Some sharks bristle with more than just primal aggression when hunting their prey at high speed. The Shortfin Mako shark uses flexible scales on its body that allow it to pull off tight underwater turns during high-speed pursuits, according to researchers.
The teeth-like scales help control flow separation — a phenomenon where turbulence and differences in pressure can cause drag that slows down a fast-moving body such as an airplane, or a hungry shark. Controlling the flow separation allows Mako sharks to move in for the kill at speeds of possibly up to 60 mph (97 kph).
"The Mako has evolved to be the cheetah of the ocean," said Amy Lang, an aerospace engineer at the University of Alabama who specializes in experimental fluid dynamics. "It has evolved to chase down tuna."
A Mako shark's side-to-side swimming motion requires flexible scales on its sides and behind the gills – especially when the shark wants to pull a quick change of direction without drag. Lang and her colleagues found that the scales can bristle at angles of 60 degrees or more.
The scales act similarly to how dimples on a golf ball control flow separation and allow the ball to travel farther in the air – though there are differences.
"Unlike a golf ball, which has no preferred separation of flow, a shark is always swimming in one direction," Lang told LiveScience.
Flow separation forms as a shark moves through the water and patches of fluid close to the body begin moving upstream against the main flow of water. But those patches end up trapped in the bristling scales, which prevents them from creating more general flow separation across the shark's body.
Such findings surprised biologists at the University of South Florida and the Mote Marine Laboratory in Florida, who worked with Lang. They had known about the shark scales, but had never considered how flexible the scales could be.
The team used pressure to manipulate the skin and scales of dead Mako sharks. That's because live Mako specimens, just like Great White sharks, tend to have a short lifespan in captivity.
Additional research could allow engineers to take a cue from some of the ocean's fastest predators and design better airplane wings, helicopter blades, or wind turbines.
But more specimens are needed beyond just Mako sharks. Lang and her colleagues hope to examine the scales of larger, slower sharks that may have a longer lineage – sharks have spent over 400 million years evolving. By contrast, the Mako is a youngster that evolved about 8 million years ago.
The research was presented on Tuesday (Nov. 23) at the American Physical Society in Long Beach, Calif.
