The only things that sharks have in common are gill slits.
Over a period of more than 400 million years sharks have adapted themselves perfectly to life in the ocean and inhabit practically all ocean realms. Several shark species even live in sweet water. Whether they live on the ocean floor, swim freely in coastal regions, the open sea or the deep sea, sharks vary greatly in appearance. Our classical image of a shark is probably best represented by the ground and mackerel sharks. Their food spectrum ranges from plankton to mussels and molluscs, up to fish, birds and marine mammals. All of these specializations require specific anatomic adaptations and explains their differences in appearance.
Yet the most reliable characteristic by which a shark can be identified are its five to seven gill slits..
© Shark Foundation
Illustration: Female Bullheade shark