The shark appears out of nowhere, charging at a school of fish many times its size.
It gets up close to the towering column, right in their fishy faces, pauses so briefly you barely notice, then whips its body backwards like a gymnast on balance bars and slaps back the water with the length of its tail.
Dinner.
The tail of the thresher shark is just as long as the animal itself. Shark biologists have caught some of the first footage of pelagic thresher sharks out on their dinner run, and recorded how they use an extra-long tail as a weapon to stun and kill fish.
"It's a cross between a bullwhip and an ancient war machine," Simon Oliver, founder of the Thresher Shark Research and Conservation Project, and a researcher at the University of Liverpool, told NBC News. "It's very violent, very dramatic."
Besides looking very shark-like, it doesn't pose any real threat to human swimmers and divers. But little fish, like schooling sardines that are accosted in the video above, have reason to worry. The thresher can use its tail to generate a shockwave that can peel off the outer swimmers of a large school, slicing through water as fast as 47 miles per hour.
Oliver and a team of shark biologists filmed feeding pelagic thresher sharks on 25 occasions in their native waters off the coast of the Philippines, during fall months in 2010, and have come to understand how they used their tails to hunt. They describe their findings in the July 10 edition of PLOS ONE.
They surmise that the sharks evolved long tails to help them hunt using this fine-tuned strategy, Oliver said. It is a more efficient way of hunting because more than just one fish is killed at a time. Threshers can also slap sideways, to put an end to tenacious fish that didn't die the first time. But those motions aren't as powerful as the vertical clap.
Once, the researchers even surveyed the casualties. "We saw broken backs, we saw imploded or exploded swim bladders, we saw flesh missing — all sorts of nasty damage, particularly in the gut cavity," Oliver said.
The footage was captured by divers swimming alongside the sharks. A human from far away can't feel much, Oliver recalls, but they can hear it. "There's usually a cracking sound or an electric-shock sound associated with the slap," he said. "You could tell something huge was going on."
The slapping motion, unsurprisingly, engages muscles in the back and all the vertebrae, the researchers write. Close analysis of the footage indicated that the shark uses its fins as brakes after the swimming up very close to the school, then uses its tail, the "caudal fin," as a whip.
Orcas — killer whales — also slap the water with their fins to stun their prey, but the way that works is quite different, Oliver explains, mostly because the orcas have wide flukes while the threshers work with their skinny upright tails.
Simon and his team are moving on to study the migratory patterns of the threshers, but said they are resting easy because "the tail slap mystery has now been solved."
In addition to Simon Oliver, the authors of "Thresher Sharks Use Tail-Slaps as a Hunting Strategy" include John Turner, Klemens Gann, Medel Silvosa and Tim D’Urban Jackson.
Nidhi Subbaraman writes about science and technology. Follow her on Facebook,Twitter and Google+.