Sharks (
superorder Selachimorpha) are a type of
fish with a full
cartilaginous skeleton and a highly
streamlined body. The earliest known sharks date from more than 420 million years ago.
Since that time, sharks have diversified into 440 species, ranging in size from the small dwarf lanternshark, ''Etmopterus perryi'', a deep sea species of only in length, to the whale shark, ''Rhincodon typus'', the largest fish, which reaches approximately and which feeds only on plankton, squid, and small fish by filter feeding. Sharks are found in all seas and are common down to depths of . They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites, and improves their fluid dynamics so the shark can move faster. They have several sets of replaceable teeth.
Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead are apex predators, at the top of the underwater food chain. Their extraordinary skills as predators fascinate and frighten humans, even as their survival is under serious threat from fishing and other human activities.
Etymology
Until the 16th century, sharks were known to mariners as "sea dogs".
The etymology of the word "shark" is uncertain. One theory is that it derives from the
Yucatec Maya word ''xok'', pronounced 'shok'.
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.
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.
Anatomy
right |440px |The major features of sharks|alt=Drawing of a shark labeling major anatomical features, including mouth, snout, nostril, eye, spiracle, dorsal fin spine, caudal keel, clasper, labial furrows, gill openings, precaudal pit and fins: first and second dorsal, anal, pectoral, caudal and pelvic.
Teeth
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 as in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species teeth are replaced one at a time, except in
cookiecutter sharks the entire row of teeth is replaced simultaneously.
Tooth shape depends on diet: sharks that feed on mollusks and crustaceans have dense flattened teeth for crushing, 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 smaller and non-functional.
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 durable, yet has about half the density of bone. This reduces the skeleton’s weight, saving energy. Sharks have no rib cage and therefore on land a shark's own weight can crush it.
Jaw
Like its relatives,
rays and
skates, the shark's
jaw is not attached to the
cranium. The jaw's surface, like 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. 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. 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. 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.
Dermal denticles
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. In the past, sharkskin has been used as
sandpaper. Their dermal teeth give them
hydrodynamic advantages as they reduce
turbulence when
swimming.
Tails
right|thumb|300px |The range of shark tail shapes|alt=Drawing of several shark tails. Varying tail shapes have evolved in sharks adapted for different environments.
Tail (
caudal fins) vary considerably between species. The tail provides thrust, making speed and acceleration dependent on tail shape. Sharks possess a
heterocercal caudal fin in which the
dorsal portion is usually noticeably larger than the
ventral portion. 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 fishes. By contrast, most bony fishes possess a homocercal caudal fin.
The tiger shark's tail has a large upper lobe which delivers maximum power for slow cruising or 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, which hunts schooling fish such as mackerel and herring has a large lower lobe to help it keep pace with its fast-swimming prey. Some tail adaptations have other purposes. The thresher feeds on fish and squid, which it herds and stuns with its powerful and elongated upper lobe.
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 the fact that cartilage is about half as dense as bone. The liver constitutes up to 30% of their body mass. The liver's effectiveness is limited, so sharks employ
dynamic lift to maintain depth, sinking when they stop swimming.
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, if at all, without sinking. However certain shark species, like the
nurse shark, are capable of pumping water across their gills, allowing them to rest on the ocean bottom.
Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely.
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
eye; the spiracle assists water intake during
respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active
pelagic sharks. 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 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.
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 arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the heart ventricle and the cycle repeats.
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 water. 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.
Osmoregulation
In contrast to bony fish, with the exception of the
Coelacanth, the blood and other tissue of sharks and
Chondrichthyes in general is
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 fresh water, and they are therefore confined to
marine environments. A few exceptions to this rule exist, such as the
bull shark which has developed a way to change its
kidney function to excrete large amounts of urea. When a shark dies the urea is broken down to ammonia by bacteria — because of this, the dead body will gradually start to smell strongly of ammonia.
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. 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.
One of the biggest differences between shark and mammalian digestion is sharks’ extremely short intestine. 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.
Senses
Smell
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.
Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is similar to the method mammals use to determine direction of sound.
They are more attracted to the chemicals found in the guts of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.
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 do. 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. 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. 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. 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 colorblind. 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. 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. In bony fish and
tetrapods the external opening into the inner ear has been lost.
Electroreception
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. This helps sharks (particularly the
hammerhead shark) find prey. 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.
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
Hz.
Life history
Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has the longest lifespan at more than 100 years. Whale sharks (''Rhincodon typus'') may also live over 100 years.
Reproduction
Unlike most
bony fishes, 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. 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.
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 behavior: 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 storage. 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. 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.
Brooding
Sharks display three ways to bear their young, varying by species,
oviparity,
viviparity and
ovoviviparity.
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.
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 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.
Dogfish have the longest known
gestation period of any shark, at 18 to 24 months.
Basking sharks and
frilled sharks appear to have even longer gestation periods, but accurate data are lacking.
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
swellshark.
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 viviparous.
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. Shark migration patterns may be even more complex than in birds, with many sharks covering entire
ocean basins.
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. Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding.
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.
Speed
In general, sharks swim ("cruise") at an average speed of but when feeding or attacking, the average shark can reach speeds upwards of . The
shortfin mako shark, the fastest shark and one of the fastest fish, can burst at speeds up to . 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
Contrary to the common wisdom that sharks are instinct-driven "eating machines", recent studies have indicated that many species possess powerful
problem-solving skills,
social skills and curiosity. The brain- to body-mass ratios of sharks are similar to mammals and birds. In 1987, near Smitswinkle Bay,
South Africa, a group of up to seven
great white sharks worked together to move a partially beached dead whale to deeper waters to feed. Sharks can engage in playful activities.
Porbeagle sharks have been seen repeatedly rolling in kelp and chasing an individual who trailed a piece of kelp behind it.
Sleep
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its
nares, but rather its
spiracles. If a shark tried to use its nares while resting on the
ocean floor, it would "inhale" sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The
spiny dogfish's
spinal cord, rather than its brain, coordinates swimming, so
spiny dogfish can continue to swim while sleeping. It is also possible that sharks sleep in a manner similar to
dolphins, one
cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.
Ecology
Feeding
Most sharks are carnivorous. Some species, including tiger sharks, eat almost anything. The vast majority seek particular prey, and rarely vary their diet. Whale, basking and megamouth sharks filter feed. These three independently evolved plankton feeding using different strategies. Whale sharks use suction to take in plankton and small fishes. Basking sharks are ram-feeders, swimming through plankton blooms with their mouth wide open. Megamouth sharks make suction feeding more efficient, using luminescent tissue inside the mouth 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 mouthfuls. Teeth in these species are comparatively small because they are not needed for feeding.
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 feeding, they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rip off flesh.
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. Many benthic sharks feed solely on crustaceans which they crush with their flat molariform teeth.
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 rostra.
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.
Range and habitat
Sharks are found in all seas. They generally do not live in freshwater, with a few exceptions such as the
bull shark and the
river shark which can swim both in seawater and freshwater. Sharks are common down to depths of , and some live even deeper, but they are almost entirely absent below . The deepest confirmed report of a shark is a
Portuguese dogfish at .
Relation to humans
Attacks
In 2006 the
International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities worldwide per year between 2001 and 2006 from unprovoked shark attacks is 4.3.
Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 360 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and bull sharks.
These sharks 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.
The perception of sharks as dangerous animals has been popularized by publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through popular fictional works about shark attacks, such as the ''Jaws'' film series. ''Jaws'' author Peter Benchley, as well as Jaws (film) director Stephen Spielberg later attempted to dispel the image of sharks as man-eating monsters.
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 more. 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. One shark that never had been successfully held in captivity for long was the great white. 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. Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far too large for typical home aquaria. Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them. 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.
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 them.
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 wrapped. 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.
Kamohoali'i is the best known and revered of the shark gods, he was the older and favored brother of Pele, and helped and journeyed with her to Hawaii. He was able to assume all human and fish forms. A summit cliff on the crater of K?lauea 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'i. 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 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 show sharks to have heightened immunity to disease.
