Open Ocean: the Blue Desert
Common Thresher Shark
Sporting a resplendent, scythe-like upper lobe of the caudal fin — as long or longer than its body — the Common Thresher (Alopias vulpinus) is among the most instantly-recognizable of sharks. The function of this hyperextended appendage has evoked much fanciful speculation and scientific debate. The matter is not completely settled, but there are a few observations that offer intriguing clues as to how this amazing structure might be used.
Just the Facts:
Habitat: Open Ocean, Deep Sea
Depth: surface to 1,200 ft (366 m)
Distribution: Central Pacific, Temperate Eastern Pacific, Tropical Eastern Pacific, Chilean, Western North Atlantic, Caribbean, Amazonian, Argentinean, Eastern North Atlantic/Mediterranean, West African, Southern African, Madagascaran, Arabian, Indian, South East Asian, Southeast Australian/New Zealand, Northern Australian, Japanese
Dramatic features, such as the caudal fin of thresher sharks, often seem to cry out for dramatic explanations. So it is perhaps unsurprising that reports of Threshers using their tails to hit or stun prey are persistent in the literature. In winter 1865 an Irish ichthyologist claimed he actually saw a Common Thresher rise up to the surface of Dublin Bay, use the tip of its caudal fin to swat a wounded diver (probably Gavia immer, the Great Northern Diver, known in North America as the Common Loon), and then swallow it. Despite the ichthyologist’s otherwise solid reputation and well-known stand on the need to back up ichthyological theory with observation in the wild, some authorities doubt his report. In 1927, two prominent American zoologists stated that the Thresher Shark’s tail is not sufficiently rigid or muscular to strike an efficient blow.
Misgivings about the Irish report notwithstanding, it has long been assumed that the Common Thresher uses the upper lobe of its long, slender caudal fin to herd its prey. Known prey of the Thresher consists mostly of small schooling fishes, such as mackerels (family Scombridae), Bluefish (Pomatomus saltatrix), herrings (Clupeidae), needlefishes (Belonidae), and lanternfishes (Myctophidae), but it also takes large, solitary fishes such as lancetfishes (Alepisauridae) as well as squids, pelagic octopuses and crustaceans. In the vastness of the open ocean, small schooling fishes and squids would certainly be easier to catch if concentrated somehow. But a prominent French shark biologist has seriously questioned the traditional assumption that the Thresher uses its tail as a food-gathering device. He points out that, when faced with a predator, the natural reaction of most schooling or herding animals is to bunch closer together; thus, a Thresher Shark could do just as well concentrating prey with an ordinary caudal fin and has no need to act like an aquatic sheepdog.
There are too many reports of Thresher Sharks herding and using their caudal fins to strike prey to discount them all. Some of these reports are buttressed by compelling circumstantial evidence. One Russian ichthyologist reported that 97% of all threshers caught on pelagic longlines in the northwestern Indian Ocean were foul-hooked through the upper lobe of the caudal fin, suggesting that the sharks were snagged as they tried to stun hooked fishes. There are also field observations of Thresher Sharks using their hyperextended caudal fins to capture fish prey, reported in the scientific literature by credible witnesses. In a 1915 paper, a well-respected American shark-watcher reported a most remarkable observation. In shallow waters of the bight of Cape Lookout, North Carolina, he actually saw a Common Thresher using its caudal fin to toss fish into its mouth; one fish the shark had failed to seize was thrown a “considerable distance”. Another account, this one by an oceanographer, seems an unimpeachable report of a Thresher Shark using its caudal fin to stun prey.
Early one morning, while collecting plankton off the pier at Scripps Institution of Oceanography (near La Jolla, California), the oceanographer heard a nearby splash. At a distance of about 100 feet (30 metres), he saw a swirl in the water like that made by a sea lion. A moment later, a three-foot- (one-metre-) long, slender, flattened tail flashed above the surface and lashed about “like a coachwhip”. A few minutes later he glimpsed a shark coming toward the surface, swimming rapidly roughly 50 feet (15 metres) from the end of the pier. Almost immediately, the oceanographer saw a small fish —which he thought might have been a California Smelt (Atherinopsis californiensis) — swimming frantically just in front of the shark. A moment later, the pursuing shark — now clearly identifiable as a six-foot (2-metre) Thresher Shark — passed partly (about half-way) ahead of its prey, turned quickly, and gave the same coach-whip lash with its caudal fin that the oceanographer had seen earlier. The whip-stroke was repeated immediately, “with very confusing speed” and it became evident that the victim was seriously injured. At this point, however, the fish was almost under the drip from the oceanographer’s net, which apparently frightened the Thresher into darting away. The injured fish sank, swimming feebly, then came to the surface and lay on its side for a while, until eventually it sank out of sight. Although the oceanographer did not actually see the Thresher Shark eat the fish it had injured, the speed and skill with which the Thresher struck its prey and the accuracy demonstrated in its lashes at a single, fast-moving target greatly impressed him. Anyone who has ever tried catching goldfish bare-handed or picking up objects with his toes cannot help but be impressed also.
If the Thresher’s caudal fin is capable of at least some of the amazing feats with which it has been credited, it must add considerably to the shark’s overall propulsive power. Although when hooked it does not perform repeated acrobatic leaps like the Shortfin Mako (Isurus oxyrinchus), the Common Thresher is ranked as a game fish by the International Game Fish Association (IGFA), putting up a strong, determined fight and even leaping on occasion. Some of these leaps are quite spectacular. One specimen was recently photographed leaping completely out of the water by a full body-length against a backdrop of the hazy, purple mountains of the Sea of Cortez. A legendary American shark fisherman once characterized the Common Thresher as “exceedingly stubborn” and, comparing this species with the Shortfin Mako, states that it is “pound for pound, a harder fish to whip”.
The Common Thresher’s strength and endurance result from some fascinating physiological adaptations. Like the Shortfin Mako and a few closely related sharks, the Thresher has a strip of red muscle along each of its flanks. This red muscle is highly aerobic, able to use oxygen efficiently so that it can contract powerfully for long periods. Further, this red muscle features a tight meshwork of tiny blood vessels that transfer metabolic heat so that it can be shunted inward, toward the body core. Retaining body heat helps the Common Thresher’s muscles function efficiently even in very cool water.
Their ability to retain body heat may explain the extraordinary range of depths over which Common Threshers have been caught — from the surface all the way down to at least 1,200 feet (365 metres). Often found close inshore and in shallow bays, young Common Threshers seem to have difficulty tolerating very cold or deep water. This may be because they are more slender with proportionately longer upper caudal lobes than adults. Therefore, young Threshers may radiate body heat to the surrounding water faster than they can produce it.