Orgin / Natural History
Sharks originated some 450 million years ago, and many species have changed little in the past 100 million years. Unlike most fishes, they have no gas bladder to keep them afloat, so many species must move constantly to keep from sinking. Their skeletons are made of light, tough cartilage instead of bone, and many have large, oil-filled livers that make them more buoyant. The tiger shark (Galeocerdo cuvier) is one of the largest and most recognizable sharks. It is covered with dark spots as a juvenile, which merge into stripes as it grows older and then eventually fade. Its powerful jaws and teeth are especially useful in hunting large animals such as loggerhead turtles (Caretta caretta), but it will eat almost anything and has the most varied diet of any shark.