Shark skin is made of a matrix of tiny, hard, tooth-like structures
called dermal denticles or placoid scales. These structures
are shaped like curved, grooved teeth and make the skin a
very tough armor with a texture like sandpaper. They have
the same structure as a tooth with an outer layer of enamel,
dentine and a central pulp cavity. Unlike the scales of scales
of bony fish (ctenoid scales) that get larger as the fish
grows, placoid scales stay the same size. As the shark grows,
it just grows more placoid scales.
These scales also help the shark swim more quickly because
their streamlined shapes helps decrease the friction of the
water flowing along the shark's body, by channeling it through
grooves. Also, the shark's skin is so rough that contact with
it can injure prey. All of the spines of the denticles point
backwards (towards the tail), so it would feel relatively
smooth it you moved your hand from head to tail (but rough
the other way).
Sharks are mostly drably countershaded. This means that the
top and bottom sides are colored differently serving to camouflage
the shark from multiple perspectives. The top (the dorsal
side) is considerably darker than the belly (the ventral side).
When the shark is viewed from above, its dark top surface
blends into the dark ocean depths or ocean floor. When viewed
from below, the light-colored belly blends in with the light
above. This helps the shark hunt in a stealthy manner, enabling
it to sneak up on prey undetected.
Bottom-dwelling sharks (like the angelshark) are camouflaged
to blend into the sand, mud, and rocks of the ocean bed.
Whale shark Sharks have very thick skin. Whale sharks have
especially thick skin, up to 4 inches (10 cm) thick.