The tail of a thresher shark (Alopias vulpinus). From Wikipedia.
Thanks to sensational documentaries and summer blockbusters, we are all familiar with the anatomy of a shark attack. The victim, unaware that they are in peril, is struck from below and behind with such speed and violence that, if they are not actually killed during the first strike, they soon find themselves a few pounds lighter in the middle of a billowing red cloud. The trouble is that this stereotyped scenario does not hold for all sharks, particularly one peculiar group of deepwater sharks which has long puzzled naturalists.
Thresher sharks, represented by three species within the genus Alopias which ran between 10 and 20 feet long, possess the classic shark body plan, but there is one feature that immediately stands out when you look at them – their ludicrously elongated tails. What could such an appendage be used for? Does it help them in propulsion, or do they use their tails as a whip to stun fish before consuming them? This latter hypothesis has been popular for years, especially since threshers are often hauled up on deep-water longlines by their tails (rather than by their mouths, like other sharks), but the only confirmation that this was correct came from anecdotal accounts. Thanks to some underwater cameras, however, scientists S.A. Aalbers, D. Bernal, and C.A. Sepulveda were able to test what has long been assumed about thresher feeding behavior, and their results have just been published in the Journal of Fish Biology.
Observational setup for the study. From Aalbers et al, 2010.
To observe how threshers hunt, the team of scientists rigged up an underwater video system which would be trained on a set of baited, but hookless, lures trolled behind the boat Malolo as it cruised the waters between Newport Beach and Point La Jolla off southern California. This was known thresher shark habitat, and, sure enough, 33 individual sharks (of the species Alopias vulpinus) showed up during 27 sampling runs between March 2007 and June 2009. Not all the sharks went after the lures or attacked them in the same way, but from that sample 14 sharks struck the bait with their tails 29 times, giving the scientists a clear look at the unique way in which threshers feed.
Sequence of video stills showing a tail strike. (For a video, see this article by the BBC.) From Aalbers et al, 2010.
When the team took the video back to the lab and analyzed it, they found two different patterns among the threshers which used the tail strike. One method involved the sharks rapidly undulated the front part of their body from side-to-side, and this created a wave down the rest of their body to the tip of their tail, flicking the tip of it towards the prey. This resulted in successful tail-strikes almost half the time. The second method, by contrast, involved the shark swimming up alongside its prey and whipping its tail sideways to strike with the distal end of the tail, and this technique was successful about 92% of the time. What had been presumed for so long turned out to be true – these thresher sharks attempted to shock or immobilize their prey with their tails before consuming it.
Aalbers, S., Bernal, D., & Sepulveda, C. (2010). The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus Journal of Fish Biology, 76 (7), 1863-1868 DOI: 10.1111/j.1095-8649.2010.02616.x