Sharks (superorder Selachimorpha) are fish
with a full cartilaginous skeleton and a streamlined body.
They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles to protect their skin from damage and parasites and
to improve fluid dynamics; they also have replaceable teeth.[1] Shark teeth are prized by
collectors for their beauty, and because they are the only surviving relics of ancient sharks that are now extinct. Sharks
include species ranging from the hand-sized pygmy shark, Euprotomicrus bispinatus, a
deep sea species of only centimetres ( in) in
length, to the whale shark, Rhincodon typus, the largest fish, which grows to a
length of approximately metres ( ft) and
which, like baleen whales, feeds only on plankton,
squid, and small fish through filter
feeding. The bull shark, Carcharhinus leucas, is the best known of several
species to swim in both salt and fresh water and in deltas.[2]
Physical characteristics
-
Skeleton
The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous
fish (skates and rays) have skeletons made from rubbery cartilage, a tissue lighter and
more flexible than bone.[1]
Like its relatives the rays and skates, the shark's jaw is not attached to the cranium. The jaw's surface, which like the vertebrae and gill arches is a skeletal element that needs extra
support due to its heavier exposure to physical stress and need for extra strength, has a layer of unique and tiny hexagonal
plates called "tesserae", crystal blocks of calcium salts arranged as a mosaic.[3] This gives these areas much of the same strength found in real and much heavier bony tissue.
The general rule is that there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the
bull shark, tiger shark, and the great white shark, have been found to be covered with both two and three layers, and even more,
depending on the body size. The jaws of a large white shark even had five layers.sharks look like fishes because they descend
from the great indian cat monkey. the great indian cat monkey was ammal that would swim great disntaces to feed on jumbp fried
shrimp at red lobster
In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
The fin skeleton are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein
resembling the horny keratin in hair and feathers.
The inner parts of the males' pelvic fins have been modified to a pair of cigar- or sausage-shaped sex organs known as
"claspers", used for internal fertilization.
Respiration
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Shark gill
slits are not covered like other fish, but are in a row behind its head. A modified slit called a spiracle is located just behind the eye; the spiracle assists the water intake
during respiration and even plays a major role in bottom dwelling sharks, but is also reduced or missing in active pelagic
sharks.[4] While moving, water passes
through the mouth of the shark and over the gills — this process is known as "ram ventilation". While at rest, most sharks pump
water over their gills to ensure a constant supply of oxygenated water. A small subset of shark species that spend their life
constantly swimming, a behavior common in pelagic sharks, have lost the ability to pump
water through their gills. These species are obligate ram ventilators and would presumably asphyxiate if unable to stay in motion. (Obligate ram ventilation is also true of some pelagic bony fish
species.)[5]
The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the
blood is pumped to the shark's gills via the ventral aorta artery where it branches off 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 ventricle of
the heart and the cycle repeats.
Buoyancy
Unlike bony fish, sharks do not have gas-filled swim bladders, but instead rely on a
large liver filled with oil that contains squalene. The liver may constitute up to 30% of their
body mass[6] for buoyancy. Its effectiveness
is limited, so sharks employ dynamic lift to maintain depth and sink when they stop
swimming. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic
immobility - researchers use this condition for handling sharks safely.[7] Sandtiger sharks are also known to gulp air from the surface and store it in their stomachs, using
the stomach as a swim bladder.
Osmoregulation
-
In contrast to bony fish, 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 oxide,
allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in fresh water, and
they are therefore confined to a marine environment. 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.[6]
Teeth
-
The teeth of carnivorous sharks are not attached to the jaw, but embedded in the flesh, and in
many species are constantly replaced throughout the shark's life; some sharks can lose 30,000 teeth in a lifetime. All sharks
have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside
the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some
sharks rows of teeth are replaced every 8–10 days, while in other species they could last several months. The lower teeth are
primarily used for holding prey, while the upper ones are used for cutting into it.[4] The teeth range from thin, needle-like
teeth for gripping fish to large, flat teeth adapted for crushing shellfish.
