Crocodile And Shark

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A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all extant members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae) Sony VAIO PCG-8131M battery, as well as the Crocodylomorpha, which include prehistoric crocodile relatives and ancestors.

Member species of the family Crocodylidae are large aquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. Crocodiles tend to congregate in freshwater habitats such as rivers, lakes, wetlands and sometimes in brackish waterSony VAIO PCG-8152M battery. They feed mostly on vertebrates - fish, reptiles, and mammals, and sometimes on invertebrates - molluscs and crustaceans, depending on species. They first appeared during the Eocene epoch, about 55 million years ago.

Etymology

The word "crocodile" comes from the Ancient Greek κροκόδιλος (crocodilos), "lizard," used in the phrase ho krokódilos ho potamós, "the lizard of the (Nile) river".

There are several variant Greek forms of the word attested, including the later form κροκόδειλος (crocodeilos) Sony VAIO PCG-31311M batteryfound cited in many English reference works.[3] In the Koine Greek of Roman times, crocodilos and crocodeilos would have been pronounced identically, and either or both may be the source of the Latinized form crocodīlus used by the ancient Romans.

Crocodilos or crocodeilos is a compound of krokè ("pebbles"), and drilos/dreilos ("worm"). It is ascribed to Herodotus, supposedly to describe the basking habits of the Egyptian crocodile.[4] However the word drilos is only attested as a colloquial term for "penis".Sony VAIO PCG-31111M battery The meaning of krokè is explained as describing the skin texture of lizards (or crocodiles) in most sources,[citation needed] but is alternately claimed to refer to a supposed habit of (lizards or crocodiles) basking on pebbly ground.

The form crocodrillus is attested in Medieval Latin.[4] It is not clear whether this is a medieval corruption or derives from alternate Greco-Latin forms (late Greek corcodrillos and corcodrillion are attested) Sony VAIO PCG-8112M battery.

A (further) corrupted form cocodrille is found in Old French and was borrowed into Middle English as cocodril(le). The Modern English form crocodile was adapted directly from the Classical Latin crocodīlus in the 16th century, replacing the earlier form.

The use of -y- in the scientific name Crocodylus (and forms derived from it) is a corruption introduced by Laurenti (1768) Sony VAIO PCG-7186M battery.

Description

Crocodiles are similar to alligators and caimans; for their common biology and differences between them, see Crocodilia.

Crocodiles, like dinosaurs, have the abdominal ribs modified into gastralia.

Crocodiles are among the more biologically complex reptiles despite their prehistoric look. Unlike other reptiles, a crocodile has a cerebral cortex, a four-chambered heart,and the functional equivalent of a diaphragmSony VAIO PCG-7171M battery, by incorporating muscles used for aquatic locomotion into respiration (e.g. m. diaphragmaticus);[6] Its external morphology, on the other hand, is a sign of its aquatic and predatory lifestyle.

A crocodile’s physical traits allow it to be a successful predator. Its streamlined body enables it to swim swiftly. It also tucks its feet to the side while swimming, which makes it faster by decreasing water resistance. Its webbed feet, though not used to propel the animal through the waterSony VAIO PCG-9Z1M battery, allow it to make fast turns and sudden moves in the water or initiate swimming. Webbed feet are an advantage in shallower water where the animal sometimes moves around by walking.

Crocodiles have a palatal flap, a rigid tissue at the back of the mouth that blocks the entry of water. The palate has a special path from the nostril to the glottis that bypasses the mouth. The nostrils are closed during submergence. Like other archosaursSony VAIO PCG-5S1M battery, crocodilians are diapsid, although their post-temporal fenestrae are reduced. The walls of the braincase are bony, but lack supratemporal and postfrontal bones.[7] Their tongues are not free, but held in place by a membrane which limits movement; as a result, crocodiles are unable to stick out their tongues. Sony VAIO PCG-5P1M battery

Crocodilian scales have pores believed to be sensory in function, analogous to the lateral line in fishes. They are particularly seen on their upper and lower jaws. Another possibility is they are secretory, as they produce an oily substance that appears to flush mud off.[7]

