Description and Habits

The typical shark body is torpedo-shaped with the upper half of the tail fin longer than the lower half. In male sharks, the pectoral fins have been modified into claspers, a pair of sexual organs containing sperm. Most sharks are black, brown, or gray, usually with lighter undersides. Some species, however, such as the leopard shark, whale shark, and swell shark, have blotches and markings on their bodies.

Sharks have many anatomical features that set them apart from most other fish. For example, the skeleton lacks bone; it is made up of cartilage. The skin is embedded with placoid scales, horny structures that, unlike the scales of most fish, do not overlap. Each bears a tiny toothlike projection called a denticle. Like most other fish, sharks breathe through gills, but unlike most other fish, which have four pairs, sharks have five to seven pairs of gills, with a corresponding number of gill slits opening at the sides of the head. Most sharks are cold-blooded. Sharks have numerous rows of teeth and produce new teeth throughout their lifetime. Teeth of the outer row periodically break off or wear away and those of the next row advance in position to take their place.

Most sharks are powerful swimmers and chase their prey. Most eat fish, mollusks, crustaceans, and occasionally, seals. Sharks have sensitive pores, called ampullae of Lorenzini, on the chin and nose. These sensory organs are used to detect weak electric currents in the water, helping the shark locate prey.

Sharks reproduce by sperm and eggs. A few species are viviparous—the females give birth to live young. Some are oviparous —the females expel the fertilized eggs. The eggs are contained in leathery cases, popularly called mermaid's purses, that are sometimes found on the seashore. Most species are ovo-viviparous —the female retains the fertilized eggs within her body until birth.

Are All Sharks Dangerous?

No. In fact, the largest shark of all is known as the “gentle giant” of the deep. It is the whale shark. A whale shark may reach a length of 40 feet (12 meters). That’s about as long as a bus. The whale shark may weigh about 13 short tons (11.8 metric tons). That’s the size of two African elephants put together! Whale sharks lay eggs that are the size of footballs!

A whale shark is dark gray or reddish in color. It has a broad, flat head and a wide mouth. It has over 300 rows of tiny, hooked teeth.

You might think such a giant of a shark would be a threat to humans. A whale shark, however, is harmless—except to the plankton that are its prey.

Why Do Whale Sharks Stand Up to Eat?

Whale sharks eat tiny plankton and fish that live near the surface of the water. To catch their prey, whale sharks swim near the surface with their mouths wide open. Whale sharks can also “stand” vertically in the water, with heads up and mouths open. These sharks take great amounts of water in through their mouths. They filter out the plankton and tiny fish through large, spongelike gills.

Unfortunately, the habit of standing has caused some whale sharks to be injured or killed by boats that accidentally ram into them.