Shark’s skin adds forward boost

A covering of sharp scales helps propel the creatures

A shark’s birthday suit not only fits perfectly, but is very fashion forward. Tiny teethlike ridges covering sharks’ skin appear to give the fish a push forward in water, a new study finds.

SHARK PROPELLERS The denticles of the bonnethead shark (Sphyrna tiburo), shown here, are about 100 microns in diameter, the thickness of a piece of printer paper. The teethlike structures give sharks extra thrust in the water, a new study shows. J. Oeffner and G.V. Lauder/Harvard Univ.

These ridges, or denticles, increase swimming speed and appeared to improve thrust, Harvard University researchers report in the March 1 Journal of Experimental Biology. Meanwhile, similar work with samples of Speedo fabric, designed to look like sharkskin, did not show a consistent improvement in swimming performance.

Much like how a golf ball is able to smoothly soar through the air because the dimpled surface distorts the surrounding airflow, the tiny teethlike structures found on some types of sharkskin are widely believed to reduce resistance in water. Until now, scientists weren’t aware of the added benefits these ridges provided.

“We show an additional function of the denticles,” says study coauthor George Lauder. “For future underwater robotic devices, there’s a real opportunity to gain an increase in performance.”

Sharkskin is well adapted for moving through water at high speeds, so engineering a similar coating could be useful in making wind and water turbines run more efficiently. To do that, scientists need to understand exactly how denticles work.

To tackle the problem, Lauder hooked up specially altered pieces of sharkskin, or skin foils, to a device that generates currents and works like an underwater treadmill. These skin foils, which were made by gluing two pieces of sharkskin together so that the denticles faced outward, mimic how a fish moves in water.

Until now, scientists “haven’t run tests on something that’s shown movement,” says Frank Fish, a biomechanics researcher at West Chester University in Pennsylvania.

The Harvard scientists found that flexible skin foils with intact denticles were able to move 12 percent faster compared with sharkskin that had been sanded smooth.

What’s more, by putting radioactive beads in the water and studying the sharkskin under a laser, Lauder and study coauthor Johannes Oeffner saw that these denticles didn’t just reduce resistance. They also appeared to distort the surrounding water to make spiral patterns that thrust the piece of skin forward.

“Sharkskin denticles seem to provide a clever boost,” says Lauder.

Also, in similar experiments using swatches of a ridged Speedo swimsuit, the researchers did not find any consistent improvements in swimming speed provided by the surface indentations. But Lauder says that the suit’s other features, like maintaining posture and keeping the swimmer’s body sleek, probably improve performance in the pool.

More Stories from Science News on Life