Great white shark
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[Photo] Great white shark, Carcharodon carcharias. Photo by Terry Goss, copyright 2006. Taken at Isla Guadalupe, Mexico, August 2006. Shot with Nikon D70s in Ikelite housing, in natural light. Animal estimated at 11-12 feet in length, age unknown.

The great white shark, Carcharodon carcharias, also known as white pointer, white shark, or white death, is an exceptionally large lamniforme shark found in coastal surface waters in all major oceans. Reaching lengths of about 6 metres (20 ft) and weighing up to 2,250 kilograms (5,000 lb), the great white shark is the world's largest known predatory fish. It is the only surviving species of its genus, Carcharodon.

Taxonomy
Carolus Linnaeus gave the great white shark its first scientific name, Squalus carcharias in 1758. Sir Andrew Smith gave it the generic name Carcharodon in 1833 and in 1873, the generic name was identified with Linnaeus specific name and the current scientific name Carcharodon carcharias was finalised. Carcharodon comes from the Greek words karcharos, which means sharp or jagged, and odous, which means tooth.


Related species
The great white is classified as a mackerel (Lamnidae) shark. There are four other living species in this family, two mako and two Lamna sharks.

Dental features and the extreme size of both the Great White and the prehistoric Megalodon lead many scientists to believe they were closely related, and the name Carcharodon megalodon was applied to the latter. At present there is considerable doubt about this hypothesis, and other scientists would place the megalodon and white shark as distant relatives - sharing the family Lamnidae but no closer relationship.

Megalodon is only known from its teeth and from a few cartillage remains, and probably reached sizes of 12 metres (40 ft) or more, considerably larger than even the largest great white sharks. From time to time it is suggested that megalodon might still exist. Megalodon teeth have supposedly been found from as recently as 10,000 to 12,000 years ago, but these results appear to be based on misinterpretation of the evidence. However, while megalodon fossils are widespread and plentiful, no evidence has surfaced that the species is anything but extinct.

Other evidence suggests that the great white shark is more closely related to the mako shark than to the megalodon. Accoding to this theory, Carcharodon orientalis and the broad tooth mako Isurus hastalis are fossil sharks that are considered ancestral to the Great White. The Carcharocles and Otodus obliquus sharks are in this case considered the ancient representatives of the extinct megalodon lineage; indeed, Carcharocles megalodon is a popular alternative classification of the megalodon.


Distribution and habitat
Great white sharks live in almost all coastal and offshore waters which have a water temperature of between 12 and 24° C (54° to 75° F), with greater concentrations off the southern coasts of Australia, off South Africa, California, Mexico's Isla Guadalupe and to a degree in the Central Mediterranean and Adriatic Seas. One of the densest known populations is found around Dyer Island, South Africa where much research on the shark is conducted. It can be also found in tropical waters like those of the Caribbean and has been recorded off Mauritius. It is a pelagic fish, but recorded or observed mostly in coastal waters in the presence of rich game like fur seals, sealions, cetaceans, other sharks and large bony fish species. It is considered an open-ocean dweller and is recorded from the surface down to depths of 1,280 metres (4,200 ft), but is most often found close to the surface.

In a recent study great white sharks from California were shown to migrate to an area between Baja California and Hawaii, where they spend at least 100 days of the year before they migrate back to Baja. On the journey out, they swim slowly and dive to up to 900 metres (3,000 ft). After they arrive, they change behaviour and do short dives to about 300 m (1,000 ft) for up to 10 minutes. It is still unknown why they migrate and what they do there; it might be seasonal feeding or possibly a mating area.

In a similar study a great white shark from South Africa was tracked swimming to the northwestern coast of Australia and back to the same location in South Africa, a journey of 20,000 kilometres (over 12000 miles) in under 9 months.


Anatomy and appearance
The Great White Shark has a robust large conical-shaped snout. It has almost the same size upper and lower lobes on the tail fin (like most mackerel sharks, but unlike most other sharks).

Great White Sharks display countershading, having a white underside and a grey dorsal area (sometimes in a brownish or bluish shade) that gives an overall "mottled" appearance. The colouration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from a lateral perspective. When viewed from above the darker shade blends in with the sea and when seen from below casts a minimal silloutte against the sunlight.

Great White Sharks, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. A Great White Shark's teeth are serrated and when the shark bites it will shake its head side to side and the teeth will act as a saw and tear off large chunks of flesh. Great White Sharks often swallow their own broken off teeth along with chunks of their prey's flesh.


Size
A typical adult Great White Shark measures 4 to 4.8 metres (13 to 16 ft) with a typical weight of 680 to 1,100 kilograms (1,500 to 2,450 lbs), females generally being larger than males. The maximum size of the Great White Shark has been subject to much debate, conjecture, and misinformation. Richard Ellis and John E. McCosker, both academic shark experts, devote a full chapter in their book, The Great White Shark (1991), to analysing various accounts of extreme size.