Tails
The tails (caudal fins) of sharks vary considerably between species and are adapted to the
lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes
have evolved in sharks adapted for different environments. Sharks possess a heterocercal caudal fin in which the dorsal portion
is usually noticeably larger than the ventral portion. This is due to the fact that the shark's vertebral column extends into
that dorsal portion, allowing for a greater surface area for muscle attachment which would then be used for more efficient
locomotion among the negatively buoyant cartilaginous fishes. This is in contrast to the bony
fishes, class osteichthyes, which possess a homocercal caudal fin.
The tiger shark's tail has a large upper lobe which delivers the maximum amount of power for slow cruising or sudden bursts of
speed. The tiger shark has a varied diet, and because of this it must be able to twist and turn in the water easily when hunting,
whereas the porbeagle, which hunts schooling fish such as mackerel and herring has a large lower lobe to provide greater speed to help
it keep pace with its fast-swimming prey. It is also believed that sharks use the upper lobe of their tails to counter the lift
generated by their pectoral fins. [8]
Some tail adaptations have purposes other than providing thrust. The cookiecutter
shark has a tail with broad lower and upper lobes of similar shape which are luminescent and may help to lure prey towards
the shark. The thresher feeds on fish and squid, which it is believed to herd, then stun
with its powerful and elongated upper lobe.
Dermal denticles
-
Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibres 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 past days the sharks skin has been used as sandpaper. Also if you happen to rub a sharks skin it is possible for it to cut
you.
Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.[9]
Body temperature
A few of the larger species, such as the shortfin mako, Isurus oxyrinchus,
and the great white, are mildly homeothermic[8] - able to
maintain their body temperature above the surrounding water temperature. This is possible because of the presence of the
rete mirabile, a counter current exchange mechanism that reduces the loss of body heat.
Muscular contraction also generates a mild amount of body heat. However, this differs significantly from true homeothermy, as
found in mammals and birds, in which heat is generated, maintained, and regulated by metabolic activity.
Lifespan
Maximum shark ages vary by species. Most sharks live for 20 to 30 years, while the spiny
dogfish lives a record lifespan of more than 100 years.[10] Whale sharks (Rhincodon typus) have been hypothesized to also
live over 100 years.[11]
Etymology
Until the 16th century,[12] sharks were known to
mariners as "sea dogs".[13] According to the
OED 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, and later as a general term for all sharks. The name may have been derived
from the Mayan word for fish, xoc, pronounced "shock" or "shawk".
Evolution
The fossil record of sharks extends back over 450 million years - before land vertebrates existed and before many plants had colonised the
continents.[14] The first sharks looked very
different from modern sharks.[15] The majority of the
modern sharks can be traced back to around 100 million years ago.[16]
Mostly only the fossilized teeth of sharks are found, although often in large numbers. In some
cases pieces of the internal skeleton or even complete fossilized sharks have been discovered. Estimates suggest that over a span
of a few years a shark may grow tens of thousands of teeth, which explains the abundance of fossils. As the teeth consist of
calcium phosphate, an apatite, they are easily
fossilized.
Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a
shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are only preserved when
rapid burial in bottom sediments occurs.
Among the most ancient and primitive sharks is Cladoselache, from about 370
million years ago,[15] which has been found within
the Paleozoic strata of Ohio, Kentucky and Tennessee. At this point in the Earth's history these rocks made up the soft sediment of the bottom of a large, shallow ocean,
which stretched across much of North America. Cladoselache was only about 1 m
long with stiff triangular fins and slender jaws.[15]
Its teeth had several pointed cusps, which would have been worn down by use. From the number of teeth found in any one place 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 pelagic makos and great white
sharks. The discovery of whole fish found tail first in their stomachs suggest that they were fast swimmers with great
agility.
