Shark

Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans.

Under this broader definition, the earliest known sharks date back to more than 420 million years ago.[2] Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length.[3] Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can be found in both seawater and freshwater.[4] Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.[5]

Well-known species such as the great white shark, tiger shark, blue shark, mako shark, thresher shark, and hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities.

Sharks
Temporal range: Ludfordian-Present, 425–0 Ma
[1]
Grey reef shark
Clockwise from top left: spiny dogfish, Japanese sawshark, whale shark, great white shark, horn shark, frilled shark, scalloped hammerhead and Australian angelshark representing the orders Squaliformes, Pristiophoriformes, Orectolobiformes, Lamniformes, Heterodontiformes, Hexanchiformes, Carcharhiniformes and Squatiniformes respectively.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Infraclass: Euselachii
Superorder: Selachimorpha
Orders

Carcharhiniformes
Heterodontiformes
Hexanchiformes
Lamniformes
Orectolobiformes
Pristiophoriformes
Squaliformes
Squatiniformes
Cladoselachiformes
Hybodontiformes
Symmoriida
Xenacanthida (Xenacantiformes) Elegestolepis

† = extinct
Synonyms
Pleurotremata

Etymology

Until the 16th century,[6] sharks were known to mariners as "sea dogs".[7] This is still evidential in several species termed "dogfish," or the porbeagle.

The etymology of the word "shark" is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning "villain, scoundrel" (cf. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour.[8]

A now disproven theory is that it derives from the Yucatec Maya word xok (pronounced 'shok'), meaning "fish".[9] Evidence for this etymology came from the Oxford English Dictionary, which notes shark first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted "sharke" to refer to the large sharks of the Caribbean Sea. However, the Middle English Dictionary records an isolated occurrence of the word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology.[10]

Evolution

CretaceousSharkTeeth061812
A collection of Cretaceous shark teeth

Evidence for the existence of sharks dates from the Ordovician period, 450–420 million years ago, before land vertebrates existed and before a variety of plants had colonized the continents.[2] Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks, suspecting that these scales are actually those of thelodont agnathans.[11] The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.[11] The first sharks looked very different from modern sharks.[12] At this time the most common shark tooth is the cladodont, a style of thin tooth with three tines like a trident, apparently to help catch fish. The majority of modern sharks can be traced back to around 100 million years ago.[13] 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,[12] 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.[12] 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.[14][15] 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. The results of a 2014 study of the gill structure of an unusually well preserved 325-million-year-old fossil suggested that sharks are not "living fossils", but rather have evolved more extensively than previously thought over the hundreds of millions of years they have been around.[16]

Megalodon scale
Megalodon (top two, estimated maximum and conservative sizes) with the whale shark, great white shark, and a human for scale

Modern sharks began to appear about 100 million years ago.[13] Fossil mackerel shark teeth date to the Early Cretaceous. One of the most recently evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene.[17] The oldest white shark teeth date from 60 to 66 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).[18][19] 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.[20] Paleontological evidence suggests that this shark was an active predator of large cetaceans.[20]

Taxonomy

Identification of the 8 extant shark orders

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 was thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks,[21] but current molecular studies support monophyly of both groups of sharks and batoids.[22][23]

The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea (or Squalomorphii). 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 Squaleans are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled shark, though some authors propose both families to be moved to separate orders. 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.[21][24]

There are more than 470 species of sharks split across twelve orders, including four orders of sharks that have gone extinct:[24]

Anatomy

Parts of a shark
General anatomical features of sharks

Teeth

Tiger 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.[25]

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.[26]

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.[27] Because sharks do not have rib cages, they can easily be crushed under their own weight on land.[28]

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.[29] 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.[27] 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.[30] 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.[28]

Dermal denticles

Denticules cutanés du requin citron Negaprion brevirostris vus au microscope électronique à balayage
The dermal denticles of a lemon shark, viewed through a scanning electron microscope

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.[31] Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.[32]

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.[33]

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.[34] 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.[31] Their liver constitutes up to 30% of their total body mass.[35] The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth while swimming. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Bottom-dwelling sharks, like the nurse shark, have negative buoyancy, allowing them to rest on the ocean floor.

Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely.[36]

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.[26] 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.[37][38]

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.[39]

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 and bigeye thresher sharks have a similar mechanism for maintaining an elevated body temperature.[40]

Osmoregulation

In contrast to bony fish, with the exception of the coelacanth,[41] the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea (up to 2.5%[42]) 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.[35] When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia.[43][44]

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.[45] 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.[46]

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.[45]

Senses

Smell

Hammerhead shark
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.[47]

Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.[48] 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

Hexanchus nakamurai JNC2615 Eye
Eye of a Bigeyed sixgill shark (Hexanchus nakamurai)

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 it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Many sharks can contract and dilate their pupils, like humans, something no teleost fish can do. 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.[49] 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.[50] [51][52]

Hearing

Although it is hard to test the hearing of sharks, they may have a sharp sense of hearing and can possibly hear prey from many miles away.[53] 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.

