Sharks are a fascinating group of fishes that strike fear into the minds of humans. This fear is really a fear of the unknown, for little evidence exists that indicates sharks are really very dangerous. Sharks have much more to fear from humans than humans have to fear from sharks. Each year thousands of tons of sharks are killed by fishermen for food and other products while very few humans are ever bitten, and fewer still die from shark bites.
Sharks belong to the class Chondrichthyes, a group which also includes rays, skates and chimeras. Unlike other vertebrates, sharks have a skeleton made of cartilage rather than bone. Only about 800 species of cartilaginous fish are known to exist compared to over 20,000 known species of bony fishes.
Sharks differ from the more common bony fishes in several ways. Besides having no bones, sharks and their relatives do not have the overlapping scales that cover many bony fish. Nor do they have a swim bladder, the air-filled balloon-like organ that keeps most fish upright. Sharks have gill slits but no gill-cover, which is common to the bony fish.
Sharks vary greatly in size from the enormous 40-foot-long whale shark to tiny two-to three foot dogfish and angel sharks. Despite its huge size, the whale shark is not an active predator. It feeds rather passively by swimming with its mouth open to collect plankton and small fish.
Most sharks are active predators and eat primarily fish, although the great white shark will also prey upon seals, sea lions and other marine mammals. Some sharks eat bottom-dwelling animals such as crabs, and others scavenge for dead animals. Some sharks even eat trash as indicated by the tin cans and boots found in sharks' stomachs.
Post new comment