Crocodiles are very fast over short distances, even out of water. Since they feed by grabbing and holding onto their preySony VAIO PCG-5N2M battery, they have evolved sharp teeth for tearing and holding onto flesh, and powerful muscles to close the jaws and hold them shut. These jaws can bite down with immense force, by far the strongest bite of any animal. The pressure of the crocodile's bite is more than 5,000 pounds per square inch (30,000 kPa),[9] compared to just 335 pounds per square inch (2,300 kPa) for a RottweilerSony VAIO PCG-3C2M battery, 400 pounds per square inch (2,800 kPa) for a large great white shark, 800 pounds per square inch (6,000 kPa) to 1,000 pounds per square inch (7,000 kPa) for a hyena, or 2,000 pounds per square inch (10,000 kPa) for a large alligator[citation needed]. The jaws are opened, however, by a very weak set of muscles. Crocodiles can thus be subdued for study or transport by taping their jaws or holding their jaws shut with large rubber bands cut from automobile inner tubesSony VAIO PCG-8161M battery. They have limited lateral (side-to-side) neck movement.

Biology and behavior

Crocodiles are ambush hunters, waiting for fish or land animals to come close, then rushing out to attack. As cold-blooded predators, they have a very slow metabolism, so they can survive long periods without food. Despite their appearance of being slow, crocodiles are top predators in their environment, and various species have been observed attacking and killing sharks. Sony VAIO PCG-8141M battery

Herodotus claimed Nile crocodiles had a symbiotic relationship with certain birds, such as the Egyptian plover, which enter the crocodile's mouth and pick leeches feeding on the crocodile's blood; with no evidence of this interaction actually occurring in any crocodile species, it is most likely mythical or allegorical fiction. Sony VAIO PCG-3J1M battery

Many large crocodilians swallow stones (called gastroliths or stomach stones) which may act as ballast to balance their bodies or assist in crushing food,[7] similar to grit in birds.

Salt glands are present in the tongues of most crocodylids and they have a pore opening on the surface of the tongue. They appear to be similar to those in marine turtles; they seem to be absent in Alligatoridae. Sony VAIO PCG-3H1M battery

Crocodilians can produce sounds during distress and in aggressive displays. They can also hear well, but their tympanic membranes are concealed by flat flaps that may be raised or lowered by muscles.[7]

A crocodile farm in Mexico

Crocodiles eat fish, birds, mammals and occasionally smaller crocodiles.

Crocodiles are protected in many parts of the world, but they also are farmed commercially. Their hides are tanned and used to make leather goods such as shoes and handbagsSony VAIO PCG-3F1M battery; crocodile meat is also considered a delicacy. The most commonly farmed species are the saltwater and Nile crocodiles, while a hybrid of the saltwater and the rare Siamese crocodile is also bred in Asian farms. Farming has resulted in an increase in the saltwater crocodile population in Australia, as eggs are usually harvested from the wild, so landowners have an incentive to conserve their habitat
Sony VAIO PCG-3C1M battery
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Distribution of crocodiles

Crocodiles are more closely related to birds and dinosaurs than to most animals classified as reptiles, the three being included in the group Archosauria ('ruling reptiles'). See Crocodilia for more information.

Crocodile embryos do not have sex chromosomes, and unlike humans, sex is not determined genetically. Sex is determined by temperature, with males produced at around 31.6 °C (89 °F), and females produced at slightly lower and higher temperaturesSony VAIO PCG-9Z2L battery. The average incubation period is around 80 days, and also is dependent on temperature.[12]

Crocodiles may possess a form of homing instinct. Three rogue saltwater crocodiles were relocated 400 kilometres by helicopter in northern Australia, but had returned to their original locations within three weeks, based on data obtained from tracking devices attached to the reptiles. Sony VAIO PCG-9Z1L battery

The land speed record for a crocodile is 17 km/h (11 mph) measured in a galloping Australian freshwater crocodile.[14] Maximum speed varies from species to species. Certain species can indeed gallop, including Cuban crocodiles, New Guinea crocodiles, African dwarf crocodiles, and even small Nile crocodilesSony VAIO PCG-9131L battery. The fastest means by which most species can move is a kind of "belly run", where the body moves in a snake-like fashion, limbs splayed out to either side paddling away frantically while the tail whips to and fro. Crocodiles can reach speeds of 10 or 11 km/h (around 7 mph) when they "belly run", and often faster if slipping down muddy riverbanks. Another form of locomotion is the "high walk", where the body is raised clear of the groundSony VAIO PCG-8161L battery.

