National Geographic

Thresher Sharks Hunt with Huge Weaponised Tails

For most sharks, the front end is the dangerous bit. Thresher sharks are the exception. They’re deadly at both ends, because they’ve managed to weaponise their tails.

The top halves of their scythe-like tail fins are so huge that they can be as long as the rest of the shark. For around a century, people have been saying that the threshers lash out at their prey with these distended fins—hence the name. But no one had ever seen them do so in the wild.

In 2010, one team showed that they can lash out at tethered bait under controlled conditions. But Simon Oliver has done better. His team spent the summer of 2010 in the Philippines, watching and filming wild pelagic thresher sharks—the smallest of the three species—hunting large shoals of sardines. The videos are spectacular and unambiguous: threshers really do hunt with their tails.

“It was absolutely extraordinary,” says Oliver, who is founder of the Thresher Shark Research and Conservation Project and based at the University of Liverpool. “We always expected this but there’s never been any solid documented evidence. This is the first time the behaviour has been observed in the sharks’ natural environment, and we observed a lot of it.”

When I first read about thresher sharks as a kid, I imaged that they would swim towards its prey, bank sharply, and lash out sideways with their tails. Oliver’s team showed that the sharks do use sideways slaps, but rarely.

Instead, here’s what usually happens. The thresher accelerates towards a ball of fish and brakes sharply by twisting its large pectoral fins. It lowers its snout, pitches its whole body forward, and flexes the base of its tail. This slings the tail tip over its head like a trebuchet, with an average speed of 30 miles per hour. (The fastest shark managed to whip its tail at an astonishing top speed of 80 miles per hour.)

“It’s fast, aggressive and violent,” says Oliver. When the tail hits sardines, the results aren’t pretty. “We saw everything from swim bladder ruptures to broken spines to parts afloat.” The sharks then swim round and swallow the pieces at their leisure.

Best scientific figure ever? From Oliver et al, 2013. PLOS

Best scientific figure ever? From Oliver et al, 2013. PLOS

The threshers are only successful on a third of their strikes but during these victories, they always kill several sardines at once. That’s far more efficient than chasing after agile individuals in a confusing shoal, and it suggests that the sharks aren’t just relying on direct hits.

During three of the hunts that Oliver filmed, he saw plumes of bubbles at the tip of the shark’s tail. That’s probably because it moves so quickly that it lowers the pressure in front of it, causing the water to boil. Small bubbles are released, and collapse again when the water pressure equalises. This process is called cavitation, and it releases huge amounts of energy. Another sea creature—the mantis shrimp—uses cavitation to attack its prey, and Oliver suspects that thresher sharks may do the same. “I think the shark’s causing a shockwave that’s strong enough to debilitate small prey,” he says. (However, he cautions that he’d need to use some physical models to prove that this is actually happening.)

Stills from a thresher shark attack video. From Oliver et al, 2013. PLOS

Stills from a thresher shark attack video. From Oliver et al, 2013. PLOS

“It’s extraordinarily rare in the animal kingdom to see animals hunt with their tails,” says Oliver. Killer whales and other dolphins sometimes do so, but the strategy is unique among sharks.

Oliver suspects that no one has witnessed this behaviour before because thresher sharks hunt in the open ocean, and usually at night. “The ocean’s a big place and studying sharks is very difficult,” he says. “You need a lot of luck. We got very lucky.” One of his team heard about a large shoal of sardines that were staying off Pescador Island in the Philippines, and the team set up a research station there. The sardines stayed around for several months, and the threshers stayed with them.

Since then, the shoals have dispersed and the sharks have also disappeared. Oliver hopes they’ll come back, but he’s also worried. “These habitats where prey can aggregate are fewer and further between,” he says. “These sharks normally hunt at night and all of our observations were during the day. It’s counter-intuitive to their normal strategy.” It’s a reminder that these astonishing animals—all three of which are classified as vulnerable—need support and protection.

Reference: Oliver, Turner, Gann, Silvosa & Jackson. 2013. Thresher Sharks Use Tail-Slaps as a Hunting Strategy. PLOS ONE. http://dx.doi.org/10.1371/journal.pone.0067380

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There are 8 Comments. Add Yours.

  1. Sharla Hardy
    July 10, 2013

    I would think that cavitation would occur on the trailing edge of the tail, rather than on the leading edge. (“That’s probably because it moves so quickly that it lowers the pressure in front of it . . .”) The moving tail would compress the water in front of it, increasing the pressure.

  2. Mark Kawakami
    July 10, 2013

    The way they use their tails reminds me of a bullwhip cracking, and if they are in fact causing cavitation then it’s actually quite similar. Threshers are turning out to be the Indiana Joneses of the ocean.

    [On Twitter, Mike Sowden offered "trebuschark", which I endorse - Ed]

  3. Michael Habib
    July 10, 2013

    Sharla Hardy: Good thought; something similar occurred to me, as well. However, there are ways that the leading edge stream can undergo cavitation. For example, lift-producing surfaces can create a low pressure zone at their leading edge, because of the vorticity around the foil (essentially it peels away from the front edge, which creates a leading edge suction). No information yet on if the thresher tail is moving with the proper kinematics to use this effect, but that might be one way to get cavitation on the leading edge. Mantis shrimp also produce cavitation bubbles on the leading surfaces of their punching organs. It is, admittedly, counter-intuitive.

  4. Richard Dashnau
    July 11, 2013

    The Snapping Shrimp uses cavitation induced in one large claw as a sort of “cannon” to stun prey. The bottom part of the claw closes quickly enough to cause a cavitation bubble in the water. This collapses and releases energy. Some video even shows a small “spark” from the energy release. http://news.nationalgeographic.com/news/2001/10/1003_SnappingShrimp.html

  5. Nathan Myers
    July 11, 2013

    “Counterintuitive” is so thoroughly exemplified by fluid dynamics that we really should be saying “fluid-dynamical” to characterize that which is incomprehensibly correct. Lots of things are counter-intuitive without the degree of induced bewilderment that shows up routinely where fluids move.

    Even for somebody used to fluid-dynamical bewilderment, plasma fluid dynamics are Lovecraft-level weird, not to say mathematically intractable. It doesn’t take extra spatial dimensions to melt your eyeballs from their sockets.

  6. Brutus
    July 19, 2013

    “The videos are spectacular and unambiguous”. This is a very misleading statement from the article. At best, the videos are grainy.

  7. kirren
    July 19, 2013

    Brilliant, informative and entertaining articles.

  8. Fiona Thompson
    August 8, 2013

    As usual I loved your article Ed! We are great fans in our household, and your articles make science very accessible to the sproglets.

    Perhaps there is more than one mode of action on the fish than suggested in the article. Thresher sharks are found in New Zealand waters and presumably would prey on New Zealand pilchards, Sardinops sagax. The New Zealand pilchard reacts to shocks like being slammed into a rock by going into a spasm that can be so severe the fish dies, perhaps it could rupture swim bladders and spines. If the New Zealand pilchard does this maybe other small shoal fish have the same reflex and this could be what the thresher shark exploits.

    The researchers filming in murky waters can’t see the exact agent of destruction of the fish, but sees the attack followed by the feeding. That means the cavitation effect of the sharks tail could have a direct effect on nearby fish but a shock effect on fish further away, a double action of killing!

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