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Shark

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

   iSharks

                    Fossil range: Early Devonian - Recent

   Oceanic whitetip shark, Carcharhinus longimanus
   Oceanic whitetip shark, Carcharhinus longimanus
                 Scientific classification

   Kingdom:    Animalia
   Phylum:     Chordata
   Subphylum:  Vertebrata
   Class:      Chondrichthyes
   Subclass:   Elasmobranchii
   Superorder: Selachimorpha

                                   Orders

   Carcharhiniformes
   Heterodontiformes
   Hexanchiformes
   Lamniformes
   Orectolobiformes
   Pristiophoriformes
   Squaliformes
   Squatiniformes

   Sharks ( superorder Selachimorpha) are fish with a full cartilaginous
   skeleton and a streamlined body. They respire with the use of five to
   seven gill slits. Sharks have a covering of dermal denticles to protect
   their skin from damage and parasites and to improve fluid dynamics.
   They have replaceable teeth. They are some of the world's most
   misunderstood predators, as they very rarely attack humans unless
   intimidated.

   Sharks include species from the hand-sized pygmy shark, Euprotomicrus
   bispinatus, a deep sea species of only 22 cm in length, to the whale
   shark, Rhincodon typus, the largest fish, which grows to a length of
   approximately 12 metres (41 feet) and which, like the great whales,
   feeds only on plankton through filter feeding. The bull shark,
   Carcharhinus leucas, is the best known of several species to swim in
   both salt and fresh water (it is found in Lake Nicaragua, in Central
   America) and in deltas.

Physical characteristics

Skeleton

   The skeleton of the shark is very different from that of bony fishes
   such as cod; it is made from cartilage, which is very light and
   flexible, although the cartilage in older sharks can sometimes be
   partly calcified, making it harder and more bone-like. The shark's jaw
   is variable and is thought to have evolved from the first gill arch. It
   is not attached to the cranium and has extra mineral deposits to give
   it greater strength.

Respiration

   The major features of sharks

   Like other fishes, sharks extract oxygen from seawater as it passes
   over their gills. Some sharks have a modified slit called a spiracle
   located just behind the eye, which is used in respiration. Due to their
   size and the nature of their metabolism, many sharks have a higher
   demand for oxygen than bony fishes and cannot rely on ambient water
   currents to provide an adequate supply of oxygenated water. If these
   sharks were to stop swimming, the water circulation would drop below
   the level necessary for respiration and the animal would suffocate. The
   process of ensuring an adequate flow of the gills by forward movement
   is known as ram ventilation. Some sharks, such as the blacktip reef
   shark, Carcharhinus melanopterus, and nurse shark, Ginglymostoma
   cirratum, can pump water over their gills as they rest.

   The respiration and circulation process begins when deoxygenated blood
   travels to the shark's two-chambered heart. Here the blood is pumped to
   the shark's gills via the ventral aorta artery where it branches off
   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 ventricle of
   the heart and the cycle repeats. This way of respiration is highly
   inefficient and if the shark were to stop moving blood would not be
   able to move through the body due to the lack of strength of the
   heart's atrium.

Buoyancy

   Unlike bony fishes, sharks do not have gas-filled swim bladders, but
   instead rely on a large liver filled with oil that contains squalene.
   The liver may constitute up to 25% of their body mass for buoyancy. Its
   effectiveness is limited, so sharks employ the dynamic lift to maintain
   depth and sink when they stop swimming. Some sharks, if inverted, enter
   a natural state of tonic immobility - researchers use this condition
   for handling sharks safely.

Osmoregulation

   In contrast to bony fishes, sharks do not drink seawater; instead they
   retain high concentrations of waste chemicals in their body to change
   the diffusion gradient so that they can absorb water directly from the
   sea. This adaptation prevents most sharks from surviving in fresh
   water, and they are therefore confined to a marine environment. A few
   exceptions to this rule exist, such as the bull shark, which has
   developed a way to change its kidney function to excrete large amounts
   of urea.

