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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, parasites and to improve fluid dynamics.^[1] They have replaceable teeth. They are some of the world's most misunderstood predators, as they never attack humans unless intimidated.

Sharks include species from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 cm in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (41 feet) and which, like the great whales, feeds only on plankton through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water (it is found in Lake Nicaragua, in Central America) and in deltas.^[2]

Physical characteristics

Skeleton

The skeleton of the shark is very different from that of bony fishes such as cod; it is made from cartilage, which is very light and flexible, although the cartilage in older sharks can sometimes be partly calcified, making it harder and more bone-like. The shark's jaw is variable and is thought to have evolved from the first gill arch. It is not attached to the cranium and has extra mineral deposits to give it greater strength.^[3]

Respiration

Like other fishes, sharks extract oxygen from seawater as it passes over their gills. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration.^[4] Due to their size and the nature of their metabolism, many sharks have a higher demand for oxygen than bony fishes and they cannot rely on ambient water currents to provide an adequate supply of oxygenated water. If these sharks were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal would suffocate. The process of ensuring an adequate flow of the gills by forward movement is known as ram ventilation. Some sharks, such as the blacktip reef shark, Carcharhinus melanopterus, and nurse shark, Ginglymostoma cirratum, can pump water over their gills as they rest.

Buoyancy

Unlike bony fishes, sharks do not have gas-filled swim bladders, but instead rely on a large oil-filled liver (which may constitute up to 25% of their body mass)^[5] for buoyancy. Its effectiveness is limited, so sharks sink when they stop swimming. Some sharks, if inverted, enter a natural state of tonic immobility - researchers use this condition for handling sharks safely.^[6]

Osmoregulation

In contrast to bony fishes, sharks do not drink seawater; instead they retain high concentrations of waste chemicals in their body to change the diffusion gradient so that they can absorb water directly from the sea. 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.^[5]

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. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks rows of teeth are replaced every 8–10 days, while in other species they could last several months. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it.^[4] The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.

Tails

The tails (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration is dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. 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 fishes such as mackerel and herring has a large lower lobe to provide greater speed to help it keep pace with its fast-swimming prey.^[7]

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.

Speed

In general, sharks swim ("cruise") at an average speed of 8 km/h (5 mph), but when feeding or attacking, the average shark can reach speeds upwards of 19 km/h (12 mph). The shortfin mako may range upwards of 50 km/h (31 mph).^[8] The shortfin mako shark is considered to be the fastest shark and one of the fastest fish. The great white shark is also capable of considerable bursts of speed. These exceptions may be due to the "warm-blooded", or homeothermic, nature of these sharks' physiology.

Dermal denticles

Unlike bony fishes, 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.

Body temperature

A few of the larger species, such as the shortfin mako, Isurus oxyrinchus, and the great white, are mildly homeothermic ^[7] - 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.

Etymology

Until the 16th century,^[9] sharks were known to mariners as "sea dogs".^[10] The name "shark" first came into use around the late 1560s 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 shark, 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.^[11] The first sharks looked very different from modern sharks.^[12] The majority of the modern sharks can be traced back to around 100 million years ago.^[13]

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 mineral apatite (calcium phosphate), they are easily fossilized.

Instead of bones, sharks have cartilagenous skeletons, with a bonelike 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,^[12] 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.^[12] 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 xenocanths, was almost exclusive to freshwater environments. 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.^[13] Fossil mackerel shark teeth occurred in the Lower Cretaceous. 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 all 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 17 cm (7 in) long and suggested a fish that could grow to a length of 25 to 30.5 m (80 to 100 ft). The reconstruction was found to be inaccurate, and estimates revised downwards to around 13 to 15.9 m (43 to 52 ft).

It is believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of the dinosaurs and the diversification of mammals. It is known that at the same time these sharks were evolving some early mammalian groups evolved into aquatic forms. Certainly, wherever the teeth of large sharks have been found, there has also been an abundance of marine mammal bones, including seals, porpoises and whales. These bones frequently show signs of shark attack. There are theories that suggest that large sharks evolved to better take advantage of larger prey.

Classification

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.

There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:

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 fulfill the role 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 than most fishes. Instead of producing huge numbers of eggs and fry (99.9% of which never reach sexual maturity in fishes which use this strategy), sharks normally produce around a dozen pups (blue sharks have been recorded as producing 135 and some species produce as few as two).^[14] These pups are either protected by egg cases or born live. No shark species are known to provide post-natal parental protection for their young, but females have a hormone that is released into their blood during the pupping season that apparently keeps them from feeding on their young.

Shark senses

Sense of smell

Sharks have keen olfactory senses, 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. The short duct between the anterior and posterior nasal openings are not fused as in bony fishes.

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.

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 fishes and tetrapods the external opening into the inner ear has been lost.

Ampullae of Lorenzini

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

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.^[16]

Behavior

Shark behaviour is inextricably linked to their senses as they influence every aspect of their lives. Studies on the behaviour of sharks have only recently been taken, leading to little information on the subject, although this is changing. The classic view of the shark is of a solitary hunter, ranging the oceans in search of food; 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.^[17] 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 in the Sea of Cortez.^[5] Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks or comparable size when feeding.

When approached too closely some sharks will perform a threat display to warn off the prospective predators.^[18] This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat. Although the display is not normally observed during feeding, it resembles the action of a hard bite, and is believed to have derived from the feeding act itself.

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.^[19]

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.^[20]

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.^[21]

Shark sleep

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. 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 with only parts of its brain in a manner similar to dolphins.^[22]

Shark attacks

Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 360 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, tiger, oceanic whitetip and bull sharks.^[23] 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.^[24]

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. In 2005, according to the International Shark Attack File, there were a total of 58 unprovoked attacks recorded worldwide, of which four were fatal.^[25] In comparison, several hundred people die annually from lightning strikes^[26] and 1.3 to 3 million people die from diseases transmitted via mosquito bites.^[27]

Sharks in captivity

Until recently only a few benthic species of shark, such as hornsharks, leopard sharks and catsharks could survive in aquaria 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.^[28]

Despite controlled feeding behaviour being considered critical for the health of the shark, very few studies on this topic have been carried out. Since food is the reward for appropriate behaviour, trainers must rely on control of feeding motivation.

Conservation

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.^[29] Stocks decline and collapse because 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.^[30] 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.

Shark fishery

Every year, an estimate states that 26 to 73 millon (median value is at 38 million) sharks are killed by people in commercial and recreational fishing.^[31] 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.^[32]

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. There have been cases where hundreds of de-finned sharks were swept up on local beaches without any way to convey themselves back into the sea. Conservationists have campaigned for changes in the law to make finning illegal in the U.S.

Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fishes 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.

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:^[33]

In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls.

Myths

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.^[35]

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