"Great white" redirects here. For other uses, see Great White (disambiguation).
| Great white shark Temporal range: 16–0 MaMiocene to Recent | |
|---|---|
 | |
| A great white off the coast of South Africa | |
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Lamniformes |
| Family: | Lamnidae |
| Genus: | Carcharodon Smith, 1838 |
| Species: | C. carcharias |
| Binomial name | |
| Carcharodon carcharias (Linnaeus, 1758) | |
 | |
| Global range highlighted in blue | |
The great white shark, Carcharodon carcharias, also known as great white, white pointer,white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. The great white shark is very well known for its size, with the largest individuals known to have approached or exceeded 6 metres (20 ft) in length and 2,268 kilograms (5,000 lb) in weight.It reaches maturity at around 15 years of age and can have a life span of over 30 years. The great white shark is arguably the world's largest known extant macropredatory fish and is one of the primary predators of marine mammals. It is also known to prey upon a variety of other marine animals including fish, pinnipeds, and seabirds. It is the only known surviving species of its genus, Carcharodon.
The best selling novel Jaws and the subsequent blockbuster film by Steven Spielberg depicted the great white shark as a "ferocious man eater". In reality, humans are not the preferred prey of the great white shark.
Anatomy and appearance
The great white shark has a robust large conical snout. The upper and lower lobes on the tail fin are approximately the same size (like some mackerel sharks).
Great whites display countershading, having a white underside and a grey dorsal area (sometimes in a brown or blue shade) that gives an overall "mottled" appearance. The coloration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from the side. From above, the darker shade blends with the sea and from below it exposes a minimal silhouette against the sunlight.
Great white sharks, like many other sharks, have rows of serrated teeth behind the main ones, ready to replace any that break off. When the shark bites it shakes its head side to side, helping the teeth saw off large chunks of flesh.
Size
Males reach maturity at 3.5–4.0 metres (11–13 ft), and females at 4.5–5.0 metres (15–16 ft). Adults on average are 4–5.2 metres (13–17.1 ft) long and have a mass of 680–1,100 kilograms(1,500–2,400 lb). Females are generally larger than males. It is widely accepted that the great white shark can approach 6.1 m (20 ft) in length and 1,900 kg (4,200 lb) in weight. However, the maximum size is still subject to hot debate because such reports are often rough estimations or speculations performed under questionable circumstances.
A number of very large great white shark specimens have been recorded. For decades, many ichthyological works, as well as the Guinness Book of World Records, listed two great white sharks as the largest individuals: a 10.9 m (36 ft) great white captured in SouthernAustralian waters near Port Fairy in the 1870s, and a 11.3 m (37 ft) shark trapped in a herringweir in New Brunswick, Canada in the 1930s. Some researchers question these measurements' reliability, noting they were much larger than any other accurately reported sighting. The New Brunswick shark may have been a misidentified basking shark, as the two have similar body shapes. The question of the Port Fairy shark was settled in the 1970s, when J. E. Randall examined the shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 ft) in length and suggested that a mistake had been made in the original record, in 1870, of the shark's length".
According to J. E. Randall, the largest white shark reliably measured was a 6.0 m (19.7 ft) individual reported from Ledge Point, WesternAustralia in 1987. Another great white specimen of similar size has been verified by the Canadian Shark Research Center: a female caught by David McKendrick of Alberton, Prince Edward Island in August 1988 in the Gulf of St. Lawrence off PEI. This female great white was 6.1 m (20 ft) long.However, a larger great white shark specimen was verified by T. C. Tricas and J. E. McCosker in 1984. This specimen was 6.4 m (21 ft) long and had a body mass of about 3,324 kg (7,330 lb).
Several great white sharks caught in modern times have been estimated to be more than 7 m (23 ft) long, but these claims have received some criticism. However, J. E. Randall believed that great white shark may have exceeded 6.1 m (20 ft) in length. A great white shark was captured near Kangaroo Island in Australia on April 1, 1987. This shark was estimated to be more than 7 m (23 ft) long by Peter Resiley, and has been designated as KANGA.Another great white shark was caught in Malta by Alfredo Cutajar on April 16, 1987. This shark was also estimated to be around 7.13 m (23.4 ft) long by John Abela and has been designated as MALTA. However, criticism occurred — Cappo used shark size estimation methods proposed by J. E. Randall to suggest that KANGA specimen was 5.8–6.4 m (19–21 ft) long. In the similar fashion, I. K. Fergusson also used shark size estimation methods proposed by J. E. Randall to suggest that MALTA specimen was 5.3–5.7 m (17–19 ft) long.However, photographic evidence suggested that these specimens were larger than the size estimations yielded through Randall's methods.Thus, a team of scientists, H. F. Mollet, G. M. Cailliet, A. P. Klimley, D. A. Ebert, A. D. Testi, and L. J. V. Compagno, reviewed the cases of KANGA and MALTA specimens in 1996 to resolve the dispute by conducting a comprehensive morphometric analysis of the remains of these sharks and re-examination of photographic evidence in an attempt to validate the original size estimations and their findings were consistent with them. The findings indicated that estimations by P. Resiley and J. Abela are reasonable and cannot be ruled out.
