'Sharks' (
superorder 'Selachimorpha') are
fish with a full
cartilaginous skeleton
[1] 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 in length, to the
whale shark, ''Rhincodon typus'', the largest fish, which grows to a length of approximately 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
Main articles: Physical characteristics of sharks
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.
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.
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
The major features of sharks
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, 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
Main articles: 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
Main articles: Shark 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 range of shark tail shapes
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
Main articles: Dermal denticle
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.
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
homeothermic8 - 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 up to almost long and suggested a fish that could grow to a length of to . The reconstruction was found to be inaccurate, and estimates revised downwards to around to .
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
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
Main articles: 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. 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
Main articles: 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]
Behaviour
Studies on the behaviour 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 , they fail to colonize deeper, putting them more easily within reach of fisheries and thus endangered status.
[32]
Shark attacks
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]
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.
[ Triple Threat: World Fin Trade May Harvest up to 73 Million Sharks per Year ] 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.
[39]
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.
[40]
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:
[41]
★ 'Kamohoali'i' - The best known and revered of the shark gods, he was the older and favoured brother of
Pele,
[42] 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.
[43]
Trivia
In 1957, after a series of shark attacks, the South African government ordered a warship to drop underwater bombs on the sharks, but it didn't work and the attacks continued.
[44]
In ancient Hawaii, prisoners were forced to fight hungry sharks--armed only with a sharks tooth.
44
The Great White shark and the Tiger shark can stick their heads out of water.
[45]
In popular culture
Films
★ ''
Jaws series'' (1975, 1978, 1983, 1987)
★ ''
Live and Let Die'' (1973)
★ ''
The Spy Who Loved Me'' (1977)
★ ''
Tintorera'' (1977)
★ ''
Great White'' (1980)
★ ''
Cruel Jaws'' (1995)
★ ''
Deep Blue Sea'' (1999)
★ ''
Shark Attack series'' (1999, 2001, 2002)
★ ''
Open Water series'' (2003, 2007)
★ ''
Red Water'' (2003)
★ ''
Finding Nemo'' (2003)
★ ''
Shark Tale'' (2004)
★ ''
The Life Aquatic with Steve Zissou'' (2004)
★ ''
The Twelve Days of Terror'' (2004)
★ ''
Megalodon'' (2004)
★ ''
Into the Blue'' (2005)
★ ''
Spring Break Shark Attack'' (2005)
★ ''
Shark Bait '' (2006)
★ ''
Sharkwater'' (2007)
Books
★ ''Megalodon'' Robin Brown (1983)
★ ''Jaws'' Peter Benchley (1974)
★ ''
Deep Wizardry'' Diane Duane (1985)
★ ''Carcharodon'' George Edward Noe (1987)
★ '' Steve Alten (1997)
★ ''
The Trench'' Steve Alten (1999)
★ '' Steve Alten (2004)
See also
★
List of sharks
References
1. The Life of Sharks, , Paul, Budker, Weidenfeld and Nicolson, , SBN 297003070
2. The Shark Almanac, , Thomas B., Allen, The Lyons Press, , ISBN 1-55821-582-4
3. Sharks, Skates and Rays: The Biology of Elasmobranch Fishes, , W. C., Hamlett, Johns Hopkins University Press, , ISBN 0-8018-6048-2
4. Zoology Laboratory Manual, , Lance, Gilbertson, McGraw-Hill Companies, Inc., , ISBN 0-07-237716-X
5. Deep Breathing William J. Bennetta
6. Sharks of the World, , Leonard, Compagno, Collins Field Guides, , ISBN 0-00-713610-2
7. Elasmobranchs as living resources: Advances in the biology, ecology, systematics, and the status of the fisheries, , H. L. Jr, Pratt, NOAA Tech Rept., ,
