Sandy Robertson, age 14, for her question:
HOW MANY KINDS OF SHARKS ARE THERE?
Sharks do not have bones. Their skeletons are made up of tough, elastic cartilage.
All sharks are meat eaters. Most eat live fish including other sharks. A shark's only natural enemy is a larger shark. Most species have mouths on their undersides and they have several rows of teeth. When an old tooth is lost, a new one grows in to replace it.
About 250 different species of sharks are found in our oceans, with most of them living in temperate areas. Largest of all types is the whale shark, measuring as long as 60 feet and weighing 15 tons. He's the largest of all fish.
Smallest of the sharks is only four inches long and he weighs less than an ounce.
The shark has developed a reputation of being a man eater. The reports, however, are greatly exaggerated. Less than 100 shark attacks are reported each year throughout the world from the 25 species who have been known to attack humans.
From the moment they are born, sharks must swim constantly or they will sink. While most fish have swim bladders, gas filled organs that help them remain at certain water depths, the sharks do not. Helping slightly is a liver larger than that found in most fish. Filled with oil that is lighter than water, it helps keep the shark from sinking.
Some species give birth to 60 or more young sharks at a time. While a few lay eggs, most young are born live. The parents never take care of the babies the minute they are born, they're on their own.
Sharks have excellent vision. When the water is clear, an object 100 feet away can be seen. And they also have sensitive organs called lateral line systems which enable them to detect vibrations in the water. The system is made of fluid filled channels that run down both sides of the sharks' body.
Another strong sense is the ability for a shark to smell. Two thirds of his brain is used in connection with smelling. He can sense blood in the water a quarter of a mile away. And he hears well, too. He can detect a sound as far as 1,000 yards away.
White sharks are those known as man eaters. They measure up to 20 feet long and are found in tropical and warm waters. They live on sea lions, tuna and other sharks but have been known to attack humans, even when they are in small boats.
The giant whale sharks are harmless to man. They live exclusively on diets of small fish and plankton.