) 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.
should stop killing sharks what have they ever done i understand they kill people but they
don't know any better"
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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 17 centimetres (6.7 in) in length, to the whale shark, Rhincodon
typus, the largest fish, which reaches approximately 12 metres (39 ft 4 in)
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 2,000 metres
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
|Zebra shark (Stegostoma fasciatum) off Mozambique
Until the 16th century,
sharks were known to mariners as "sea dogs".
The word "shark" almost certainly 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 to refer to the large sharks of the Caribbean Sea.
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.
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.
|Jaws Great White Shark South Australia
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.
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
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.
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
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.
In the past, sharkskin has been used as sandpaper. Their dermal teeth give them
hydrodynamic advantages as they reduce turbulence when swimming.
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.
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.
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
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
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.
Click image to see the different parts of a shark
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
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
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
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 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
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.
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
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
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.
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
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
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.
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.
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.
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
|A collection of fossilised shark teeth
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.
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
Sharks mature slowly relative to many other fish. For example, lemon sharks
reach sexual maturity at around age 13–15.
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
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
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.
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
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
|Egg case of Port Jackson shark - found on Vincentia beach, Jervis Bay
Sharks display three ways to bear their young, varying by species, oviparity,
viviparity and 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.
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.
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.
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.
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.
|Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the
head of a shark.
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.
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).
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). 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.
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.
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.
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
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.
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
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 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).
Relation to humans
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.
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.
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
A popular myth is that sharks are immune to disease and cancer; however, this
remains unproven. 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.
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 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.
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.
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 1 metre (3.3 ft) 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
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
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.
Paleontological evidence suggests that this shark was an active predator of
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
- 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.
|they should stop killing sharks what have they ever done i understand they kill people but they
don't know any better