Tales of the Thresher Shark

The family of Alopiidae comprise of three Thresher shark species: the Common Thresher (Alopias vulpinus), the Bigeye Thresher (Alopias supercilious), and the Pelagic Thresher (Alopias pelagic us).

As part of a study conducted off Pescador Island in the Philippines, a team of scientists from the School of Ocean Sciences of the Bangor University in Wales sent scuba divers down between the hours of 9am and 6pm to film the hunting strategies of the Pelagic Thresher (Alopias pelagicus). Thresher sharks are easily distinguishable by their long ribbon-like caudal fin, which can be the same length as the shark’s body, and it has been thought that they used their tail to hunt.

Image: Photographer Rafn Ingi Finnsson

Pelagic Thresher sharks reach up to a maximum of around 3.5m, and prey mostly on Indian sardines which are found surrounding Pescador Island. The Thresher shark's unique technique involves swimming at speed directly into a bait ball, where they use an overhead whipping technique, whereby both pectoral fins are drawn together, pushing the front of their body down. The back half is thrown upward and the tail is slapped into the bait ball. A diagram is below to help make this a little easier to picture.

Image: Journal Article: Thresher sharks use tail-slaps as a hunting strategy (cited below)

After careful analysis of the footage that was taken of the sharks during these activities, it was found that the tip of the tail moved so quickly that the frame rates of the cameras used were unable to keep up with the motion, giving only a blurred image of the tail tip. Bubbles were also seen surrounding the tail during this particular moment of movement, suggesting that the velocity was so high that it caused pressure changes in the surrounding water column resulting in gas particles diffusing and forming small air bubbles, which grew in size. This pressure change stuns the fish, giving the shark time to turn around and grab their prey.

The unusual hunting tactics of this species of shark, and the specific prey that it targets suggests that such a high energy demand during the overhead tail whip must require a high return in food. On averages, the studied sharks collected around 3 fish on average after each tail slap; the most successful catching 7 in one go. As stated in the paper, it appears that the Thresher shark’s pectoral and caudal fins were evolved for the purpose of hunting, and using the tail whipping technique.

For those who are wondering what this all looks like put together, here is some video footage from the original study provided by Youtube channel: Animal Wire.

Reference:

Oliver, S. P., Turner, J. R., Gann, K., Silvosa, M., & Jackson, U. (2013). Thresher sharks use tail-slaps as a hunting strategy. PLoS One, 8(7) doi:http://dx.doi.org/10.1371/journal.pone.0067380