Main article: Physical characteristics of sharks
Teeth
Main article: Shark teeth
The teeth of tiger
sharks are oblique and serrated to saw through
flesh
Shark teeth are embedded in the gums rather than directly affixed to the jaw, and
are constantly replaced throughout life. Multiple rows of replacement teeth grow
in a groove on the inside of the jaw and steadily move forward in comparison to
a conveyor belt; some sharks lose 30,000 or more
teeth in their lifetime. The rate of tooth replacement varies from once every 8
to 10 days to several months. In most species, teeth are replaced one at a time
as opposed to the simultaneous replacement of an entire row, which is observed
in the cookiecutter shark.[8]
Tooth shape depends on the shark's diet: those that feed on mollusks
and crustaceans have dense and flattened teeth used
for crushing, those that feed on fish have needle-like teeth for 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.[9]
Skeleton
Shark skeletons are very different from those of bony
fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and
durable, yet is about half the normal density of bone. This reduces the
skeleton’s weight, saving energy.[10]
Because sharks do not have rib cages, they can easily be crushed under their own
weight on land.[11]
Jaw
Jaws
of sharks, like those of rays and skates, are not attached to the cranium.
The jaw's surface (in comparison to the shark's vertebrae and gill arches) needs extra support
due to its heavy exposure to physical stress and its need for strength. It has a
layer of tiny hexagonal plates called "tesserae",
which are crystal blocks of calcium salts arranged as a mosaic.[12] This
gives these areas much of the same strength found in the bony tissue found in
other animals.
Generally sharks have only one layer of tesserae, but the jaws of large
specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or
more, depending on body size. The jaws of a large great white shark may have up to five layers.[10] In
the rostrum (snout), the cartilage can be spongy and
flexible to absorb the power of impacts.
Fins
Fin skeletons are elongated and supported with soft and unsegmented rays
named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.[13] Most
sharks have eight fins. Sharks can only drift away from objects directly in
front of them because their fins do not allow them to move in the tail-first
direction.[11]
Dermal
denticles
Main article: Dermal denticle
Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This
works as an outer skeleton, providing attachment for their swimming muscles and
thus saving energy.[14] Their
dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.[8]
Tails
Tails
provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark
species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than
the ventral 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 fish. By contrast, most
bony fish possess a homocercal caudal fin.[15]
Tiger sharks have a large upper lobe, which allows for slow cruising and sudden
bursts of speed. The tiger shark must be able to twist and turn in the water
easily when hunting to support its varied diet, whereas the porbeagle
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.[16] Other
tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated
upper lobe to stun fish and squid.
Physiology
Buoyancy
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy.
Instead, sharks rely on a large liver filled with oil that contains squalene, and their cartilage, which is about
half the normal density of bone.[14] Their
liver constitutes up to 30% of their total body mass.[17] The
liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth when not 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 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.[18]
Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition
to handle sharks safely.[19]
Respiration
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are
not covered, but lie in a row behind the head. A modified slit called a spiracle
lies just behind the 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.[9] While
the shark is moving, water passes through the mouth and over the gills in a
process known as "ram ventilation". While at rest, most sharks pump water over
their gills to ensure a constant supply of oxygenated 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.[20]
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.[21]
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[not in citation
given].[22]
Osmoregulation
In contrast to bony fish, with the exception of the coelacanth,[23] the
blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of
the high concentration of urea and trimethylamine N-oxide (TMAO), allowing them to
be in osmotic balance with the seawater. This
adaptation prevents most sharks from surviving in 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.[17] When
a shark dies, the urea is broken down to ammonia by bacteria, causing the dead
body to gradually smell strongly of ammonia.[24][25]
Digestion
Digestion can take a long time. The food moves from the mouth to a J-shaped
stomach, where it is stored and initial digestion occurs.[26]
Unwanted items may never get past the stomach, and instead the shark either
vomits or turns its stomachs inside out and ejects unwanted items from its
mouth.
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.[26]
Senses
Smell
The shape of the hammerhead shark's head may enhance olfaction by
spacing the nostrils further apart.
Sharks have keen olfactory senses, located in the short duct
(which is not fused, unlike bony fish) between the anterior and posterior nasal
openings, with some species able to detect as little as one part per million of blood in seawater.[27]
Sharks have the ability to determine the direction of a given scent based on
the timing of scent detection in each nostril.[28] This
is similar to the method mammals use to determine direction of sound.
