Debbie Jenkins, age 12, of Cape Elizabeth, Me., for her question:
DO SHARKS LAY EGGS?
Most fish have what is called a swim bladder. This is a gas filled organ that helps the fish stay at a certain depth without sinking. Sharks don't have this organ. Instead, they have large livers that are filled with oil. This oil is lighter than water and it helps to keep sharks from sinking. Even so, most sharks must swim constantly or else they will sink.
A shark is a meat eating fish that many people fear. There are about 250 species in the oceans of the world, with most of them found in warm seas.
Most sharks eat live fish, including other fish. They have a reputation of being man eaters, but actually there are less than 100 shark attacks throughout the world each year. Only 25 species of sharks have been known to attack human beings.
Unlike most fish, sharks have no bones. Their skeletons are made of a tough, elastic substance called cartilage. But like most other fish, sharks do produce eggs.
Shark eggs, unlike those of most fish, are fertilized inside the female's body. The male has two organs called claspers, which release sperm into the female where it fertilizes the eggs.
In most of the species of sharks, the eggs are hatched inside the female and the pups, as baby sharks are called, are born alive. A few species do lay their eggs and the babies are hatched outside the mother's body.
The body of a newborn pup resembles that of an adult in almost every way except size.Sharks have fewer babies at a time than do most other fish. Some may give birth to 60 or more pups in a litter, but most have far fewer. Parents do not take care of the newborn pups. In fact, often they eat many of them.
In the olden days it was thought that a shark ate constantly and that it would therefore attack any human being immediately. But scientists have since learned that these fish can go for many weeks and even months without feeding. During this period of time, the oil stored in their livers nourishes them.
Sharks can swim swiftly. A blue shark was recorded recently traveling 43 miles per hour. Now, that's fast for a fish!
Most sharks have crescent shaped tails that provide power for swimming. The upper part of the tail is longer than the lower part. Stiff side fins, called pectoral fins, help lift and balance the front of the body.
Sharks have several rows of teeth. When old teeth wear out or are lost, new ones grow in and replace them.