AFTER AN UNCIVILISED early start, Malcolm found himself in the dawn twilight at 30m, atop an uninspiring rubble reef shoal. But the hour, the surroundings and the early light didn’t temper an excitement that had been building for years;
“I’ll never forget my first sighting of a thresher shark,” he says. “I was both thrilled and stunned. With its disproportionately large eyes and huge, slender scythe-like tail, half the length of its body, its appearance is unlike that of any other shark. I’ve been privileged to see and photograph many sharks, but this species is particularly special.”
The three species of threshers (although recent DNA studies suggest that there might be a fourth) are all rather large, partly warm-blooded sharks, most closely akin to the great white and its cousins.
If you ignore the length of that enormous, elegant tapering tail, threshers are similar in size and build to the larger and bulkier requiem sharks – maybe most similar to an oceanic whitetip or silky – or to the threshers’ slightly closer cousins the makos and porbeagles.
The smallest species, the pelagic thresher found at Monad Shoal in the Philippines and right through the warm Indo-Pacific, matures at about the same size and bulk as a human, at about 12 years old. A large adult female might be 2m long to the fork in her tail and weigh 100kg, and with the long upper tail more than 3.5m in total.
The basic body is that of the stocky, muscular, active pelagic shark, but then attached is that gorgeous, bizarre tail.
Basically the oversized top lobe of the tail, just shy of the same length of the entire body again, has become a giant whip. The shark uses it to stun its prey.
In recent years threshers have been observed a few times hunting under water, and have been seen flicking their tail both over the top of their heads and around to the side into shoals of small fish, stunning them.
The tail flows behind the animal like a ribbon as it swims relaxed, but snaps with incredible force when used in hunting. Orcas use their rather different tails to stun herring in a similar way. If you can stun your prey rather than chase it down, you don’t need quite such a mouthful of teeth as other similarly sized hunters.
The small down-turned mouth combined with the huge eyes – threshers hunt rather deep in dark waters – gives a somewhat comical, cartoonish startled and scared look to these sharks.
They are all rather small-toothed for such large sharks, and their prey consists mainly of moderately small fish.
The pelagic threshers Malcolm saw in the Philippines have large eyes, the temperate common thresher smaller, more normal-sized eyes and the bigeye thresher very odd, giant eyes that wrap around to the top of its head, enabling it to look upwards at depth for prey silhouetted against the dim light flittering down.
The pelagic thresher that visits Monad Shoal off Malapascua Island seems to live a similar life to its closest cousin, the better studied, slightly larger bigeye thresher found in warm seas worldwide.
Both prefer tropical seas, reaching into warm temperate areas as large adults, when their body size retains heat, and their partial warm-bloodedness allows them to tolerate slightly cooler seas, where food is more plentiful. Pelagic threshers live in the tropical Indo-Pacific.
Threshers stay in warm shallow water at night, straying occasionally into diving depths, then dive deeper, far beyond the range of divers, to hunt during the day in colder, richer waters.
Bigeyes have been known to dive more than 700m deep, into water at just 5- 6°C, though 250m and 10° is more normal. Pelagic threshers seem to live their lives a little shallower, and a little warmer.