Sharks have tiny, coarse scales called placoid scales, which reduce drag from the surrounding water. Placoid scales are also known as dermal denticles and structurally resemble teeth. The texture of a shark is similar to sandpaper when rubbing from tail to head and smooth in the opposite direction.
Sharks are cartilaginous fish closely related to skates and rays. Over 470 species of shark are known to exist. Sharks live in all oceans worldwide. Some sharks, such as the bull shark, are able to survive in fresh water. Due to their lack of swim bladders, sharks depend on their enlarged livers and their cartilaginous bodies to stay buoyant. Most species must keep swimming in order to breathe.