Sharks ：characterized by a cartilaginous skeleton

http://www.all-keyboard.com/,http://www.keyboards-shop.com/,http://www.laptop-fan-shop.com/,http://www.laptopkeyboard-shop.com/ 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 (orSelachii), and are the sister group to the rays. Sony  VGP-BPS13 Battery However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.^[1] Since that time, sharks have diversified into over 470 species.Sony  VAIO VGP-BPS13A/B Battery They range in size from the small dwarf lanternshark(Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft). Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft). Sony  VAIO VGP-BPS13B/B Battery They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive 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. Sony  VAIO VGP-BPS13/S Battery They also have several sets of replaceable teeth.^[3] 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 activities. Sony  VAIO VGP-BPS13A/S Battery 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 theYucatec 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 Sea. Sony  VAIO VGP-BPS13B/S 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. Evidence for the existence of sharks dates from the Ordovician period, over 450–420 million years ago, before landvertebrates existed and before many plants had colonized the continents. Sony  VAIO VGP-BPS13/Q Battery Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks.^[8] The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.^[8] The first sharks looked very different from modern sharks.^[9] The majority of modern sharks can be traced back to around 100 million years ago.^[10] Sony  VAIO VGP-BPS13A/Q 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 prisms. Sony  VAIO VGP-BPS13B/Q Battery Preservation requires rapid burial in bottom sediments. Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago,^[9] which has been found within Paleozoic strata in Ohio, Kentuckyand Tennessee. At that point in Earth's history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America. Sony  VGP-BPS21 Battery Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws.^[9] Its teeth had several pointed cusps, which wore down from use. From the small number of teeth found together, it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. Sony  VGP-BPS21A 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.^[11][12] By the time this group became extinct about 220 million years ago, they had spread worldwide. Sony  VGP-BPS21B Battery The other group, the hybodonts, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater. Modern sharks began to appear about 100 million years ago.^[10] Fossil mackerel shark teeth date to the Early Cretaceous. One of the most recently evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene.^[13] Sony  VGP-BPS21/S 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 teeth. Sony  VGP-BPS21A/b 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).^[14][15] C. megalodon may have approached a maxima of 20.3 metres (67 ft) in total length and 103metric tons (114 short tons) in mass.^[16] Sony VGP-BPS26 Battery Paleontological evidence suggests that this shark was an active predator of largecetaceans. 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. Sony VGP-BPL26 Battery 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.^[17] The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea. The Galeans are theHeterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Sony VGP-BPS26A Battery 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 Squalean. The Squalea is divided into Hexanchoidei and Squalomorpha. The Hexanchoidei includes the Hexanchiformes and Chlamydoselachiformes. Sony VGP-BPS22 Battery The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes theSquatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and theBatoidea. More than 440 species of sharks split across eight orders, listed below in roughly their evolutionary relationship from ancient to modern:^[18] Sony VGP-BPL22 Battery

• Hexanchiformes: Examples from this group include the cow sharks and frilled shark, which somewhat resembles a marine snake.
• Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and prickly shark.

Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing their prey. Sony VGP-BPS22A Battery