Electroreceptors in a sharks head
Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark

Electroreception

The ampullae of Lorenzini are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce.[54] 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.[55]

Lateral line

This system is found in most fish, including sharks. It is a tactile sensory system which allows the organism to detect water speed and pressure changes near by.[56] The main component of the system is the neuromast, a cell similar to hair cells present in the vertebrate ear that interact with the surrounding aquatic environment. This helps sharks distinguish between the currents around them, obstacles off on their periphery, and struggling prey out of visual view. The shark can sense frequencies in the range of 25 to 50 Hz.[57]

Life history

Wobbegong claspers
The claspers of male spotted wobbegong
Scyliorhinus canicula foetus in an egg
Shark egg

Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has one of the longest lifespans at more than 100 years.[58] Whale sharks (Rhincodon typus) may also live over 100 years.[59] Earlier estimates suggested the Greenland shark (Somniosus microcephalus) could reach about 200 years, but a recent study found that a 5.02-metre-long (16.5 ft) specimen was 392 ± 120 years old (i.e., at least 272 years old), making it the longest-lived vertebrate known.[60][61]

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.[62] Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity at around age 13–15.[63]

Sexual

Sharks practice internal fertilization.[64] 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.[65]

Mating has rarely been observed in sharks.[66] 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.[65]

Asexual

There have been a number of 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.[67][68] 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. Mammals are now the only major vertebrate group in which asexual reproduction has not been observed.

Scientists say 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.[69]

Brooding

Sharks display three ways to bear their young, varying by species, oviparity, viviparity and ovoviviparity.[70][71]

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. Taking this a step further, sand tiger shark pups cannibalistically consume neighboring 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.[70]

Oviparity

Some species are oviparous, laying their fertilized 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. The egg case is commonly called a mermaid's purse. Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swellshark.[70][72]

Viviparity

Viviparity is the gestation of young without the use of a traditional egg, and results in live birth.[73] Viviparity in sharks can be placental or aplacental.[73] Young are born fully formed and self-sufficient.[73] Hammerheads, the requiem sharks (such as the bull and blue sharks), and smoothhounds are viviparous.[62][70]

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 social, sedentary, benthic lives, and appear likely to have their own distinct personalities.[74] Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year.[75] 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.[35] Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding.[62]

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.[76]

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).[77] 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. Sharks can travel 70 to 80 km in a day.[78]

Intelligence

Sharks possess brain-to-body mass ratios that are similar to mammals and birds,[79] and have exhibited apparent curiosity and behavior resembling play in the wild.[80][81]

There is evidence that juvenile lemon sharks can use observational learning in their investigation of novel objects in their environment.[82]

Sleep

All sharks need to keep water flowing over their gills in order for them to breathe; however, not all species need to be moving to do this. Those that are able to breathe while not swimming do so by using their spiracles to force water over their gills, thereby allowing them to extract oxygen from the water. It has been recorded that their eyes remain open while in this state and actively follow the movements of divers swimming around them[83] and as such they are not truly asleep.

Species that do need to swim continuously to breathe go through a process known as sleep swimming, in which the shark is essentially unconscious. It is known from experiments conducted on the spiny dogfish that its spinal cord, rather than its brain, coordinates swimming, so spiny dogfish can continue to swim while sleeping, and this also may be the case in larger shark species.[83]

Ecology

Feeding

Most sharks are carnivorous.[84] Basking sharks, whale sharks, and megamouth 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.[84]

Surfacing great white shark
Unlike many other sharks, the great white shark is not actually an apex predator in all of its natural environments, as it is sometimes hunted by orcas

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.[35]

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.[85] 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.[86]

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.[87] 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).[88]

Relationship with humans

Attacks

Shark warning - Salt Rock South Africa
A sign warning about the presence of sharks in Salt Rock, South Africa
Snorkeler with blacktip reef shark
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.

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.[89]

Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 470 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.[90][91] 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.[92]

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.[93]

To help avoid an unprovoked attack, humans should not wear jewelry or metal that is shiny and refrain from splashing around too much.[94]

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, along with safer transportation techniques that have enabled long distance transportation.[95] For a long time, the great white shark had never been successfully held in captivity for long periods of time, 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.[96] Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far too large for typical home aquaria.[96] Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them.[96] Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of 3 feet (90 cm) and can live up to 25 years.[96]

In culture

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.[97]

Kamohoali'i is the best known and revered of the shark gods, he was the older and favored brother of Pele,[98] 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 Molokai. Kamohoali'i was an ancestral god, not a human who became a shark and banned the eating of humans after eating one herself.[99][100] In Fijian mythology, Dakuwaqa was a shark god who was the eater of lost souls.