From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the
acanthuses, was almost exclusive to freshwater environments.[17][18] By
the time this group became extinct (about 220 million years ago) they had achieved worldwide distribution. The other group, the
hybodonts, appeared about 320 million years ago and was mostly found in the oceans, but also in freshwater.
Modern sharks began to appear about 100 million years ago.[16] Fossil mackerel shark teeth occurred in the Lower
Cretaceous. One of the most recent families of sharks that evolved is the hammerhead
sharks (family Sphyrnidae), which emerged in Eocene.[19] 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 with coarsely serrated teeth that probably gave rise to the modern great white shark, and another
with finely serrated teeth and a tendency to attain gigantic proportions. This group includes the extinct megalodon, Carcharodon megalodon, which like most extinct sharks is only known from its teeth. A
reproduction of its jaws was based on some of the largest teeth which were up to almost centimetres ( in) long and
suggested a fish that could grow to a length of metres ( ft) to metres ( ft). The reconstruction was found
to be inaccurate, and estimates revised downwards to around metres ( ft) to metres ( ft).
It is believed that the immense size of predatory sharks such as the great white
may have arisen from the extinction of the dinosaurs and the diversification of
mammals. It is known that at the same time these sharks were evolving some early mammalian groups
evolved into aquatic forms. Certainly, wherever the teeth of large sharks have been found, there has also been an abundance of
marine mammal bones, including seals, porpoises and
whales. These bones frequently show signs of shark attack. There are theories that suggest that
large sharks evolved to better take advantage of larger prey.
Classification
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: in particular, some sharks are more closely related to rays than they are to some other sharks.
There are more than 360 described species of sharks split across are eight orders of
sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:
- Hexanchiformes: Examples from this group include the cow
sharks, frilled shark and even a shark that looks on first inspection to be 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 the fish that they eat.
- 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: These are commonly referred to as the groundsharks, and some of the species include the blue,
tiger, bull, 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.
Reproduction
Claspers of male spotted wobbegong,
Orectolobus maculatus
The sex of a shark can be easily determined. The males have modified
pelvic fins which have become a pair of claspers. The name is
somewhat misleading as they are not used to hold on to the female, but fulfil the role of the mammalian penis.
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 behaviour:
the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these
bites.
Sharks have a different reproductive strategy from most fish. Instead of producing huge numbers of eggs and fry (a strategy
which can result in a survival rate of less than .01%), sharks normally produce around a dozen pups (blue sharks have been recorded as producing 135 and some species produce as few as two).[20] These pups are either protected by egg cases or born
live.
There are three ways in which shark pups are born:
- Oviparity - Some sharks lay eggs. In most of these species, the developing embryo
is protected by an egg case with the consistency of leather. Sometimes these cases are corkscrewed into crevices for protection.
The mermaid's purse, found washed-up on beaches, is an empty egg case. Oviparous sharks
include the horn shark, catshark, Port Jackson shark, and swellshark.[21]
- Viviparity - These sharks maintain a placental link to the developing young,
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 tiger sharks), the
basking shark and the smooth dogfish fall into this
category. Dogfish have the longest known gestation period of any shark, at 18 to 24 months. Basking sharks and frilled sharks are
likely to have even longer gestation periods, but accurate data is lacking.[20]
- Ovoviviparity - Most sharks utilize this method. The young are nourished by the
yolk of their egg and by fluids secreted by glands in the walls of the oviduct. The eggs hatch within the oviduct, and 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. Some species practice oophagy, where the first embryos to hatch eat the
remaining eggs in the oviduct. This practice is believed to be present in all lamniforme sharks, while the developing pups of the
grey nurse shark take this a stage further and consume other developing embryos
(intrauterine cannibalism). The survival strategy for the species that are
ovoviviparous is that the young are able to grow to a comparatively larger size before being born. The whale shark is now
considered to be in this category after long having been classified as oviparous. Whale
shark eggs found 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 because of the protection from predators (mainly other sharks) and
the abundance of food.
