It’s the skin of a Wobbegong shark. Fish are covered with scales, but shark skin (and ray skin) is covered with dermal denticles. Dermal denticles are like small teeth: they have a soft centre supplied with blood and nerves, and are coated in a hard, enamel-like material. The denticles don’t grow in size as the animal does (unlike fish scales), instead more denticles are added.
The denticles face backward from the head; this improves hydrodynamics by creating vortices as the shark moves forward through the water. It’s what creates that rough feel when you rub a shark up the wrong way: the animal is smoother to pat if you go front-to-back, which conveniently has your hand moving away from the mouth… be a bit wary, though, as the Wobbegong is reportedly the only shark that can reach around and bite its own tail 😉
—Josh & Liz