Selasa, 13 Desember 2011


Out of all the species walking, flying, slithering or swimming, there aren't many who have been around as long, survived as well, or come in so many shapes and kinds as the shark. The earliest evidences of sharks are isolated spines, teeth and scales that appeared about 430 million years ago in the Silurian Period, known as the ''Age of Fishes". Sharks have a sleek, streamlined design which helps them swim without using up a tot of energy. They certainly need to conserve their energy because they never really sleep and most of them never stop swimming. Some sharks are fierce predators, and would be happy to eat you if they encountered you. 

Almost any shark six feet or longer is a potential danger, but three species have been identified repeatedly in attacks: the Great White Shark, the Tiger Shark and the Bull Shark. All three live world wide, reach large sizes and eat large prey such as marine mammals or sea turtles. But most sharks never grow longer than five feet and never even see anyone with legs and arms anyway.

People kill thousands more sharks every year than sharks kill people. Sharks take about as long to mature as we do. Some of them become adults in their teens. A mother shark carries her babies inside her body while they develop, sometimes for more than a year. Even so, some sharks are born inside an egg which they have to crack open. They spend early portions of their, lives in nursery grounds. Some of the advantages sharks have over people is that they keep growing new teeth, they don't have breakable bones, and they are not prone to get cancer. Sometimes sharks are referred to as swimming computers because of the six senses which they possess: vision, hearing, vibration, smell, taste and electro-perception.

Shark Diversity
Sharks come in many shapes, sizes, and colors. They have different personalities, and they live in oceans all over the world. Not much you can say about sharks is true for all of them, but there are a few things you can say about the anatomy of most sharks. They breathe through gill slits, have rigid fins and they don't have a bladder.

The Whale Shark is the biggest creature in the ocean, about eleven times taller than your Mom or Dad, and weighing in at about 2,000 pounds. A Whale Shark wouldn't be too interested in eating you, but you could offer him about half a ton of teeny fish, crustaceans and little drifting invertebrates and he would be very happy. The Spined Pygmy Shark only grows to the size of a medium cucumber, and sleeps all day at the bottom of the ocean.

Some sharks only like warm water, and some only like cold water. Some of them like to travel all over the world and some of them never leave the neighborhood they were corn in. There are even sharks in freshwater rivers. Most sharks like to eat fish, but none of them have ever tasted any tartar sauce. Of course there are a few of them who would eat you in less than a minute if you went to pay them a visit, but it wouldn't be anything personal. The majority of sharks feed on live fish. Most of them make their homes in very deep water.

Scientists divide all the species of sharks into eight groups, or orders accord-ing to certain features which they have in common. The biggest groups of sharks are the Ground Sharks. They are found all around the world in temperate and tropi¬cal waters. Saw Sharks all have long, blade like snouts. Dogfish Sharks live on the bottom of the ocean bear live young and are not considered dangerous to humans. Angel Sharks look a lot like Rays and are found most often en the continental shelves. Carpet Sharks like warm water and live on the bottom of the ocean.

Mackerel Sharks grow up to be very big, and can be found anywhere between tropical waters and Arctic waters. Bullheads Sharks like rocky reefs, and have only been found in the Pacific and Indian Oceans. And finally, Frilled Sharks are the only ones with six or seven gill slits. They like deep water' and usually stay near the bottom.
All the creatures living under water have certain characteristics in common. They all need a way to get oxygen. They all need to reproduce to make sure their species continues. They all need to eat to live. But each species also has special features which are unique.

Sharks have highly sensitive senses, a special liver which helps them to float, several rows of teeth, and eyes which aren't so different from yours. Like rays, shark skeletons are made of cartilage.
The shape of a shark is specially designed to help in navigate long distances and maneuver around its prey with ease. Its several pairs of fins help it navigate through the water, kind of like our legs get us humans around, and our arms help us keep our balance.

A shark has several pairs of gills on either side of its head, unlike other fish who only have one gill on each side. You probably breathe through your nose and mouth, but a shark only uses its nostrils for smelling. When people don't want to be seen by their enemies they sometimes wear fatigue suits. Sharks are born with their own special colors to protect i.hem from predators above and below them in the water.

The bones of a fish are made mostly of calcium, but a shark doesn't really have any bones. A shark skeleton is made of cartilage. Bony fish have a gas-filled swim bladder which enables them to float in the water, but sharks have no such bladder. Since cartilage is lighter than bone, it helps to keep a shark from just sinking to the bottom of the ocean. A shark doesn't have as many moveable parts as a bony fish, which in some ways makes a shark a little more clumsy. But cartilage is more flexible than bone, so a shark can turn around in a smaller space than a bony fish. Sharks keep growing cartilage as long as they live, and have extra mineral deposits in their jaws where they need extra strength, (the better to bite you with).

