Friday, 7 March 2014

shark, sea

The great white shark averages 4.5 m (15 ft.) in length, but some have been recorded as large as 6 m (20 ft.) long! They generally weigh up to 2250 kg (5000 lb.).
Great white sharks are blue-gray on the dorsal, or top, part of their bodies. This helps them blend in with the bottom of the ocean when viewed from above. The belly, or ventral, part of the body, is white. This makes it difficult to see the sharks from below, with sunlight shining in around them. They have strong, torpedo-shaped bodies and powerful tails that help them swim. Great whites can reach speeds up to 24 km/hr (15 mph).
Great whites use their speed and coloring to help them hunt. They search for prey at the surface of the ocean while swimming below. Once they spot a target, they use a burst of speed to bump their prey while simultaneously biting it. They have several rows of teeth that can number into the thousands. As teeth fall out, they are rapidly replaced by those in the row behind them. These sharp, serrated teeth can be devastating. A single, large bite can be fatal.
When great white sharks are young, they feed on smaller prey, like fish and rays. As they grow larger, they feed more exclusively on marine mammals, such as sea lions, seals and small whales.
The great white is at the top of the food chain and has few threats in the ocean. Only orcas and larger sharks can pose a risk. The only other risk to the great white shark is human interaction. They are sometimes caught by accident in fishing nets or intentionally sought out by sport fisherman. Their jaws and fins are sold for considerable amounts of money.
Not much is known about the mating habits of great white sharks. What is known is that after mating the female develops several eggs which hatch in her womb. The newly-hatched shark pups feed on unfertilized eggs in the womb as they develop before being born. In general, the mother gives birth to a litter of two to ten pups, each of which average 1.5 m (5 ft.) in length. Male great whites reach maturity at 9-10 years of age. Females mature even later, between 14 and 16 years of age. Female sharks are thought to give birth once every couple years, but even that is uncertain.
Many paleontologists are very interested in living groups, because the study of the living organisms can both unlock their evolutionary history and provide important keys towards interpreting their fossil record. Some living groups have ancient histories. For example, sharks have existed as a group for over 350 million years! Today, sharks and rays (elasmobranchs) are represented by over 600 species that show a remarkable range of ecological and morphological diversity. Unlike the true fishes, sharks do not have internal bone, but instead have a cartilaginous skeleton. Although many people are told that sharks are primitive in comparison to other groups, this is not true. Many sharks are efficient and specialized hunters that have thrived for millions of years.
This small exhibit shows an amazing predator: the great white shark (Carcharodon carcharias). The white shark is found in temperate waters throughout the world's oceans, and it is an important, though not common, predator in California's coastal habitats. Scientists from several organizations throughout California including the Point Reyes Bird Observatory, the Marine Mammal Center, the California Academy of Sciences, Bodega Bay Marine Laboratory, California Department of Fish and Game, and the Museum of Vertebrate Zoology at UC Berkeley, have been studying white sharks and their prey for several years in hopes to better understand their behavior and ecology.
White sharks are predatory animals that begin life by feeding on fish, rays, and other sharks, and as they grow, switch to feeding on marine mammals and scavenging on large animal carcasses. Their first mammalian prey are usually the small harbor seal, but as the sharks increase in size, they become large enough to eat sea lions, elephant seals, and small toothed whales. Attack strategy consists of a swift, surprise attack from below, inflicting a large, potentially fatal bite. The pinniped often dies from massive trauma or blood loss, but the bites may be superficial or misplaced on the body, allowing the seal to escape and survive the attacks with their scars as witness. Large white sharks will also scavenge on the carcasses of whale sharks, and on the fat-rich blubber layer of dead whales. They will occasionally feed on sea turtles and sea otters, and are known to attack, but not eat, humans.
The great white shark came into existence during the mid-Miocene epoch.
 The earliest known fossils of the great white shark are about 16 million years old. However, the phylogeny of the great white is still in dispute. The original hypothesis for the great white's origins is that it shares a common ancestor with a prehistoric shark, such as the C. megalodon. Similarities among the physical remains and the extreme size of both the great white andC. megalodon led many scientists to believe these sharks were closely related, and the name Carcharodon megalodon was applied to the latter. However, a new hypothesis proposes that the C. megalodon and the great white are distant relatives (albeit sharing the family Lamnidae). The great white is also more closely related to an ancient mako shark, Isurus hastalis, than to the C. megalodon, a theory that seems to be supported with the discovery of a complete set of jaws with 222 teeth and 45 vertebrae of the extinct transitional species Carcharodon hubbelli in 1988 and published on 14 November 2012.[ In addition, the new hypothesis assigns C. megalodon to the genus Carcharocles, which also comprises the other megatoothed sharks; Otodus obliquus is the ancient representative of the extinct Carcharocles lineage.

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