Thresher sharks derive their genus and family name, Alopias, from the Greek word ἀλώπηξ (alṓpēx), meaning fox. This association arises from their long-tailed or caudal fin, reminiscent of the tail of a fox. The common thresher shark, Alopias vulpinus, also goes by the name "fox shark" due to this characteristic feature.
The family Alopiidae comprises three extant species within the genus Alopias:
It is worth mentioning that a potential fourth species was discovered in the eastern Pacific off Baja California, initially misidentified as the bigeye thresher. However, further research is needed to confirm its existence and document its morphology.
Thresher sharks are primarily pelagic creatures, preferring the vast openness of the ocean. While occasionally sighted in shallow, inshore waters, they are most commonly found in depths of 500 meters (1,600 feet) or less. Common threshers are prevalent in coastal waters along the continental shelves of North America and Asia in the North Pacific. However, they are rare in the Central and Western Pacific. On the other hand, bigeye and pelagic threshers thrive in the warmer waters of the Central and Western Pacific. Recent observations have even detected thresher sharks at depths exceeding the previously believed limit of 500 meters, indicating their adaptability to diverse environments.
Thresher sharks possess remarkable physical attributes that contribute to their survival and predatory prowess. With their elongated, thresher-like heterocercal tails or caudal fins, they exhibit an extraordinary adaptation. These tails, often as long as their entire bodies, serve as formidable weapons used to stun prey. Thresher sharks have short heads and cone-shaped noses, with relatively small mouths containing teeth of varying sizes.
There are three extant species of thresher sharks, each with distinct characteristics:
Thresher sharks have a slender body shape, small dorsal fins, and large, recurved pectoral fins. Their eye size varies, with the exception of the bigeye thresher, which has larger eyes positioned forward on the head. The coloration of thresher sharks ranges from brownish, bluish, or purplish gray dorsally, with lighter shades ventrally. Lighting conditions and water clarity can affect their appearance, but the primary color of the dorsal surface provides a reliable indicator for identifying the different species.
Thresher sharks are active predators, preying mainly on pelagic schooling fish. Their diet consists of species such as bluefish, juvenile tuna, and mackerel. Thresher sharks are known to follow these fish into shallow waters. Additionally, they feed on squid, cuttlefish, crustaceans, and occasionally seabirds. Their elongated tails serve as effective weapons, stunning their prey before consumption. At times, thresher sharks even slice their prey in half to facilitate consumption.
Thresher sharks are typically solitary creatures, preferring to keep to themselves. However, there have been occasional sightings of two or three individuals hunting together, deviating from their solitary nature. These sharks exhibit highly migratory behavior, constantly moving across the oceans. When hunting, they employ a unique technique known as "whipping" the water, using their elongated tails to swat smaller fish and stun them. Breaching, a behavior where thresher sharks jump fully out of the water, is another remarkable sight, reminiscent of dolphins.
Thresher sharks do not adhere to a distinct breeding season. They reproduce through an ovoviviparous or live-bearing method. Female thresher sharks give birth to a small litter of two to four large, well-developed pups. These pups can measure up to 150 centimeters (59 inches) at birth in thintail threshers. Interestingly, the young fish consume the mother's unfertilized eggs after depleting their yolk sacs, a behavior known as oophagy.
Thresher sharks have a slow maturation process, with males reaching sexual maturity between seven and 13 years of age, and females between eight and 14 years. They can live for over 20 years.
In October 2013, an extraordinary event was captured off the coast of the Philippines—an image of a thresher shark giving birth, providing a rare glimpse into the reproductive process of these magnificent creatures.
Due to their low fecundity, thresher sharks are highly susceptible to overfishing. All three species of thresher sharks have been listed as vulnerable to extinction by the International Union for Conservation of Nature (IUCN) since 2007. The demand for their meat, livers (for shark liver oil), skin (for leather), and fins (for shark-fin soup) poses significant threats to their populations.