In American Samoa

On the island of Tutuila in American Samoa (a U.S. territory), there is a location called Turtle and Shark (Laumei ma Malie) which is important in Samoan culture — the location is the site of a legend called O Le Tala I Le Laumei Ma Le Malie, in which two humans are said to have transformed into a turtle and a shark.[101][102][103] According to the U.S. National Park Service, "Villagers from nearby Vaitogi continue to reenact an important aspect of the legend at Turtle and Shark by performing a ritual song intended to summon the legendary animals to the ocean surface, and visitors are frequently amazed to see one or both of these creatures emerge from the sea in apparent response to this call."[101]

In popular culture

In contrast to the complex portrayals by Hawaiians and other Pacific Islanders, the European and Western view of sharks has historically been mostly of fear and malevolence.[104] Sharks are used in popular culture commonly as eating machines, notably in the Jaws novel and the film of the same name, along with its sequels.[105] Sharks are threats in other films such as Deep Blue Sea, The Reef, and others, although they are sometimes used for comedic effect such as in Finding Nemo and the Austin Powers series. Sharks tend to be seen quite often in cartoons whenever a scene involves the ocean. Such examples include the Tom and Jerry cartoons, Jabberjaw, and other shows produced by Hanna-Barbera. They also are used commonly as a clichéd means of killing off a character that is held up by a rope or some similar object as the sharks swim right below them, or the character may be standing on a plank above shark infested waters.

Popular misconceptions

A popular myth is that sharks are immune to disease and cancer, but this is not scientifically supported. Sharks have been known to get cancer.[106][107] 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.[108] Other apparently false claims are that fins prevent cancer[109] and treat osteoarthritis.[110] No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment.[111]

Threats to sharks

Sharksfin
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.
Global shark catch
The annual shark catch has increased rapidly over the last 60 years.
Tiger shark, Hawaii Aii
A 14-foot (4.3 m), 1,200-pound (540 kg) tiger shark caught in Kāne'ohe Bay, Oahu in 1966

Fishery

It is estimated that 100 million sharks are killed by people every year, due to commercial and recreational fishing.[112][113] Shark finning yields are estimated at 1.44 million metric tons for 2000, and 1.41 million tons for 2010. Based on an analysis of average shark weights, this translates into a total annual mortality estimate of about 100 million sharks in 2000, and about 97 million sharks in 2010, with a total range of possible values between 63 and 273 million sharks per year.[114][115] 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 a delicacy called hákarl.[116] During a four-year period from 1996 to 2000, an estimated 26 to 73 million sharks were killed and traded annually in commercial markets.[117]

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.[113] The resulting immobile shark soon dies from suffocation or predators.[118] Shark fin has become a major trade within black markets all over the world. Fins sell for about $300/lb in 2009.[119] Poachers illegally fin millions each year. Few governments enforce laws that protect them.[115] In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins.[120] From 1996 to 2000, an estimated 38 million sharks had been killed per year for harvesting shark fins.[117] It is estimated by TRAFFIC that over 14,000 tonnes of shark fins were exported into Singapore between 2005–2007 and 2012–2014.[121]

Shark fin soup is a status symbol in Asian countries, and is erroneously 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.[122] 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. Capture induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished.[64] Capture-induced parturition is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).[64]

The majority of shark fisheries have little monitoring or management. The rise in demand for shark products increases pressure on fisheries.[36] 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.[123] A study by the International Union for Conservation of Nature suggests that one quarter of all known species of sharks and rays are threatened by extinction and 25 species were classified as critically endangered.[124][125]

Shark culling

In 2014, a shark cull in Western Australia killed dozens of sharks (mostly tiger sharks) using drum lines,[126] until it was cancelled after public protests and a decision by the Western Australia EPA; from 2014 to 2017, there was an "imminent threat" policy in Western Australia in which sharks that "threatened" humans in the ocean were shot and killed.[127] This "imminent threat" policy was criticized by senator Rachel Siewart for killing endangered sharks.[128] The "imminent threat" policy was cancelled in March 2017.[129] In August 2018, the Western Australia government announced a plan to re-introduce drum lines (though, this time the drum lines are "SMART" drum lines).[130]

From 1962 to the present,[131] the government of Queensland has targeted and killed sharks in large numbers by using drum lines, under a "shark control" program—this program has also inadvertently killed large numbers of other animals such as dolphins; it has also killed endangered hammerhead sharks.[132][133][134][135] Queensland's drum line program has been called "outdated, cruel and ineffective".[135] From 2001 to 2018, a total of 10,480 sharks were killed on lethal drum lines in Queensland, including in the Great Barrier Reef.[136] From 1962 to 2018, roughly 50,000 sharks were killed by Queensland authorities.[137]

