There are over 400 species of shark in the oceans today. Historically, there were many more: sharks first evolved 400 million years ago. Since the days of the megalodon, a vast array of species have evolved, from the hammerhead to the speedy mako and the famous great white. At an average of 10 metres long, the whale shark is the largest species of fish in the world. Sharks' skeletons are made of cartilage instead of bone, like their cousins, the rays.