Other apparently false claims are that fins prevent
cancer and treat
osteoarthritis. No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment.
Conservation
Fishery
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, 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). 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.
Sharks are often killed for shark fin soup. Fishermen capture live sharks, fin them, and dump the finless animal back into the water. Finning involves removing the fin with a hot metal blade. The resulting immobile shark soon dies from suffocation or predators. Shark fin has become a major trade within black markets all over the world. Fins sell for about $300/lb in 2009. Poachers illegally fin millions each year. Few governments enforce laws that protect them. In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins.
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. 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 women.
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. 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. 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.
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.
Shark finning attracts much controversy and regulations are being enacted to prevent it from occurring. The 2007 documentary, ''
Sharkwater'' exposed how sharks are being hunted to extinction.
Protection
In 1991 South Africa was the first country in the world to declare Great White sharks a legally protected species.
In 2009, the Shark Conservation Act of 2009, passed the U.S. House of Representatives. A similar bill is pending in the 2010 U.S. Senate. The bill would strengthen existing shark finning laws.
In 2010, the Convention on International Trade in Endangered Species 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 delegates, 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. Advocacy group Shark Trust campaigns to limit shark fishing. Advocacy group Seafood Watch directs American consumers to not eat sharks.
Evolution
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. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period. The first sharks looked very different from modern sharks. The majority of modern sharks can be traced back to around 100 million years ago. 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. Preservation requires rapid burial in bottom sediments.
Among the most ancient and primitive sharks is ''Cladoselache'', from about 370 million years ago, 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 America. ''Cladoselache'' was only about long with stiff triangular fins and slender jaws. 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. 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 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. 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. 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. 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 . ''C. megalodon'' may have approached a maxima of in total length and in mass. 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.
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 Squalean. 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 Batoidea.
More than 440 species of sharks split across eight orders, listed below in roughly their evolutionary relationship from ancient to modern:
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 shark.
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 shark.
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.
Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes include the extinct megalodon, ''Carcharodon megalodon''.
See also
List of sharks
List of prehistoric cartilaginous fish
Marine vertebrates
Outline of sharks
Shark finning
Shark sanctuary
Shark Week
References
;General references
Clover, Charles. 2004. ''The End of the Line: How overfishing is changing the world and what we eat''. Ebury Press, London. ISBN 0-09-189780-7
Owen, David, ''Shark: In Peril in the Sea'', New South Wales, Allen and Unwin, 2009. ISBN 978-1-74175-032-4
Moss, Jillian. 2010. "The Invention of Sharks, AKA: I Read It On the Internet". University of North Carolina Publishing, Raleigh, USA. (Publishing Pending)
External links
WikiAnswers: questions and answers about Sharks
Oceana's International Shark Campaign
South African Marine Predator Lab a research institute studying marine top predators.
Shark Research Institute
Greenland Shark and Elasmobranch Education and Research Group
The International Shark Attack File
Shark Trust Organization
ReefQuest Centre for Shark Research
ECOCEAN Whale Shark Photo-identification Library
Updated list of all known shark species
Global Shark Attack File
Shark Research Committee
Shark Pictures and Elasmobranch Field Guide
Shark pictures on Morphbank
Shark pictures by Wolfgang Leander
Shark Diversity by The Smithsonian Institution: Ocean Portal
Sharks.org.za A detailed study
BBC Nature: Shark news, and video clips from BBC programmes past and present.
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Category:Ichthyology
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![The egg capsule of a swell shark (on the right).The swell shark is oviparous and they lay two green or amber[7] flattened eggs which are attached to the reef with tendrils.](http://web.archive.org./web/20120419075016/http://cdn3.wn.com/pd/94/1a/592fdbab62b581362dcba7771837_medium.jpg "The egg capsule of a swell shark (on the right).The swell shark is oviparous and they lay two green or amber[7] flattened eggs which are attached to the reef with tendrils.")
