Oceanic whitetip shark, Carcharhinus longimanus
Shark is the common name for any member of several orders of cartilaginous fish comprising the taxonomic group Selachimorpha (generally a superorder) of the subclass Elasmobranchii of the class Chondrichthyes. Sharks are characterized by a streamlined body, five to seven gill slits, replaceable teeth, and a covering of dermal denticles (toothlike scales) to protect their skin from damage and parasites and to improve fluid dynamics (Budker 1971). Unlike the closely related rays, sharks have lateral gill openings, pectoral girdle halves not joined dorsally, and the anterior edge of the pectoral fin is not attached to the side of the head (Nelson 1994).
Due to a short history of shark attacks on humans, most sharks inspire fear in many people. But the reality is that only a few of over 350 species of shark are responsible for most attacks, and even these are rare and often provoked, such as a diver grabbing or feeding a shark, or a fisherman handling a caught shark (Burgess 2006).
Furthermore, like all organisms, sharks not only advance their own individual purpose of survival, reproduction, maintenance, and so forth, but also provide value for the ecosystem and for human beings. As apex predators, sharks help maintain the delicate balance in the world's oceans. As carnivores at the top of the food chain, it is understood that they would have mechanisms that would also translate to a threat to humans, and there have been some highly publicized unprovoked attacks. But without their role, prey species could overwhelm the ocean ecosystems. On the other hand, their own low birthrate and slow maturation assures that their numbers remain in balance. In addition, sharks provide food for human beings and, some would say, aesthetic value, adding to the human fascination with nature.
Despite the importance of sharks, their reputation and location makes them difficult to conserve. Humans have placed great pressure on their numbers from commercial fisheries, such as harvesting shark fins for shark fin soup, as well as from recreational fishing and as by-catch in other fisheries. Humans kill between 100 million and 200 million sharks each year, while human deaths are about five per year.
The Chondrichthyes or "cartilaginous fishes" are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. They are divided into two subclasses: Elasmobranchii (sharks, rays, and skates) and Holocephali (chimaera, sometimes called ghost sharks). The Elasmobranchii are sometimes divided into two superorders, Selachimorpha (sharks) and Batoidea (rays, skates, sawfish). Nelson (1994) notes that there is growing acceptance of the view that sharks and rays form a monophyletic group (superorder Euselachii), and sharks without rays are a paraphyletic group.
The extant (living) orders of Elasmobranchii that are typically considered sharks are Hexanchiformes, Squaliformes, Squatiniformes, Pristiophoriformes, Heterodontiformes, Orectolobiformes, Lamniformes, and Carchariniformes (Nelson 1994; Murch 2007). The Squatiniformes (angel sharks) have a ray-like body (Nelson 1994).
Sharks include species ranging from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 centimeters (9 inches) in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 meters (41 feet).
Sharks are mostly marine and mostly carnivorous. However, the whale shark, 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 freshwater and in deltas (Allen 1999).
A December 10, 2006 report by the Census of Marine Life group reveals that 70 percent of the world's oceans are shark-free. They have discovered that although many sharks live up to depths as low as 1,500 meters (5,000 feet), they fail to colonize deeper, putting them more easily within reach of fisheries and thus endangered status (CML 2006).
Until the sixteenth century, sharks were known to mariners as "sea dogs" (Marx 1990). According to the Oxford English Dictionary, the name "shark" first came into use to refer to the large sharks of the Caribbean Sea after Sir John Hawkins' sailors exhibited one in London in 1569; later it became a general term for all sharks. The name may have been derived from the Mayan word for fish, xoc, pronounced "shock" or "shawk."
The skeleton of a shark is very different from that of bony fish such as cod or salmon. Sharks and their relatives, skates and rays, have skeletons made from rubbery cartilage, which is very light and flexible. But the cartilage in older sharks can sometimes be partly calcified (mineralized with calcium), 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 (Hamlett 1999).
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Shark gill slits are not covered like other fish, but are in a row behind its head. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration (Gilbertson 1999).
While moving, water passes through the mouth of the shark and over the gills: This process is known as ram ventilation. While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small subset of shark species that spend their life constantly swimming, a behavior common in pelagic (open ocean) sharks, have lost the ability to pump water through their gills. These species are obligate ram ventilators and would presumably asphyxiate if unable to stay in motion. (Obligate ram ventilation is also true of some pelagic fish species.)
The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there, blood enters the ventricle of the heart and the cycle repeats.
Unlike bony fish, sharks do not have gas-filled swim bladders. Since they also lack lungs, sharks lack the natural buoyancy of gas-filled structures. Part of the buoyancy problem is addressed by the fact that sharks have skeletons made of cartilage, which is lighter than bone. They also rely on a large liver filled with oil that contains squalene, an organic compound. The liver may constitute up to 25 percent of their body mass (Compagno et al. 2005).