A close contender in size is the Tiger shark, Galeocerdo cuvier, with largest specimen reported to have been 7.4 metres (24 ft) in length along with a mass of 3,110 kilograms (6,900 lb) Some other macropredatory sharks such as Greenland Shark, Somniosus microcephalus, andPacific sleeper shark, Somniosus pacificus, are also known to rival these sharks in length.The question of maximum weight is complicated by the unresolved question of whether or not to include the shark's stomach contents when weighing the shark. With a single bite a great white can take in up to 14 kg (31 lb) of flesh, and can consume several hundred kilograms of food.
The largest great white recognized by the International Game Fish Association (IGFA) is one landed by Alf Dean in south Australian waters in 1959, weighing 1,208 kg (2,660 lb).Several larger great whites 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. Even heart beats emit a very faint electrical pulse. If close enough, the shark can detect even that faint electrical pulse. Most fish have a less-developed but similar sense using their body's lateral line.
To more successfully hunt fast and agile prey such as sea lions, the great white has adapted 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 the stomach) at temperatures up to 14 °C (25 °F) above that of the surrounding water, while the heart and gills remain at sea-temperature. When conserving energy 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 anendothermic poikilotherm, because its body temperature is not constant but is internally regulated.
Bite force
A 2007 study from the University of New South Wales in Sydney, Australia, used CT scans of a shark's skull and computer models to measure maximum bite force. The study reveals the forces and behaviors its skull is adapted to handle and resolves competing theories about its feeding behaviour. In 2008, a team of scientists led by Stephen Wroe conducted an experiment to determine great white shark's jaw power and findings indicated that a specimen more than 6.1 m (20 ft) long could exert a bite force of over 18,000 newtons (4,000 lbf).
Ecology and behavior
This shark's behavior and social structure is not well understood. In South Africa, white sharks have a dominance hierarchy depending on size, sex and squatter's rights: Females dominate males, larger sharks dominate smaller sharks, and residents dominate newcomers. When hunting, great whites tend to separate and resolve conflicts with rituals and displays.White sharks rarely resort to combat although some individuals have been found with bite marks that match those of other white sharks. This suggests that when another shark approaches too closely, great whites react with a warning bite. Another possibility is that white sharks bite to show dominance.
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 learned from interaction with humans (it is theorized that the shark may also be able to smell better this way, because smell travels through air faster than through water). The white sharks are generally very curious animals, display intelligence and may also turn to socializing if situation demands such.At Seal Island, white sharks have been observed arriving and departing in stable "clans" of two to six individuals on a yearly basis. Whether clan members are related is unknown, but they get along peacefully enough. In fact, the social structure of a clan is probably most aptly compared to that of a wolf pack: each member has a clearly established rank, and each clan has an alpha leader. When members of different clans meet, they establish social rank nonviolently through any of a fascinating variety of interactions.
Diet
Great white sharks are carnivorous, and prey upon fish (e.g. tuna, rays, other sharks),cetaceans (i.e., dolphins, porpoises, whales), pinnipeds (e.g. seals, fur seals,and sea lions),sea turtles, sea otters, and seabirds.Great whites have also been known to eat objects that they are unable to digest. Upon approaching a length of nearly 4 metres (13 ft), great white sharks begin to target predominately marine mammals for food. These sharks prefer prey with a high content 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 sheep carcass.
Great white sharks' reputation as ferocious predators is well-earned, yet they are not (as was once believed) indiscriminate "eating machines". They are ambush hunters, taking prey by surprise from below. Near the now famous Seal Island, in South Africa's False Bay, shark attacks most often occur in the morning, within 2 hours after sunrise, when visibility is poor. Their success rate is 55% in the first 2 hours, falling to 40% in late morning, after which hunting stops.
Hunting techniques vary by prey species. Off Seal Island the shark ambush cape fur seals from below at high speeds, hitting the seal mid-body. They go so fast that they can completely leave the water. The peak burst speed of these sharks is largely accepted in the scientific community to be above 40 kilometres per hour (25 mph). However further precision is still speculative.They have also been observed chasing prey after a missed attack. Prey is usually attacked at the surface.
Aucun commentaire:
Enregistrer un commentaire