The government of New South Wales has a program that deliberately kills sharks using nets.[134][138] The current net program in New South Wales has been described as being "extremely destructive" to marine life, including sharks.[139] Between 1950 and 2008, 352 tiger sharks and 577 great white sharks were killed in the nets in New South Wales — also during this period, a total of 15,135 marine animals were killed in the nets, including dolphins, whales, turtles, dugongs, and critically endangered grey nurse sharks.[140] There has been a very large decrease in the number of sharks in eastern Australia, and the shark-killing programs in Queensland and New South Wales are partly responsible for this decrease.[137]

Kwazulu-Natal, an area of South Africa, has a shark-killing program using nets and drum lines—these nets and drum lines have killed turtles and dolphins, and have been criticized for killing wildlife.[141] During a 30-year period, more than 33,000 sharks have been killed in KwaZulu-Natal's shark-killing program — during the same 30-year period, 2,211 turtles, 8,448 rays, and 2,310 dolphins were killed in KwaZulu-Natal.[141] Authorities on the French island of Réunion kill about 100 sharks per year.[142]

Killing sharks negatively affects the marine ecosystem.[143][144] Jessica Morris of Humane Society International calls shark culling a "knee-jerk reaction" and says, "sharks are top order predators that play an important role in the functioning of marine ecosystems. We need them for healthy oceans."[145]

George H. Burgess, the former[146] director of the International Shark Attack File, "describes [shark] culling as a form of revenge, satisfying a public demand for blood and little else";[147] he also said shark culling is a "retro-type move reminiscent of what people would have done in the 1940s and 50s, back when we didn't have an ecological conscience and before we knew the consequences of our actions."[147] Jane Williamson, an associate professor in marine ecology at Macquarie University, says "There is no scientific support for the concept that culling sharks in a particular area will lead to a decrease in shark attacks and increase ocean safety."[148]

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.[149] The 2007 documentary Sharkwater exposed how sharks are being hunted to extinction.[150]

Conservation

In 1991, South Africa was the first country in the world to declare Great White sharks a legally protected species[151] (however, the KwaZulu-Natal Sharks Board is allowed to kill great white sharks in its "shark control" program in eastern South Africa).[141]

Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000.[152] 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.[153] 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.[154][155]

In 2003, the European Union introduced a general shark finning ban for all vessels of all nationalities in Union waters and for all vessels flying a flag of one of its member states.[156] This prohibition was amended in June 2013 to close remaining loopholes.[157]

In 2009, the International Union for Conservation of Nature's 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.[158][159]

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.[160][161] In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip and porbeagle were added to Appendix 2 of CITES, bringing shark fishing and commerce of these species under licensing and regulation.[162]

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.[163] Advocacy group Shark Trust campaigns to limit shark fishing. Advocacy group Seafood Watch directs American consumers to not eat sharks.[164]

Under the auspices of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), also known as the Bonn Convention, the Memorandum of Understanding on the Conservation of Migratory Sharks was concluded and came into effect in March 2010. It was the first global instrument concluded under CMS and aims at facilitating international coordination for the protection, conservation and management of migratory sharks, through multilateral, intergovernmental discussion and scientific research.

In July 2013, New York state, a major market and entry point for shark fins, banned the shark fin trade joining seven other states of the United States and the three Pacific U.S territories in providing legal protection to sharks.[165]

As of September 2018, 12 U.S. states (California, Delaware, Hawaii, Illinois, Maryland, Massachusetts, Nevada, New York, Oregon, Rhode Island, Texas, and Washington) and 3 U.S. territories (American Samoa, Guam and the Northern Mariana Islands) have banned the sale or possession of shark fins.[166]

Several regions now have shark sanctuaries or have banned shark fishing — these regions include American Samoa, the Bahamas, the Cook Islands, French Polynesia, Guam, the Maldives, the Marshall Islands, Micronesia, the Northern Mariana Islands, and Palau.[167][168]

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General references
  • Castro, Jose (1983). The Sharks of North American Waters. College Station: Texas A&M University Press. ISBN 978-0-89096-143-8. OCLC 183037060.
  • Stevens, John D. (1987). Sharks. New York: NY Facts on File Publications. ISBN 978-0-8160-1800-0. OCLC 15163749.
  • Pough, F. H.; Janis, C. M.; Heiser, J. B. (2005). Vertebrate Life (7th ed.). New Jersey: Pearson Education Ltd. ISBN 978-0-13-127836-3. OCLC 54822028.
  • Clover, Charles (2004). The End of the Line: How overfishing is changing the world and what we eat. London: Ebury Press. ISBN 978-0-09-189780-2.
  • Owen, David (2009). Shark: In Peril in the Sea. New South Wales: Allen and Unwin. ISBN 978-1-74175-032-4.

Further reading

  • Musick, Jogn A and Musick, Susanna (2011) "Sharks" In: Review of the state of world marine fishery resources, pages 245–254, FAO Fisheries technical paper 569, FAO, Rome. ISBN 978-92-5-107023-9.
  • Sharks Falling Prey To Humans' Appetites National Geographic, 28 October 2010.
Basking shark

The basking shark (Cetorhinus maximus) is the second-largest living shark, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach 6–8 m (20–26 ft) in length. They are usually greyish-brown, with mottled skin. The caudal fin has a strong lateral keel and a crescent shape.

