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.
Some sharks only eat each other.
Besides his or her own relatives, the shark's biggest predator is man's appetite. Many sharks are endangered because they have been overfished. While they may be speedy feeders, they are slow to reproduce. It generally takes sharks about 10 to 20 years to mature. Large sharks eat only once every week or two, smaller sharks eat every day, according to Wilga, who notes that most sharks are picky eaters and don't hanker for humans.