Siamese crocodile sleeping with its mouth open to pant

Crocodiles do not have sweat glands and release heat through their mouths. They often sleep with their mouths open and may even pant like a dog.[15]

The BBC TV[16] reported thee Nile crocodile which has lurked a long time underwater to catch prey, has built up a large oxygen debt. When it has caught and eaten that prey, it closes its right aortic arch and uses its left aortic arch to flush blood loaded with carbon dioxide from its muscles directly to its stomachSony VAIO PCG-8152L battery; the resulting excess acidity in its blood supply makes it much easier for the stomach lining to secrete more stomach acid to quickly dissolve bulks of swallowed prey flesh and bone.

Size

A large saltwater crocodile in captivity in Australia

Size greatly varies between species, from the dwarf crocodile to the saltwater crocodile. Species of Palaeosuchus and Osteolaemus grow to an adult size of just 1 metre (3.3 ft) to 1.5 metres (4.9 ft) Sony VAIO PCG-8141L battery. Larger species can reach over 4.85 metres (15.9 ft) long and weigh well over 1,200 kilograms (2,600 lb). Crocodilians show pronounced sexual dimorphism, with males growing much larger and more rapidly than females.[7] Despite their large adult sizes, crocodiles start their lives at around 20 centimetres (7.9 in) long. The largest species of crocodile is the saltwater crocodile, found in eastern India, northern Australia, throughout South-east Asia, and in the surrounding watersSony VAIO PCG-8131L battery.

Two larger certifiable records are both of 6.2 metres (20 ft) crocodiles. The first was shot in the Mary River in the Northern Territory of Australia in 1974 by poachers, and measured by wildlife rangers.[citation needed] The second crocodile was killed in 1983 in the Fly River, Papua New Guinea. In the case of the second crocodile it was actually the skin that was measured by zoologist Jerome MontagueSony VAIO PCG-81312L battery, and as skins are known to underestimate the size of the actual animal, it is possible this crocodile was at least another 10 cm longer.[citation needed]

The largest crocodile ever held in captivity is an estuarine–Siamese hybrid named Yai (Thai: ใหญ่, meaning big) (born 10 June 1972) at the famous Samutprakarn Crocodile Farm and Zoo, Thailand. This animal measures 6 m (19.69 ft) (19 ft 8 in) in length and weighs 1114.27 kg. Sony VAIO PCG-81214L battery

Meanwhile, the longest crocodile captured alive is Lolong which was measured at 6.096 metres (20 ft 3 in) by a National Geographic team in Agusan del Sur Province, Philippines.[18]

Wildlife experts, however, argue the largest crocodile so far found in the Bhitarkanika was almost 23 feet (7.0 m) long, which could be traced from the skull preserved by the Kanika royal familySony VAIO PCG-81115L battery. The crocodile was shot near Dhamara in 1926 and later its skull was preserved by the then Kanika king. Crocodile experts estimated the animal was between 20 feet (6.1 m) and 23 feet (7.0 m) long, as the size of the skull was measured one-ninth of the total length of the body.

