Alopias vulpinus

The common thresher (Alopias vulpinus), also known by many names such as Atlantic thresher, big-eye thresher, fox shark, green thresher, swingletail, slasher, swiveltail, thintail thresher, whip-tailed shark and Zorro thresher shark, 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 lower 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 has an aplacental viviparous mode of reproduction, with oophagous embryos that feed on undeveloped eggs ovulated 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 is minimally dangerous 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, a case in point being 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 has assessed this species as vulnerable. The first scientific description of the common thresher, as Squalus vulpinus, was written by French naturalist Pierre Joseph Bonnaterre in the 1788 Tableau encyclopédique et méthodique des trois règnes de la nature. In 1810, Constantine Samuel Rafinesque described Alopias macrourus from a thresher shark caught off Sicily. Later authors recognized the genus Alopias as valid, while synonymizing A. macrourus with S. vulpinus, thus the common thresher's scientific name became Alopias vulpinus. The specific epithet vulpinus is derived from the Latin vulpes meaning 'fox', and in some older literature the species name was given incorrectly as Alopias vulpes. 'Fox shark' is the earliest known English name for this species and is rooted in classical antiquity, from a belief that it was especially cunning. In the mid-19th century, the name 'fox' was mostly superseded by 'thresher', referencing the shark's flail-like use of its tail. This species is often known simply as thresher shark or thresher; Henry Bigelow and William Schroeder introduced the name 'common thresher' in 1945 to differentiate it from the bigeye thresher (A. superciliosus). It is also known by many other common names, including Atlantic thresher, grayfish, green thresher, long-tailed shark, sea ape, sea fox, slasher, swiveltail, thintail thresher, thrasher shark, and whiptail shark. Morphological and allozyme analyses have agreed that the common thresher is basal to the clade formed by the bigeye thresher and the pelagic thresher (A. pelagicus). The closest relative of this species within the family may be a fourth, unrecognized thresher shark species off Baja California, reported from allozyme evidence by Blaise Eitner in 1995. However, the existence of this fourth species has yet to be confirmed by other sources. The common thresher is a fairly robust shark with a torpedo-shaped trunk and a short, broad head. The dorsal profile of the head curves evenly down to the pointed, conical snout. The eyes are moderately large and lack nictitating membranes. The small mouth is arched and, unlike in other thresher sharks, has furrows at the corners. The species has 32-53 upper and 25-50 lower tooth rows; the teeth are small, triangular, and smooth-edged, lacking lateral cusplets. The five pairs of gill slits are short, with the fourth and fifth pairs located over the pectoral fin bases. The long, falcate (sickle-shaped) pectoral fins taper to narrowly pointed tips. The first dorsal fin is tall and positioned slightly closer to the pectoral fins than the pelvic fins. The pelvic fins are almost as large as the first dorsal fin and bear long, thin claspers in males. The second dorsal and anal fins are tiny, with the former positioned ahead of the latter. Crescent-shaped notches occur on the caudal peduncle at the upper and lower origins of the caudal fin. The upper caudal fin lobe is enormously elongated as is characteristic of threshers, measuring about as long as the rest of the shark; the thin, gently curving lobe is held at a steep upward angle and has a notch in the trailing margin near the tip.