Asexual reproduction
In December 2001, a pup was born from a female hammerhead shark who had not been in contact with a male shark for over three
years. This has led scientists to believe that sharks can produce without the mating process.
After three years of research, this assumption was confirmed on May 23 2007, after determining the shark born had no paternal DNA, ruling out any sperm-storage theory as previous
thought. It is unknown as to the extent of this behaviour in the wild, and how many species of shark are capable of
parthenogenesis. This observation in sharks made mammals the only remaining major
vertabrate group in which the phenomenon of asexual reproduction has not been observed.
Scientists warned that this type of behaviour in the wild is rare, and probably a last ditch effort of a species to reproduce
when a mate isn't present. This leads to a lack of genetic diversity, required to
build defenses againsts natural threats, and if a species of shark were to rely solely on asexual reproduction, it would probably
be a road to extinction and maybe attribute to the decline of blue sharks off the Irish coast.[22] [23]
[24]
Shark senses
Sense of smell
Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike
bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. They are attracted to the chemicals found in the guts of many
species, and as a result often linger near or in sewage outfalls. Some species, such as nurse
sharks, have external barbels that greatly increase their ability to sense
prey.
Sharks generally rely on their superior sense of smell to find prey, but at closer range they also use the lateral lines running along their sides to sense movement in the water, and also employ special sensory
pores on their heads (Ampullae of Lorenzini) to detect electrical fields created by prey and the ambient electric fields of the ocean.
Sense of 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 tissue is behind the retina and reflects light back to the retina, 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 have nictitating membranes.
This membrane covers the eyes during predation, 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 behaviour 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 degree to which
sight is used probably varies with species and water conditions.
Sense of hearing
Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their heads (not
to be confused with the spiracle) leads directly into the inner ear through a thin channel. The
lateral line shows a similar arrangement, as it 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
-
Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the head of a shark.
The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they
vary in number from a couple of hundred to thousands in an individual. Sharks use the Ampullae of Lorenzini to detect the
electromagnetic fields that all living things produce. This helps sharks find its prey (mostly the hammer head). The shark has
the greatest electricity sensitivity known in all animals. This sense is used to find prey hidden in sand by detecting the
electric fields inadvertently produced by all fish. It is this sense that sometimes confuses a shark into attacking a boat: when
the metal interacts with salt water, the electrochemical potentials generated by the rusting metal are similar to the weak fields
of prey, or in some cases, much stronger than the prey's electrical fields: strong enough to attract sharks from miles away. The
oceanic currents moving in the magnetic field of the Earth also generate electric
fields that can be used by the sharks for orientation and navigation.
Lateral line
-
This system is found in most fish, including sharks. It is used to detect motion or vibrations in the water. The shark uses
this to detect the movements of other organisms, especially wounded fish. The shark can sense frequencies in the range of 25 to
50 Hz.[25]
Behavior
Studies on the behavior of sharks have only recently been carried out leading to little information on the subject, although
this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, this
is only true for a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or on
rich hunting grounds, which may lead them to cover thousands of miles in a year.[26] Migration patterns in sharks may be even more complex than in birds, with many sharks covering
entire ocean basins.
Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals for scalloped hammerheads congregating around seamounts and islands
e.g. in the Gulf of California.[6] Cross-species social hierarchies exist with oceanic
whitetip sharks dominating silky sharks of comparable size when feeding.
