What tiny tooth-like scales cover shark skin?

What tiny tooth-like scales cover shark skin? They are called dermal denticles.

Dermal denticles are tiny, tooth-like structures that cover the skin of sharks. They are also known as placoid scales, but “dermal denticles” is the precise term for the shark’s individual skin projections. Their structure resembles a tooth, with dentine and an enamel-like outer surface over a pulp-containing center.

These denticles do more than protect the shark. Their backward-pointing shape and microscopic riblets help control water flow over the body, reducing turbulence and drag. That streamlined surface can make swimming more efficient and quieter. Shark skin therefore feels smooth when rubbed from head to tail but rough in the opposite direction.

Dermal denticles are not the same as the ganoid, ctenoid, or cycloid scales found among various bony fishes. Sharks are cartilaginous fish, and their placoid covering reflects that different evolutionary anatomy. Similar denticles also occur in rays and other cartilaginous fishes, although coverage and form vary among species.