Today, most experts contend that the Great White Shark's "normal" maximum size is about 6 metres (20 ft), with a "normal" maximum weight of about 1,900 kilograms (4,200 lb).

For several decades, many ichthyological works, as well as the Guinness Book of World Records, listed two great white sharks as the largest individuals caught: an 11 metre (36 ft) great white captured in South Australian waters near Port Fairy in the 1870s, and an 11.3 metre (37.6 ft) shark trapped in a herring weir in New Brunswick, Canada in the 1930s. While this was the commonly accepted maximum size, reports of 7.5 to 10 metre (25 to 33.3 ft) Great White Sharks were common and often deemed credible.

Some researchers questioned the reliability of both measurements, noting they were much larger than any other accurately-reported Great White Shark. The New Brunswick shark may have been a misidentified basking shark, as both sharks have similar body shapes. The question of the Port Fairy shark was settled in the 1970s, when J.E. Reynolds examined the shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 feet) in length and suggested that a mistake had been made in the original record, in 1870, of the shark's length.

Ellis and McCosker write that "the largest White Sharks accurately measured range between 19 and 21 ft [about 5.8 to 6.4 m], and there are some questionable 23-footers [about 7 m] in the popular ??? but not the scientific ??? literature". Furthermore, they add that "these giants seem to disappear when a responsible observer approaches with a tape measure." (For more about legendary exaggerated shark measurements, see the submarine).

The largest specimen Ellis and McCosker endorse as reliably measured was 6.4 metres (21.3 ft) long, caught in Cuban waters in 1945; though confident in their opinion, Ellis and McCosker note other experts have argued this individual might have been a few feet shorter. There have since been claims of larger Great White Sharks, but, as Ellis and McCosker note, verification is often lacking and these extraordinarily large great white sharks have, upon examination, all proved under the 20-21 ft limit. For example, a much-publicized female Great White said to be 7.13 metres (over 23 ft) was fished in Malta in 1987 by Alfredo Cutajar. In their book, Ellis and McCosker agree this shark seemed to be larger than average, but they did not endorse the 7.13 metres measurement. In the years since, experts eventually found reason to doubt the claim, due in no small part to conflicting accounts offered by Cutajar and others. A BBC photo analyst concluded that even "allowing for error ... the shark is concluded to be in the 18.3 ft [5.5 m] range and in no way approaches the 23 ft [7 m] reported by Abela." (as in original)

According to the Canadian Shark Research Centre, the largest accurately measured Great White Shark was a female caught in August 1988 at Prince Edward Island off the Canadian (North Atlantic) coast and measured 6.1 metres (20.3 ft). The shark was caught by David McKendrick, a local resident from Alberton, West Prince.

The question of maximum weight is complicated by an unresolved question: when weighing a Great White Shark, does one account for the weight of the shark's recent meals? With a single bite, a Great White can take in up to 14 kilograms (30 lb) of flesh, and can gorge on several hundred kilograms or pounds of food.

Ellis and McCosker write in regards to modern Great White Sharks that "it is likely that [Great White] sharks can weigh as much as 2 tons", but also note that the largest recent scientifically measured examples weigh in at about 2 tonnes (1.75 short tons).

The largest Great White Shark recognized by the International Game Fish Association (IGFA) is one landed by Alf Dean in south Australian waters in 1959, weighing 1,208 kilograms (2,664 lb). Several larger Great White Sharks caught by anglers have since been verified, but were later disallowed from formal recognition by IGFA monitors for rules violations.


Adaptations
Great white sharks, like all other sharks, have an extra sense given by the Ampullae of Lorenzini, which enables them to detect the electromagnetic field emitted by the movement of living animals. Every time a living creature moves it generates an electrical field and great whites are so sensitive they can detect half a billionth of a volt. Most fish have a less developed but similar ability in the horizontal line along their body.

To more successfully hunt fast moving and agile prey such as sea lions, the poikilothermic great white shark has developed adaptations that allow it to maintain a body temperature warmer than the surrounding water. One of these adaptations is a "rete mirabile" (Latin for "wonderful net"). This close web-like structure of veins and arteries, located along each lateral side of the shark, conserves heat by warming the cooler arterial blood with the venous blood that has been warmed by the working muscles. This keeps certain parts of the body (particularly their brain) at temperatures up to 14°C above the surrounding water, while the heart and gills remain at sea-temperature. When conserving energy (a great white shark can go weeks between meals), the core body temperature can drop to match the surroundings. A great white shark's success in raising its core temperature is an example of gigantothermy. Therefore, the great white shark can be considered an endothermic poikilotherm, because its body temperature is not constant but is internally regulated.