They are more attracted to the chemicals found in the intestines of many
species, and as a result often linger near or in sewage outfalls. Some species, such as nurse
sharks, have external barbels that greatly increase their ability to
sense 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.[29] 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.[30] [31][32]
Hearing
Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many
miles away.[33] A
small opening on each side of their heads (not the spiracle) leads directly into the inner
ear through a thin 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
Main article: Electroreception
Electromagnetic field receptors
(ampullae of Lorenzini) and motion detecting canals in the head of a
shark
The ampullae of Lorenzini are the electroreceptor organs. They number in the
hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things
produce.[34] This
helps sharks (particularly the hammerhead shark) find 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.[35]
Lateral line
Main article: Lateral line
This system is found in most fish, including sharks. It detects motion or
vibrations in water. The shark can sense frequencies in the range of 25 to 50 Hz.[36]
Life history
The claspers of male spotted wobbegong
Shark lifespans vary by species. Most live 20 to 30 years. The spiny
dogfish has the longest lifespan at more than 100 years.[37] Whale sharks (Rhincodon typus) may also
live over 100 years.[38]
Reproduction
Unlike most bony fish, sharks are K-selected
reproducers, meaning that they produce a small number of well-developed young as
opposed to a large number of poorly developed young. Fecundity in sharks ranges from 2 to over 100
young per reproductive cycle.[39]
Sharks mature slowly relative to many other fish. For example, lemon
sharks reach sexual maturity at around age 13–15.[40]
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.[41]
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.[41]
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.[42][43] 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.[44]
Brooding
Sharks display three ways to bear their young, varying by species, oviparity,
viviparity and ovoviviparity.[45]
The spiral egg case of a Port Jackson shark
Ovoviviparity
Most sharks are ovoviviparous, meaning that the eggs hatch in the
oviduct
within the mother's body and that the egg's yolk and fluids secreted by glands in the walls
of the oviduct nourishes the embryos. The young continue
to be nourished by the remnants of the yolk and the oviduct's fluids. As in
viviparity, the young are born alive and fully functional. 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.[45]
Oviparity
Some species are oviparous like most other fish, laying their eggs
in the water. In most oviparous shark species, an egg case with the consistency of leather protects the developing embryo(s). These
cases may be corkscrewed into crevices for protection. Once empty, the egg case
is known as the mermaid's purse, and can wash up on shore.
Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swellshark.[45][46]
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.[39][45]
Behavior
The classic view describes a solitary hunter, ranging the oceans in search of
food. However, this applies to only a few species. Most live far more sedentary,
benthic lives. Even solitary sharks meet for
breeding or at rich hunting grounds, which may lead them to cover thousands of
miles in a year.[47] Shark
migration patterns may be even more complex than in birds, with many sharks
covering entire ocean 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.[17]
Cross-species social hierarchies exist. For example, oceanic
whitetip sharks dominate silky sharks of comparable size during
feeding.[39]
When approached too closely some sharks perform a threat
display. This usually consists of exaggerated swimming movements,
and can vary in intensity according to the threat level.[48]
Speed
In general, sharks swim ("cruise") at an average speed of 8 kilometres per
hour (5.0 mph) but when feeding or attacking, the average shark can reach speeds
upwards of 19 kilometres per hour (12 mph). The shortfin mako shark, the fastest shark and one of
the fastest fish, can burst at speeds up to 50 kilometres per hour (31 mph).[49] The
great white shark is also capable of speed
bursts. These exceptions may be due to the warm-blooded, or homeothermic, nature of these sharks'
physiology.
Intelligence
Sharks possess brain-to-body mass ratios that are similar to mammals and
birds,[50] and
have exhibited apparent curiosity and behavior resembling play in the wild.[51][52]
Sleep
Some sharks can lie on the bottom while actively pumping water over their
gills, but their eyes remain open and actively follow divers.[53] When
a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares
while resting on the ocean floor, it would "inhale" sand rather than
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.[53] It is
also possible that sharks sleep in a manner similar to dolphins,[53] one
cerebral hemisphere at a time, thus maintaining
some consciousness and cerebral activity at all times.
Ecology
Feeding
This section is about shark feeding. For the sport of
shark feeding, see Shark baiting.
Like many sharks, the great white shark is an
apex
predator in its environment.