The basking shark is a cosmopolitan migratory species, found in all the world's temperate oceans. A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has anatomical adaptations for filter-feeding, such as a greatly enlarged mouth and highly developed gill rakers. Its snout is conical and the gill slits extend around the top and bottom of its head. The gill rakers, dark and bristle-like, are used to catch plankton as water filters through the mouth and over the gills. The teeth are numerous and very small, and often number 100 per row. The teeth have a single conical cusp, are curved backwards, and are the same on both the upper and lower jaws. This species has the smallest weight-for-weight brain size of any shark, reflecting its relatively passive lifestyle.Basking sharks have been shown from satellite tracking to overwinter in both continental shelf (less than 200 m or 660 ft) and deeper waters. They may be found in either small shoals or alone. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans.

The basking shark has long been a commercially important fish, as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point where some have disappeared and others need protection.

Blue shark

The blue shark (Prionace glauca) is a species of requiem shark, in the family Carcharhinidae, that inhabits deep waters in the world's temperate and tropical oceans. Preferring cooler waters, blue sharks migrate long distances, such as from New England to South America. The species is listed as Near Threatened by the IUCN.

Although generally lethargic, they can move very quickly. Blue sharks are viviparous and are noted for large litters of 25 to over 100 pups. They feed primarily on small fish and squid, although they can take larger prey. Maximum lifespan is still unknown, but it is believed that they can live up to 20 years.

Bull shark

The bull shark (Carcharhinus leucas), also known as the Zambezi shark (informally "zambi") in Africa, and Lake Nicaragua shark in Nicaragua, is a requiem shark commonly found worldwide in warm, shallow waters along coasts and in rivers. The bull shark is known for its aggressive nature, and presence in warm, shallow brackish and freshwater systems including estuaries and rivers.

Bull sharks can thrive in both salt and fresh water and can travel far up rivers. They have been known to travel up the Mississippi River as far as Alton, Illinois, about 700 miles (1100 km) from the ocean. However, few freshwater human-shark interactions have been recorded. Larger-sized bull sharks are probably responsible for the majority of near-shore shark attacks, including many bites attributed to other species.Unlike the river sharks of the genus Glyphis, bull sharks are not true freshwater sharks, despite their ability to survive in freshwater habitats.

Great white shark

The great white shark (Carcharodon carcharias), also known as the great white, white shark or white pointer, is a species of large mackerel shark which can be found in the coastal surface waters of all the major oceans. The great white shark is notable for its size, with larger female individuals growing to 6.1 m (20 ft) in length and 1,905 kg (4,200 lb) in weight at maturity. However, most are smaller; males measure 3.4 to 4.0 m (11 to 13 ft), and females measure 4.6 to 4.9 m (15 to 16 ft) on average. According to a 2014 study, the lifespan of great white sharks is estimated to be as long as 70 years or more, well above previous estimates, making it one of the longest lived cartilaginous fish currently known. According to the same study, male great white sharks take 26 years to reach sexual maturity, while the females take 33 years to be ready to produce offspring. Great white sharks can swim at speeds of over 56 km/h (35 mph), and can swim to depths of 1,200 m (3,900 ft).The great white shark has no known natural predators other than, on very rare occasions, the killer whale. The great white shark is arguably the world's largest known extant macropredatory fish, and is one of the primary predators of marine mammals. It is also known to prey upon a variety of other marine animals, including fish and seabirds. It is the only known surviving species of its genus Carcharodon, and is responsible for more recorded human bite incidents than any other shark.The species faces numerous ecological challenges which has resulted in international protection. The IUCN lists the great white shark as a vulnerable species, and it is included in Appendix II of CITES. It is also protected by several national governments such as Australia (as of 2018).The novel Jaws by Peter Benchley and its subsequent film adaptation by Steven Spielberg depicted the great white shark as a "ferocious man eater". Humans are not the preferred prey of the great white shark, but the great white is nevertheless responsible for the largest number of reported and identified fatal unprovoked shark attacks on humans.

Greenland shark

The Greenland shark (Somniosus microcephalus), also known as the gurry shark, grey shark, or by the Kalaallisut name eqalussuaq, is a large shark of the family Somniosidae ("sleeper sharks"), closely related to the Pacific and southern sleeper sharks. The distribution of this species is mostly restricted to the waters of the North Atlantic Ocean and Arctic Ocean.

Greenland sharks have the longest known lifespan of all vertebrate species (estimated to be between 300–500 years), and the species is among the largest extant species of shark. As an adaptation to living at depth, it has a high concentration of trimethylamine N-oxide in its tissues, which causes the meat to be toxic. Greenland shark flesh treated to reduce toxin levels is eaten in Iceland as a delicacy known as kæstur hákarl.

