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Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton and a streamlined body, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some species, a modified slit called a spiracle, is located just behind the eye), dermal denticles covering the body to protect from damage, parasites and improve fluid dynamics, and rows of replaceable teeth in the mouth.

Characteristics

Sharks have keen olfactory senses, with some species able to detect as little as one part per million of blood in seawater. They are even more attracted to the chemicals found in the gut of many species, and 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 like in bony fish.

Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to their 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 it 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.

Sharks generally rely on their superior sense of smell to find prey, but once they are close 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.

The teeth of carnivorous sharks are not attached to the jaw, but embedded in their flesh, and in many species are constantly replaced throughout the shark's life. The lower teeth are primarily used for holding prey, while the top are used for cutting into it. (Gilbertson, 7.3)

Note that carnivorous sharks jaws are not attached to the skull, so when a shark bites, first the lower jaw "catches" the prey and springs upwards, at which point the upper jaw springs down and both sink into the prey, biting in different fashions depending on the species.

Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their head (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 is grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost.

There are exceptions to the "large", "marine" (as in 'ocean-going') and "predatory" portions of the characterization. Sharks include everything from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species, to the whale shark, Rhincodon typus, which is known to grow to a maximum length of approximately 15 metres (49 feet) and which, like the great whales, feeds only on plankton. Although not unique among sharks, the bull shark, Carcharhinus leucas, is the best known of several species to regularly swim in both salt and fresh water environments (most famously in Lake Nicaragua, in Central America) and in most deltas. A few of the larger species, the shortfin mako, Isurus oxyrinchus, and the great white, are mildly homeothermic, able to maintain their body temperature at a level above the ocean's temperature. This is possible because of the presence of the rete mirabile, a counter current exchange mechanism that reduces the loss of body heat.

Like other fish, sharks extract their oxygen from seawater as it passes over their gills. Due to their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they cannot rely on ambient water current to provide an adequate supply of oxygenated water. If a shark were to stop swimming, the necessary water circulation for respiration would become too low and the animal could suffocate, although some sharks have been known to "nap" on the bottom. Some sharks, like the blacktip reef shark, Carcharhinus melanopterus, and nurse shark, Ginglymostoma cirratum, can pump water over their gills as they rest. There are also known instances, such as in certain caves along the Yucatan coast, where sharks of varying species rest on the cave floors and allow the fresh water outflow to pass over them. The outflow is strong enough to allow for respiration, and it is believed that the reason for this behaviour is that the fresh water helps remove certain parasites. Also, unlike other fish, sharks do not have gas-filled swim bladders, but rather rely on an oil-filled liver for (limited) buoyancy, so they sink when they stop swimming; a resting shark always sinks to the sea bed. Grey nurse sharks, Carcharias taurus, are known to gulp air at the surface and store it in their stomachs to provide buoyancy.

Unlike bony fish, the sharks have a complex dermal corset arranged as a helical network and made of flexible collagenous fibres surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. A similar arrangement of collagen fibres has been discovered in dolphins and squids.

Their dermal teeth gives them hydrodynamic advantages as they are reducing the turbulence when swimming.

Some sharks, if inverted, enter a natural state of temporary paralysis. Researchers use this condition for handling sharks safely.

A popular myth is that sharks are immune to disease and cancer, however, this is untrue and 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. *

Urine in sharks accumulates in the blood and is then secreted through the skin.

Speed

In general, sharks swim ("cruise") at an average speed of 8 kilometers (5 miles) per hour, but when feeding or attacking, the average shark reaches speeds upwards of 19 kilometers (12 miles) per hour. One exception to this generality is the shortfin mako, whose speed may range upwards of 48 kilometers (30 miles) per hour. The shortfin mako shark is considered to be the fastest shark and one of the fastest fish. Also the great white shark is capable of surprising bursts of speed. These exceptions may be due to the "warm-blooded", or homeothermic, nature of their physiology.

Etymology

Until the late 16th century sharks were usually referred to in the English language 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 to all sharks in general. The name may have been derived from the Mayan word for shark, xoc, pronounced "shock" or "shawk".

Classification

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.

The first sharks appeared in the oceans 400 to 350 million years ago. Most of the species we know today are as old as the Jurassic period. There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:

The Lamniformes include the extinct megalodon (Carcharodon megalodon), which like all extinct sharks is only known from its teeth (the only bone found in these cartilaginous fishes, and therefore the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 17 cm (7 inches) in length) and suggested a fish that could grow 25 metres (80 feet) long to 30.5 metres (100 feet). The jaw was realized to be inaccurate, and estimates revised downwards to around 13 metres (43 feet) to 15.9 meters (52 feet).

Reproduction

The sex of a shark can be easily determined. The males all have their pelvic fins modified into a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but are the shark's version of the mammalian penis. (As a side note, Class Chondrichthyes has the distinction of having the animal with the largest intromittent organ — an organ used for transmitting sperm — in relation to body length. This animal is the clearnose skate (Raja eglanteria) which has claspers of 15 cm (6 in) in size on a fish that reaches 1 m (3 feet) in length.)

