Great White Shark - biology.

Great White Shark - biology. I INTRODUCTION Great White Shark, also known as white pointer and white death, the largest meat-eating shark. It can reach over 6 m (20 ft) in length and is one of the top predators in the ocean. It hunts marine mammals such as seals, sea lions, and dolphins, as well as fish. Although attacks on humans are rare, the great white is responsible for more human deaths than any other shark species. II HABITAT AND RANGE The great white shark lives in temperate coastal waters. It is a rare species, most commonly found along the coasts of North America, the Mediterranean, South Africa, and Australia. Some populations of great whites migrate to different areas at particular times of the year to find prey such as sea lions. Recent research using radio tags and satellite tracking shows that some great whites also spend time in the open sea--one tagged great white swam between South Africa and Australia, a distance of 12,000 miles, in nine months. The great white typically hunts in the upper part of the ocean where seals and other usual prey swim, but it may dive for prey in deeper waters in the open sea. III PHYSICAL DESCRIPTION The great white shark has a conical snout, black eyes, and large, serrated, arrowhead-shaped teeth. The upper and lower lobes of the tail are almost equal in size, and the body is blue- or brown-gray, not white, except on the underside. Most adults are from 3.7 to 5 m (12 to 16 ft) long. Individuals measuring 6 m (20 ft) have been caught and even larger sizes are possible. Recorded weights have reached 2,043 kg (4,500 lb). Life spans are estimated to be up to 30 years. As in all sharks, the skeleton of the great white is made of cartilage rather than bone. Only its teeth are made of a hard, bony substance. Its jaws are not attached to its skull and can be pushed forward in an attack to embed rows of serrated teeth viciously into a victim. The great white can tear away up to 13.6 kg (30 lb) of flesh in a single bite. The great white's tail is powered by a thick cylinder of red muscle on each side of its vertebral column running nearly the length of its body. The largest organ in its body is its liver, which contains a mixture of oil and fat that is much lighter than seawater and gives the shark buoyancy. The liver of a great white can weigh up to 454 kg (1,000 lbs) and makes up 25 percent of its total body weight. The shark has a two-chambered heart. Like other sharks, the great white needs to swim constantly to push water through its gills to obtain oxygen. Although fish are typically considered "cold-blooded," the great white and closely related lamnid sharks can keep their body temperatures up to 15°C (27°F) warmer than surrounding seawater by retaining heat generated from metabolism with a special system of arteries and veins. The warm blood is supplied to the brain and eyes, to the stomach, and to the muscles. The higher temperature likely keeps the animal's senses sharp and makes digestion and swimming more efficient. The great white shark's sense of smell is thought to be exceptionally well developed compared to that of other types of sharks. Its vision may also be acute--great whites are the only fish known to lift their heads out of water to survey their surroundings, a behavior called "spy-hopping." Like other sharks, great whites have special sense organs to detect electric currents generated by other animals. Like other fish, great whites also have lateral line sense organs along their bodies that can pick up vibrations in water caused by sounds or animal movements. IV HUNTING AND DIET The preferred prey for the great white shark are marine mammals that have large stores of fat or blubber, a rich source of energy. Seals and sea lions are the main targets in regions such as southern California and South Africa. A great white may also attack dolphins and elephant seals, and scavenge dead whales. However, it will also eat less energy-rich meals such as sea otters, birds, turtles, squid, and fish. Curiously, dissected great whites have been found with human garbage in their stomachs such as shoes, tin cans, and even cuckoo clocks. The great white and related sharks can reportedly push their stomachs out of their mouths to rinse the organs inside-out in seawater, a behavior known as stomach eversion. When hunting, the great white uses its keen sense of smell to find prey at long distances in water. As it approaches a target, the shark relies on its eyesight. For the final, up-close attack, it likely relies on its electrosensors. Before contact, it rolls back its eyes to protect them from sharp bone fragments as its jaws rip into the prey. After delivering a powerful bite, the shark may retreat to let its victim bleed to death before eating. Although the great white has been portrayed as a mindless killing machine, research shows that it appears to pull back from attacking prey nearly half the time. It can learn from experience and may be able to judge the risk of injury to itself if the prey appears large enough to struggle hard or fight back. Most authorities agree that the great white shark's reputation for seeking out human prey is undeserved, despite its portrayal as a notorious villain in fiction and in motion pictures. The documented cases of great white attacks on humans worldwide from 1876 to 2006 come to 245, resulting in 64 deaths. Unprovoked attacks on divers do happen, however. In some encounters with humans, the shark may deliver a vicious but not lethal bite, perhaps out of curiosity or to test the prospective prey before returning for a stronger bite. Because the great white shark prefers prey rich in fat or blubber, it may even lose interest in the relatively lean body of a human as a food item. According to experts, it is important to fight back if attacked by a great white, striking at the sensitive gills or eyes, since the shark will usually abandon a target in such cases. Some attacks on people are likely cases of mistaken identity--a human lying on a surfboard resembles the outline of a sea lion when seen from below. V REPRODUCTION Female great whites are larger than males. Male great whites reach sexual maturity at 9 to 10 years of age, females at from 14 to 16 years. Not much is known about the mating behavior of great whites. However, scars seen on the gills and head region of females may result from males biting the females during courtship rituals. The males have special modified pelvic fins called claspers that are used to hold the females during mating. Females are ovoviviparous, meaning they give birth to live young from eggs that hatch inside their bodies. The gestation period for the young, called pups, is not known for certain but it may be about 14 months. The pups survive inside their mothers after hatching by eating undeveloped eggs in the uterus, a behavior called oophagy. A female great white may give birth to from 2 to 17 pups. The pups are about 100 to 150 cm (39 to 59 in) long at birth and immediately swim off to fend for themselves. VI CONSERVATION The great white shark is a rare species that takes long to reach sexual maturity and that reproduces slowly. Populations cannot easily increase if depleted. The biggest threat to great whites comes from being unintentionally caught in fishing nets. Some targeted fisheries and sport fishing for great white sharks also occur. Trade in great white shark products includes the jaws (which can sell for thousands of dollars as curios), individual teeth, and leather from its skin. Fins from all sharks, including great whites, are a high-demand food item for shark-fin soup in Asia. As a result, great whites that are accidentally caught are apt to be killed and butchered rather than set free alive. As a predator at the top of the marine ecosystem, the great white shark is important to health of the oceans. South Africa granted the species protected status in 1991. Australia and the state of California imposed similar protections in 1994. International protection came in 2000 when the species was put on the Red List of Threatened Species compiled by the World Conservation Union (IUCN), with the status of vulnerable. The Convention on International Trade in Endangered Species (CITES) placed the great white shark on Appendix II in 2004, meaning it could be threatened with extinction if trade is not closely controlled. Humans clearly are much more of a threat to the great white shark than the shark has ever been to humans. Ongoing fascination with the great white shark has led to attempts at contact that avoid harming the shark. Cage diving with great whites has become a popular type of ecotourism in recent years in places such as South Africa, Australia, and Mexico. Divers are lowered in protective steel cages and bait is used to draw in great white sharks. Some critics are concerned that the sharks will associate people with food, thus increasing the risk of attacks on unprotected humans. Scientific classification: The great white shark belongs to the family Lamnidae. It is classified as Carcharodon carcharias. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.