Greg Norman

Gregory John Norman AO (born 10 February 1955) is an Australian professional golfer and entrepreneur who spent 331 weeks as the world's Number 1 Official World Golf Rankings ranked golfer in the 1980s and 1990s. He has won 91 international tournaments, including 20 PGA Tour tournaments and two majors: The Open Championships in 1986 and 1993. Norman also earned thirty top-10 finishes and was the runner-up 8 times in majors throughout his career. He was inducted into the World Golf Hall of Fame in 2001 with the highest percentage of votes (80%) of any golfer to date. In a reference to his blond hair, size, aggressive golf style and his birthplace's native coastal animal, Norman's nickname is "The Great White Shark" (often shortened to just "The Shark"), which he earned after his play at the 1981 Masters.During and after his playing career, Norman engaged in numerous entrepreneurial and philanthropic endeavors. He currently serves as the chairman and CEO of the Greg Norman Company, a global corporation with a portfolio of companies in fields like apparel, interior design, real estate, private equity, golf course design, and more. Norman has also donated to and established numerous charities and charity events like the QBE Shootout which benefits the CureSearch for Children's Cancer fund. He became a Trustee of the Environmental Institute for Golf in 2004 and received the Golf Writers Association of America's Bartlett Award in 2008 for his philanthropic endeavors.In 2009 as part of the Q150 celebrations, Greg Norman was announced as one of the Q150 Icons of Queensland for his role as a "sports legend".

Hammerhead shark

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Most hammerhead species are placed in the genus Sphyrna, while the winghead shark is placed in its own genus, Eusphyra. Many, but not necessarily mutually exclusive, functions have been postulated for the cephalofoil, including sensory reception, manoeuvering, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in schools during the day, becoming solitary hunters at night. Some of these schools can be found near Malpelo Island in Colombia, the Galapagos Islands in Ecuador, Cocos Island off Costa Rica, and near Molokai in Hawaii. Large schools are also seen in the waters off southern and eastern Africa.

Jaws (film)

Jaws is a 1975 American thriller film directed by Steven Spielberg and based on Peter Benchley's 1974 novel of the same name. In the film, a giant man-eating great white shark attacks beachgoers on Amity Island, a fictional New England summer resort town, prompting police chief Martin Brody (Roy Scheider) to hunt it with the help of a marine biologist (Richard Dreyfuss) and a professional shark hunter (Robert Shaw). Murray Hamilton plays the mayor, and Lorraine Gary portrays Brody's wife. The screenplay is credited to Benchley, who wrote the first drafts, and actor-writer Carl Gottlieb, who rewrote the script during principal photography.

Shot mostly on location on Martha's Vineyard in Massachusetts, Jaws had a troubled production, going over budget and past schedule. As the art department's mechanical sharks often malfunctioned, Spielberg decided to mostly suggest the shark's presence, employing an ominous and minimalistic theme created by composer John Williams to indicate its impending appearances. Spielberg and others have compared this suggestive approach to that of thriller director Alfred Hitchcock. Universal Pictures gave the film what was then an exceptionally wide release for a major studio picture, on over 450 screens, accompanied by an extensive marketing campaign with a heavy emphasis on television spots and tie-in merchandise.

Considered one of the greatest films ever made, Jaws was the prototypical summer blockbuster, with its release regarded as a watershed moment in motion picture history, and it won several awards for its music and editing. It became the highest-grossing film of all time until the release of Star Wars in 1977. Both films were pivotal in establishing the modern Hollywood business model, which revolves around high box-office returns from action and adventure pictures with simple high-concept premises released during the summer in thousands of theaters and heavily advertised. It was followed by three sequels, all without Spielberg or Benchley, and many imitative thrillers. In 2001, it was selected by the Library of Congress for preservation in the United States National Film Registry, being deemed "culturally, historically, or aesthetically significant".

Jumping the shark

Jumping the shark is the moment when something that was once popular that no longer warrants the attention it previously received makes an attempt at publicity, which only serves to highlight its irrelevance. This is especially applicable to television series or other entertainment outlets.The idiom "jumping the shark" is pejorative, most commonly used in reference to unsuccessful gimmicks for promoting something. It is similar to "past its peak" but more specifically suggests an unwillingness to acknowledge the failing. Originally, the phrase was used to describe an episode of a television comedy with a gimmick or unlikely occurrence desperately attempting to keep viewers' interest. Moments labeled as "jumping the shark" are considered indications that writers have exhausted their focus; that the show has strayed irretrievably from an older and better formula; or that the series as a whole is declining in quality.

Popularized by radio personality Jon Hein in the 1990s and early 2000s, the phrase derives from a scene in a Season 5 episode of the 1970s sitcom Happy Days in which the character Fonzie jumps over a shark while on water-skis. This gimmick strayed absurdly outside the original storyline of the sitcom.