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

Sharks have a much different reproductive strategy than most fishes. Instead of producing huge numbers of eggs and larvae (99.9% of which never reach sexual maturity in fishes that use this strategy) sharks normally produce around a dozen pups, some species up to 70-80 and some as few as 2-3. These pups are either protected by egg cases or born live. No known sharks provide 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.

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. Some of these cases are corkscrewed into crevices for protection. When they wash up empty on beaches, the egg cases are sometimes called mermaid's purses. Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swell shark.
  • Viviparity - These sharks actually maintain a placental link to the developing young, more analogous to mammals than other fishes. The young are born alive and fully functional. Hammerheads, the requiem sharks (like the bull and tiger sharks), the basking shark and the smooth dogfishes fall into this category. Dogfishes also have the longest known gestation period of any shark, 22 months. The blue shark produces the most young of sharks that have had the number of pups recorded, the maximum reported being 82.
  • 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. Sometimes they are functional even before being born, as some species practice oophagy, where the first to hatch eat the remaining eggs in the oviduct. grey nurse sharks, shortfin makos, threshers, porbeagles and possibly great whites have oophagous young. The survival strategy for the species that do this is that the young are able to grow to an even larger size before being born. The whale shark is now considered to be in this category after having been classified as oviparous for a long time. Whale shark eggs found are now thought to have been aborted. Most ovoviviparous sharks generally give birth in sheltered areas, including bays, river mouths, and shallow reefs. They choose such areas mainly because of the protection from predators (mainly other sharks) and the abundance of food.

Shark senses

Sharks have two senses that many animals do not have:
  • Electroreception: 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 in bottom feeding sharks, 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 a prey, or in some cases, much stronger than the prey electric fields, 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 'sound' in the water. The shark uses this to detect other organisms moving, especially wounded fish. The shark can 'hear' frequencies in the range of 25 to 50 Hz using this sense.

Shark intelligence

Despite the common myth that sharks are largely 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 in size to those of mammals and other higher vertebrate species. This is surprising when considering that more advanced groups of fishes like teleosts typically do not display the same degree of mental development *.

In 1987, near Smitswinkle Bay, South Africa, an estimated group of seven Great White sharks worked together to relocate the partially beached body of a dead whale to deeper waters to feed *. This is an example of a cooperative and intelligent effort that directly contradicts more the typically reported behaviors like feeding frenzies.

Sharks have even been known to engage in playful activities (a trait typical to animals like whales or primates). One group of porbeagle sharks developed a game similar to tag in which they would chase a loner carrying a piece of kelp between its teeth, attempting to take it for themselves. If another shark succeeded in capturing the plant it would became the next target.

Shark attacks

The fear of sharks has been fueled by a few rare instances of unprovoked attack, such as the Jersey Shore Shark Attacks of 1916, and by sensationalized fiction and film, such as the Jaws series. The creator of the Jaws phenomenon, the late Peter Benchley, has himself in recent years attempted to dispel the myth of sharks being man-eating monsters. In 2005 there were worldwide a total of 58 unprovoked recorded attacks of which four were fatalISAF 2005 Worldwide Shark Attack Summary.. In comparison, several hundred people die annually from lightning strikesFlorida Museum of Natural History, University of Florida. A Comparison with the Number of Lightning Fatalities in Coastal United States: 1959-2004. and 1.3 to 3 millionCampbell, Neil A. et al. "Biology" Seventh edition. Menlo Park, CA: Addison Wesley Longman, Inc. 2005. people die from diseases transmitted via mosquito bites.

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 amount of fatal unprovoked attacks on humans: the great white, tiger, oceanic whitetip and bull sharksISAF Statistics on Attacking Species of Shark.. These sharks, being large, powerful predators may sometimes attack and kill people, but all of the above sharks, even the great white, have been filmed in open water, with no cageHawaiian newspaper article with pictures of cageless diver with great white shark.The 1992 Cageless shark-diving expedition by Ron and Valerie Taylor., time and time again, without incident.

Shark fishery

Every year, 100 million sharks are killed by people in commercial and recreational fishing. In the past they were killed simply for the sport of landing a good fighting fish (shortfin mako sharks for instance). Sharkskin is covered with dermal denticles, which are similar to tiny teeth. It 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, in which many sharks are hunted for their fins, which are cut off with a hot metal blade before the live animal is tossed back into the water. There have been cases where hundreds of de-finned animals 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 desired, and for any length. 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. A listing of them follows:

  • Kamohoali'i - The most well 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 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.

In ancient Greece, shark flesh was forbidden to be eaten at women's festivals.

See also

Films

References

=General references
=
  • Gilbertson, Lance (1999). Zoology Laboratory Manual. New York, McGraw-Hill Companies, Inc.
  • Castro, Jose. The Sharks of North American Waters. College Station: Texas A&M University Press, 1983.
  • Stevens, John D. Sharks. New York: NY Facts on File Publications, 1987.

External links

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This article is licensed under the GNU Free Documentation License. It uses material from the "Shark".

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