Teeth

   Tiger shark teeth
   Enlarge
   Tiger shark teeth

   The teeth of carnivorous sharks are not attached to the jaw, but
   embedded in the flesh, and in many species are constantly replaced
   throughout the shark's life. All sharks have multiple rows of teeth
   along the edges of their upper and lower jaws. New teeth grow
   continuously in a groove just inside the mouth and move forward from
   inside the mouth on a "conveyor belt" formed by the skin in which they
   are anchored. In some sharks rows of teeth are replaced every 8–10
   days, while in other species they could last several months. The lower
   teeth are primarily used for holding prey, while the upper ones are
   used for cutting into it. The teeth range from thin, needle-like teeth
   for gripping fish to large, flat teeth adapted for crushing shellfish.

Tails

   The range of shark tail shapes

   The tails (caudal fins) of sharks vary considerably between species and
   are adapted to the lifestyle of the shark. The tail provides thrust and
   so speed and acceleration are dependent on tail shape. Different tail
   shapes have evolved in sharks adapted for different environments. The
   tiger shark's tail has a large upper lobe which delivers the maximum
   amount of power for slow cruising or sudden bursts of speed. The tiger
   shark has a varied diet, and because of this it must be able to twist
   and turn in the water easily when hunting, whereas the porbeagle, which
   hunts schooling fishes such as mackerel and herring has a large lower
   lobe to provide greater speed to help it keep pace with its
   fast-swimming prey. It is also believed that sharks use the upper lobe
   of their tails to counter the lift generated by their pectoral fins.

   Some tail adaptations have purposes other than providing thrust. The
   cookiecutter shark has a tail with broad lower and upper lobes of
   similar shape which are luminescent and may help to lure prey towards
   the shark. The thresher feeds on fish and squid, which it is believed
   to herd, then stun with its powerful and elongated upper lobe.

Speed

   In general, sharks swim ("cruise") at an average speed of 8 km/h
   (5 mph), but when feeding or attacking, the average shark can reach
   speeds upwards of 19 km/h (12 mph). The shortfin mako may range upwards
   of 50 km/h (31 mph). The shortfin mako shark is considered to be the
   fastest shark and one of the fastest fish. The great white shark is
   also capable of considerable bursts of speed. These exceptions may be
   due to the " warm-blooded", or homeothermic, nature of these sharks'
   physiology.

Dermal denticles

   Unlike bony fishes, sharks have a complex dermal corset made of
   flexible collagenous fibres 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.

Body temperature

   A few of the larger species, such as the shortfin mako, Isurus
   oxyrinchus, and the great white, are mildly homeothermic - able to
   maintain their body temperature above the surrounding water
   temperature. This is possible because of the presence of the rete
   mirabile, a counter current exchange mechanism that reduces the loss of
   body heat.

Etymology

   Until the 16th century, sharks were known to mariners as "sea dogs".
   The name "shark" first came into use around the late 1560s to refer to
   the large sharks of the Caribbean Sea, and later as a general term for
   all sharks. The name may have been derived from the Mayan word for
   shark, xoc, pronounced "shock" or "shawk".

Evolution

   A collection of fossilised shark teeth
   Enlarge
   A collection of fossilised shark teeth

   The fossil record of sharks extends back over 450 million years -
   before land vertebrates existed and before many plants had colonised
   the continents. The first sharks looked very different from modern
   sharks. The majority of the modern sharks can be traced back to around
   100 million years ago.

   Mostly only the fossilized teeth of sharks are found, although often in
   large numbers. In some cases pieces of the internal skeleton or even
   complete fossilized sharks have been discovered. Estimates suggest that
   over a span of a few years a shark may grow tens of thousands of teeth,
   which explains the abundance of fossils. As the teeth consist of
   mineral apatite (calcium phosphate), they are easily fossilized.

