Hunting strategy like no other; the thresher shark
The thresher shark (Alopias vulpinus) is found under the Lamniformes order of sharks, meaning that is related to species like the rarely seen megamouth shark, and the megalodon which has been extinct for some time. Usually Lamniformes are large and active predators, that are found in coastal and epipelagic habitats.
The diet of this species primarily consists of shoaling fish like mackerel, sardines and herring, but during the winter months, it has been found that the stomach contents of thresher sharks can often contain cephalopods such as squid, showing that these sharks are able to alternate diets in order to cope with a shortage of one type of food resource.
The thresher shark is usually identified by the abnormally long upper lobe of the caudal fin, which in some cases, can grow to be half of their body length and also make up a third of their body weight. This long and slender tail adaptation they have evolved gives them a ‘whip-like’ appearance (Figure 1).
In regards to eyes, the common thresher shark has a moderately sized eye, which is ideal for the type of habitat in which it feeds, as this species doesn’t hunt at large depths where light is unable to reach.
Unlike its close relative, the bigeye thresher (Alopias superciliosus) and like the name suggests, it has a very large vertically oval eye, suggesting that they are found much lower down in the water column where light penetration is weak. They also have a longer snout and much fewer teeth than the A. vulpinus.
As thresher sharks have a large range of distribution, the species as a whole has not been widely researched, and so, it is unknown when exactly they evolved and when this elongated caudal fin was first formed.
The knowledge, or lack thereof, of thresher shark distribution across the globe is few and far between. However, it is known for sure that thresher sharks are considered a highly migratory species.
Their overall habitat is found mainly off-shore, away from human disturbances, but they have been seen inland in search of food. Adult threshers are situated across continental shelves, whereas the juveniles tend to stick to coastal bays and shallower waters. Depth range for this species varies greatly, from 5m from the surface, down to 550m in the water column.
As previously mentioned, distribution of this species is extremely patchy and can be found in a large range of regions. They are most commonly seen around South Africa, Tanzania, Somalia, Maldives, Chagos Archipelago, Gulf of Aden , Pakistan, India, Sri Lanka, Sumatra, Japan , Republic of Korea, Australia, New Zealand, and New Caledonia (Figure 2).
Although this whip-like tail adaptation is very unusual, it does allow for a very unique method of hunting that has possibly never been seen before.
What thresher sharks lack in very tiny teeth, they make up for in their agility and long caudal fin. It has been recorded that thresher shark tails are able to move at 45-75mph and can throw it completely over their head (Figure 3). The primary use for such adaptation is for hunting down prey like sardines and herring, and allows a paralysing effect on the prey. As shown in figure 3, thresher sharks will dart towards shoaling fish, stop abruptly and then lift its tail over its head at speed, creating a ‘whip-like’ motion. However, not every hit is successful, it is said that roughly only a third of each tail slap is successful. But the advantage of this hunting method means that they are able to stun or even kill, up to 3 or 4 fish at one time and then circle back to feed again.
Figure 4: Thresher shark demonstrating the tail slap hunting behaviour in order to stun and capture prey efficiently.
As these sharks are able to consume more than one fish at any given time, it suggests that this tail-slapping is a very effective method of foraging, which has not been seen before, especially in shark species (Figure 4). From this, it is clear to see that thresher sharks really do rely on their tails in order to thrive here.
These creatures may be proficient hunters of the deep blue sea, but, like many other shark species, threshers in particular are under immense anthropogenic threats from fisheries. It was once thought that thresher sharks were only really active at night, in order to find their food. However, due to an increase in the amount of fisheries and their productivity, these sharks have became opportunistic feeders and even hunting shoals of fish during the daylight hours. This has had major disadvantages for the species as a whole. A study found that there was a sudden spike in the amount of threshers being culled due to being hooked on fishing gear when foraging for food. Something more interesting came to light from that study as it was also found that thresher sharks caught from long-lining were most frequently hooked by the tail, emphasising further the threat that sharks face in search for food and just how much they rely on that elongated caudal fin.