Most sharks are carnivorous.[54] Basking sharks, whale sharks, and megamouth sharks sharks have independently
evolved different strategies for filter feeding plankton: basking sharks practice ram feeding, whale sharks use suction to take in
plankton and small fishes, and megamouth sharks make suction feeding more efficient by using the luminescent tissue inside of their mouths to
attract prey in the deep ocean. This type of feeding requires gill
rakers—long, slender filaments that form a very efficient sieve—analogous to the baleen plates of the great whales. The shark traps the plankton in
these filaments and swallows from time to time in huge mouthfuls. Teeth in these
species are comparatively small because they are not needed for feeding.[54]
Other highly specialized feeders include cookiecutter sharks, which feed on flesh sliced
out of other larger fish and marine mammals. Cookiecutter teeth are enormous
compared to the animal's size. The lower teeth are particularly sharp. Although
they have never been observed 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.[17]
Some seabed–dwelling species are highly effective ambush predators. Angel
sharks and wobbegongs use camouflage to lie in wait and suck
prey into their mouths.[55] Many
benthic sharks feed solely on crustaceans which they crush with their flat molariform 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.[56]
Range and
habitat
Sharks are found in all seas. They generally do not live in fresh water, with
a few exceptions such as the bull shark and the river shark which can swim both in seawater and
freshwater.[57]
Sharks are common down to depths of 2,000 metres (7,000 ft), and some live even
deeper, but they are almost entirely absent below 3,000 metres (10,000 ft). The
deepest confirmed report of a shark is a Portuguese dogfish at 3,700 metres (12,100
ft).[58]
Relationship with
humans
Attacks
A sign warning about the presence of sharks in
Salt Rock, South Africa
Snorkeler swims near blacktip reef shark. In rare circumstances
involving poor visibility, blacktips may bite a human, mistaking it for prey.
Under normal conditions they are harmless and shy.
Main article: Shark
attack
In 2006 the International Shark Attack File (ISAF) undertook
an investigation into 96 alleged shark 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.[59]
Contrary to popular belief, only a few sharks are dangerous to humans. Out of
more than 360 species, only four have been involved in a significant number of
fatal, unprovoked attacks on humans: the great white, oceanic
whitetip, tiger, and bull sharks.[60] [61] These
sharks are large, powerful predators, and may sometimes attack and kill people.
Despite being responsible for attacks on humans they have all been filmed
without using a protective cage.[62]
The perception of sharks as dangerous animals has been popularized by
publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through
popular fictional works about shark attacks, such as the Jaws film series. Jaws author Peter Benchley, as well as Jaws director Steven Spielberg later attempted to dispel the
image of sharks as man-eating monsters.[63]
In captivity
A whale shark in Georgia Aquarium
Main article: Sharks in captivity
Until recently only a few benthic species of shark, such as hornsharks, leopard
sharks and catsharks had survived in aquarium conditions for
a year or 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.[64] 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.[65]
Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far
too large for typical home aquaria.[65]
Public aquaria generally do not accept donated specimens that have outgrown
their housing. Some owners have been tempted to release them.[65]
Species appropriate to home aquaria represent considerable spatial and financial
investments as they generally approach adult lengths of 3 feet and can live up
to 25 years.[65]
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.[66]
Kamohoali'i is the best known and revered of the
shark gods, he was the older and favored brother of Pele,[67] and
helped and journeyed with her to Hawaii. He was able to assume all human and fish
forms. A summit cliff on the crater of Kilauea is one of his most sacred spots. At one
point he had a heiau (temple or shrine) dedicated to him on
every piece of land that jutted into the ocean on the island of Moloka'i.
Kamohoali'i was an ancestral god, not a human who became a shark and banned the
eating of humans after eating one herself.[68][69] 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.[70][71] 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.[72] Other
apparently false claims are that fins prevent cancer[73] and
treat osteoarthritis.[74] No
scientific proof supports these claims; at least one study has shown shark
cartilage of no value in cancer treatment.[75]
Conservation
Further information: Shark sanctuary
The value of shark fins for shark fin soup has led to an increase in shark
catches. Usually only the fins are taken, while the rest of the shark is
discarded, usually into the sea.
Fishery
The annual shark catch has increased rapidly
over the last 50 years.