When approached too closely some sharks will perform a threat display to warn
off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to
the level of threat.[27]
Shark intelligence
Despite the common myth that sharks are instinct-driven "eating machines", recent studies have indicated that many species
possess powerful problem-solving skills, social complexity and curiosity. The brain-mass-to-body-mass ratios of sharks are
similar to those of mammals and other higher vertebrate species.[28]
In 1987, near Smitswinkle Bay, South Africa, a group of up to seven great white sharks worked together to relocate the partially beached body of a dead whale to deeper
waters to feed.[29]
Sharks have even been known to engage in playful activities (a trait also observed in cetaceans and primates). Porbeagle sharks
have been seen repeatedly rolling in kelp and have even been observed chasing an individual trailing a piece behind them.[30]
Shark sleep
Some say a shark never sleeps. It is unclear how sharks sleep. Some sharks can lie on the bottom while actively pumping water
over their gills, but their eyes remain open and actively follow divers. When a shark is resting, they do not use their
nares, but rather their spiracles. If a shark tried to use their
nares while resting on the ocean floor, they would be sucking up 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 it is possible for a spiny dogfish to continue to swim while sleeping.
It is also possible that a shark can sleep in a manner similar to dolphins.[31] In this situation, one half of the brain sleeps at a time,
thereby allowing the shark to be half conscious while sleeping.
Habitat
A December 10, 2006 report by the Census of Marine Life group reveals that 70%
of the world's oceans are shark-free. They have discovered that although many sharks live up to depths as low as metres
( ft), they fail to colonize deeper, putting them more easily within reach of fisheries and thus endangered status.[32]
Shark attacks
Snorkeler with
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.
-
Sharks rarely attack humans unless provoked. 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 per year between 2001 and 2006 from unprovoked shark attacks is
4.3.[33]
Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 360 species, only three have been
involved in a significant number of fatal, unprovoked attacks on humans: the great
white, tiger and bull sharks.[34] These sharks, being large, powerful predators, may
sometimes attack and kill people, but all of these sharks have been filmed in open water, without the use of a protective
cage.[35]
The perception of sharks as dangerous animals has been popularised 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. The author of
Jaws, Peter Benchley, had in his later years attempted to dispel the image of
sharks as man-eating monsters.
Sharks in captivity
Until recently only a few benthic species of shark, such as hornsharks, leopard sharks and catsharks could survive in aquarium conditions for up to 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. A better knowledge of sharks has led to
more species (including the large pelagic sharks) being able to be kept for far longer. At
the same time, transportation techniques have improved and now provide a way for the long distance movement of sharks.[36] The only species of shark to have never been sucsessfully
held in captivity is the great white.
Despite being considered critical for the health of the shark, very few studies on feeding have been carried out. Since food
is the reward for appropriate behaviour, trainers must rely on control of feeding motivation.
Conservation
The number of sharks being caught has increased rapidly over the last 50 years.
The majority of shark fisheries around the globe have little monitoring or management. With the rise in demand of shark
products there is a greater pressure on fisheries.[37]
Stocks decline and collapse because sharks are long-lived apex predators with comparatively small populations, which makes it
difficult for them breed rapidly enough to maintain population levels. Major declines in shark stocks have been recorded in
recent years - some species have been depleted by over 90% over the past 20-30 years with a population decline of 70% not being
unusual.[38] Many governments and the UN have acknowledged the need for shark fisheries management, but due to the low economic value of shark
fisheries, the small volumes of products produced and the poor public image of sharks, little progress has been made.
Many other threats to sharks include habitat alteration, damage and loss from coastal developments, pollution and the impact
of fisheries on the seabed and prey species.
A Canadian-made documentary, Sharkwater is raising awareness of the depletion of the
world's shark population.
Shark fishery
Every year, an estimate states that 26 to 73 million (median value is at 38 million) sharks are killed by people in commercial
and recreational fishing.[39] In the past, sharks
were killed simply for the sport of landing a good fighting fish (such as the shortfin mako sharks). Shark skin is covered with
dermal denticles, which are similar to tiny teeth, and was used for purposes similar to sandpaper. Other sharks are hunted for
food (Atlantic thresher, shortfin mako and others), and some species for other products.[40]
Sharks are a common seafood in many places around the world, 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 and served alongside chips. When served in fish and chip shops, it is called flake.
