Spilling The Secret To A Mako Shark’s Speed

Two big eyes lock onto the animal it hopes to have in its jaws soon. There is an explosion of movement as the predator pursues their prey, with both performing acrobatic feats that would be graceful were it not for this being a life-or-death battle. With a well-timed CHOMP! the animal claims their prize and slows down to enjoy the meal.

Did you think this was a cheetah? Sure sounds like it. But there is another fast animal that most people forget about: shortfin mako sharks (Isurus oxyrinchus). Found worldwide in tropical to temperate waters, they are known as the peregrine falcons of the seas and seem to effortlessly swimming through the water at estimated speeds of at least 45 miles per hour (74 kilometers per hour). Shortfin mako sharks can reach lengths of up to 12 feet (3.8 m) and weigh at least 1200 pounds (545 kg), making them a large predatory shark. This species’ athleticism allows them to hunt fast-moving prey such as bony fishes (such as large tunas), squid, other sharks, small marine mammals, and sea turtles. Being at the top of the pelagic food web means they can give chase to animals both in and out of the water, leaping to extreme heights out of the water if need be.

But how do they pull off this feat? While the shortfin mako shark has a specialized blood vessel structure – known as the rete mirable – that allows them to maintain a warmer body temperature to the surrounding water and gives them an advantage when hunting the colder waters… this isn’t the ‘secret sauce’ to their speed. Aeronautical engineer Amy Lang of the University of Alabama and her team were interested in seeing just how these sharks pull off being so fast and began looking at their skin.

“Sharks, skates and rays (of the subclass Elasmobranchii) possess small, tooth-like scales known as denticles. Evidence from the fossil record indicates these ‘skin-teeth’ arose roughly 450 million years ago. Denticles have evolved to fulfill a diverse range of roles, meaning their shape varies both within and between different species.” Scientist Rory Cooper, from the University of Sheffield, told The Fins United Initiative. Also known as dermal denticles, they cover the bodies of these animals and are unusual in that they are similar to the teeth of other vertebrates! Like other teeth, they consist of a pulp cavity that is surrounded by layers of dentine and enamel-like tissue. The denticles have a variety of functions, including making the shark more hydrodynamic against the back-flow of water from their fast swimming. This reduces drag and helps make swimming more efficient, meaning they use less energy.

Keeping all of this in mind, the researchers wanted to look at the effect of these dermal denticles in specific locations on their body; these regions were specifically the flank (side) and the fins. If you have ever touched a shark, you may have realized that they feel a bit smoother if you run your hand from their head to their tail instead of the other way around. This is because the denticles are angled back from the front of the fish. Lang and her team discovered that some of those translucent 0.2-mm-long dermal denticle scales are actually capable of flexing up to an angle of 40 degrees out from the body in the opposite direction of flow. “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.” Lang told New Scientist. This phenomenon, known as flow separation, causes resistance and is controlled by a “passive bristling” made possible by the surface geometry on their scales. The most flexible of the dermal denticle scales are seen on the flank behind the gills and the trailing edges of their pectoral (side) fins.

“We set up an experiment in the tunnel with a measured amount of flow separation induced on a smooth surface. Then we replaced the smooth surface with shark skin and re-quantified the flow separation. In all cases with the flank skin, we saw the size of the separated flow region reduced significantly by the presence of the skin,” explained Lang to Earth.

But why was an aeronautical engineer interested in shark skin in the first place? Understanding dermal denticles could actually help improve human technology. Commented Cooper: “The drag reductive properties of shark denticles are attractive to industries aiming to improve the efficiency of locomotion. For example, shark skin inspired materials have been manufactured by Speedo for use in swimming competitions. Scientists and engineers are also examining whether denticle inspired design could improve the efficiency of airplanes and boats, by reducing fuel costs and increasing speed.” The researchers have already produced 3-D printed models of the flexible scales and have demonstrated that they work in both the air and water!

Shortfin mako sharks have a high quality meat and are important to many fisheries around the world, but are also taken as bycatch from tuna and swordfish longline fisheries globally. Currently they are listed as Vulnerable by the IUCN Red List… seems extinction may be something they can’t outswim.