Why Sharks Aren't Covered in Scales

Dermal denticles (placoid scales) are tough "scales" that cover the skin of elasmobranchs (sharks and rays). Even though denticles are similar to scales, they are actually just modified teeth and are covered with hard enamel. These structures are packed tightly together and grow with their tips facing backward, giving the skin a rough feel if you run your finger from tail to head, and a smooth feel from head to tail.

What Dermal Denticles Do

The main function of these denticles is for protection against predators, kind of like a naturally occurring chainmail armor, although in some sharks they have a hydrodynamic function. The denticles reduce turbulence and drag which allows the shark to swim faster and covertly. Some swimsuit manufacturers are trying to replicate shark's denticles in swimsuit material in order to help swimmers cut through the water faster. 

Like our teeth, dermal denticles have an inner core of pulp (made up of connective tissues, blood vessels, and nerves), covered by a layer of dentine (hard calcareous material). This is covered with an enamel-like vitrodentine, which provides a hard outer casing.

While scales in bony fish grow as the fish gets large, dermal denticles stop growing after they reach a certain size. More denticles are added later as the fish grows.