Sharks are often killed for shark fin soup: the finning process involves capture of a
live shark, the removal of the fin with a hot metal blade, and the release of the live animal back into the water. Sharks are
also killed for their meat. Conservationists have campaigned for changes in the law to make finning illegal in the U.S. The meat
of dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and rays are in high demand by European
consumers.[41]
Shark cartilage has been advocated as effective against cancer and for treatment of osteoarthritis. (This is because many people
believe that sharks cannot get cancer and that taking it will prevent people from getting these diseases, which is untrue.)
However, a trial by Mayo Clinic found no effect in advanced cancer patients.
Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fish that are harvested.
This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species
are considered to be threatened.
Some organizations, such as the Shark Trust, campaign to limit shark fishing.
Sharks in mythology
Sharks figure prominently in the Hawaiian mythology. There are stories of shark
men who have shark jaws on their back. They could change form between shark and human at any time they desired. A common theme in
the stories was that the shark men would warn beach-goers that sharks were in the waters. The beach-goers would laugh and ignore
the warnings and go swimming, subsequently being eaten by the same shark man who warned them not to enter the water.
Hawaiian mythology also contained many shark gods. They believed that sharks
were guardians of the sea, and called them Aumakua:[42]
- Kamohoali'i - The best known and revered of the shark gods, he was the older and favoured brother of Pele,[43] and helped and journeyed
with her to Hawaii. He was able to take on all human and fish forms. A summit cliff on the crater
of Kilauea is considered to be one of his most sacred spots. At one point he had a he'iau (temple or shrine) dedicated to
him on every piece of land that jutted into the ocean on the island of Moloka'i.
- Ka'ahupahau - This goddess was born human, with her defining characteristic being her red hair. She was later
transformed into shark form and was believed to protect the people who lived on O'ahu from sharks. She was also believed to live
near Pearl Harbor.
- Kaholia Kane - This was the shark god of the ali'i Kalaniopu'u and he was believed to live in a cave at Puhi,
Kaua'i.
- Kane'ae - The shark goddess who transformed into a human in order to experience the joy of dancing.
- Kane'apua - Most commonly, he was the brother of Pele and Kamohoali'i. He was a trickster god who performed many
heroic feats, including the calming of two legendary colliding hills that destroyed canoes trying to pass between.
- Kawelomahamahai'a - Another human, he was transformed into a shark.
- Keali'ikau 'o Ka'u - He was the cousin of Pele and son of Kua. He was called the protector of the Ka'u people. He had
an affair with a human girl, who gave birth to a helpful green shark.
- Kua - This was the main shark god of the people of Ka'u, and believed to be their ancestor.
- Kuhaimoana - He was the brother of Pele and lived in the Ka'ula islet. He was said to be 30 fathoms (55 m)
long and was the husband of Ka'ahupahau.
- Kauhuhu - He was a fierce king shark that lived in a cave in Kipahulu on the island of Maui. He sometimes moved to
another cave on the windward side of island of Moloka'i.
- Kane-i-kokala - A kind shark god that saved shipwrecked people by taking them to shore. The people who worshipped him
feared to eat, touch or cross the smoke of the kokala, his sacred fish.
In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls.
Sharks in cultural tradition
In ancient Greece, it was forbidden to eat shark flesh at women's festivals.
A popular myth is that sharks are immune to disease and cancer; however, this is untrue. There
are both diseases and parasites that 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 have shown sharks to have heightened immunity to disease.[44]
In popular culture
Films
Books
See also
References
- ^ a b
- ^ Allen, Thomas B. (1999).
The Shark Almanac. New York: The Lyons Press. ISBN 1-55821-582-4.
- ^ Hamlett, W. C. (1999).
Sharks, Skates and Rays: The Biology of Elasmobranch Fishes. Johns Hopkins University Press. ISBN
0-8018-6048-2.
- ^ a b Gilbertson, Lance (1999).
Zoology Laboratory Manual. New York: McGraw-Hill Companies, Inc.. ISBN 0-07-237
