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.
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.)
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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