Two Kinds of Skin
Most sharks' skin is no thicker than the heel of your shoe, but the whale shark can have a skin that is almost as thick as a mattress! Its skin is so thick it can stop a harpoon. If the whales, dolphins, and seals had skins like this, it would make it a lot more difficult for the sharks to get their meals. The denticles in the shark's skin fall out just like its teeth do. Immediately, a new one pops up to fill the hole in its skin. Do you wish that you had this type of skin when you fall and scrape your knee?
How thick is your skin? Well, it can vary from the thickness of a paper to the rind of an orange. It never gets thicker, because the outer layer falls off. When you take your bath, you can rub off part of this outer layer with a sponge. Our skin also is made up of little building blocks that all fit together.
A Missing Piece
One common feature of most sharks is what appears to be a cut or notch in their tails. Some of these notches are rather large while others are fairly small.
Rough or Smooth Skin?
Now that you have heard that sharks are covered with teeth, you might have a picture in your mind of a shark with these three-inch-long teeth covering its body! The denticles really don't look like teeth from a distance, though. Even if you could get close enough to a shark to see its skin, you still might not know that the denticles were there unless you touched them.
A shark's skin is so sharp you could scrape your hand on it. People have also used shark's skin to smooth wood and metal instead of using sandpaper. But if you stroke the shark's skin in the other direction, it feels smooth.
Although sharks are famous for the single dorsal fin most people see sticking up out of the water, most sharks have a second dorsal fin farther down their backs, behind the first.
To get an idea of how this feels, try running your fingers through your hair, starting at your neck moving upward. This works best if you've just had your hair cut short. (If not, try this on someone else who has really short hair.)
Your hair, on the upward stroke, would feel like sandpaper. Now try patting your hair back down. Like a shark's skin, it should feel smooth as silk. In fact, there is one shark whose skin feels so smooth that they call it the silky shark, but not all sharkskin feels that smooth!
Some sharks, like the bramble and the prickly dogfish, are covered with spikes that look like spurs or thorns. Other sharks, like the Port Jackson shark, the lantern shark, and the pygmy shark, have enormous denticles right in front of their dorsal fins that look a lot like a big horn or spine.