The usage of "jump the shark" has subsequently broadened beyond television, indicating the moment when a brand, design, franchise, or creative effort's evolution declines, or when it changes notably in style into something unwelcome.

List of fish common names

This is a list of common fish names. While some common names refer to a single species or family, others have been used for a confusing variety of types; the articles listed here should explain the possibilities if the name is ambiguous.

Loan shark

A loan shark is a person who offers loans at extremely high interest rates, has strict terms of collection upon failure, and operates outside off the street (outside of local authority). The term usually refers to illegal activity, but may also refer to predatory lending with extremely high interest rates such as payday or title loans.An unintended consequence of poverty alleviation initiatives can be that loan sharks borrow from formal microfinance lenders and lend on to poor borrowers. Loan sharks sometimes enforce repayment by blackmail or threats of violence. Historically, many moneylenders skirted between legal and criminal activity. In the recent western world, loan sharks have been a feature of the criminal underworld.

Megalodon

Megalodon (Carcharocles megalodon), meaning "big tooth", is an extinct species of shark that lived approximately 23 to 3.6 million years ago (mya), during the Early Miocene to the end of the Pliocene. It was formerly thought to be a member of the family Lamnidae, making it closely related to the great white shark (Carcharodon carcharias). However presently there is near unanimous consensus that it belongs to the extinct family Otodontidae, which diverged from the ancestry of the great white shark during the Early Cretaceous. Its genus placement is still debated, authors placing it in either Carcharocles, Megaselachus, Otodus, or Procarcharodon.

Scientists suggest that megalodon looked like a stockier version of the great white shark, though some experts believe it may have looked similar to the basking shark (Cetorhinus maximus) or the sand tiger shark (Carcharias taurus). Regarded as one of the largest and most powerful predators to have ever lived, fossil remains of megalodon suggest that this giant shark reached a maximum length of 18 meters (59 ft) with the average size being 10.5 meters (34 ft). Their large jaws could exert a bite force of up to 110,000 to 180,000 newtons (25,000 to 40,000 lbf). Their teeth were thick and robust, built for grabbing prey and breaking bone.

Megalodon probably had a major impact on the structure of marine communities. The fossil record indicates that it had a cosmopolitan distribution. It probably targeted large prey, such as whales, seals, and sea turtles. Juveniles inhabited warm coastal waters and fed on fish and small whales. Unlike the great white, which attacks prey from the soft underside, megalodon probably used its strong jaws to break through the chest cavity and puncture the heart and lungs of its prey.

The animal faced competition from whale-eating cetaceans, such as Livyatan and other macroraptorial sperm whales, and smaller ancestral killer whales such as Orcinus citoniensis. As the shark preferred warmer waters, it is thought that oceanic cooling associated with the onset of the ice ages, coupled with the lowering of sea levels and resulting loss of suitable nursery areas, may have also contributed to its decline. A reduction in the diversity of baleen whales and a shift in their distribution toward polar regions may have reduced megalodon's primary food source. More recently, evidence has come forward that competition from the modern great white shark may have also contributed to the extinction of megalodon, coupled with range fragmentation resulting in a gradual, asynchronous extinction as a result of cooling oceans around 3.6-4 million years ago, far earlier than previously assumed. The extinction of the shark appeared to affect other animals; for example, the size of baleen whales increased significantly after the shark had disappeared.

Oceanic whitetip shark

The oceanic whitetip shark (Carcharhinus longimanus), also known as Brown Milbert's sand bar shark, brown shark, lesser white shark, nigano shark, oceanic white-tipped whaler, and silvertip shark, is a large pelagic requiem shark inhabiting tropical and warm temperate seas. Its stocky body is most notable for its long, white-tipped, rounded fins.

This slow-moving but aggressive fish dominates feeding frenzies, and is a danger to shipwreck or air crash survivors. Recent studies show steeply declining populations because its large fins are highly valued as the chief ingredient of shark fin soup, and as with other shark species, the whitetip faces mounting fishing pressure throughout its range.

Shark Tale

Shark Tale is a 2004 American computer-animated comedy film produced by DreamWorks Animation and directed by Vicky Jenson, Bibo Bergeron and Rob Letterman. The first computer-animated film by DreamWorks Animation to be produced at their Glendale studio, the film stars the voices of Will Smith, Robert De Niro, Renée Zellweger, Angelina Jolie, Jack Black, and Martin Scorsese. Other voices were provided by Ziggy Marley, Doug E. Doug, Michael Imperioli, Vincent Pastore and Peter Falk. It tells the story of a fish named Oscar (Smith) who falsely claims to have killed the son of a shark mob boss (De Niro) to advance his own community standing.

Shark Tale opened at #1 with $47.6 million, which was the second-highest opening for a DreamWorks Animation film at the time, behind Shrek 2 ($108 million). It remained as the #1 film in the U.S. and Canada for its second and third weekends, and made $367 million worldwide against its $75 million budget, despite generally mixed critical reception. It was also nominated for the Academy Award for Best Animated Feature, losing to The Incredibles.