When engineers came up with the shape of a DC-9 airplane, they were thinking about the fastest way to travel using the least amount of energy. If you compare the DC-9 to 3 sharks, you will see that their shapes are amazingly similar. They both have round bodies tapering off at both ends. This shape allows the plane to glide through the air, and the shark to glide through the water, without using up all their fuel before they get where they want to go.

Sharks are generally a dark color on top, which is called their dorsal side, and a light color on the bottom, which is called their ventral side. Predators looking down might not see the dark top of the countershaded shark, because it blends in with the dark ocean depths. But if a predator looks up from below, the light bottom of the shark blends in with the lighter surface of the sea where the sun shines. Either way, the shark manages very well to blend in with its environment and avoid being seen when it doesn't want to be seen.

Sharks do have noses, but they only use them for smelling not for breathing. You'll find their pair of nostrils on the, underside of their snouts. Some species, like the Nurse Shark, even have some extra smellers near called nasal barbels, which stick out near the nostrils and mouth.

Ampullae Of Lorenzini
These are electro receptive organs. The ampullae are jelly-filled pores shaped like ampullae (clay jars used by the Romans and others to store grain, oil, and wine). The function of these organs was not discovered until fairly recently. They are electrical field sensing devices, and as everything living produces an electrical field, the shark can detect the presence of another living creature even if all its other senses were deactivated. The ampullae are distributed abound the head, and the external openings and are large enough to be seen with the naked eye.

In an experiment, a group of sharks and rays were trained to eat in an area directly over a pair of electrodes buried in the sand bottom. When the sharks were fed fish, the current was turned on and emitted four-tenths of a microvolt Then when the food was withheld, but the current turned on, the sharks and rays swarmed about the electrodes', uncovering them and snapping at them Thus it war shown that the animals could sense minute changes of electricity and could trace it to its source.

Sharks, as all fish, use their body and tail in a side to side motion to move through the water. Shark fins are rigid not flexible, and are supported by rods made of cartilage.
Sharks have five different kinds of fins:
  • Paired pectoral fins lift the shark as it swims.
  • Paired pelvic fins stabilize the shark. One or two dorsal fins also stabilize the shark.
  • In some species, dorsal fins have spines.
  • Not all sharks have an anal fin, but it provides stability for the sharks that do have one.
  • The caudal or tail fin moves the shark forward. 
Sharks have five to seven gill slits on each side of their head, unlike bony fish which have one gill on each side. As water passes over their gills, oxygen is absorbed by the blood in the gills and transported from there to the rest of the body.

Some sharks have small openings called spiracles behind their eyes, at the top of the head. Spiracles are sort of like baby gill slits. The more active, fist swimming sharks seem to have outgrown the need for these spiracles, and only have very tiny ones or don't have them at all.

Threat To Shark
Despite their superior physiology and hurting skills, many shark species are now threatened with extinction due to human activity in the ocean. The main threats to sharks are over-fishing and accidental by catch. In many parts of the world, sharks are in very high demand, for their meat, skin and cartilage, which is used in several medicines. Thieve shark products sell at very high prices, making them an attractive catch to fishermen.

Sharks mate only rarely and have a relatively small number of babies at a time. Consequently, they can't replenish their population quickly. Sharks also have fairly long lifespan - on average, sharks live 25 to 30 years, and some sharks live 100 years or more. If left alone, a female will mate marry times in its life. With this reproductive pattern, the death of every single shirk obviously has a significant effect on the shark population.
Over-fishing is actually a problem for both sharks and humans. If humans kill too many sharks in a given amount of time, the population will dwindle and they won't be able to catch many sharks in the future. The only way to maintain profitable shark fishing over time is to allow sharks to continue to reproduce, which means decreasing shark fishing significantly. Sharks are also killed accidentally, primarily by long lines used to catch other fish. Researchers suggest we must ban certain fishing methods, or some shark species will die out at some point in the near future.

Sharks have persevered for hundreds of millions of years, while thousands of other animals have come and gone. When you consider this incredible history, and the unique physiological characteristics found in sharks, it's clear that it would be a great tragedy to lose any shark species. They are among the most remarkable animals en earth, and there is still so much we don't know about them.

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