VI

SHARKS

 

Great white shark

     Men are afraid of sharks, perhaps because they are the only creatures left in nature that will attack men without provocation.

     Closely related to the fearsome Great White, and an even faster swimmer, is the Mako shark, a deep-water species. Another fierce variety, the Hammerhead, has a weird T-shaped head that looks like a hammer, which it apparently uses as a kind of rudder for rapid turning.

The largest group of man-killers includes the Tiger shark, a striped species (hence the name) feared equally in the West Indies and Australia, which can reach a length of 18' with a weight of at least 1,500 pounds. The Lemon shark, like the Tiger, is often seen around docks and anchored ships. Out at sea the best-known man-killers are the blue and white-tipped sharks.

     Sharks inhabit nearly all latitudes, but the vast majority of sharks are found in temperate and tropical seas, and nearly all shark attacks on humans have taken place when the water temperature has been over 70 degrees Fahrenheit. Some sharks also seem to have the curious trait of picking out a single person in a group and limiting their attacks to him.

     Sharks represent a very ancient group of animals. The first sharks lived more than 400 million years ago - 200 million years before the first dinosaurs walked the earth. In fact, the first sharks swam in the ocean before animals of any kind walked on the land. Today, there are more than 350 different species (or kinds) of sharks.

     Sharks are different from most fishes in several odd ways. Their skeletons are not made of bones, as are those of other fishes, but rather of cartilage hardened by lime. They have skin as rough as a file, for they are the only fishes with spiny scales. The most dangerous sharks are the great whites and the tigers.  Unlike most bony fish, sharks sink when they stop swimming. Most fish can stay up in the water without swimming because they have swim bladders. A swim bladder is like a balloon inside a fish's body, which can be inflated or deflated.

     Sharks don't have swim bladders to help them stay up in the water. To keep from sinking, they must always swim in a slightly upward direction. If they stop swimming, the weight of their bodies pulls them down to the bottom.

     To swim, most fish have to wriggle their entire body from side to side. But a shark can get most of the power it needs from the rear caudal fin. Some large sharks can swim at speeds up to 40 miles per hour.

     Sharks breathe under water in the same way fish do. They use their gills to get oxygen out of the water. To begin the process, a shark draws water into its mouth and lets the water pass over its gills.

     Most fish have only one gill opening to let the water out, but sharks usually have five openings. Some have six or seven. As the water passes over the gills, it comes into contact with many small blood vessels.

     Most of the water in the sea has oxygen in it. As the water contacts the blood vessels, the oxygen moves from the water into the shark's blood. This is similar to the way that human lungs put oxygen into the blood.

 

     The front fins of most fish are flexible and can be moved in many different directions. But the front fins of most sharks are stiff and can be moved in only a few directions. These fins are called pectoral and are stiffened with rods of cartilage and are extremely tough. By changing the angle of these fins, a shark can swim up or down.

     Sharks teeth are made the same way as human teeth. There is a pulp cavity in the middle, covered by dentine, with hard enamel on the outside.

     Even in murky water, or in complete darkness, a shark's senses can tell where prey is. It can tell whether the prey is hurt or not. Sharks are very sensitive to everything that is going on in the water around them and they can react very quickly to the information that their senses bring to them.

     Hearing is probably the best of all a shark's senses. Some sharks can hear prey in the water fiom 3,000 feet away. They use their ears to find out the direction the sound is coming from and then turn to swim toward it.

     Sharks have a keen sense of smell and some kinds can smell one part of blood in 100 million parts of water. By turning their heads from side to side, they can tell the direction that a smell is coming from.  The jaws of a Great White shark are filled with very sharp teeth. There are 26 teeth on top that are shaped like triangles, and 24 narrow teeth on the bottom. The narrow teeth grab and hold prey, and the triangle-shaped teeth are for cutting. The edges of the teeth have serrations on them, like a saw, to help them cut more easily. Sharks donít chew their food, they swallow it whole.

 

 

Tiger Shark

 

From: THE PIRATE HUNTERS

 

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