Pelagic thresher

What is a pelagic thresher?

Like great whites, pelagic threshers are a type of mackerel shark. However, unlike the shark that inspired Jaws, which is known for its ferocious teeth, the thresher shark’s secret weapon is its whip-like tail which can immobilize multiple prey in an instant.

These cartoonish, torpedo-shaped fish have big eyes and a small, downturned mouth. Smaller and lighter in color than the other thresher species—the bigeye (Alopias superciliosus) and common thresher sharks (Alopias vulpinus)—pelagic threshers (Alopias pelagicus) have a blue-gray back, light blue-gray sides, and a white underbelly. Their deadly and flexible tails can be longer than their bodies.

A deadly whip-like tail

Its distinctive tail is the thresher shark’s secret weapon when hunting prey, which includes sardines, mackerel, herring, bluefish, shrimp, and squid. Many sharks hunt by separating an individual fish from the safety of the group and chasing it. For this reason, fish often pack together in a spherical formation known as a bait ball to try to protect themselves. However, the thresher shark has found a way to use this bait ball to its advantage.

To catch its prey, the shark rushes toward the tight cluster of fish then uses its pectoral fins like an emergency brake. As it slams to a stop, the shark whips its tail over its head toward the fish, sometimes following this with a sideways slap of its tail. Reaching speeds of up to 45 miles per hour, the force of the tail-slap sends a shockwave through the water so powerful that dissolved gas may diffuse out of the water, forming bubbles. The shockwave stuns the prey and allows the shark to collect its reward—eating between two and seven sardines after each blow. This hunting method is more energy efficient and has a higher chance of success than chasing prey one by one.

Habitat

While bigeye and common thresher sharks can be found all around the world, pelagic threshers are only found in the Indian and Pacific Oceans. They live in the upper zone of the open ocean but can reach depths from 1,640 to 2,460 feet. Living mainly offshore, there are very few places in the world they can be reliably spotted by scuba divers—most famously at Monad Shoal off the shores of Malapascua, an island in the Philippines—making them challenging to study.

Reproduction

Researchers do know these mysterious sharks have two pups in each litter and females give birth to around 40 pups during their lifetime. In the womb, pups are not connected to the mother via a placenta. Instead, cannibalism occurs as they feed on her unfertilized eggs. The mother then gives birth to unusually large live young: the free-swimming pups are from around three to five feet—meaning they can be almost half as long as their mother. Researchers theorize that their surprising size could be to minimize their chances of being preyed upon by other sharks.  

Threats 

Despite being harmless to humans, this species is endangered due to high demand from global and local fisheries for their meat. Their livers are also used for vitamins and cosmetic products, their skins in leather production, and their fins prized for shark fin soup. Sports fishermen also target threshers, which they catch by the tail. Being captured can kill them, even if they are released quickly, as they must keep swimming to pass oxygenated water over their gills. Thresher sharks are also often caught as bycatch because their habitats are heavily fished.

Because of their low reproduction rate, it is difficult for thresher shark populations to recover from exploitation and threats. However, conservation efforts are underway. Notably, in 2017, all three thresher shark species gained protections under the Convention on International Trade in Endangered Species (CITES), the international treaty which aims to protect threatened animals from the wildlife trade.