Shape-shifter in action Richard Robinson/Getty

Millions of tiny “loose teeth” covering the mako shark’s skin could be the secret to its incredible speed. Mako sharks are known as the cheetahs of the ocean, rocketing through the water at speeds of up to 68 kilometres per hour. New research shows that patches of flexible, scale-like denticles on the shark’s skin allow it to glide more efficiently through the water.

Stroke a shark from nose to tail, and its skin feels smooth. Rub it up the wrong way, however, and a shark feels sandpaper rough. That is due to being covered in millions of the tiny, protruding denticles.

“The mako has translucent denticles about 0.2 millimetres in size,” says Amy Lang at the University of Alabama. “It turns out that the mako has very flexible denticles. These sit like little loose teeth. If water flow begins to reverse, the scales pop up.”


Less of a drag

The streamlining effect of denticles has already been copied for applications such as Speedo’s famous “shark skin” LZR swimsuit. Lang suspected that the highly flexible denticles were key to reducing another kind of drag: flow separation. After passing the widest part of a shark’s body – typically its gills – the flow of water slows, which leads to a pressure drop and can result in eddies and vortices. The same phenomenon explains why whirlpools appear at the edges of paddling oars.

By studying the flow of water over shark skin in the lab, Lang saw that the loose denticles prevent this flow separation happening by bristling up in the swirling water. “It’s entirely passive, and happens in about 0.2 milliseconds,” she says. The most flexible scales are seen in areas that experience the most flow separation: the flank behind the gills, and the trailing edges of a shark’s pectoral fins.

The shape-shifting skin could have applications in aeronautics. For instance, flow separation on top of helicopter rotor blades makes them less efficient. These blades and the wings of fighter jets could benefit from shark-inspired microstructures.

Lang and her team have already 3D printed models of the flexible mako denticles and shown they work as well in air as they do in water. “We can probably manufacture these on the order of shark skin, 0.2 millimetres in size,” says Lang. “We’re definitely moving forward to being able to replicate it.”

Lang will present the findings this week at the American Physical Society March Meeting in Boston.