Age

Measuring crocodile age is unreliable, although several techniques are used to derive a reasonable guess. The most common method is to measure lamellar growth rings in bones and teeth—Sony VAIO PCG-81114L batteryeach ring corresponds to a change in growth rate which typically occurs once a year between dry and wet seasons.[19] Bearing these inaccuracies in mind, the oldest crocodilians appear to be the largest species. C. porosus is estimated to live around 70 years on average, with limited evidence of some individuals exceeding 100 years. One of the oldest crocodiles recorded died in a zoo in RussiaSony VAIO PCG-81113L battery. A male freshwater crocodile at the Australia Zoo is estimated to be 130 years old. He was rescued from the wild by Bob Irwin and Steve Irwin after being shot twice by hunters. As a result of the shootings, this crocodile (known affectionately as "Mr. Freshy") has lost his right eye. Sony VAIO PCG-7142L battery

Skin

Crocodiles have smooth skin on their bellies and sides, while their dorsal surfaces are armoured with large osteoderms. The armoured skin has scales and is thick and rugged, providing some protection. They are still able to absorb heat through this armour, as a network of small capillaries allow blood through the scales to absorb heatSony VAIO PCG-7141L battery.

Taxonomy of the Crocodylidae

Crocodiles and humans

Danger to humans

Main article: Crocodile attacks

Crocodile warning sign, Trinity Beach, Queensland, Australia

The larger species of crocodiles are very dangerous to humans, mainly not from their ability to run after a person, but their ability to strike before the person can react. The saltwater crocodile and Nile crocodile are the most dangerous, killing hundreds of people each year in parts of Southeast Asia and Africa. The mugger crocodile, American crocodile, American alligator and black caiman are also dangerous to humansSony VAIO PCG-71111L battery.

Crocodile products

Main article: Crocodile farm

Crocodile leather wallets from a Bangkok crocodile farm

Chiang Mai crocodile leather belt

Crocodile leather can be made into goods such as wallets, briefcases, purses, handbags, belts, hats, and shoes.

Crocodile meat is consumed in some countries, such as Australia, Ethiopia, Thailand, South Africa and also Cuba (in pickled form); it can also be found in specialty restaurants in some parts of the United StatesSony VAIO PCG-61411L battery. The meat is white and its nutritional composition compares favourably with that of other meats.[citation needed] It tends to have a slightly higher cholesterol level than other meats. Crocodile meat has a delicate flavour; some describe it as a cross between chicken and crab.[citation needed] Cuts of meat include backstrap and tail filletSony VAIO PCG-61112L battery.

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 suborder Elasmobranchii outside the SelachimorphaSony VAIO PCG-61111L battery, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.[1]

Since that time, sharks have diversified into over 400 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)Sony VAIO PCG-5T4L battery, the largest fish in the world, which reaches approximately 12 metres (39 ft). Despite its size, the whale shark feeds only on plankton, squid, and small fish by filter feeding. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptionsSony VAIO PCG-5T3L battery, such as the bull shark and the river shark that can survive in both seawater and freshwater.[2] 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 also have several sets of replaceable teeth. Sony VAIO PCG-5T2L battery

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. Their predatory skill fascinates and frightens humans, even though their survival is threatened by human-related activitiesSony VAIO PCG-5S3L battery.

Etymology

Until the 16th century,[4] sharks were known to mariners as "sea dogs".[5] The etymology of the word "shark" is uncertain. One theory is that it derives from the Yucatec Maya word xok, pronounced 'shok'.[6] Evidence for this etymology comes from the OED, which notes the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and used the word "sharke" to refer to the large sharks of the Caribbean SeaSony VAIO PCG-5S2L battery.

An alternate etymology states that the original sense of the word was that of "predator, one who preys on others" from the German Schorck, a variant of Schurke "villain, scoundrel" (cf. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour. Sony VAIO PCG-5S1L battery

Anatomy

Teeth

Main article: Shark teeth

The teeth of tiger sharks are oblique and serrated to saw through flesh

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 lifetimeSony VAIO PCG-5R2L battery. 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.[8]

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 grippingSony VAIO PCG-5R1L battery, 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.[9]

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 durableSony VAIO PCG-5P4L battery, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy.[10] Because sharks do not have rib cages, they can easily be crushed under their own weight on land.[11]

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 strengthSony VAIO PCG-5P2L battery. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic.[12] 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. Sony VAIO PCG-5N4L battery 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.[13] 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. Sony VAIO PCG-5N2L battery

Dermal denticles

Main article: Dermal denticle

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.[14] Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming. Sony VAIO PCG-51513L battery

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 portionSony VAIO PCG-51511L battery. 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.[15]

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 sharkSony VAIO PCG-51412L battery, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey.[16] 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 squidSony VAIO PCG-51411L battery.

