale sharks, and megamouth sharks have independently evolved 
different strategies for filter feeding plankton:
basking sharks practice ram feeding, whale sharks 
use suction to take in plankton and small fishes, 
and megamouth sharks make suction feeding 
more efficient by using the luminescent tissue inside 
of their mouths to attract prey in the deep ocean.
This type of feeding requires gill rakers—long, 
slender filaments that form a very efficient sieve—analogous 
to the baleen plates of the great whales. 
The shark traps the plankton in these filaments 
and swallows from time to time in huge mouthfuls. 
Teeth in these species are comparatively small 
because they are not needed for feeding.

Other highly specialized feeders include cookiecutter sharks, 
which feed on flesh sliced out of other larger fish and marine mammals. 
Cookiecutter teeth are enormous compared to the animal's size. 
The lower teeth are particularly sharp. 
Although they have never been observed feeding, they are believed 
to latch onto their prey and use their thick lips to make a seal, 
twisting their bodies to rip off flesh.

Some seabed–dwelling species are highly effective ambush predators. 
Angel sharks and wobbegongs use camouflage to lie in wait and 
suck prey into their mouths. 
Many benthic sharks feed solely on crustaceans which they 
crush with their flat molariform teeth.

Other sharks feed on squid or fish, which they swallow whole. 
The viper dogfish has teeth it can point outwards to strike and 
capture prey that it then swallows intact. 
The great white and other large predators either swallow small 
prey whole or take huge bites out of large animals. 
Thresher sharks use their long tails to stun shoaling fishes, 
and sawsharks either stir prey from the seabed or 
slash at swimming prey with their tooth-studded rostra.

Many sharks, including the whitetip reef shark are cooperative 
feeders and hunt in packs to herd and capture elusive prey. 
These social sharks are often migratory, 
traveling huge distances around ocean basins in large schools. 
These migrations may be partly necessary to find new food sources.

------------------------------------
Anatomy

Teeth
Shark teeth are embedded in the gums rather than directly affixed to the jaw, 
and are constantly replaced throughout life. 
Multiple rows of replacement teeth grow in a groove on the inside of the jaw 
and steadily move forward in comparison to a conveyor belt; 
some sharks lose 30,000 or more teeth in their lifetime. 
The rate of tooth replacement varies from once every 8 to 10 days to several months. 
In most species, teeth are replaced one at a time as 
opposed to the simultaneous replacement of an entire row, 
which is observed in the cookiecutter shark.

Tooth shape depends on the shark's diet: those that feed on mollusks 
and crustaceans have dense and flattened teeth used for crushing, 
those that feed on fish have needle-like teeth for gripping, and those that feed on 
larger prey such as mammals have pointed lower teeth for gripping and 
triangular upper teeth with serrated edges for cutting. 
The teeth of plankton-feeders such as the basking shark are small and non-functional.

Skeleton
Shark skeletons are very different from those of bony fish and terrestrial vertebrates. 
Sharks and other cartilaginous fish (skates and rays) 
have skeletons made of cartilage and connective tissue. 
Cartilage is flexible and durable, yet is about half the normal density of bone. 
This reduces the skeleton's weight, saving energy. 
Because sharks do not have rib cages, 
they can easily be crushed under their own weight on land.

Jaw
Jaws of sharks, like those of rays and skates, 
are not attached to the cranium. 
The jaw's surface (in comparison to the shark's 
vertebrae and gill arches) needs extra support due to its heavy exposure to 
physical stress and its need for strength. 
It has a layer of tiny hexagonal plates called "tesserae", 
which are crystal blocks of calcium salts arranged as a mosaic. 
This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, 
such as the bull shark, tiger shark, and the great white shark, 
have two to three layers or more, depending on body size. 
The jaws of a large great white shark may have up to five layers. 
In the rostrum (snout), 
the cartilage can be spongy and flexible to absorb the power of impacts.

Fins
Fin skeletons are elongated and supported with soft and 
unsegmented rays named ceratotrichia, filaments of elastic protein 
resembling the horny keratin in hair and feathers. 
Most sharks have eight fins. Sharks can only drift away from 
objects directly in front of them because their fins do not allow 
them to move in the tail-first direction.

Dermal denticles
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.

------------------------------------
Evolution

Evidence for the existence of sharks dates from the Ordovician period, 450–420 million years ago, 
before land vertebrates existed and before many plants had colonized the continents. 
Only scales have been recovered from the first sharks and 
not all paleontologists agree that these are from true sharks, 
suspecting that these scales are actually those of thelodont agnathans. 
The oldest generally accepted shark scales are from about 
420 million years ago, in the Silurian period. 
The first sharks looked very different from modern sharks. 
The majority of modern sharks can be traced back to around 100 million years ago. 
Most fossils are of teeth, often in large numbers. 
Partial skeletons and even complete fossilized remains have been discovered. 
Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, 
which explains the abundant fossils. 
The teeth consist of easily fossilized calcium phosphate, an apatite. 
When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms. 
Preservation requires rapid burial in bottom sediments.

Among the most ancient and primitive sharks is Cladoselache, 
from about 370 million years ago, which has been 
found within Paleozoic strata in Ohio, Kentucky, and Tennessee. 
At that point in Earth's history these rocks 
made up the soft bottom sediments of a large, shallow ocean, 
which stretched across much of North America. 
Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws. 
Its teeth had several pointed cusps, which wore down from use. 
From the small number of teeth found together, it is most likely 
that Cladoselache did not replace its teeth as regularly as modern sharks. 
Its caudal fins had a similar shape to the great white sharks 
and the pelagic shortfin and longfin makos. 
The presence of whole fish arranged tail-first in their stomachs 
suggest that they were fast swimmers with great agility.

Most fossil sharks from about 300 to 150 million years ago 
can be assigned to one of two groups. 
The Xenacanthida was almost exclusive to freshwater environments. 
By the time this group became extinct about 220 million years ago, 
they had spread worldwide. 
The other group, the hybodonts, appeared about 320 million years ago 
and lived mostly in the oceans, but also in freshwater. 
The results of a 2014 study of the gill structure of an unusually well-preserved 
325 million year old fossil suggested that sharks are not "living fossils", 
but rather have evolved more extensively than previously 
thought over the hundreds of millions of years they have been around.

Modern sharks began to appear about 100 million years ago. 
Fossil mackerel shark teeth date to the Early Cretaceous. 
One of the most recently evolved families is the hammerhead shark 
(family Sphyrnidae), which emerged in the Eocene. 
The oldest white shark teeth date from 60 to 66 million years ago, 
around the time of the extinction of the dinosaurs. 
In early white shark evolution there are at least two lineages: 
one lineage is of white sharks with coarsely serrated teeth and 
it probably gave rise to the modern great white shark, 
and another lineage is of white sharks with finely serrated teeth. 
These sharks attained gigantic proportions and include the extinct megatoothed shark, C. megalodon. 
Like most extinct sharks, C. megalodon is also primarily known from its fossil teeth and vertebrae. 
This giant shark reached a total length (TL) of more than 16 metres (52 ft).
C. megalodon may have approached a maxima of 20.3 metres (67 ft) 
in total length and 103 metric tons (114 short tons) in mass. 
Paleontological evidence suggests that this shark was an active predator of large cetaceans.

------------------------------------
I left out some parts of the article in here. full article: http://en.wikipedia.org/wiki/Shark
or visit the shark portal for all shark-related articles: http://en.wikipedia.org/wiki/Portal:Sharks