Highly successful predators, sharks have been around for about 450 million years. There are about 800 species of sharks worldwide, including rays and skates that are close relatives and believed to have evolved from the shark. Sharks have been clocked up to 60 M.P.H. and perhaps travel as fast as 70 m.p.h. Wilga says she studies sharks because they have evolved many different mechanisms for feeding as well as swimming and have been understudied, mostly due to the difficulty of working with large predators. They are unique.
Sharks have no bones and are composed of cartilage.
They breathe differently too. Their gill structure has five slits on the side while bony fish have one. Their liver is filled with oil and, not surprisingly, their pearly whites are different. Since it's heavier than water and doesn't have a swim bladder like bony fish, the shark must keep moving or bottom out. Some sharks only eat fish, others only eat crabs, still others only seals.