Researchers have captured video footage of thresher sharks using their tails to swat smaller fish. This seems to be part of their hunting strategy. It might also explain why thresher sharks are often caught by their tails on long-lines.
Thresher shark tails can be up to half the length of the shark and until now there has been no documented evidence of why they had evolved to such a size. Recently a team from the Pfleger Institute of Environmental Research in California and the University of Massachusetts in Dartmouth towed 2 baited lines behind a boat. Over 27 separate days they filmed 33 common thresher sharks near the bait. 14 of these sharks attempted to hit the bait, with a success rate of 65%. This seems to confirm that the sharks use their tails to stun their prey, making it easier to catch.
So far this has only been observed in common thresher sharks, but it seems likely that the other two species of threshers use a similar strategy.
There is an article and some video from the BBC’s website.
I’ve been lucky enough to dive with them several times down at Malapascua with Trevor and Andreas of Thresher Shark Divers, from whose website I borrowed the photos above.