   Instead of bones, sharks have cartilagenous skeletons, with a bonelike
   layer broken up into thousands of isolated apatite prisms. When a shark
   dies, the decomposing skeleton breaks up and the apatite prisms
   scatter. Complete shark skeletons are only preserved when rapid burial
   in bottom sediments occurs.

   Among the most ancient and primitive sharks is Cladoselache, from about
   370 million years ago, which has been found within the Paleozoic strata
   of Ohio, Kentucky and Tennessee. At this point in the earth's history
   these rocks made up the soft sediment of the bottom of a large, shallow
   ocean, which stretched across much of North America. Cladoselache was
   only about 1 m long with stiff triangular fins and slender jaws. Its
   teeth had several pointed cusps, which would have been worn down by
   use. From the number of teeth found in any one place 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 pelagic makos and
   great white sharks. The discovery of whole fish found tail first in
   their stomachs suggest that they were fast swimmers with great agility.

   From about 300 to 150 million years ago, most fossil sharks can be
   assigned to one of two groups. One of these, the xenacanths, was almost
   exclusive to freshwater environments., By the time this group became
   extinct (about 220 million years ago) they had achieved worldwide
   distribution. The other group, the hybodonts, appeared about 320
   million years ago and was mostly found in the oceans, but also in
   freshwater.

   Modern sharks began to appear about 100 million years ago. Fossil
   mackerel shark teeth occurred in the Lower Cretaceous. The oldest white
   shark teeth date from 60 to 65 million years ago, around the time of
   the extinction of the dinosaurs. In early white shark evolution there
   are at least two lineages: one with coarsely serrated teeth that
   probably gave rise to the modern great white shark, and another with
   finely serrated teeth and a tendency to attain gigantic proportions.
   This group includes the extinct megalodon, Carcharodon megalodon, which
   like all extinct sharks is only known from its teeth. A reproduction of
   its jaws was based on some of the largest teeth which up to almost
   17 cm (7 in) long and suggested a fish that could grow to a length of
   25 to 30.5 m (80 to 100 ft). The reconstruction was found to be
   inaccurate, and estimates revised downwards to around 13 to 15.9 m (43
   to 52 ft).

   It is believed that the immense size of predatory sharks such as the
   great white may have arisen from the extinction of the dinosaurs and
   the diversification of mammals. It is known that at the same time these
   sharks were evolving some early mammalian groups evolved into aquatic
   forms. Certainly, wherever the teeth of large sharks have been found,
   there has also been an abundance of marine mammal bones, including
   seals, porpoises and whales. These bones frequently show signs of shark
   attack. There are theories that suggest that large sharks evolved to
   better take advantage of larger prey.

Classification

   Identification of the 8 extant shark orders

   Sharks belong to the superorder Selachimorpha in the subclass
   Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also
   include rays and skates; the Chondrichthyes also include Chimaeras. It
   is currently thought that the sharks form a polyphyletic group: in
   particular, some sharks are more closely related to rays than they are
   to some other sharks.

   There are more than 360 described species of sharks.

   There are eight orders of sharks, listed below in roughly their
   evolutionary relationship from more primitive to more modern species:
     * Hexanchiformes: Examples from this group include the cow sharks,
       frilled shark and even a shark that looks on first inspection to be
       a marine snake.
     * Squaliformes: This group includes the bramble sharks, dogfish and
       roughsharks, and prickly shark.
     * Pristiophoriformes: These are the sawsharks, with an elongated,
       toothed snout that they use for slashing the fish that they eat.
     * Squatiniformes: Angel sharks.
     * Heterodontiformes: They are generally referred to as the bullhead
       or horn sharks.
     * Orectolobiformes: They are commonly referred to as the carpet
       sharks, including zebra sharks, nurse sharks, wobbegongs and the
       whale shark.
     * Carcharhiniformes: These are commonly referred to as the
       groundsharks, and some of the species include the blue, tiger,
       bull, reef and oceanic whitetip sharks (collectively called the
       requiem sharks) along with the houndsharks, catsharks and
       hammerhead sharks. They are distinguished by an elongated snout and
       a nictitating membrane which protects the eyes during an attack.
     * Lamniformes: They are commonly known as the mackerel sharks. They
       include the goblin shark, basking shark, megamouth shark, the
       thresher sharks, shortfin and longfin mako sharks, and great white
       shark. They are distinguished by their large jaws and ovoviviparous
       reproduction. The Lamniformes include the extinct megalodon,
       Carcharodon megalodon.

