Thresher sharks stun prey with tail slaps
Thresher sharks use their impressive tails to stun lots of prey at once with sharp overhead "tail slaps", scientists have confirmed.
The sharks' scythe-like tails make up half the length of their body, so it had long been suspected that the animals used them as hunting weapons.
Footage of the predators using their tail to stun and kill sardines has finally confirmed this.
The findings are reported in the journal Plos One.
Dr Simon Oliver, founder of the Thresher Shark Research and Conservation Project and a researcher based at University of Liverpool, filmed pelagic thresher sharks as they hunted off Pescador Island in the Philippines.
The vast sardine shoals there are feeding hot spots for these sharks, and the researchers encountered and filmed more than 60 occasions where the sharks "bullwhipped" their tails in an effort to kill the fish.
"It was a massively exciting moment for me to come across his behaviour," said Dr Oliver.
Back in 2010, another team managed to film the common thresher shark swiping at and making contact with tethered bait using their tails.
But Dr Oliver said that these new observations provided the first clear evidence that "thresher sharks really do hunt with their tails".
'Medieval war machine'
Dr Oliver said that the tail slap happened so quickly that it was often difficult to track on camera.
"It's a cross between a bull whip and a medieval war machine - it's extremely violent, extremely quick and it's incredible to observe," he said.
As part of their study, the researchers analysed their footage, breaking down each attack into stages in order to record the animals' exact movements.
They noted that the sharks accelerated through the water and then "stalled themselves" by pulling their pectoral fins together. They then thrashed out by dipping their nose and swiftly bringing their tail upwards and over their heads.
"The interesting thing about it was that these tail slaps were only successful about 60% of the time," said Dr Oliver, "but when they were successful they managed to kill more than one prey item.
"So it seems the strategy is efficient in that the shark is able to consume more than one fish at a time to balance out the times when it wasn't successful."
Dr Oliver also speculated that the speed and movement of the strikes might create shock waves in the water.
"We saw bubbles coming out of the water at the apex of the tail whip; an explanation for that is that it's caused by pressure differentials [that create] an explosion in the water [and] break down water molecules."
Prof David Sims, deputy director of research at the UK's Marine Biological Association in Plymouth, said that the discovery was very interesting.
He told BBC News that it was an extension of the discovery made two years ago in common thresher sharks, but that while that shark made contact with its prey using a "sideways slap", "this species [the pelagic thresher shark] does an impressive overhead kick with its tail fin".
"This team has really nailed it in terms of showing not only that the sharks do this, but that it leads to prey capture."
Prof Sims added that studying shark behaviour was crucial for conservation.
"Some 70 million sharks are extracted from the ocean every year," he said.
Although sharks are actively fished, particularly for their fins, many are caught accidentally by fishing vessels looking for other species.
"Understanding their behaviour is vital in order to find out where the feeding hot spots are, so that we can do something about protecting those areas."