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.[14] Their liver constitutes up to 30% of their total body mass.[17] The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth when not swimmingSony VAIO PCG-51312L battery. 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. Sony VAIO PCG-51311L battery

Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely.[19]

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 eyeSony VAIO PCG-51211L battery, 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.[9] 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 waterSony VAIO PCG-41112L battery. 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. Sony VAIO PCG-3A4L battery

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 arteriesSony VAIO PCG-3A3L battery, 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. Sony VAIO PCG-3A2L battery

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 waterSony VAIO PCG-3A1L battery. 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]. Sony VAIO PCG-394L battery

Osmoregulation

In contrast to bony fish, with the exception of the coelacanth,[23] the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea and trimethylamine N-oxide (TMAO), allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in freshwaterSony VAIO PCG-393L battery, 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.[17] When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammoniaSony VAIO PCG-391L battery.

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.[26] 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 mouthSony VAIO PCG-384L battery.

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. Sony VAIO PCG-383L battery

Senses

Smell

The shape of the hammerhead shark's head may enhance olfaction by spacing the nostrils further apart.

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.Sony VAIO PCG-382L battery

Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.[28] 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 preySony VAIO PCG-381L battery.

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 means that sharks can contract and dilate their pupils, like humans, something no teleost fish can doSony VAIO PCG-7185L battery. 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. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranesSony VAIO PCG-7184L battery. 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 debatedSony VAIO PCG-7183L battery. 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. Sony VAIO PCG-7182L battery 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 colorblindSony VAIO PCG-7181L battery. 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 sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away.[33] A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channelSony VAIO PCG-7174L battery. 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 lostSony VAIO PCG-7173L battery.

Electroreception

Main article: Electroreception

Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark

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.[34] This helps sharks (particularly the hammerhead shark) find preySony VAIO PCG-7172L battery. 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.[35]

Lateral line

Main article: 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 HzSony VAIO PCG-7171L battery.

Life history

The claspers of male spotted wobbegong

Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has the longest lifespan at more than 100 years.[37] Whale sharks (Rhincodon typus) may also live over 100 years.[38]

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. Sony VAIO PCG-7162L battery 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. Sony VAIO PCG-7161L battery

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 behaviorSony VAIO PCG-7154L battery: the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these bites.

Asexual

There are two documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through parthenogenesis. The details of this process are not well understood, but genetic fingerprinting showed that the pups had no paternal genetic contribution, ruling out sperm storageSony VAIO PCG-7153L battery. The extent of this behavior in the wild is unknown, as is whether other species have this capability. Mammals are now the only major vertebrate group in which asexual reproduction has not been observed.

Scientists assert that asexual reproduction in the wild is rare, and probably a last-ditch effort to reproduce when a mate is not presentSony VAIO PCG-7152L battery. Asexual reproduction diminishes genetic diversity, which helps build defenses against threats to the species. Species that rely solely on it risk extinction. Asexual reproduction may have contributed to the blue shark's decline off the Irish coast.[44]

Brooding

Sharks display three ways to bear their young, varying by species, oviparity, viviparity and ovoviviparity. Sony VAIO PCG-7151L battery

The spiral egg case of a Port Jackson shark

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
Sony VAIO PCG-7148L battery
. Lamniforme sharks practice oophagy, where the first embryos to hatch eat the remaining eggs. Grey nurse shark pups intrauterine cannibalistically take this a step further and consume other developing 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 oviparousSony VPCW21M2E/WI battery, 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 monthsSony VPCW21C7E battery. Basking sharks and frilled sharks appear to have even longer gestation periods, but accurate data are lacking.[45]

Oviparity

Some species are oviparous like most other fish, laying their eggs in the water. In most oviparous shark species, an egg case with the consistency of leather protects the developing embryo(s). These cases may be corkscrewed into crevices for protection. Once empty, the egg case is known as the mermaid's purse, and can wash up on shore. Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swellsharkSony VPCW12S1E/WZ battery.