Reproduction

   Claspers of male spotted wobbegong, Orectolobus maculatus
   Enlarge
   Claspers of male spotted wobbegong, Orectolobus maculatus

   The sex of a shark can be easily determined. The males have modified
   pelvic fins which have become a pair of claspers. The name is somewhat
   misleading as they are not used to hold on to the female, but fulfill
   the role the mammalian penis.

   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 behaviour: the male may bite the female to show his
   interest. In some species, females have evolved thicker skin to
   withstand these bites.

   Sharks have a different reproductive strategy from most fish. Instead
   of producing huge numbers of eggs and fry (99.9% of which never reach
   sexual maturity in fishes which use this strategy), sharks normally
   produce around a dozen pups (blue sharks have been recorded as
   producing 135 and some species produce as few as two). These pups are
   either protected by egg cases or born live. No shark species are known
   to provide post-natal parental protection for their young, but females
   have a hormone that is released into their blood during the pupping
   season that apparently keeps them from feeding on their young.
   Egg case of Port Jackson shark - found on Vincentia beach, Jervis Bay
   Territory, Australia
   Enlarge
   Egg case of Port Jackson shark - found on Vincentia beach, Jervis Bay
   Territory, Australia

   There are three ways in which shark pups are born:
     * Oviparity - Some sharks lay eggs. In most of these species, the
       developing embryo is protected by an egg case with the consistency
       of leather. Sometimes these cases are corkscrewed into crevices for
       protection. The mermaid's purse, found washed-up on beaches, is an
       empty egg case. Oviparous sharks include the horn shark, catshark,
       Port Jackson shark, and swellshark.
     * Viviparity - These sharks maintain a placental link to the
       developing young, more analogous to mammalian gestation than that
       of other fishes. The young are born alive and fully functional.
       Hammerheads, the requiem sharks (such as the bull and tiger
       sharks), the basking shark and the smooth dogfish fall into this
       category. Dogfish have the longest known gestation period of any
       shark, at 18 to 24 months. Basking sharks and frilled sharks are
       likely to have even longer gestation periods, but accurate data is
       lacking.
     * Ovoviviparity - Most sharks utilize this method. The young are
       nourished by the yolk of their egg and by fluids secreted by glands
       in the walls of the oviduct. The eggs hatch within the oviduct, and
       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. Some species practice oophagy, where the
       first embryos to hatch eat the remaining eggs in the oviduct. This
       practice is believed to be present in all lamniforme sharks, while
       the developing pups of the grey nurse shark take this a stage
       further and consume other developing embryos ( intrauterine
       cannibalism). The survival strategy for the species that are
       ovoviviparous is that the young are able to grow to a comparatively
       larger size before being born. The whale shark is now considered to
       be in this category after long having been classified as oviparous.
       Whale shark eggs found 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 because of
       the protection from predators (mainly other sharks) and the
       abundance of food.

Shark senses

Sense of smell

   Sharks have keen olfactory senses, with some species able to detect as
   little as one part per million of blood in seawater. They are attracted
   to the chemicals found in the guts 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. The short duct between the anterior and posterior nasal
   openings are not fused as in bony fishes.

   Sharks generally rely on their superior sense of smell to find prey,
   but at closer range they also use the lateral lines running along their
   sides to sense movement in the water, and also employ special sensory
   pores on their heads ( Ampullae of Lorenzini) to detect electrical
   fields created by prey and the ambient electric fields of the ocean.

