The general rule is that there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have been found to be covered with two to three layers or more, depending on the body size. The jaws of a large white shark may even have up to five layers.
In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
The 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.
The inner parts of the males' pelvic fins have been modified to a pair of cigar- or dicks-shaped sexayy organs known as "claspers," used for internal fertilization.
Because of this, most sharks need to constantly swim in order to breathe and can't sleep very long, if at all, or they will sink. However certain shark species, like the nurse shark, have spiracles that force water across their gills allowing them for stationary rest on the ocean bottom.
Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers have used this condition to handle sharks safely..
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. They stick out of their mouth at angles of up to thirty degrees. 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. 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. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion. This is due to the fact that the shark's vertebral column extends into that dorsal portion, allowing for a greater surface area for muscle attachment which would then be used for more efficient locomotion among the negatively buoyant cartilaginous fishes. This is in contrast to the bony fishes, class osteichthyes, which possess a homocercal caudal fin.
The tiger shark's tail has a large upper lobe which delivers the maximum amount of power for slow cruising or sudden bursts of speed. The tiger shark has a varied diet, and because of this it must be able to twist and turn in the water easily when hunting, whereas the porbeagle, which hunts schooling fish such as mackerel and herring has a large lower lobe to provide greater speed to help it keep pace with its fast-swimming prey. It is also believed that sharks use the upper lobe of their tails to counter the lift generated by their pectoral fins.
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.
Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.
Evidence for the existence of sharks extends back over 450–420 million years, into the Ordovician period, before land vertebrates existed and before many plants had colonised the continents. All that has been recovered from the first sharks are some scales. The oldest shark teeth are from 400 million years ago. The first sharks looked very different from modern sharks. The majority of the modern sharks can be traced back to around 100 million years ago.
Mostly only the fossilized teeth of sharks are found, although often in large numbers. In some cases pieces of the internal skeleton or even complete fossilized sharks have been discovered. Estimates suggest that over a span of a few years a shark may grow tens of thousands of teeth, which explains the abundance of fossils. As the teeth consist of calcium phosphate, an apatite, they are easily fossilized.
Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are only preserved when rapid burial in bottom sediments occurs.
Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, 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. 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 great white sharks and the pelagic shortfin and longfin makos. 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 Acanthodii, 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. Fossil mackerel shark teeth occurred in the Lower Cretaceous. One of the most recent families of sharks that evolved is the hammerhead sharks (family Sphyrnidae), which emerged in Eocene. 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 and a few vertebrae. This shark could grow to more than long and is recognized as the biggest known carnivorous fish to have ever existed. Fossil records reveal that this shark preyed upon whales and other large marine mammals.
It is believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of giant marine reptiles, such as the mosasaurs 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 hypotheses 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 split across eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:
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 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 evolved thicker skin to withstand these bites.
Sharks have a different reproductive strategy from most fish. Instead of producing huge numbers of eggs and fry (a strategy which can result in a survival rate of less than 0.01%), sharks normally produce around a dozen pups (blue sharks have been recorded as producing 135 and some species produce as few as two). These pups are either protected by egg cases or born live.
There are three ways in which shark pups are born:
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 isn't 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 may have contributed to the decline of blue sharks off the Irish coast.
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 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 nictating membrane as evidence that sight is important. (Presumably, the shark would not protect its eyes were they unimportant.) The degree to which sight is used probably varies with species and water conditions. In effect the shark's field of vision can swap between monnocular and stereoscopic at any time.
The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they vary in number from a couple of hundred to thousands in an individual. Sharks use the Ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (mostly the hammer head) find its prey. The shark has the greatest electrical 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 may be used in navigation.
Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals of scalloped hammerheads congregating around seamounts and islands e.g. in the Gulf of California. 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.
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.
It is also possible that a shark can sleep in a manner similar to dolphins. In this situation, one half of the brain sleeps at a time, thereby allowing the shark to be half conscious while sleeping.
In 2006 the International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities per year between 2001 and 2006 from unprovoked shark attacks is 4.3.
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, oceanic whitetip, tiger, and bull sharks. 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.
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.
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. The only species of shark to have never been successfully held in captivity was the great white, until September 2004 when the Monterey Bay Aquarium successfully kept a young female great white shark for 198 days before releasing her back into the wild.
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. Stocks decline and collapse because sharks are long-lived apex predators with comparatively small populations, which makes it difficult for them breed rapidly enough to maintain population levels. Major declines in shark stocks have been recorded in recent years - some species have been depleted by over 90% over the past 20-30 years with a population decline of 70% not being unusual. 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.
The practice of shark finning, cutting the fin from a shark and discarding the live animal, attracts much controversy and regulations are being enacted to prevent it from occurring.
A Canadian-made documentary, Sharkwater is raising awareness of the depletion of the world's shark population.
An estimate states that, every year, 26 to 73 million (median value is at 38 million) sharks are killed by people in commercial and recreational fishing. 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.
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. In India small sharks or baby sharks (called sora in Tamil language) are caught by fishermen routinely and are sold in the local markets. Since the flesh is not developed completely it just breaks into powder once boiled and this is then fried in oil and spices (called sora puttu). Even the bones are soft and these can be easily chewed and considered a delicacy in coastal Tamil Nadu. In Iceland, Greenland sharks are fished to produce hákarl or fermented shark, which is widely regarded as a national dish.
Sharks are often killed for shark fin soup: the finning process involves the removal of the fin with a hot metal blade. Fishermen will capture live sharks, fin them, and release the finless animal back into the water. The immobile shark soon dies from suffocation or predators. Despite claims that this practice is rare, it has become a major trade within black markets all over the world with shark fins going at about $220/ lbs. Millions of sharks a year are being illegally poached for their fins and not many governments are enforcing the laws of protecting these apex predators. The dish is considered a status symbol in Asian countries, and is considered healthy and full of nutrients, with some even claiming they prevent cancer and other ailments. There is no scientific proof that supports these claims; at least one study has shown shark cartilage of no value in cancer treatment. The shark fin trade is a major problem and has gained international controversy.
Sharks are also killed for their meat. Conservationists have campaigned for changes in the law to make finning illegal in the U.S. The meat of dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and rays are in high demand by European consumers. However, the U.S. FDA lists sharks as one of four fish (with swordfish, king mackerel, and tilefish,) that children and women who are or may be pregnant should refrain from eating. For details see mercury poisoning.
Shark cartilage has been advocated as effective against cancer and for treatment of osteoarthritis. (This is because many people believe that sharks cannot get cancer and that taking it will prevent people from getting these diseases, which is untrue.) However, a trial by Mayo Clinic found no effect in advanced cancer patients.
Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fish that are harvested. This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species are considered to be threatened.
Some organizations, such as the Shark Trust, campaign to limit shark fishing. According to Seafood Watch, sharks are currently on the list of fish that American consumers, who are sustainability minded, should avoid.
In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls.
A popular myth is that sharks are immune to disease and cancer; however, this is untrue, sharks do get cancer. 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.