Viviparity

Finally some sharks maintain a placental link to the developing young, this method is called viviparity. This is 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 blue sharks), and smoothhounds are viviparousSony VPCW12S1E/W battery.

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 sedentary, benthic lives. Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year.[47] Shark migration patterns may be even more complex than in birds, with many sharks covering entire ocean basinsSony VPCW12S1E/T battery.

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.[17] Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding. Sony VPCW12S1E/P battery

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.[48]

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) Sony VPCW11S1E/W battery. 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).[49] 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.

Intelligence

Sharks possess brain-to-body mass ratios that are similar to mammals and birds,[50] and have exhibited apparent curiosity and behavior resembling play in the wildSony VPCW11S1E/T battery.

Sleep

Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers.[53] 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 "inhale" sand rather than waterSony VPCW11S1E/P battery. 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 spiny dogfish can continue to swim while sleeping.[53] It is also possible that sharks sleep in a manner similar to dolphins,[53] one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all timesSony VPCYA1S1E/B battery.

Ecology

Feeding

This section is about shark feeding. For the sport of shark feeding, see Shark baiting.

Like many sharks, the great white shark is an apex predator in its environment.

Most sharks are carnivorous.[54] Basking sharks, whale sharks, and megamouth sharks 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 fishesSony VPCYA1V9E/B battery, 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 mouthfulsSony VPCY21S1E/SI battery. Teeth in these species are comparatively small because they are not needed for feeding.[54]

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 feedingSony VPCY21S1E/L battery, they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rip off flesh.[17]

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.[55] Many benthic sharks feed solely on crustaceans which they crush with their flat molariform teethSony VPCY21S1E/G battery.

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 rostraSony VPCY11S1E/S battery.

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. Sony VPCY11S1E battery

Range and habitat

Sharks are found in all seas. They generally do not live in fresh water, with a few exceptions such as the bull shark and the river shark which can swim both in seawater and freshwater.[57] Sharks are common down to depths of 2,000 metres (7,000 ft), and some live even deeper, but they are almost entirely absent below 3,000 metres (10,000 ft). The deepest confirmed report of a shark is a Portuguese dogfish at 3,700 metres (12,100 ft) Sony VPCZ11X9E/B battery.

Relationship with humans

Attacks

A sign warning about the presence of sharks in Salt Rock, South Africa

Snorkeler swims near 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 attacksSony VPCY11S1E battery, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities worldwide per year between 2001 and 2006 from unprovoked shark attacks is 4.3.[59]

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. Sony VPCZ11X9E/B battery These sharks are large, powerful predators, and may sometimes attack and kill people. Despite being responsible for attacks on humans they have all been filmed without using a protective cage.[62]

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 1916Sony VPCZ11Z9E/B battery, and through popular fictional works about shark attacks, such as the Jaws film series. Jaws author Peter Benchley, as well as Jaws director Steven Spielberg later attempted to dispel the image of sharks as man-eating monsters.[63]

In captivity

A whale shark in Georgia Aquarium

Main article: Sharks in captivity

Until recently only a few benthic species of shark, such as hornsharks, leopard sharks and catsharks had survived in aquarium conditions for a year or moreSony VPCZ12M9E/B battery. This gave rise to the belief that sharks, as well as being difficult to capture and transport, were difficult to care for. More knowledge has led to more species (including the large pelagic sharks) living far longer in captivity. At the same time, safer transportation techniques have enabled long distance movement.[64] One shark that never had been successfully held in captivity for long was the great whiteSony VPCZ12X9E/X battery. But in September 2004 the Monterey Bay Aquarium successfully kept a young female for 198 days before releasing her.