It is estimated that 100 million sharks are killed by people every year, due
to commercial and recreational fishing.[76][77]
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.[citation needed]
A 14-foot (4.3 m), 1,200-pound (540 kg)
tiger shark caught in Kāne‘ohe Bay, Oahu in 1966
Sharks are often killed for shark fin soup. Fishermen capture live sharks,
fin them, and dump the finless animal back into the water. Shark
finning involves removing the fin with a hot metal blade.[77] The
resulting immobile shark soon dies from suffocation or predators.[78] Shark
fin has become a major trade within black markets all over the world. Fins sell
for about $300/lb in 2009.[79]
Poachers illegally fin millions each year. Few governments enforce laws that
protect them.[citation needed] In 2010
Hawaii
became the first U.S. state to prohibit the possession, sale, trade or
distribution of shark fins.[80]
Shark fin soup is a status symbol in Asian countries, and is
considered healthy and full of nutrients. Sharks are also killed for meat.
European diners consume dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and
rays.[81]
However, the U.S. FDA
lists sharks as one of four fish (with swordfish, king mackerel, and tilefish) whose high mercury content is hazardous to children and
pregnant 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.[19] Major
declines in shark stocks have been recorded—some species have been depleted by
over 90% over the past 20–30 years with population declines of 70% not
unusual.[82] Many
governments and the UN have acknowledged the need for shark fisheries
management, but little progress has been made due to their low
economic value, the small volumes of products produced and sharks' poor public
image.[citation needed]
Other threats
Other threats include habitat alteration, damage and loss from coastal
development, pollution and the impact of fisheries on the seabed and prey
species.[83] The
2007 documentary, Sharkwater exposed how sharks are being
hunted to extinction.[84]
Protection
In 1991 South Africa was the first country in the world to declare Great
White sharks a legally protected species.[85]
Intending to ban the practice of shark finning while at sea, the United
States Congress passed the Shark Finning Prohibition Act in 2000.[86] Two
years later the Act saw its first legal challenge in United States v. Approximately 64,695 Pounds of Shark
Fins. In 2008 a Federal
Appeals Court ruled that a loophole in the law allowed non-fishing vessels
to purchase shark fins from fishing vessels while on the high seas.[87]
Seeking to close the loophole, the Shark Conservation Act was passed by Congress in
December 2010, and it was signed into law in January 2011.[88][89]
In 2009, the International Union for the Conservation of
Nature (IUCN) Red List of Endangered Species named 64
species, one-third of all oceanic shark species, as being at risk of extinction
due to fishing and shark finning.[90][91]
In 2010, the Convention on International Trade in Endangered
Species (CITES) rejected proposals from the United States and Palau that would have required countries to
strictly regulate trade in several species of scalloped
hammerhead, oceanic whitetip and spiny dogfish sharks. The majority, but not the
required two-thirds of voting 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.[92][93]
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.[94]
Advocacy group Shark Trust campaigns to limit shark fishing.
Advocacy group Seafood Watch directs American consumers to not
eat sharks.[95]
Evolution
A collection of Cretaceous shark
teeth
Evidence for the existence of sharks dates from the Ordovician period, over 450–420 million years
ago, before land vertebrates existed and before many plants
had colonized the continents.[1] Only
scales have been recovered from the first sharks and not all paleontologists
agree that these are from true sharks.[96] The
oldest generally accepted shark scales are from about 420 million years ago, in
the Silurian period.[96] The
first sharks looked very different from modern sharks.[97] The
majority of modern sharks can be traced back to around 100 million years
ago.[98] 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,[97] which
has been found within Paleozoic strata in Ohio, Kentucky and Tennessee. At that point in Earth's history these rocks made up the soft
bottom sediments of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about
1 metre (3.3 ft) long with stiff triangular fins and slender jaws.[97] Its
teeth had several pointed cusps, which wore down from use. From the small number
of teeth found together, it is most likely that Cladoselache did not
replace its teeth as regularly as modern sharks. Its caudal fins had a similar
shape to the great white sharks and the pelagic shortfin and longfin 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.[99][100] 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.
