SHARK

Wikipedia.- Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics; they also have replaceable teeth. Sharks range in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres (7 in) in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (39 ft) and which, like baleen whales (the only larger extant animals), feeds only on plankton, squid, and small fish through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt, freshwater and in deltas.

Shark senses
Sense of smell
Scarface tiger sharkSharks 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. 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.

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
Oceanic whitetip shark with a small school of pilot fishShark 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. 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 degree to which sight is used probably varies with species and water conditions. In effect the shark's field of vision can swap between monnocular and stereoscopic at any time.

Sense of hearing
Although it is hard to test the hearing of sharks, there are indications that suggest that they have a sharp sense of hearing and can possibly 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 fish and tetrapods the external opening into the inner ear has been lost.

Electroreception
Main article: Electroreception
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. Sharks use the Ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (mostly the hammer head) find its prey. The shark has the greatest electrical 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.[citation needed] 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 may be used in navigation.

Lateral line
Main article: 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.

Behavior
Studies on the behaviour of sharks have only recently been carried out leading to little information on the subject, although this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, 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 of scalloped hammerheads congregating around seamounts and islands e.g. in the Gulf of California. Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks of 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.

Aversion to Dolphins
There are many stories of dolphins protecting humans from shark attacks. This phenomenon was investigated in an episode of Discovery Channel's Mythbusters, in which a feeding great white shark did not attack either a seal cutout or raw bait when a mechanical dolphin was placed in the water nearby. There has been no conclusive scientific study to explain this behaviour.

Feeding
The Great white shark is one of only four kind of sharks that have been involved in a significant number of fatal unprovoked attacks on humans.All sharks are carnivorous, some like the whale shark are filter feeders and sieve plankton and small animals, others like the great white shark eat fish and even marine mammals, many bottom living shark like the zebra shark eat mollusks and crustaceans.

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. When a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. 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 in a manner similar to dolphins. In this situation, one half of the brain sleeps at a time, thereby allowing the shark to be half conscious while sleeping.

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.Main article: Shark attack
In 2006 the International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities per year between 2001 and 2006 from unprovoked shark attacks is 4.3.

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, oceanic whitetip, tiger, 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 popularized 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.

Sharks in captivity
A whale shark in Georgia Aquarium.Until recently only a few benthic species of shark, such as hornsharks, leopard sharks and catsharks could survive in aquarium 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. The only species of shark to have never been successfully held in captivity was the great white, until September 2004 when the Monterey Bay Aquarium successfully kept a young female great white shark for 198 days before releasing her back into the wild.

Habitat
A December 10, 2006 report by the Census of Marine Life group reveals that 70% of the world's oceans are shark-free. They have discovered that although many sharks live up to depths as low as 1,500 metres (5,000 ft), they fail to colonize below 3,000 metres (10,000 ft), putting them more easily within reach of fisheries and thus endangered status.

Conservation
The value of shark fins for shark fin soup has led to an increase in shark catches. Usually only the fin is taken, and the rest of the shark discarded, usually back into the sea. The shark can't swim and is left to die.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.

The practice of shark finning, cutting the fin from a shark and discarding the live animal, attracts much controversy and regulations are being enacted to prevent it from occurring. A Canadian-made documentary, Sharkwater is raising awareness of the depletion of the world's shark population.

Shark fishery
The number of sharks being caught has increased rapidly over the last 50 years.An estimate states that, every year, 26 to 73 million (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. In India small sharks or baby sharks (called sora in Tamil language) are caught by fishermen routinely and are sold in the local markets. Since the flesh is not developed completely it just breaks into powder once boiled and this is then fried in oil and spices (called sora puttu). Even the bones are soft and these can be easily chewed and considered a delicacy in coastal Tamil Nadu. In Iceland, Greenland sharks are fished to produce hákarl or fermented shark, which is widely regarded as a national dish.

A 14-foot (4 m), 544 kg (1200 pound) Tiger shark caught in Kaneohe Bay, Oahu in 1966.Sharks are often killed for shark fin soup: the finning process involves the removal of the fin with a hot metal blade. Fishermen will capture live sharks, fin them, and release the finless animal back into the water. The immobile shark soon dies from suffocation or predators. Despite claims that this practice is rare, it has become a major trade within black markets all over the world with shark fins going at about $220/ lbs. Millions of sharks a year are being illegally poached for their fins and not many governments are enforcing the laws of protecting these apex predators. The dish is considered a status symbol in Asian countries, and is considered healthy and full of nutrients, with some even claiming they prevent cancer and other ailments. There is no scientific proof that supports these claims; at least one study has shown shark cartilage of no value in cancer treatment. The shark fin trade is a major problem and has gained international controversy.

Sharks are also killed for their meat. Conservationists have campaigned for changes in the law to make finning illegal in the U.S. The meat of dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and rays are in high demand by European consumers. However, the U.S. FDA lists sharks as one of four fish (with swordfish, king mackerel, and tilefish,) that children and women who are or may be pregnant should refrain from eating. For details see mercury poisoning.

Shark cartilage has been advocated as effective against cancer and for treatment of osteoarthritis. (This is because many people believe that sharks cannot get cancer and that taking it will prevent people from getting these diseases, which is untrue.) However, a trial by Mayo Clinic found no effect in advanced cancer patients.

Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fish 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. According to Seafood Watch, sharks are currently on the list of fish that American consumers, who are sustainability minded, should avoid.

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. Some families' Aumakua, or deified ancestor guardians, were sharks who protected family members.

Kamohoali'i - The best known and revered of the shark gods,
he was the older and favoured 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 heiau (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 Harbor.
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..

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

A popular myth is that sharks are immune to disease and cancer; however, this remains to be proved. sharks may get cancer. 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.