Sense of 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 the retina, thereby increasing visibility in the dark waters.
   The effectiveness of the tissue varies, with some sharks having
   stronger nocturnal adaptations. Sharks have eyelids, but they do not
   blink because the surrounding water cleans their eyes. To protect their
   eyes some have nictitating membranes. This membrane covers the eyes
   during predation, 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.

Sense of hearing

   Sharks also have a sharp sense of hearing and can hear prey many miles
   away. A small opening on each side of their heads (not to be confused
   with the spiracle) leads directly into the inner ear through a thin
   channel. The lateral line shows a similar arrangement, as it 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 fishes and tetrapods the external
   opening into the inner ear has been lost.

Ampullae of Lorenzini

   Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the
   head of a shark.
   Enlarge
   Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the
   head of a shark.

   The Ampullae of Lorenzini are the electroreceptor organs of the shark,
   and they vary in number from a couple of hundred to thousands in an
   individual. The shark has the greatest electricity sensitivity known in
   all animals. This sense is used to find prey hidden in sand by
   detecting the electric fields inadvertently produced by all fish. It is
   this sense that sometimes confuses a shark into attacking a boat: when
   the metal interacts with salt water, the electrochemical potentials
   generated by the rusting metal are similar to the weak fields of prey,
   or in some cases, much stronger than the prey's electrical fields:
   strong enough to attract sharks from miles away. The oceanic currents
   moving in the magnetic field of the earth also generate electric fields
   that can be used by the sharks for orientation and navigation.

Lateral line

   This system is found in most fish, including sharks. It is used to
   detect motion or vibrations in the water. The shark uses this to detect
   the movements of other organisms, especially wounded fish. The shark
   can sense frequencies in the range of 25 to 50 Hz.

Behaviour

   Shark behaviour is inextricably linked to their senses as they
   influence every aspect of their lives. Studies on the behaviour of
   sharks have only recently been taken, leading to little information on
   the subject, although this is changing. The classic view of the shark
   is of a solitary hunter, ranging the oceans in search of food; this is
   only true for a few species, with most living far more sedentary,
   benthic lives. Even solitary sharks meet for breeding or on rich
   hunting grounds, which may lead them to cover thousands of miles in a
   year. Migration patterns in sharks may be even more complex than in
   birds, with many sharks covering entire ocean basins.

   Some sharks can be highly social, remaining in large schools, sometimes
   up to over 100 individuals for scalloped hammerheads congregating
   around seamounts and islands e.g. in the Sea of Cortez. Cross-species
   social hierarchies exist with oceanic whitetip sharks dominating silky
   sharks or comparable size when feeding.

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

Shark intelligence

   Despite the common myth that sharks are instinct-driven "eating
   machines", recent studies have indicated that many species possess
   powerful problem-solving skills, social complexity and curiosity. The
   brain-mass-to-body-mass ratios of sharks are similar to those of
   mammals and other higher vertebrate species.

   In 1987, near Smitswinkle Bay, South Africa, a group of up to seven
   great white sharks worked together to relocate the partially beached
   body of a dead whale to deeper waters to feed.

   Sharks have even been known to engage in playful activities (a trait
   also observed in cetaceans and primates). Porbeagle sharks have been
   seen repeatedly rolling in kelp and have even been observed chasing an
   individual trailing a piece behind them.

Shark sleep

   It is unclear how sharks sleep. Some sharks can lie on the bottom while
   actively pumping water over their gills, but their eyes remain open and
   actively follow divers. The spiny dogfish's spinal cord, rather than
   its brain, coordinates swimming, so it is possible for a spiny dogfish
   to continue to swim while sleeping. It is also possible that a shark
   can sleep with only parts of its brain in a manner similar to dolphins.

Shark attacks

   Snorkeler with blacktip reef shark. In rare circumstances involving
   poor visibility, blacktips may bite a human, mistaking it for prey.
   Under normal conditions they are harmless and shy.
   Enlarge
   Snorkeler with blacktip reef shark. In rare circumstances involving
   poor visibility, blacktips may bite a human, mistaking it for prey.
   Under normal conditions they are harmless and shy.

