A Slap Upside the Head
Thresher sharks are among the most impressive of elasmobranchs, their scythe-like tails nearly equaling their bodies in length. Many hypotheses have been proposed for the function of these elongated tails. Do they aid in swimming? Attract potential mates? Serve as a warning to predators? Perhaps they are hunting tools, used to stun prey much as the sawfish uses his saw to slice and dice his next meal. This last hypothesis receives support from a new study published by Oliver et al in the journal PLoS One, which provides the first detailed analysis of thresher sharks using their tails as hunting tools.
The authors observed pelagic thresher sharks (Alopias pelagicus) hunting near Pescador Island in the Philippines, where the animals gather to feed on schools of sardines. In many recorded hunting attempts, the sharks were seen to wield their tails as a weapon, slapping and stunning sardines that were then quickly eaten. The sharks swam forward into a bait ball, then brought their pectoral fins together beneath their bodies, this motion raising their tails in the water. The tail was then snapped rapidly down towards the head, forming a sharp arc in the water and striking any fish in its path.
The researchers were able to measure the speed of the average tail slap to 14 meters/second, more than 30 mph, with the fastest slaps measuring nearly 50 mph! Slap speed was correlated with the overall size of the shark, and therefore the length of the tail, with larger sharks having faster tail speeds. The tail snap was followed by a quick turn in the water, so the shark could eat the fish it had stunned. Roughly one third of all tail slaps were successful in stunning prey, with several fish often incapacitated by a single slap. The thresher sharks would sometimes also employ a sideways tail slap, depending on their position relative to the bait ball.
Marine mammals such as dolphins and orcas are known to use their tails to corral and stun schools of fish. The paper by Oliver et al is the most detailed observation to date of sharks employing this same hunting strategy.
The paper is: Oliver SP, Turner JR, Gann K, Silvosa M, D'Urban Jackson T (2013) Thresher Sharks Use Tail-Slaps as a Hunting Strategy. PLoS ONE 8(7): e67380.
It is available open access at PLoS One, and includes several terrific videos of thresher sharks employing the tail snap hunting strategy:
(Photo from http://www.arkive.org)