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Sharks are amazing fish that have been around since long before the dinosaurs existed. They live in waters all over the world, in every ocean, and even in some rivers and lakes.
Sharks belong to the group of fish called Elasmobranchii, which also includes the rays, skates, and ratfish. The Elasmobranchii are fish that have no bones; their skeleton is made of cartilage, which is a tough, fibrous substance, not nearly as hard as the bone (our nose and ears are made of cartilage). Some parts of their skeleton, like their vertebrae, are calcified. Sharks also have no swim bladder (like bony fish do).
Varieties of sharks
There are about 368 different species of sharks, which are divided into 30 families. These are very different in the way they look, live, and eat. They have different shapes, sizes, colour, fins, teeth, habitat, diet, personality, method of reproduction, and other attributes. Some species of shark are very rare (like the great white shark and the megamouth) and some are quite common (like the dogfish shark and bull shark).
There are many different species of sharks that range from the size of a person's hand to bigger than a bus. Fully-grown sharks range in size from 18cm long (the Spined Pygmy shark), up to 15m long (the Whale shark). Most sharks are intermediate in size, and are about the same size as humans, 1.5-2.1m long. Half of the 368 shark species are under one metre long.
Sharks have a variety of body shapes. Most sharks have streamlined, torpedo-shaped bodies that glide easily through the water. Some bottom-dwelling sharks (e.g. the angel shark) have flattened bodies that allow them to hide in the sand of the ocean bed. Some sharks have an elongated body (e.g., cookiecutter sharks and wobbegongs). Sawsharks have elongated snouts, thresher sharks have a tremendously elongated upper tail fin which they use to stun the prey, and hammerheads have extraordinarily wide heads. The goblin shark has a large, pointed protuberance on its head, though its purpose is unknown.
Sharks may have up to 3,000 teeth at one time. Most sharks do not chew their food, but gulp it down in large pieces. The teeth are arranged in rows; when one tooth is damaged or lost, it is replaced by another. Most sharks have about five rows of teeth at any time. The front set is the largest and does most of the work.
Sharks vary greatly in their diets, but they are all carnivores. Some (like the great white, mako, tiger, and hammerhead) are swift predators that eat fish, squid, other sharks, and marine mammals.
Some (like the zebra horn shark, angelshark, and wobbegong) are slow-swimming predators that crush and eat shellfish (crabs and clams) from the ocean floor.
Others (like the whale shark, the basking shark, and the megamouth) are filter feeders that sieve tiny bits of plankton and small animals from the water as they swim with open mouths. They eat huge amounts of these tiny animals and plants.
Do sharks sleep?
Fish don't sleep in the same way that we do, but they have active and inactive periods. Some sharks (like the nurse shark) have been observed resting motionless on the sea floor. Others have to keep moving in order to breathe.
When some sharks (like the Great White or the Gray Reef shark) turn aggressive prior to an attack, they arch their back and throw back their head. This places their mouth in a better position for taking a big bite. They also move their tail more acutely (probably in preparation for a chase).
Sharks do not normally attack people, and only about 25 species of sharks are known to attack people. Sharks attack fewer than 100 people each year. Many more people are killed by bees or lightning.
The sharks that are the most dangerous to people are the great white shark, the tiger shark, the bull shark, and the oceanic whitetip shark. The bull shark is the most frequent attacker of people as it swims in very shallow waters where people swim and is a very plentiful shark. Some of the other sharks that are known to have attacked people include the gray shark, blue shark, hammerhead shark, mako shark, nurse shark, lemon shark, blacktip reef shark, wobbegongs, sandtiger, spitting sharks, and the porbeagle. Some people believe that sharks mistake people (specially people swimming on surf boards) for seals and sea lions, some of their favourite foods.
Occasionally, a group of sharks will attack a food source (for example, a school of fish) in a maniacal fashion. They will wildly attack the food and anything in the area, even each other, sometimes wounding or eating fellow sharks.
Scientists have shown that sharks are relatively intelligent and can learn at a rate similar to that of rats and birds.
Sharks live in oceans and seas all over the world, and even in some rivers and lakes, specially in warmer waters. Some sharks live near the surface, some live deep in the water, and others live on or near the ocean floor. Pelagic sharks (living in the open ocean) include the great white shark, the basking shark, etc. Benthic sharks (living at the ocean floor) include the zebra horn shark, the wobbegongs, and the angelshark, which usually have flattened, camouflaged bodies that let them hide in the sea bed. Some sharks even venture many miles up into the fresh water of rivers like the Mississippi in the USA and the Amazon in Brazil. The bull shark (Carcharhinus leucas) sometimes ventures into fresh water.
Some sharks live in relatively warm waters (hammerheads, bull sharks, and tiger sharks). Other sharks, such as the thresher, mako, basking and blue shark, live in temperate water (which is neither hot nor cold). Others, including the dogfish, Greenland, and goblin, live in cool waters. Some sharks stay in the same region for their entire lives while others travel across oceans.
Migration of sharks
There are three different types of sharks when it come to migratory patterns:
Local sharks - these sharks do not migrate, and range only about a hundred miles from their habitat. Examples include the bonnethead shark, and the nurse shark.
Coastal pelagic sharks - these sharks can migrate over 1,600 km. Examples include the dusky shark, the blacktip sharks, the tiger shark, and the sandbar shark.
Highly pelagic sharks - these sharks migrate across oceans. Examples include the blue shark and the mako.
Evolution of sharks
Sharks have existed for over 350 million years. They evolved over 100 million years before the dinosaurs did. This was long before people evolved. Most fossil evidence of early sharks is from fossilized teeth and a few skin impressions. Cladodonts, primitive sharks, had double-pointed teeth, were up to one metre long fish-eaters and lived about 400 million years ago.
Megalodon (Carcharodon megalodon) was an ancient, meat-eating shark, living between 25-1.6 million years ago; it has become extinct now. It was over 12 m long, but this is only an estimate from fossil teeth that have been found. Its teeth resemble those of the great white shark but are almost three times larger; these teeth are each the size of a person's hand!
Shark fossils are rare because sharks have no bones, only cartilage, which does not fossilize well. Fossilized shark teeth are very hard and fossilize well.
Endangered and protected species
The largest sharks are decreasing in numbers around the world because they are being hunted by people. The Great white shark, the Basking shark, and the Whale shark are all waning. The Great white is protected along the coast of California and South Africa.