Megalodon with the whale shark, great white
shark, and a human for scale
Modern sharks began to appear about 100 million years ago.[98]
Fossil mackerel shark teeth date to the Lower Cretaceous. One of the most recently
evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene.[101] 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).[102][103]
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.[104]
Paleontological evidence suggests that this shark was an active predator of
large cetaceans.[104]
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.[105]
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.[105][106]
More than 440 species of sharks split across eight orders, listed below in roughly their
evolutionary relationship from ancient to modern:[106]
See also
Teeth
Main article: Shark teeth
The teeth of tiger
sharks are oblique and serrated to saw through
flesh
Shark teeth are embedded in the gums rather than directly affixed to the jaw, and
are constantly replaced throughout life. Multiple rows of replacement teeth grow
in a groove on the inside of the jaw and steadily move forward in comparison to
a conveyor belt; some sharks lose 30,000 or more
teeth in their lifetime. The rate of tooth replacement varies from once every 8
to 10 days to several months. In most species, teeth are replaced one at a time
as opposed to the simultaneous replacement of an entire row, which is observed
in the cookiecutter shark.[8]
Tooth shape depends on the shark's diet: those that feed on mollusks
and crustaceans have dense and flattened teeth used
for crushing, those that feed on fish have needle-like teeth for 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.[9]
Skeleton
Shark skeletons are very different from those of bony
fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and
durable, yet is about half the normal density of bone. This reduces the
skeleton’s weight, saving energy.[10]
Because sharks do not have rib cages, they can easily be crushed under their own
weight on land.[11]
Jaw
Jaws
of sharks, like those of rays and skates, are not attached to the cranium.
The jaw's surface (in comparison to the shark's vertebrae and gill arches) needs extra support
due to its heavy exposure to physical stress and its need for strength. It has a
layer of tiny hexagonal plates called "tesserae",
which are crystal blocks of calcium salts arranged as a mosaic.[12] This
gives these areas much of the same strength found in the bony tissue found in
other animals.
Generally sharks have only one layer of tesserae, but the jaws of large
specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or
more, depending on body size. The jaws of a large great white shark may have up to five layers.[10] In
the rostrum (snout), the cartilage can be spongy and
flexible to absorb the power of impacts.
Fins
Fin skeletons are elongated and supported with soft and unsegmented rays
named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.[13] Most
sharks have eight fins. Sharks can only drift away from objects directly in
front of them because their fins do not allow them to move in the tail-first
direction.[11]
Dermal
denticles
Main article: Dermal denticle
Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This
works as an outer skeleton, providing attachment for their swimming muscles and
thus saving energy.[14] Their
dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.[8]
Tails
Tails
provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark
species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than
the ventral 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 fish. By contrast, most
bony fish possess a homocercal caudal fin.[15]
Tiger sharks have a large upper lobe, which allows for slow cruising and sudden
bursts of speed. The tiger shark must be able to twist and turn in the water
easily when hunting to support its varied diet, whereas the porbeagle
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.[16] Other
tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated
upper lobe to stun fish and squid.
Physiology
Buoyancy
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy.
Instead, sharks rely on a large liver filled with oil that contains squalene, and their cartilage, which is about
half the normal density of bone.[14] Their
liver constitutes up to 30% of their total body mass.[17] The
liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth when not 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 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.[18]
Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition
to handle sharks safely.[19]
Respiration
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are
not covered, but lie in a row behind the head. A modified slit called a spiracle
lies just behind the 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.[9] While
the shark is moving, water passes through the mouth and over the gills in a
process known as "ram ventilation". While at rest, most sharks pump water over
their gills to ensure a constant supply of oxygenated 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.[20]
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.[21]
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[not in citation
given].[22]
Osmoregulation
In contrast to bony fish, with the exception of the coelacanth,[23] the
blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of
the high concentration of urea and trimethylamine N-oxide (TMAO), allowing them to
be in osmotic balance with the seawater. This
adaptation prevents most sharks from surviving in 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.[17] When
a shark dies, the urea is broken down to ammonia by bacteria, causing the dead
body to gradually smell strongly of ammonia.[24][25]
Digestion
Digestion can take a long time. The food moves from the mouth to a J-shaped
stomach, where it is stored and initial digestion occurs.[26]
Unwanted items may never get past the stomach, and instead the shark either
vomits or turns its stomachs inside out and ejects unwanted items from its
mouth.
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.[26]
Senses
Smell
The shape of the hammerhead shark's head may enhance olfaction by
spacing the nostrils further apart.
Sharks have keen olfactory senses, located in the short duct
(which is not fused, unlike bony fish) between the anterior and posterior nasal
openings, with some species able to detect as little as one part per million of blood in seawater.[27]
Sharks have the ability to determine the direction of a given scent based on
the timing of scent detection in each nostril.[28] This
is similar to the method mammals use to determine direction of sound.
