Shortfin mako sharks can shoot through the ocean at up to 50 miles per hour (80 kilometres an hour). Now a trick that helps them to reach such speeds has been discovered – the sharks can raise their scales to create tiny wells across the surface of their skin, reducing drag like the dimples on a golf ball.
The minute scales – just 200 micrometers long – are made from tough enamel, such as that found on teeth, giving the skin a rough texture like sandpaper. Lying flat, they had previously been found to reduce drag as the shark swims.
Some reports had also suggested that sharks can bristle their scales, causing them to stand up on end, so Amy Lang from the University of Alabama in Tuscaloosa and colleagues decided to investigate whether this too could help sharks travel at high speeds.
The team created artificial shark skin with a 16 x 24 array of synthetic scales, each 2 centimetres in length and angled at 90° to the surface of the "skin".
They then placed the arrangement in a stream of water travelling at a steady 20 centimetres per second. The water contained silver-coated nanospheres, which a laser illuminated to reveal the nature of the flow around the scales.
The experiments revealed that tiny vortices or whirlpools formed within the cavities between the scales. These vortices form a kind of "buffer layer" between the skin's surface and the fast moving fluid, preventing a turbulent wake from forming behind the shark.
Since a wake has a lower pressure than the rest of the fluid, it exerts a backwards pull on an object, decreasing its speed and making it harder to change direction.
Eliminating this wake decreases the overall drag on the shark, allowing it to travel faster and move more easily without the thick, syrupy feeling humans get as they try to move through water.
"It's like the difference between pushing a box over ball bearings instead of dragging it along the floor," says Lang. The same principle explains the dimples on golf balls, which also create mini vortices to reduce drag in this way, she says.
Ultimately, the team hope further investigations could be used to design torpedoes, underwater vehicles, and even aircraft inspired by shark skin that can move more quickly through water and change direction more easily.
Sergei Chernyshenko, an aeronautical engineer from Imperial College London, UK, describes the research as fascinating. However, he points out that while the team have shown the existence of vortices, they haven't yet quantified the extent of the effect on the shark's drag, which he thinks could be minimal.