Melissa Sandaval, age 12, of Miami, Fla., for her question:
ARE THERE MANY KINDS OF SHARKS?
The shark is a meat eating fish and one of the most feared animals of the sea. Scientists tell us there are about 250 species of sharks. They live in oceans throughout the world but are most common in warm seas.
Sharks differ from most other kinds of fish in a number of ways. First of all, they have no bones. Their skeletons are made of a tough, elastic substance called cartilage.
Only about a tenth of the 250 species of sharks are considered dangerous to human beings. But most of the shark attacks that do occur result in death or serious injury. Swimmers should take special care in areas known to have sharks.
Sharks are versatile and keen sensed fish, many species of which are able to hunt and eat nearly all the larger marine animals in both shallow and deep seas. These two features account for their long evolutionary history.
Many of the shark species living today are quite similar to abundant species that swam in seas of the Cretaceous period more than 100 million years ago.
Sharks reveal great diversity in behavior and size. The whale shark is the largest shark and also the largest fish in the sea, measuring up to 49 feet in length. Squalius is the smallest of the sharks, measuring only six to eight inches to length.
Sharks are especially abundant in tropical and subtropical waters. Generally speaking, they are best known as aggressive carnivores that even attack members of their own species. But two of the largest sharks, the whale shark and the basking shark, are docile feeders on plankton, which they strain from the water with sieve like gill rakers.
Most sharks are gray in color and have leathery skin covered with small, sharp, pointed placoid scales, which, unlike those of bony fish, do not enlarge during the animals' growth. usually five gill slits lie behind the head.
Many species of sharks have several rows of sharp teeth embedded in fibrous membranes instead of in the jawbones. The teeth, which are frequently lost in the flesh of prey, are quickly replaced by other teeth that shift into position.
The fins and tails of sharks are rigid instead of erectile, like those of the bony fish. Contrary to a popular image, the dorsal fins rarely protrude above water when the fish are close to the surface.
Sharks do not have swim bladders and, when motionless, sink to the bottom. They have strong digestive enzymes and a specialized epithelial fold that spirals the length of the small intestine, enabling sharks to absorb a great diversity of foods.
Sharks, to a large extent, are scavengers. They eat injured fish, garbage and other waste from passenger and fishing ships, in addition to eating animals such as seals, turtles, birds, whales, crabs and a wide range of fish.