鯊魚既食物[10分][勁急]

Feeding and Digestion of Sharks

All sharks are carnivorous and many people believe that sharks will eat just about anything; for a few species, such as the tiger shark, this is true. The vast majority of sharks, however, are far more specialised for particular prey items, and rarely stray from these. Some of the most specialised sharks have developed a filter feeding technique, which is employed by the whale, basking and megamouth sharks. These three shark species have evolved plankton feeding independently and use different strategies. Whale sharks feed using suction to take in large concentrations of plankton and small fishes. Basking sharks are ram-feeders, swimming steadily, with their mouth wide open, through plankton blooms. Megamouth sharks may make their suction feeding extra efficient with the use of luminescent tissue inside the mouth the attract prey in the deep ocean. This type of feeding was only possible through the evolution of gill rakers, long slender filaments that form a very efficient sieve, analogous to the baleen plates of the great whales. Plankton is trapped in these filaments and swallowed from time to time in huge mouthfuls. Teeth in these species are very small compared to the size of the animal, because they are not needed for feeding.

Other highly specialist feeders include the cookiecutter sharks, which feed on the flesh sliced out of other larger fish and marine mammals. The teeth in these sharks are enormous, compared to their size, with the teeth of the lower jaw being particularly sharp. Although they have never been observed feeding they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rasp of flesh.

Some seabed dwelling species are highly effective as ambush predators. Angel sharks and wobbegongs are perfectly camouflaged for lying in wait in order to suck prey into their mouths. Many benthic sharks feed solely on crustaceans which they crush up with their flat molariform teeth.

Other sharks feed on squid or fishes, which are swallowed whole. The viper shark has teeth which can be pointed outwards to strike at and capture prey that is then swallowed intact. The great white and other large predators can either swallow small prey whole or take huge bites out of large animals. Thresher sharks use their long tails to stun shoaling fishes, and sawsharks may either stir prey up from the seabed or slash at swimming prey with their tooth-studded rostra.

Many sharks, including the whitetip reef shark are cooperative feeders and hunt in packs in order to herd and capture elusive prey. These social sharks are often highly migratory, travelling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.

Digestion of the food can take a long time in sharks, particularly in the cold-blooded species. The food moves from the mouth to the 'J' shaped stomach, where it is stored and initial digestion occurs. Unwanted items may never get any further than the stomach, and are coughed up again. Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of any unwanted contents.

One of the biggest differences in digestion in sharks when compared to mammals is the extremely short intestine. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a very long tube-like intestine. The valve provides a very long surface area for the digestion of food, requiring it to pass around inside the apparently short gut until fully digested., when remaining waste products pass into the cloaca and vent.

normal are fish, seal,sea dog or turtle

Sharks (superorder Selachimorpha) are fish with a full cartilaginous skeleton[1] and a streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles to protect their skin from damage and parasites and to improve fluid dynamics.[1] They have replaceable teeth. They are some of the world's most misunderstood predators, as they very rarely attack humans unless intimidated.

Sharks include species from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 cm in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (41 feet) and which, like the great whales, feeds only on plankton through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water (it is found in Lake Nicaragua, in Central America) and in deltas.[2]


Skeleton
The skeleton of the shark is very different from that of bony fishes such as cod; it is made from cartilage, which is very light and flexible, although the cartilage in older sharks can sometimes be partly calcified, making it harder and more bone-like. The shark's jaw is variable and is thought to have evolved from the first gill arch. It is not attached to the cranium and has extra mineral deposits to give it greater strength.[3]


The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats. This way of respiration is highly inefficient and if the shark were to stop moving blood would not be able to move through the body due to the lack of strength of the heart's atrium.[citation needed]


All sharks are carnivorous and many people believe that sharks will eat just about anything; for a few species, such as the tiger shark, this is true. The vast majority of sharks, however, are far more specialised for particular prey items, and rarely stray from these. Some of the most specialised sharks have developed a filter feeding technique, which is employed by the whale, basking and megamouth sharks. These three shark species have evolved plankton feeding independently and use different strategies. Whale sharks feed using suction to take in large concentrations of plankton and small fishes. Basking sharks are ram-feeders, swimming steadily, with their mouth wide open, through plankton blooms. Megamouth sharks may make their suction feeding extra efficient with the use of luminescent tissue inside the mouth the attract prey in the deep ocean. This type of feeding was only possible through the evolution of gill rakers, long slender filaments that form a very efficient sieve, analogous to the baleen plates of the great whales. Plankton is trapped in these filaments and swallowed from time to time in huge mouthfuls. Teeth in these species are very small compared to the size of the animal, because they are not needed for feeding.

all food in the sea