Diet and hunting
Great white sharks are carnivorous, and primarily eat fish (including rays and smaller sharks), dolphins, porpoises, whale carcasses and pinnipeds such as earless seals, fur seals and sea lions. Sea otters and sea turtles are also taken at times. Great whites have also been known to eat objects that they are unable to digest. In great white sharks above 3.41 metres (11 ft, 2 in) a diet consisting of a higher proportion of mammals has been observed. These sharks prefer prey with high contents of energy-rich fat. Shark expert Peter Klimley used a rod-and-reel rig and trolled carcasses of a seal, a pig, and a sheep to his boat in the South Farallons. The sharks attacked all three baits but rejected the lower fat content sheep carcass.

The great white is regarded as an apex predator with its only real threats from humans and, in at least one incident, the Orca. Although their diets overlap greatly, there are few reports of encounters between orcas and great whites, and they don't seem to directly compete with each other. Great whites are also sometimes preyed on by larger specimens.

A great white shark primarily uses its extra senses (i.e, electrosense and mechanosense) to locate prey from far off. Then, the shark uses smell and hearing to further verify that its target is food. At close range, the shark utilizes sight for the attack.

Great white sharks' reputation as ferocious predators is well-earned, yet they are not (as was once believed) indiscriminate "eating machines". They typically hunt using an "ambush" technique, taking their prey by surprise from below. Near the now-famous Seal Island, in South Africa's False Bay; studies have shown that the shark attacks most often in the morning, within 2 hours after sunrise. The reason for this is that it is hard to see a shark close to the bottom at this time. The success rate of attacks is 55% in the first 2 hours, it falls to 40% in late morning and after that the sharks stop hunting.

The hunting technique of the white shark varies with the species it hunts. When hunting Cape fur seals off Seal Island, South Africa; the shark will ambush it from below at high speeds and hit the seal at mid-body. They go so fast that they actually breach out of the water. They have also been observed chasing their prey after a missed attack. The prey is usually attacked at the surface.

When hunting Northern elephant seals off California, the shark immobilizes the prey with a large bite to the hindquarters (which is the main source of the seal's mobility) and waits for the seal to bleed to death. This technique is especially used on adults which are large and dangerous. Prey is normally attacked sub-surface. Harbour seals are simply grabbed from the surface and pulled down until they stop struggling. They are then eaten near the bottom. California sea lions are ambushed from below and struck in mid-body before being dragged and eaten.

When hunting dolphins and porpoises, white sharks attack them from above, behind or below to avoid being detected by their echolocation.

A new study from the University of New South Wales in Sydney, Australia, is using CT scans of a shark's skull and complex computer models to measure the maximum bite force of the great white. The study will reveal what forces and behaviours the carnivore's skull is adapted to handle and will help resolve competing theories about its feeding behaviour.


Behaviour
The behaviour and social structure of the white shark is not well understood but recent research shows that white sharks are more social than previously thought. In South Africa, white sharks seem to have a pecking order depending on size, sex and squatter's rights. Females dominate over males, larger sharks dominate smaller sharks, and residents dominate new comers. When hunting the white sharks tend to space out between each other and resolve conflicts with rituals and displays. White sharks rarely resort to combat although some individuals have been found with bite marks that match that of other white sharks. This suggests that when their personal space is intruded upon, a white shark will give the intruder a warning bite. Another possibility is that white sharks may softly bite other individuals as a way of showing their dominance. Also, as noted above, white sharks can be cannibalistic.

The great white shark is one of only a few sharks known to regularly lift its head above the sea surface to gaze at other objects such as prey; this is known as "spy-hopping". This behaviour has also been seen in at least one group of blacktip reef sharks, but this might be a behaviour learned from interaction with humans (it is theorized that the shark may also be able to smell better this way, because smells travel through air faster than through water). They are very curious animals, and can display a high degree of intelligence and personality when conditions permit (such as in the clear waters off of Isla Guadalupe, Mexico).


Reproduction
There is still a great deal that is unknown about great white shark behaviour, such as their mating habits. Birth has never been observed, but several pregnant females have been examined. Great white sharks are ovoviviparous, the eggs developing in the female's uterus, hatching there and continuing to develop until they are born, at which point they are perfectly capable predators. The embryos can feed off unfecundated eggs. The delivery takes place in the period transitioning spring and summer. When giving birth, the female has to fast to prevent herself from eating her young after they are born.

The young, which number 8 or 9 (with a maximum of perhaps 14) for a single delivery, are about 1.5 metres (5 ft) long when born. Their teeth are provided with small side cusps. They grow rapidly, reaching 2 metres of length in the first year of life. Almost nothing, however, is known about how and where the great white mates. There is some evidence that points to the near-soporific effect resulting from a large feast (such as a whale carcass) possibly inducing mating.

A great white shark can reproduce when a male's length is around 3.8 metres (12 ft) and a female's length is around 4 to 4.8 metres (13.3 to 15.8 ft). Their lifespan has not been definitively established, though many sources estimate 30 to 40 years. It would not be unreasonable to expect such a slow maturing animal to live longer however.

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