8. Fishes of the World, , Joseph S., Nelson, John Wiley and Sons, , ISBN 0-471-54713-1
9. Skin of the Teeth R. Aidan Martin
10. Mote Marine Laboratory, "Shark Notes"
11. Florida Museum of Natural History Ichthyology Department, "National Shark Research Consortium - Shark Basics"
12. Online Etymology Dictionary
13. The History of Underwater Exploration, , Robert F., Marx, Courier Dover Publications, , ISBN 0-486-26487-4
14. Geologic Time Martin, R. Aidan.
15. Ancient Sharks Martin, R. Aidan.
16. The Origin of Modern Sharks Martin, R. Aidan.
17. http://hoopermuseum.earthsci.carleton.ca/sharks/P2-3.htm "Xenacanth". Retrieved on 11/26/06.
18. http://www.elasmo-research.org/education/evolution/earliest.htm "Biology of Sharks and Rays: 'The Earliest Sharks'". Retrieved on 11/26/06.
19. The Rise of Modern Sharks R. Aidan Martin
20. Sharks of the World: An annotated and illustrated catalogue of shark species known to date, Leonard J. V. Compagno, , , Food and Agriculture Organization of the United Nations, , ISBN 92-5-104543-7
21. Marine Biology notes
22. Female sharks reproduce without male DNA, scientists say
23. No need for dad: Female shark reproduces without sex
24. Virgin birth in a hammerhead shark Demian D. Chapman1, Mahmood S. Shivji, Ed Louis, Julie Sommer, Hugh Fletcher and Paulo A. Prodöhl
25. Inner ear and lateral line, , A.N., Popper, The Physiology of Fishes,
26. Scientists track shark's 12,000 mile round-trip
27. Jaws: The natural history of sharks
28. Smart sharks
29. Is the White Shark Intelligent
30. Biology of the Porbeagle
31. How Do Sharks Swim When Asleep?
32. Extreme Life, Marine Style, Highlights 2006 Ocean Census
33. Worldwide shark attack summary
34. Statistics on Attacking Species of Shark
35. Great white shark spotted off Hale'iwa with pictures of cageless diver with great white shark.
36. Whale Sharks in Captivity
37. Elasmobranchs as living resources: Advances in the biology, ecology, systematics, and the status of the fisheries, , H. L. Jr., Pratt, NOAA Tech Rept. (90), ,
38. Shark Fisheries Management and Biology, , T.I., Walker, , ,
39. FAO Shark Fisheries
40. Shark fisheries and trade in Europe:
Fact sheet on Italy
41. Hawaiian Mythology
42. Pele, Goddess of Fire
43. Do Sharks Hold Secret to Human Cancer Fight?
44. Angry Animals (Horrible Science), , Nick, Arnold, Scholastic Hippo, , ISBN 0439963648
45. Shark Trivia
;General references
★
The Sharks of North American Waters, , Jose, Castro, Texas A&M University Press, , ISBN 0-89096-143-3
★
Sharks, , John D., Stevens, NY Facts on File Publications, , ISBN 0-8160-1800-6
★
Vertebrate Life. 7th Ed., , F. H., Pough, Pearson Education Ltd., , ISBN 0-13-127836-3
★ Clover, Charles. 2004. ''The End of the Line: How overfishing is changing the world and what we eat''. Ebury Press, London. ISBN 0-09-189780-7
External links
★
Shark Research Institute
★
Shark Conservation
★
Greenland Shark and Elasmobranch Education and Research Group
★
The International Shark Attack File
★
Shark Trust Organization
★
ReefQuest Centre for Shark Research
★
ECOCEAN Whale Shark Photo-identification Library
★
Photographs of sharks
★
Sharks Photo gallery
★
Sharkology
★
Updated list of all known shark species
★
Bite-Back.com
★
Global Shark Attack File
★
The Ocean Conservancy: Sharks
★
The Shark Alliance
★
Shark Research Committee
★
Shark Pictures and Elasmobranch Field Guide
★
Shark pictures on Morphbank
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