   Contrary to popular belief, only a few sharks are dangerous to humans.
   Out of more than 360 species, only four have been involved in a
   significant number of fatal, unprovoked attacks on humans: the great
   white, tiger, oceanic whitetip and bull sharks. These sharks, being
   large, powerful predators, may sometimes attack and kill people, but
   all of these sharks have been filmed in open water, without the use of
   a protective cage.

   The perception of sharks as dangerous animals has been popularised by
   publicity given to a few isolated unprovoked attacks, such as the
   Jersey Shore Shark Attacks of 1916, and through popular fictional works
   about shark attacks, such as the Jaws film series. The author of Jaws,
   Peter Benchley, had in his later years attempted to dispel the image of
   sharks as man-eating monsters. In 2005, according to the International
   Shark Attack File, there were a total of 58 unprovoked attacks recorded
   worldwide, of which four were fatal. In comparison, several hundred
   people die annually from lightning strikes and 1.3 to 3 million people
   die from diseases transmitted via mosquito bites.

   In 2005 the International Shark Attack File (ISAF) took an
   investigation on 105 shark attacks. Out of those 105, 58 of the attacks
   were unprovoked.

Sharks in captivity

   Two whale sharks in the Okinawa Churaumi Aquarium
   Enlarge
   Two whale sharks in the Okinawa Churaumi Aquarium

   Until recently only a few benthic species of shark, such as hornsharks,
   leopard sharks and catsharks could survive in aquaria conditions for up
   to a year or more. This gave rise to the belief that sharks, as well as
   being difficult to capture and transport, were difficult to care for. A
   better knowledge of sharks has led to more species (including the large
   pelagic sharks) being able to be kept for far longer. At the same time,
   transportation techniques have improved and now provide a way for the
   long distance movement of sharks.

   Despite controlled feeding behaviour being considered critical for the
   health of the shark, very few studies on this topic have been carried
   out. Since food is the reward for appropriate behaviour, trainers must
   rely on control of feeding motivation.

Conservation

   The number of sharks being caught has increased rapidly over the last
   50 years.
   Enlarge
   The number of sharks being caught has increased rapidly over the last
   50 years.

   The majority of shark fisheries around the globe have little monitoring
   or management. With the rise in demand of shark products there is a
   greater pressure on fisheries. Stocks decline and collapse because
   sharks are long-lived apex predators with comparatively small
   populations, which makes it difficult for them breed rapidly enough to
   maintain population levels. Major declines in shark stocks have been
   recorded in recent years - some species have been depleted by over 90%
   over the past 20-30 years with a population decline of 70% not being
   unusual. Many governments and the UN have acknowledged the need for
   shark fisheries management, but due to the low economic value of shark
   fisheries, the small volumes of products produced and the poor public
   image of sharks, little progress has been made.

   Many other threats to sharks include habitat alteration, damage and
   loss from coastal developments, pollution and the impact of fisheries
   on the seabed and prey species.

Shark fishery

   A 14-foot, 544 kg (1200 pound) Tiger shark caught in Kaneohe Bay, Oahu
   in 1966
   Enlarge
   A 14-foot, 544 kg (1200 pound) Tiger shark caught in Kaneohe Bay, Oahu
   in 1966

   Every year, an estimate states that 26 to 73 millon (median value is at
   38 million) sharks are killed by people in commercial and recreational
   fishing. In the past, sharks were killed simply for the sport of
   landing a good fighting fish (such as the shortfin mako sharks). Shark
   skin is covered with dermal denticles, which are similar to tiny teeth,
   and was used for purposes similar to sandpaper. Other sharks are hunted
   for food (Atlantic thresher, shortfin mako and others), and some
   species for other products.

   Sharks are a common seafood in many places around the world, including
   Japan and Australia. In the Australian State of Victoria shark is the
   most commonly used fish in fish and chips, in which fillets are
   battered and deep-fried or crumbed and grilled and served alongside
   chips. When served in fish and chip shops, it is called flake.