The cartilage and oil-filled liver only addresses part of the problem, so sharks also employ dynamic lift to maintain depth, by moving and utilizing their large pectoral fins and upward curved tail. They sink when they stop swimming.
Some sharks, if inverted, enter a natural state of tonic immobility—researchers use this condition for handling sharks safely (Pratt et al. 1990).
In contrast to bony fish, 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 (Compagno et al. 2005).
The teeth of carnivorous sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life. Some sharks can lose 30,000 teeth in a lifetime.
All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks, rows of teeth are replaced every 8–10 days, while in other species they could last several months. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it (Gilbertson 1999). The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.
The tails (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. 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. On the other hand, the porbeagle, which hunts schooling fishes such as mackerel and herring, has a large lower lobe to provide greater speed and help it keep pace with its fast-swimming prey.
It is also believed that sharks use the upper lobe of their tails to counter the lift generated by their pectoral fins (Nelson 1994).
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.
Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers 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.
While sharks, like other fish, are poikilothermic in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature, certain species of shark do maintain elevated body temperatures to varying degrees. These include all sharks in the family Lamnidae—shortfin mako, long fin mako, white, porbeagle, and salmon sharks—and evidence suggests the trait exists in family Alopiidae (thresher sharks). The porbeagle sharks, like some tunas, can elevate body temperatures in excess of 20°C above ambient water temperatures.
This is possible because of the presence of the rete mirabile, a counter current exchange mechanism that reduces the loss of body heat. Essentially, warmer blood being returned to the gills in small veins runs close to colder, oxygenated blood in narrow arteries leaving the gills. Muscular contraction also generates a mild amount of body heat. This ability to have elevated temperatures allows fish to be active in colder waters and to have enhanced swimming ability because of the warmer muscles. However, this differs significantly from true homeothermy, as found in mammals and birds, in which heat is generated, maintained, and regulated by metabolic activity.
The sex of a shark can be easily determined. The males have modified pelvic fins that 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 of the mammalian penis.
Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. In less flexible species, the two sharks swim parallel to each other while the male inserts a clasper into the female's oviduct. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during mating. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. In some species, females have thicker skin to withstand these bites.
Sharks have a different reproductive strategy from most fish. Instead of producing huge numbers of eggs and fry (99.9% percent of which never reach sexual maturity in fishes that 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) (Campagno 1984). 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.
There are three main ways in which shark pups are born:
In December 2001, a pup was born from a female hammerhead shark that had not been in contact with a male shark for over three years. This has led scientists to believe that sharks can produce without the mating process.
After three years of research, this assumption was confirmed in a 2007 publication, after determining the shark born had no paternal DNA, ruling out any sperm-storage theory as previous thought (Chapman et al. 2007). It is unknown as to the extent of this behavior in the wild, and how many species of shark are capable of reproducing without a mate. This observation in sharks made mammals the only remaining major vertebrate group in which the phenomenon of asexual reproduction has not been observed (Chapman et al. 2007).
Scientists warned that this type of behavior in the wild is rare, and probably a last ditch effort of a species to reproduce when a mate is not present. This leads to a lack of genetic diversity, required to build defenses against natural threats, and if a species of shark were to rely solely on asexual reproduction, it would probably be a road to extinction and maybe attribute to the decline of blue sharks off the Irish coast (Pogatchnik 2007; Chapman et al. 2007).
The fossil record of sharks extends back over 450 million years—before land vertebrates existed and before many plants had colonized the continents (Martin 2007a). The first sharks looked very different from modern sharks (Martin 2007b). The majority of the modern sharks can be traced back to around 100 million years ago (Martin 2007c).
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. The abundance of such fossils of teeth is attributed to the fact that sharks may grow tens of thousands of teeth over a few years, and that the teeth consist of mineral apatite (calcium phosphate), making them easily fossilized.
Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are believe to be preserved only when rapid burial in bottom sediments occurs.
Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago (Martin 2007b), which has been found within the Paleozoic strata of the U.S. states 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 meter long with stiff triangular fins and slender jaws (Martin 2007b). 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 suggests that they were fast swimmers with great agility.
From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the acanthuses, was almost exclusive to freshwater environments (Martin 2007d; Harris 2007). 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 (Martin 2007c). 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 most extinct sharks, is only known from its teeth. A reproduction of its jaws was based on some of the largest teeth, which were almost 17 centimeters (7 inches) long and suggested a fish that could grow to a length of 25 to 30.5 meters (80 to 100 feet). The reconstruction was found to be inaccurate, and estimates revised downwards to around 13 to 15.9 meters (43 to 52 feet).
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.
Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.
There are more than 360 described species of sharks. Murch (2007) states that conservative estimates among shark taxonomists place the number of known shark species to be approaching 500 (and that there are more than 700 or more species of rays and skates).
