Energy efficient hammerhead sharks swim sideways to cut down on drag

A mallet-shaped noggin isn't the only thing about hammerhead sharks that's a little different

They've had hundreds of millions of years to practise, so you'd be right to think that sharks know a thing or two about navigating the ocean depths. But a clever manoeuvre used by one particular species has surprised marine biologists, who discovered that hammerhead sharks spend much of their time swimming sideways to conserve energy.

The dorsal and wing-like pectorial fins play a part in helping sharks turn, control vertical position and move steadily through the water. But a mallet-shaped noggin isn't the only thing about the great hammerhead shark that's different. This particular type of hammerhead also features pointy dorsal fins that are longer than their pectorial fins, the only shark known to possess this anatomical trait. And as it turns out, they use it to their benefit in quite a crafty way.

An international team of scientists tagged wild hammerhead sharks with accelerometers and attached video cameras to their dorsal fins to observe how they move through the water. They found that the sharks give their dorsal fins a new purpose by rolling onto their side to boost the area of their lifting surface.

Using the dorsal fin, rather than the pectorial fin, for this activity seems to streamline swimming for the sharks, with hydrodynamic modeling showing that it reduces drag by around 10 percent. The researchers say that the tagged sharks spent as much as 90 percent of their time swimming at angles ranging from 50 to 75 degrees.

There has been sightings of hammerheads swimming sideways in public aquariums and the wild (you can even see it in action towards of the end of this GoPro video from last year), but what inspires this peculiar way of swimming has been unclear with "no precedent in literature," according to the researchers. They say the findings could challenge our understanding of how sharks use their fins to glide through the water.

"These findings question the paradigm of the division of labour in shark fins, and highlight that efficient travel is a strong selective agent in driving the evolution of animals," they write in their research paper.

Source: Nature

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