You might assume that sharks are all quite similar - that iconic dorsal fin on top, 2 pectoral fins on the sides, a slender body, with teeth at the front - but in fact there is enormous variation in body forms even just within the sharks. Some sharks look more like rays (the angel sharks) some have armoured swords projecting from their snouts (the sawsharks) and some look more like a carpet than a shark (seriously, google 'wobbegongs' and you'll understand what I'm talking about). One group, known as the 'thresher sharks' (Family Alopiidae) are undeniably unusual... their caudal fins (aka tails) are long, slender and whip like; they can even be as long as the rest of the shark's body! So why have thresher sharks evolved this way? And what is that tail for?
A Family Affair
It is thought that thresher sharks evolved some 55 million years ago. They are part of the order of Lamniform sharks, so are relatively closely related to mako sharks (Isurus species) and the great white (Carcharodon carcharias) (Compagno, 1984; Naylor et al, 2012).
There are currently 3 living species of thresher sharks:
pelagic thresher shark (Alopias pelagicus),
common thresher (A. vulpinus) &
bigeye thresher (A. supercilious).
Whilst they all look quote similar, the different species can be told apart based on the relative length of their tail and the size of their eyes. The tail of the common thresher shark is comparatively longer than the tails of its cousins and (unsurprisingly) bigeye threshers have comparably large eyes. Bigeye threshers are also relatively brutish in shape and have pronounced ridges running down their faces. It can be difficult to tell pelagic and common threshers apart, but if you look very closely, the dorsal fin of the pelagic thresher forms an almost perfect triangle between the pectoral and pelvic fins, where the dorsal of the common thresher is closer to the front of the shark (Compagno, 1984; Naylor et al, 2012).
Survival of the Flickiest
Evolution does not act randomly, but only causes new features to arise if they confer some benefit to the animal... Mutations in the genetic code, leading to changes in morphology, can either be advantageous (meaning the animal lives long enough to breed and pass on its genes) or detrimental (meaning it dies or does not reproduce). Subsequently a beneficial trait becomes more prevelent in the population and subtle changes can become enhanced, until we see extreme features arise: like the remarkably long neck of the giraffe, the ostentatious feathers of birds of paradise, or the awesome tails of the thresher sharks.
It is thought that one reason the thresher sharks' tails have evolved to become so long is that they are handy when hunting; thresher sharks use their tails like a whip to stun their prey (Aalbers et al, 2010; Oliver et al, 2013).
Slap Dash
Whipping their long tail through the water generates a considerable shockwave, which can startle and immobile fish, allowing the thresher shark to snack on them without having to give chase. Not only is this energy saving, but it is thought that this hunting method is especially efficient because it allows the shark to stun multiple prey items at once, which massively increases their hunting success (Aalbers et al, 2010; Oliver et al, 2013).
For example, the pelagic thresher shark has been shown to perform both overhead and sideways "tail slaps", to stun multiple sardines at a time. The whipping motion can be so fast that enormous, localised changes in water pressure actually cause bubbles of air to form! Whatsmore, these slaps can be extremely acrobatic, with the shark bending its body almost 90° to throw its tail over its head! Quite a sight! That is, if you are able to see the move without the aid of slow motion cinematography- the tail slaps of common thresher sharks can be so fast, that they only last a total of 33 milliseconds! That is just 0.033 seconds (Aalbers et al, 2010; Oliver et al, 2013).
Shark Bait
Thresher sharks predominately hunt small fish like sardines and mackerel, which form dense schools called "bait balls". These fish behave in this way because it offers them some protection from predators; swimming closely alongside many others makes it more difficult for a hunter to single out and catch an individual (this is known as the "dilution effect"). In response, predators must adapt novel features or behaviours to allow them to win a meal (Aalbers et al, 2010; Oliver et al, 2013).
The thresher sharks have evolved their long tails and whipping technique in order to target bait balls for their next meal, but this did not happen overnight. It was a very long and slow process... At some point, a thresher ancestor will have had a random mutation causing the tail to be longer, which lead to some hunting advantage. This individual will have lived a successful life, producing many offspring, with the same genes. The next generation may have had slightly longer tails, and the following generation, even longer still... until, eventually (millions of years later), evolution gave rise to the wonderful tail we know today. In my opinion, it was well worth the wait!
References
Aalbers SA, Bernal D & Sepulveda CA (2010). The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus. Journal of Fish Biology, 76, 1863–1868. Access online.
Compagno LJV (1984). Sharks of the World: An Annotated and illustrated Catalogue of Shark Species Known to Date. FAO Species Catalogue: Volume 4 Part 1 - Hexanchiformes to Lamniformes. Access online.
Naylor GJP, Caira JN, Jensen K, Rosana KAM, Straube N & Lakner C (2012). Elasmobranch phylogeny: a mitochondrial estimate based on 595 species, p. 31–57. In: The Biology of Sharks and Their Relatives. J. C. Carrier, J. A. Musick, and M. R. Heithaus (Eds.). CRC Press, Boca Raton, Florida.
Oliver SP, Turner JR, Gann K, Silvosa M & Jackson TDU (2013). Thresher sharks use tail-slaps as a hunting strategy. PLoS One, 8:7, e67380. Access online.
By Sophie A. Maycock for SharkSpeak