There are eight extant orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:
Sharks have keen olfactory senses, with some species able to detect as little as one part per million of blood in seawater, up to a quarter of a mile away. 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 is not fused as in bony fish.
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.
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 (nighttime) adaptations.
Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes, some have nictitating membranes. This membrane covers the eyes during predation, and when the shark is being attacked. However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey.
The importance of sight in shark hunting behavior is debated. Some believe that electro and chemoreception are more significant, while others point to the nictitating membrane as evidence that sight is important. (Presumably, the shark would not protect its eyes were they unimportant.) The degree to which sight is used probably varies with species and water conditions.
Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their heads (not to be confused with the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, as it is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and tetrapods (four-legged vertebrates), the external opening into the inner ear has been lost.
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.
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 (Popper and Platt 1993).
Few studies on the behavior of sharks have only been carried out, leading to little information on the subject, although this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, this is only true for a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or on rich hunting grounds, which may lead them to cover thousands of miles in a year (Ravilious 2005). Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins.
Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals for scalloped hammerheads congregating around seamounts and islands, e.g. in the Gulf of California (Compagno et al. 2005). Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks of comparable size when feeding.
When approached too closely, some sharks will perform a threat display to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat (Bright 2007).
Despite the common view that sharks are simple, instinct-driven "eating machines," recent studies have indicated that many species are more complex, possessing 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 (Martin 2003).
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 (Martin 2007e).
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 (Martin 2007f).
Some say a shark never sleeps. It is unclear how sharks sleep. Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, they do not use their nares, but rather their spiracles. If a shark tried to use their nares while resting on the ocean floor, they would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish's spinal cord, rather than its brain, coordinates swimming, so it is possible for a spiny dogfish to continue to swim while sleeping. It is also possible that a shark can sleep with only parts of its brain in a manner similar to dolphins (Martin 2007g).
Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 360 species, only three have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, tiger, and bull sharks, with notable attacks from other Carcharhinus species as well (besides the bull shark), and the hammerhead and sand tiger (ISAF 2007). About 38 species have been implicated in attacks between 1580 and 2006, and one more if attacks on boats are counted, but identification of species is often imprecise (ISAF 2007). The "Big Three" of great white, tiger, and bull sharks, large species capable of inflicting serious injuries, are often found where humans enter the water, and have shearing teeth, rather than teeth designed to hold (ISAF 2007). All of these sharks have been filmed in open water, without the use of a protective cage.
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. 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 (Burgess 2006) In 2005, the International Shark Attack File (ISAF) undertook an investigation into 105 shark attacks. Out of those 105, 58 of the attacks were unprovoked (Burgess 2006).
Until recently, only a few benthic species of shark, such as hornsharks, leopard sharks, and catsharks could survive in aquarium conditions for up to a year or more. This gave rise to the belief that sharks, as well as being difficult to capture and transport, were difficult to care for. A better knowledge of sharks has led to more species (including the large pelagic sharks) being able to be kept for far longer. At the same time, transportation techniques have improved and now provide a way for the long distance movement of sharks (MCS 2005).
Despite being considered critical for the health of the shark, very few studies on feeding have been carried out. Since food is the reward for appropriate behavior, trainers must rely on control of feeding motivation.
Sharks are hunted in commercial fisheries and for recreational purposes.
It is estimated that every year 26 to 73 million (median value of 38 million) sharks are killed by people in commercial endeavors aimed at harvesting their fins, which are used in shark fin soup and are the most economically valuable part of the sharks (PEW 2006). Shark fin soup is a delicacy in China and Southeast Asia and Pacific Rim nations. The finning process may include removal of the fin of a live shark, 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.
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) (FAO 1998). 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 also killed for their meat. The meat of dogfishes, smoothhounds, catsharks, skates, and rays are in high demand in some locations.
Sharks have been a popular recreational target, with a reputation as a good fighting fish (such as the shortfin mako sharks and blue shark).
Shark cartilage has a popular perception as being effective against cancer and for treatment of osteoarthritis. This may be traceable to a common view that sharks cannot get cancer and that taking it will prevent people from getting these diseases. The evidence that sharks are resistant to cancer is mostly anecdotal and there have been few, if any, systematic scientific studies that have shown sharks to have heightened immunity to this disease (Handwerk 2003). A trial by the Mayo Clinic found no effect in advanced cancer patients.
Threats to shark populations include fishing, habitat alteration and loss (such as from coastal developments), pollution, and the impact of fisheries on the seabed and prey species. 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 (Pratt et al. 1990).
Sharks are long-lived apex predators with comparatively small populations, which make it difficult for them to breed rapidly enough to maintain population levels. They 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 efforts applied to catching sharks over time, and many species are considered to be threatened.
Major declines in shark stocks have been recorded in recent years—some species have been depleted by over 90 percent over the past 20–30 years with a population decline of 70 percent not being unusual (Walker 1998). Many governments and the United Nations 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.
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