They are more attracted to the chemicals found in the intestines of many
species, and as a result often linger near or in sewage outfalls. Some species, such as nurse
sharks, have external barbels that greatly increase their ability to
sense 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.[29] 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.[30] [31][32]
Hearing
Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many
miles away.[33] A
small opening on each side of their heads (not the spiracle) leads directly into the inner
ear through a thin 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
Main article: Electroreception
Electromagnetic field receptors
(ampullae of Lorenzini) and motion detecting canals in the head of a
shark
The ampullae of Lorenzini are the electroreceptor organs. They number in the
hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things
produce.[34] This
helps sharks (particularly the hammerhead shark) find 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.[35]
Lateral line
Main article: Lateral line
This system is found in most fish, including sharks. It detects motion or
vibrations in water. The shark can sense frequencies in the range of 25 to 50 Hz.[36]
Life history
The claspers of male spotted wobbegong
Shark lifespans vary by species. Most live 20 to 30 years. The spiny
dogfish has the longest lifespan at more than 100 years.[37] Whale sharks (Rhincodon typus) may also
live over 100 years.[38]
Reproduction
Unlike most bony fish, sharks are K-selected
reproducers, meaning that they produce a small number of well-developed young as
opposed to a large number of poorly developed young. Fecundity in sharks ranges from 2 to over 100
young per reproductive cycle.[39]
Sharks mature slowly relative to many other fish. For example, lemon
sharks reach sexual maturity at around age 13–15.[40]
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.[41]
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.[41]
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.[42][43] 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.[44]
Brooding
Sharks display three ways to bear their young, varying by species, oviparity,
viviparity and ovoviviparity.[45]
The spiral egg case of a Port Jackson shark
Ovoviviparity
Most sharks are ovoviviparous, meaning that the eggs hatch in the
oviduct
within the mother's body and that the egg's yolk and fluids secreted by glands in the walls
of the oviduct nourishes the embryos. The young continue
to be nourished by the remnants of the yolk and the oviduct's fluids. As in
viviparity, the young are born alive and fully functional. 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.[45]
Oviparity
Some species are oviparous like most other fish, laying their eggs
in the water. In most oviparous shark species, an egg case with the consistency of leather protects the developing embryo(s). These
cases may be corkscrewed into crevices for protection. Once empty, the egg case
is known as the mermaid's purse, and can wash up on shore.
Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swellshark.[45][46]
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.[39][45]
Behavior
The classic view describes a solitary hunter, ranging the oceans in search of
food. However, this applies to only a few species. Most live far more sedentary,
benthic lives. Even solitary sharks meet for
breeding or at rich hunting grounds, which may lead them to cover thousands of
miles in a year.[47] Shark
migration patterns may be even more complex than in birds, with many sharks
covering entire ocean 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.[17]
Cross-species social hierarchies exist. For example, oceanic
whitetip sharks dominate silky sharks of comparable size during
feeding.[39]
When approached too closely some sharks perform a threat
display. This usually consists of exaggerated swimming movements,
and can vary in intensity according to the threat level.[48]
Speed
In general, sharks swim ("cruise") at an average speed of 8 kilometres per
hour (5.0 mph) but when feeding or attacking, the average shark can reach speeds
upwards of 19 kilometres per hour (12 mph). The shortfin mako shark, the fastest shark and one of
the fastest fish, can burst at speeds up to 50 kilometres per hour (31 mph).[49] The
great white shark is also capable of speed
bursts. These exceptions may be due to the warm-blooded, or homeothermic, nature of these sharks'
physiology.
Intelligence
Sharks possess brain-to-body mass ratios that are similar to mammals and
birds,[50] and
have exhibited apparent curiosity and behavior resembling play in the wild.[51][52]
Sleep
Some sharks can lie on the bottom while actively pumping water over their
gills, but their eyes remain open and actively follow divers.[53] When
a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares
while resting on the ocean floor, it would "inhale" sand rather than
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.[53] It is
also possible that sharks sleep in a manner similar to dolphins,[53] one
cerebral hemisphere at a time, thus maintaining
some consciousness and cerebral activity at all times.
Ecology
Feeding
This section is about shark feeding. For the sport of
shark feeding, see Shark baiting.
Like many sharks, the great white shark is an
apex
predator in its environment.
