The common thresher shark (Alopias vulpinus) is the largest species of thresher shark, family Alopiidae, reaching some 6 m (20 ft) in length. About half of its length consists of the elongated upper lobe of its caudal fin. With a streamlined body, short pointed snout, and modestly sized eyes, the common thresher resembles (and has often been confused with) the pelagic thresher (A. pelagicus). It can be distinguished from the latter species by the white of its belly extending in a band over the bases of its pectoral fins. The common thresher is distributed worldwide in tropical and temperate waters, though it prefers cooler temperatures. It can be found both close to shore and in the open ocean, from the surface to a depth of 550 m (1,800 ft). It is seasonally migratory and spends summers at higher latitudes.

The long tail of the common thresher, the source of many fanciful tales through history, is used in a whip-like fashion to deliver incapacitating blows to its prey. This species feeds mainly on small schooling forage fishes such as herrings and anchovies. It is a fast, strong swimmer that has been known to leap clear of the water, and possesses physiological adaptations that allow it to maintain an internal body temperature warmer than that of the surrounding sea water. The common thresher gives birth to live young. The developing embryos are oophagous, feeding on unfertilized eggs provided by their mother. Females typically give birth to four pups at a time, following a gestation period of nine months.

Despite its size, the common thresher poses little danger to humans due to its relatively small teeth and timid disposition. It is highly valued by commercial fishers for its meat, fins, hide, and liver oil; large numbers are taken by longline and gillnet fisheries throughout its range. This shark is also esteemed by recreational anglers for the exceptional fight it offers on hook-and-line. The common thresher has a low rate of reproduction and cannot withstand heavy fishing pressure for long, which is exemplified by the rapid collapse of the thresher shark fishery off California in the 1980s. With commercial exploitation increasing in many parts of the world, the International Union for Conservation of Nature (IUCN) has assessed this species as Vulnerable.

Feeding
Northern anchovies are the most important food source for common threshers off California.

The common thresher is a rather specialized predator feeding mostly on small schooling bony fishes of the open ocean, such as anchovies, herring, and mackerel. It also eats larger pelagic fishes such as lancetfish and bluefish, bottom-dwelling fishes such as flounder, and invertebrates including squid, octopus, and swimming crustaceans. In the California Current, the most important prey species is the northern anchovy (Engraulis mordax); other common prey are Pacific hake (Merluccius productus), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), market squid (Loligo opalescens), and pelagic red crab (Pleuroncodes planipes). This species concentrates on a few preferred prey species during cold years, but becomes less discriminating during less productive, warmer El Niño periods.

When hunting schooling prey, the common thresher first compacts them into a tight ball by swimming in ever-smaller circles and splashing the water with its tail, often in pairs or small groups. The shark then stuns individual prey with blows from the long, flexible upper lobe of its caudal fin. There are two general ways by which these strikes are delivered. In the first, the shark rapidly undulates the front of its body, sending a rippling motion backwards along its length into its tail. In the second, the shark maneuvers parallel to its target and whips its tail sideways; this method is used less often than the first but has a higher success rate. Multiple strikes may be made over a matter of seconds, and the prey can be hit by almost any point along the caudal lobe from the base to the tip.

Many eyewitness accounts attest to the incredible power and control with which the common thresher directs its tail strikes. In the winter of 1865, Irish ichthyologist Harry Blake-Knox claimed to have seen a thresher shark in Dublin Bay use its tail to strike a wounded loon (probably a great northern diver, Gavia immer), which it then swallowed. Blake-Knox's account was doubted by other ichthyologists such as Charles Breder, who asserted that the thresher's tail is not rigid or muscular enough to effect such a blow. In July 1914, shark watcher Russell J. Coles reported seeing a thresher shark off Cape Lookout, North Carolina use its tail to flip fish into its mouth. In April 1923, oceanographer Winfred E. Allen observed a common thresher pursuing a California smelt (Atherinopsis californiensis) from a pier at the Scripps Institution of Oceanography in La Jolla, California. The shark overtook the small fish and swung its tail above the water twice "with very confusing speed", severely injuring its target.

Life History
Embryos of the common thresher are nourished by eggs during development.

Like other mackerel sharks, common threshers are aplacental viviparous. They give birth to litters of two to four (rarely six) pups in the eastern Pacific, and three to seven pups in the eastern Atlantic. They are believed to reproduce throughout their range; one known nursery area is the Southern California Bight. Breeding occurs in the summer, usually July or August, and parturition occurs from March to June following a gestation period of nine months. The developing embryos are oophagous, feeding on eggs ovulated by the mother. The teeth of small embryos are peg-like and non-functional, being covered by a sheath of soft tissues. As the embryos mature, their series of teeth become progressively more like those of adults in shape, though they remain depressed and hidden until shortly before birth.

Newborn pups usually measure 114–160 cm (3.74–5.25 ft) long and weigh 5–6 kg (11–13 lb), depending on the size of the mother. The juveniles grow about 50 cm (1.6 ft) a year while adults grow about 10 cm (0.33 ft) a year. The size at maturation appears to vary between populations. In the eastern North Pacific males mature at 3.3 m (11 ft) and five years old, and females at around 2.6–4.5 m (8.5–14.8 ft) and seven years old. They are known to live to at least 15 years of age and their maximum lifespan has been estimated to be 45–50 years.

Human Interactions
While any large shark is capable of inflicting injury and thus merits respect, the common thresher poses little danger to humans. Most divers report that they are shy and difficult to approach underwater. The International Shark Attack File lists a single provoked attack by the thresher shark and four attacks on boats, which were probably incidental from individuals fighting capture. There is an unsubstantiated report of a common thresher acting aggressively towards a spearfisherman off New Zealand.

Famed big-game angler Frank Mundus, in his book Sportsfishing for Sharks, recounted a tale in which a longline fisherman off the Carolinas leaned over the side of his boat to examine something large that he had hooked, and was decapitated by the caudal fin of a thresher shark estimated to be 5 m (16 ft) long. The head supposedly fell into the water and was never recovered. This account is considered highly improbable by most authors.