   Sharks are often killed for shark fin soup: the finning process
   involves capture of a live shark, the removal of the fin with a hot
   metal blade, and the release of the live animal back into the water.
   There have been cases where hundreds of de-finned sharks were swept up
   on local beaches without any way to convey themselves back into the
   sea. Conservationists have campaigned for changes in the law to make
   finning illegal in the U.S.

   Sharks generally reach sexual maturity slowly and produce very few
   offspring in comparison to other fishes that are harvested. This has
   caused concern among biologists regarding the increase in effort
   applied to catching sharks over time, and many species are considered
   to be threatened.

   Some organizations, such as the Shark Trust, campaign to limit shark
   fishing.

Sharks in mythology

   Sharks figure prominently in the Hawaiian mythology. There are stories
   of shark men who have shark jaws on their back. They could change form
   between shark and human at any time they desired. A common theme in the
   stories was that the shark men would warn beach-goers that sharks were
   in the waters. The beach-goers would laugh and ignore the warnings and
   go swimming, subsequently being eaten by the same shark man who warned
   them not to enter the water.

   Hawaiian mythology also contained many shark gods. They believed that
   sharks were guardians of the sea, and called them Aumakua:
     * Kamohoali'i - The best known and revered of the shark gods, he was
       the older and favored brother of Pele, and helped and journeyed
       with her to Hawaii. He was able to take on all human and fish
       forms. A summit cliff on the crater of Kilauea is considered to be
       one of his most sacred spots. At one point he had a he'iau (temple
       or shrine) dedicated to him on every piece of land that jutted into
       the ocean on the island of Moloka'i.
     * Ka'ahupahau - This goddess was born human, with her defining
       characteristic being her red hair. She was later transformed into
       shark form and was believed to protect the people who lived on
       O'ahu from sharks. She was also believed to live near Pearl
       Harbour.
     * Kaholia Kane - This was the shark god of the ali'i Kalaniopu'u and
       he was believed to live in a cave at Puhi, Kaua'i.
     * Kane'ae - The shark goddess who transformed into a human in order
       to experience the joy of dancing.
     * Kane'apua - Most commonly, he was the brother of Pele and
       Kamohoali'i. He was a trickster god who performed many heroic
       feats, including the calming of two legendary colliding hills that
       destroyed canoes trying to pass between.
     * Kawelomahamahai'a - Another human, he was transformed into a shark.
     * Keali'ikau 'o Ka'u - He was the cousin of Pele and son of Kua. He
       was called the protector of the Ka'u people. He had an affair with
       a human girl, who gave birth to a helpful green shark.
     * Kua - This was the main shark god of the people of Ka'u, and
       believed to be their ancestor.
     * Kuhaimoana - He was the brother of Pele and lived in the Ka'ula
       islet. He was said to be 30 fathoms (55 m) long and was the husband
       of Ka'ahupahau.
     * Kauhuhu - He was a fierce king shark that lived in a cave in
       Kipahulu on the island of Maui. He sometimes moved to another cave
       on the windward side of island of Moloka'i.
     * Kane-i-kokala - A kind shark god that saved shipwrecked people by
       taking them to shore. The people who worshipped him feared to eat,
       touch or cross the smoke of the kokala, his sacred fish.

   In other Pacific Ocean cultures, Dakuwanga was a shark god who was the
   eater of lost souls.

   In ancient Greece, it was forbidden to eat shark flesh at women's
   festivals.

Myths

   A popular myth is that sharks are immune to disease and cancer;
   however, this is untrue. There are both diseases and parasites that
   affect sharks. The evidence that sharks are at least resistant to
   cancer and disease is mostly anecdotal and there have been few, if any,
   scientific or statistical studies that have shown sharks to have
   heightened immunity to disease.

   Retrieved from " http://en.wikipedia.org/wiki/Shark"
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   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