Most sharks are carnivorous.[54] Basking sharks, whale sharks, and megamouth sharks sharks have independently
evolved different strategies for filter feeding plankton: basking sharks practice ram feeding, whale sharks use suction to take in
plankton and small fishes, and megamouth sharks make suction feeding more efficient by using the luminescent tissue inside of their mouths to
attract prey in the deep ocean. This type of feeding requires gill
rakers—long, slender filaments that form a very efficient sieve—analogous to the baleen plates of the great whales. The shark traps the plankton in
these filaments and swallows from time to time in huge mouthfuls. Teeth in these
species are comparatively small because they are not needed for feeding.[54]
Other highly specialized feeders include cookiecutter sharks, which feed on flesh sliced
out of other larger fish and marine mammals. Cookiecutter teeth are enormous
compared to the animal's size. The lower teeth are particularly sharp. Although
they have never been observed 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.[17]
Some seabed–dwelling species are highly effective ambush predators. Angel
sharks and wobbegongs use camouflage to lie in wait and suck
prey into their mouths.[55] Many
benthic sharks feed solely on crustaceans which they crush with their flat molariform 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.[56]
Range and
habitat
Sharks are found in all seas. They generally do not live in fresh water, with
a few exceptions such as the bull shark and the river shark which can swim both in seawater and
freshwater.[57]
Sharks are common down to depths of 2,000 metres (7,000 ft), and some live even
deeper, but they are almost entirely absent below 3,000 metres (10,000 ft). The
deepest confirmed report of a shark is a Portuguese dogfish at 3,700 metres (12,100
ft).[58]
Relationship with
humans
Attacks
A sign warning about the presence of sharks in
Salt Rock, South Africa
Snorkeler swims near blacktip reef shark. In rare circumstances
involving poor visibility, blacktips may bite a human, mistaking it for prey.
Under normal conditions they are harmless and shy.
Main article: Shark
attack
In 2006 the International Shark Attack File (ISAF) undertook
an investigation into 96 alleged shark 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.[59]
Contrary to popular belief, only a few sharks are dangerous to humans. Out of
more than 360 species, only four have been involved in a significant number of
fatal, unprovoked attacks on humans: the great white, oceanic
whitetip, tiger, and bull sharks.[60] [61] These
sharks are large, powerful predators, and may sometimes attack and kill people.
Despite being responsible for attacks on humans they have all been filmed
without using a protective cage.[62]
The perception of sharks as dangerous animals has been popularized by
publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through
popular fictional works about shark attacks, such as the Jaws film series. Jaws author Peter Benchley, as well as Jaws director Steven Spielberg later attempted to dispel the
image of sharks as man-eating monsters.[63]
In captivity
A whale shark in Georgia Aquarium
Main article: Sharks in captivity
Until recently only a few benthic species of shark, such as hornsharks, leopard
sharks and catsharks had survived in aquarium conditions for
a year or 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.[64] 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.[65]
Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far
too large for typical home aquaria.[65]
Public aquaria generally do not accept donated specimens that have outgrown
their housing. Some owners have been tempted to release them.[65]
Species appropriate to home aquaria represent considerable spatial and financial
investments as they generally approach adult lengths of 3 feet and can live up
to 25 years.[65]
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.[66]
Kamohoali'i is the best known and revered of the
shark gods, he was the older and favored brother of Pele,[67] and
helped and journeyed with her to Hawaii. He was able to assume all human and fish
forms. A summit cliff on the crater of Kilauea is one of his most sacred spots. At one
point he had a heiau (temple or shrine) dedicated to him on
every piece of land that jutted into the ocean on the island of Moloka'i.
Kamohoali'i was an ancestral god, not a human who became a shark and banned the
eating of humans after eating one herself.[68][69] 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.[70][71] 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.[72] Other
apparently false claims are that fins prevent cancer[73] and
treat osteoarthritis.[74] No
scientific proof supports these claims; at least one study has shown shark
cartilage of no value in cancer treatment.[75]
Conservation
Further information: Shark sanctuary
The value of shark fins for shark fin soup has led to an increase in shark
catches. Usually only the fins are taken, while the rest of the shark is
discarded, usually into the sea.
Fishery
The annual shark catch has increased rapidly
over the last 50 years.