Most species are not suitable for home aquaria and not every species sold by pet stores are appropriate. Some species can flourish in home saltwater aquaria.[65] Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse sharkSony VPCZ12Z9E/X battery, which upon reaching adulthood is far too large for typical home aquaria.[65] Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them.[65] Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of 3 feet and can live up to 25 years. Sony VPCZ13M9E/B battery

In Hawaii

Sharks figure prominently in Hawaiian mythology. Stories tell of men with shark jaws on their back who could change between shark and human form. A common theme was that a shark-man would warn beach-goers of sharks in the waters. The beach-goers would laugh and ignore the warnings and get eaten by the shark-man who warned themSony VPCZ13M9E/X battery. Hawaiian mythology also includes many shark gods. Among a fishing people, the most popular of all aumakua, or deified ancestor guardians, are shark aumakua. Kamaku describes in detail how to offer a corpse to become a shark. The body transforms gradually until the kahuna can point the awe-struck family to the markings on the shark's body that correspond to the clothing in which the beloved's body had been wrappedSony VPCZ13V9E battery. Such a shark aumakua becomes the family pet, receiving food, and driving fish into the family net and warding off danger. Like all aumakua it had evil uses such as helping kill enemies. The ruling chiefs typically forbade such sorcery. Many Native Hawaiian families claim such an aumakua, who is known by name to the whole community. Sony VPCZ13V9E/X battery

Kamohoali'i is the best known and revered of the shark gods, he was the older and favored brother of Pele,[67] and helped and journeyed with her to Hawaii. He was able to assume all human and fish forms. A summit cliff on the crater of Kilauea is 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'iSony VPCZ13Z9E/X battery. Kamohoali'i was an ancestral god, not a human who became a shark and banned the eating of humans after eating one herself. In Fijian mytholog, Dakuwanga was a shark god who was the eater of lost souls.

Popular misconceptions

A popular myth is that sharks are immune to disease and cancer; this is not scientifically supported. Sharks may get cancer. Both diseases and parasites affect sharksSony VPCZ21M9E battery. 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 show sharks to have heightened immunity to disease.[72] Other apparently false claims are that fins prevent cancer[73] and treat osteoarthritis.[74] No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment. Sony VPCZ21Q9E battery

Conservation

Further information: Shark sanctuary

The value of shark fins for shark fin soup has led to an increase in shark catches. Usually only the fins are taken, while the rest of the shark is discarded, usually into the sea.

Fishery

The annual shark catch has increased rapidly over the last 50 years.

It is estimated that 100 million sharks are killed by people every year, due to commercial and recreational fishing. Sharks are a common seafood in many places, including Japan and Australia. In the Australian state of Victoria

>Sony VPCZ21V9E battery, shark is the most commonly used fish in fish and chips, in which fillets are battered and deep-fried or crumbed and grilled. In fish and chip shops, shark is called flake. In India, small sharks or baby sharks (called sora in Tamil language, Telugu language) are sold in local markets. Since the flesh is not developed, cooking the flesh breaks it into powder, which is then fried in oil and spices (called sora puttu/sora poratu) Sony VPCEH3T9E battery. The soft bones can be easily chewed. They are considered a delicacy in coastal Tamil Nadu. Icelanders ferment Greenland sharks to produce hákarl, which is widely regarded as a national dish.[citation needed]

A 14-foot (4.3 m), 1,200-pound (540 kg) tiger shark caught in Kāne‘ohe Bay, Oahu in 1966

Sharks are often killed for shark fin soup. Fishermen capture live sharks, fin them, and dump the finless animal back into the water. Shark finning involves removing the fin with a hot metal blade. Sony VPCEH3N6E battery The resulting immobile shark soon dies from suffocation or predators.[78] Shark fin has become a major trade within black markets all over the world. Fins sell for about $300/lb in 2009.[79] Poachers illegally fin millions each year. Few governments enforce laws that protect them.[citation needed] In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins. Sony VPCEH3N1E battery

Shark fin soup is a status symbol in Asian countries, and is considered healthy and full of nutrients. Sharks are also killed for meat. European diners consume dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and rays.[81] However, the U.S. FDA lists sharks as one of four fish (with swordfish, king mackerel, and tilefish) whose high mercury content is hazardous to children and pregnant womenSony VPCEH3D0E battery.

Sharks generally reach sexual maturity only after many years and produce few offspring in comparison to other harvested fish. Harvesting sharks before they reproduce severely impacts future populations.

