Scientists believe the oceans support thousands of species of these flat fish, which use their wing-shaped bodies to soar through the depths. Still, it is the sharks that fascinate and terrify us. Ironically, however, that fascination is not matched by a clear understanding of how sharks live. We lack some of the most basic facts of their life stories.
Researchers do know that sharks and rays are some of Earth's oldest fish.
Once, more than 400 million years ago, the two groups shared a common ancestor that swam through the planet's oceans long before the first dinosaur appeared. Today, to the untrained eye, that common origin isn't obvious, because most sharks and rays look so different from one another. But the fish are, in fact, relatives with structural similarities. Some sharks are reputed to be able to detect minute quantities of substances such as blood in water from several hundred feet away. Some sharks grow to be only a foot long.