• 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 shark. Sony VAIO PCG-3B1M 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 attack. Sony VAIO PCG-3D1M Battery Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin andlongfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes include the extinctmegalodon, Carcharodon megalodon. Sony VAIO PCG-3G2M Battery Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. Sony VAIO PCG-5R1M 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.^[19] Tooth shape depends on the shark's diet: those that feed on mollusks and crustaceanshave dense and flattened teeth used for crushing, Sony VAIO PCG-7162M Battery those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional. Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sony VAIO PCG-7181M Battery Sharks and other cartilaginous fish(skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy.^[21] Because sharks do not have rib cages, they can easily be crushed under their own weight on land. Sony VAIO PCG-41112M Battery Jaws of sharks, like those of rays and skates, are not attached to the cranium. The jaw's surface (in comparison to the shark's vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic.^[23] Sony VAIO PCG-7153M Battery 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.^[21] Sony VAIO PCG-71312M Battery In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts. Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hairand feathers.^[24] Most sharks have eight fins. Sony VAIO PCG-7144M Battery 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. 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.^[25Sony VAIO PCG-7191L Battery ^] Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming. Tails provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion.  Sony VAIO PCG-3C1M 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 efficientlocomotion among these negatively buoyant cartilaginous fish. By contrast, most bony fish possess a homocercal caudal fin.^[26] Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. Sony VAIO PCG-3F1M Battery The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey.^[27] Other tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid. Sony VAIO PCG-3H1M Battery 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.^[25] Their liver constitutes up to 30% of their total body mass.^[28] The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth when not swimming. Sony VAIO PCG-3J1M 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.^[29] Sony VAIO PCG-8141M 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. 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. Sony VAIO PCG-8161M Battery A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagic sharks.^[20] While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation".Sony VAIO PCG-3C2M Battery While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.^[31] Sony VAIO PCG-5N2M 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 theefferent brachial arteries, which come together to form the dorsal aorta. Sony VAIO PCG-5P1M Battery 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 heartventricle and the cycle repeats. Most sharks are "cold-blooded" or, more precisely, poikilothermic, meaning that their internal body temperature matches that of their ambient environment. Sony VAIO PCG-5S1M Battery Members of the family Lamnidae (such as the shortfin mako shark and the great white shark) are homeothermic and maintain a higher body temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, Sony VAIO PCG-9Z1M Battery which the body retains via acountercurrent 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. Sony VAIO PCG-7171M Battery In contrast to bony fish, with the exception of the coelacanth,^[34] 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. Sony VAIO PCG-7186M Battery This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments. A few exceptions exist, such as the bull shark, which has developed a way to change its kidney function to excrete large amounts of urea.^[28] When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia. Sony VAIO PCG-81112M Battery 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.^[37] Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth. Sony VAIO PCG-31111M 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-31311M Battery 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.^[38] Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. Sony VAIO PCG-8152M Battery 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 insewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.Sony VAIO PCG-61111M Battery 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 do. Sony PCG-71213M 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 membranes. Sony PCG-71311M 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 debated. Sony PCG-71312M 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 andstereoscopic at any time. Sony PCG-71212M 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 ofcone photoreceptor sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. Sony PCG-71811M Battery The study indicates that an object's contrast against the background, rather than colour, may be more important for object detection. Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away.Sony PCG-71911M Battery A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, and is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. Sony PCG-91211M Battery In bony fish and tetrapods the external opening into the inner ear has been lost. 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.^[45] This helps sharks (particularly the hammerhead shark) find prey. Sony PCG-91111M 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. Sony VAIO PCG-5J4M Battery 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.^[50] Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity at around age 13–15. Sony VAIO PCG-5K1M Battery Sharks practice internal fertilization. The posterior part of a male shark's pelvic fins are modified into a pair ofintromittent organs called claspers, analogous to a mammalian penis, of which one is used to deliver sperm into the female.^[52] Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. Sony VAIO PCG-5K2M Battery In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female's oviduct. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during mating. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. Sony VAIO PCG-5J5M Battery In some species, females have evolved thicker skin to withstand these bites. 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 storage. Sony VAIO PCG-5L2M 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 present. Sony VAIO PCG-6S4M 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. Sony VAIO PCG-6W1M Battery 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-6W2M Battery Lamniforme sharks practice oophagy, where the first embryos to hatch eat the remaining eggs. Taking this a step further,sand tiger shark pups cannibalistically consume neighboring embryos. The survival strategy for ovoviviparous species is to brood the young to a comparatively large size before birth. Sony VAIO PCG-7Z1M Battery The whale shark is now classified as ovoviviparous rather than oviparous, because extrauterine eggs are now thought to have been aborted. Most ovoviviparous sharks give birth in sheltered areas, including bays, river mouths and shallow reefs. They choose such areas for protection from predators (mainly other sharks) and the abundance of food. Sony VAIO PCG-8Y2M Battery Dogfish have the longest known gestation period of any shark, at 18 to 24 months. Basking sharks andfrilled sharks appear to have even longer gestation periods, but accurate data are lacking. Some species are oviparous like most other fish, laying their eggs in the water. In most oviparous shark species, an egg casewith the consistency of leather protects the developing embryo(s). Sony VAIO PCG-8Y3M Battery These cases may be corkscrewed into crevices for protection. Once empty, the egg case is known as a mermaid's purse, and can wash up on shore. Oviparous sharks include thehorn shark, catshark, Port Jackson shark, and swellshark. Finally some sharks maintain a placental link to the developing young, this method is called viviparity. Sony VAIO PCG-8Z1M Battery This is more analogous to mammalian gestation than that of other fishes. The young are born alive and fully functional. Hammerheads, therequiem sharks (such as the bull and blue sharks), and smoothhounds are viviparous. The classic view describes a solitary hunter, ranging the oceans in search of food. Sony VAIO PCG-8Z2M Battery 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.^[58] Shark migration patterns may be even more complex than in birds, with many sharks covering entire ocean basins. Sony VAIO PCG-8Z3M Battery Sharks can be highly social, remaining in large schools. Sometimes more than 100 scalloped hammerheads congregate around seamounts and islands, e.g., in theGulf of California.^[28] Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding.^[50] Sony VAIO PCG-7112M 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. 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 VGP-BPL15/B 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).^[60] 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.