Sharks differ from most other kinds of fish in a number of ways. For example, sharks have a boneless skeleton made of a tough, elastic substance called cartilage. Most species of sharks have a rounded body, shaped somewhat like a torpedo. This streamlined shape helps sharks to swim. Angel sharks, which live near the ocean bottom, have a flat body similar to that of skates and rays.
Sharks have fewer young at a time than most fish do.
Some species give birth to 60 or more pups in a litter, but most have far fewer. Shark eggs, unlike those of most fish, are fertilized inside the female's body. The male shark has two organs called claspers, which release sperm into the female, where it fertilizes the eggs. Among most species of sharks, the eggs hatch inside the female, and the pups are born alive. At least 40 species lay their eggs outside their bodies.