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
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.
From: THE PIRATE HUNTERS