It is estimated that 100 million sharks are killed by people every year, due
to commercial and recreational fishing.[76][77]
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.[citation needed]
A 14-foot (4.3 m), 1,200-pound (540 kg)
tiger shark caught in Kāne‘ohe Bay, Oahu in 1966
Sharks are often killed for shark fin soup. Fishermen capture live sharks,
fin them, and dump the finless animal back into the water. Shark
finning involves removing the fin with a hot metal blade.[77] The
resulting immobile shark soon dies from suffocation or predators.[78] Shark
fin has become a major trade within black markets all over the world. Fins sell
for about $300/lb in 2009.[79]
Poachers illegally fin millions each year. Few governments enforce laws that
protect them.[citation needed] In 2010
Hawaii
became the first U.S. state to prohibit the possession, sale, trade or
distribution of shark fins.[80]
Shark fin soup is a status symbol in Asian countries, and is
considered healthy and full of nutrients. Sharks are also killed for meat.
European diners consume dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and
rays.[81]
However, the U.S. FDA
lists sharks as one of four fish (with swordfish, king mackerel, and tilefish) whose high mercury content is hazardous to children and
pregnant 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.[19] Major
declines in shark stocks have been recorded—some species have been depleted by
over 90% over the past 20–30 years with population declines of 70% not
unusual.[82] Many
governments and the UN have acknowledged the need for shark fisheries
management, but little progress has been made due to their low
economic value, the small volumes of products produced and sharks' poor public
image.[citation needed]
Other threats
Other threats include habitat alteration, damage and loss from coastal
development, pollution and the impact of fisheries on the seabed and prey
species.[83] The
2007 documentary, Sharkwater exposed how sharks are being
hunted to extinction.[84]
Protection
In 1991 South Africa was the first country in the world to declare Great
White sharks a legally protected species.[85]
Intending to ban the practice of shark finning while at sea, the United
States Congress passed the Shark Finning Prohibition Act in 2000.[86] Two
years later the Act saw its first legal challenge in United States v. Approximately 64,695 Pounds of Shark
Fins. In 2008 a Federal
Appeals Court ruled that a loophole in the law allowed non-fishing vessels
to purchase shark fins from fishing vessels while on the high seas.[87]
Seeking to close the loophole, the Shark Conservation Act was passed by Congress in
December 2010, and it was signed into law in January 2011.[88][89]
In 2009, the International Union for the Conservation of
Nature (IUCN) Red List of Endangered Species named 64
species, one-third of all oceanic shark species, as being at risk of extinction
due to fishing and shark finning.[90][91]
In 2010, the Convention on International Trade in Endangered
Species (CITES) rejected proposals from the United States and Palau that would have required countries to
strictly regulate trade in several species of scalloped
hammerhead, oceanic whitetip and spiny dogfish sharks. The majority, but not the
required two-thirds of voting 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.[92][93]
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.[94]
Advocacy group Shark Trust campaigns to limit shark fishing.
Advocacy group Seafood Watch directs American consumers to not
eat sharks.[95]
Evolution
A collection of Cretaceous shark
teeth
Evidence for the existence of sharks dates from the Ordovician period, over 450–420 million years
ago, before land vertebrates existed and before many plants
had colonized the continents.[1] Only
scales have been recovered from the first sharks and not all paleontologists
agree that these are from true sharks.[96] The
oldest generally accepted shark scales are from about 420 million years ago, in
the Silurian period.[96] The
first sharks looked very different from modern sharks.[97] The
majority of modern sharks can be traced back to around 100 million years
ago.[98] 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,[97] which
has been found within Paleozoic strata in Ohio, Kentucky and Tennessee. At that point in Earth's history these rocks made up the soft
bottom sediments of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about
1 metre (3.3 ft) long with stiff triangular fins and slender jaws.[97] Its
teeth had several pointed cusps, which wore down from use. From the small number
of teeth found together, it is most likely that Cladoselache did not
replace its teeth as regularly as modern sharks. Its caudal fins had a similar
shape to the great white sharks and the pelagic shortfin and longfin 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.[99][100] 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.
Megalodon with the whale shark, great white
shark, and a human for scale
Modern sharks began to appear about 100 million years ago.[98]
Fossil mackerel shark teeth date to the Lower Cretaceous. One of the most recently
evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene.[101] 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).[102][103]
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.[104]
Paleontological evidence suggests that this shark was an active predator of
large cetaceans.[104]
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.[105]
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.[105][106]
More than 440 species of sharks split across eight orders, listed below in roughly their
evolutionary relationship from ancient to modern:[106]
- Hexanchiformes: Examples from this group include
the cow sharks, frilled shark and even a shark that resembles a
marine snake. - Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and prickly 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