Shark Tank

Shark Tank is an American business-related reality television series on ABC that premiered on August 9, 2009. The show is the American franchise of the international format Dragons' Den, which originated in Japan in 2001. It shows aspiring entrepreneurs as they make business presentations to a panel of five investors or "sharks", who then choose whether to invest in their company as business partners.

The series has been a ratings success in its time slot and has won the Primetime Emmy Award for Outstanding Structured Reality Program four times (2014 through 2017), in all four years of that category's existence. Prior to that (2012 to 2013), it won Outstanding Reality Program.On February 5, 2019, ABC renewed the series for an eleventh season.

Shark attack

A shark attack is an attack on a human by a shark. Every year, around 80 unprovoked attacks are reported worldwide. Despite their relative rarity, many people fear shark attacks after occasional serial attacks, such as the Jersey Shore shark attacks of 1916, and horror fiction and films such as the Jaws series. Out of more than 489 shark species, only three are responsible for a double-digit number of fatal, unprovoked attacks on humans: the great white, tiger, and bull. The oceanic whitetip has probably killed many more castaways, but these are not recorded in the statistics.

Shark finning

Shark finning is the act of removing fins from sharks and discarding the rest of the shark. The sharks are often still alive when discarded, but without their fins. Unable to swim effectively, they sink to the bottom of the ocean and die of suffocation or are eaten by other predators. Shark finning at sea enables fishing vessels to increase profitability and increase the number of sharks harvested, as they only have to store and transport the fins, by far the most profitable part of the shark; the shark meat is bulky to transport. Some countries have banned this practice and require the whole shark to be brought back to port before removing the fins.

Shark finning increased since 1997 largely due to the increasing demand for shark fins for shark fin soup and traditional cures, particularly in China and its territories, and as a result of improved fishing technology and market economics. The International Union for Conservation of Nature's Shark Specialist Group say that shark finning is widespread, and that "the rapidly expanding and largely unregulated shark fin trade represents one of the most serious threats to shark populations worldwide". Estimates of the global value of the shark fin trade range from US$540 million to US$1.2 billion (2007). Shark fins are among the most expensive seafood products, commonly retailing at US$400 per kg. In the United States, where finning is prohibited, some buyers regard the whale shark and the basking shark as trophy species, and pay $10,000 to $20,000 for a fin.The regulated global catch of sharks reported to the Food and Agriculture Organization of the United Nations has been stable in recent years at an annual average just over 500,000 tonnes. Additional unregulated and unreported catches are thought to be common.Shark finning has caused catastrophic harm to the marine ecosystem. Roughly 73 million sharks are killed each year by finning, though some estimate that finning kills 100 million sharks each year. A variety of shark species are threatened by shark finning, including the endangered scalloped hammerhead shark.

Tiger shark

The tiger shark (Galeocerdo cuvier) is a species of requiem shark and the only extant member of the genus Galeocerdo. It is a large macropredator, capable of attaining a length over 5 m (16 ft 5 in). Populations are found in many tropical and temperate waters, especially around central Pacific islands. Its name derives from the dark stripes down its body, which resemble a tiger's pattern, but fade as the shark matures.The tiger shark is a solitary, mostly nocturnal hunter. It is notable for having the widest food spectrum of all sharks, with a range of prey that includes crustaceans, fish, seals, birds, squid, turtles, sea snakes, dolphins, and even other smaller sharks. It also has a reputation as a "garbage eater", consuming a variety of inedible, man-made objects that linger in its stomach. Though apex predators, tiger sharks are sometimes taken by groups of killer whales. It is considered a near threatened species due to finning and fishing by humans.The tiger shark is second only to the great white in recorded fatal attacks on humans.

Whale shark

The whale shark (Rhincodon typus) is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of 18.8 m (62 ft) The whale shark holds many records for size in the animal kingdom, most notably being by far the largest living nonmammalian vertebrate. It is the sole member of the genus Rhincodon and the only extant member of the family Rhincodontidae, which belongs to the subclass Elasmobranchii in the class Chondrichthyes. Before 1984 it was classified as Rhiniodon into Rhinodontidae.

The whale shark is found in open waters of the tropical oceans and is rarely found in the water below 21 °C (70 °F). Modeling suggests a lifespan of about 70 years, and while measurements have proven difficult, estimates from field data suggest they may live as long as 130 years Whale sharks have very large mouths and are filter feeders, which is a feeding mode that occurs in only two other sharks, the megamouth shark and the basking shark. They feed almost exclusively on plankton and small fishes, and pose no threat to humans.

The species was distinguished in April 1828 after the harpooning of a 4.6 m (15 ft) specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town, described it the following year. The name "whale shark" refers to the fish's size, being as large as some species of whales, and also to its being a filter feeder like baleen whales.

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