The majority of shark fisheries have little monitoring or management. The rise in demand for shark products increases pressure on fisheries. Sony VPCEH3B1E battery Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with population declines of 70% not unusual.[82] Many governments and the UN have acknowledged the need for shark fisheries management, but little progress has been made due to their low economic value, the small volumes of products produced and sharks' poor public image. Sony VPCEH2Z1E battery

Other threats

Other threats include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species.[83] The 2007 documentary, Sharkwater exposed how sharks are being hunted to extinction.[84]

Protection

In 1991 South Africa was the first country in the world to declare Great White sharks a legally protected species. Sony VPCEH2S9E battery

Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000.[86] Two years later the Act saw its first legal challenge in United States v. Approximately 64,695 Pounds of Shark Fins. In 2008 a Federal Appeals Court ruled that a loophole in the law allowed non-fishing vessels to purchase shark fins from fishing vessels while on the high seas. Sony VPCEH2Q1E battery Seeking to close the loophole, the Shark Conservation Act was passed by Congress in December 2010, and it was signed into law in January 2011.[88][89]

In 2009, the International Union for the Conservation of Nature (IUCN) Red List of Endangered Species named 64 species, one-third of all oceanic shark species, as being at risk of extinction due to fishing and shark finningSony VPCEH2P0E battery.

In 2010, the Convention on International Trade in Endangered Species (CITES) rejected proposals from the United States and Palau that would have required countries to strictly regulate trade in several species of scalloped hammerhead, oceanic whitetip and spiny dogfish sharks. The majority, but not the required two-thirds of voting delegatesSony VPCEH2N1E battery, approved the proposal. China, by far the world’s largest shark market, and Japan, which battles all attempts to extend the convention to marine species, led the opposition.

In 2010, Greenpeace International added the school shark, shortfin mako shark, mackerel shark, tiger shark and spiny dogfish to its seafood red list, a list of common supermarket fish that are often sourced from unsustainable fisheries.Sony VPCEH2M9E battery Advocacy group Shark Trust campaigns to limit shark fishing. Advocacy group Seafood Watch directs American consumers to not eat sharks.[95]

Evolution

A collection of Cretaceous shark teeth

Evidence for the existence of sharks dates from the Ordovician period, over 450–420 million years ago, before land vertebrates existed and before many plants had colonized the continents. Sony VPCEH2M1E battery Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks.[96] The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.[96] The first sharks looked very different from modern sharks.[97] The majority of modern sharks can be traced back to around 100 million years ago. Sony VPCEH2L9E battery 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 prismsSony VPCEH2J1E battery. Preservation requires rapid burial in bottom sediments.

Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago,[97] 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 AmericaSony VPCEH2H1E battery. Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws.[97] 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 makosSony VPCEH2F1E battery. 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 hybodontsSony VPCEH2E0E battery, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater.

Megalodon with the whale shark, great white shark, and a human for scale

Modern sharks began to appear about 100 million years ago.[98] Fossil mackerel shark teeth date to the Lower Cretaceous. One of the most recently evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene. Sony VPCEH2D0E battery 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 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 teethSony VPCEH2C0E battery. 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. Sony VPCEH1Z1E battery Paleontological evidence suggests that this shark was an active predator of large cetaceans.

Taxonomy

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: some sharks are more closely related to rays than they are to some other sharks.Sony VPCEH1S9E battery

The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea. The Galeans are the Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and Carcharhinoids are usually placed in one clade, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be SqualeanSony VPCEH1S8E battery. The Squalea is divided into Hexanchoidei and Squalomorpha. The Hexanchoidei includes the Hexanchiformes and Chlamydoselachiformes. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the BatoideaSony VPCEH1S1E battery.

More than 440 species of sharks split across eight orders, listed below in roughly their evolutionary relationship from ancient to modern:[106]

Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even a shark that resembles a marine snake.

Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and prickly sharkSony VPCEH1S0E battery.

Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing their prey.

Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong resemblance to stingrays and skates.

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 sharkSony VPCEH1M9E battery.

Carcharhiniformes: Commonly known as groundsharks, the species include the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip 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 attackSony VPCEH1M1E battery.

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 megalodonSony VPCEH1L9E battery.

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