The Common Thresher (Alopias vulpinus) is the largest species of thresher shark, family Alopiidae. 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 (IUCN) has assessed this species as Vulnerable.
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. There are 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. There are crescent-shaped notches 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.
The skin is covered by small, overlapping dermal denticles, each with three horizontal ridges and three to five marginal teeth. This species is metallic purplish brown to gray above, becoming more bluish on the flanks. The underside is white, which extends over the pectoral and pelvic fin bases; this pattern is in contrast to the pelagic thresher, which is solidly colored over these fins. The meeting line between the dorsal and ventral coloration is often irregular. There may be a white spot at the tips of the pectoral fins. The common thresher is the largest thresher shark species, commonly reaching 5 m (16 ft) long and 230 kg (510 lb) in weight.
Common threshers are inhabitants of both continental waters and the open ocean. They tend to be most abundant in proximity to land, particularly the juveniles which frequent near-coastal habitats such as bays. Most individuals are encountered near the surface, but this species has been recorded to at least a depth of 550 m (1,800 ft).
Some 97% of the common thresher's diet is composed of bony fishes, mostly small schooling? forage fish such as mackerel, bluefish, herring, needlefish, and? lanternfish. Before striking, the sharks compact schools of prey by swimming around them and splashing the water with its tail, often in pairs or small groups. Threshers are also known to take large, solitary fishes such as lancetfish, as well as squid and other pelagic invertebrates. Off California, common threshers feed mostly on the northern anchovy (Engraulis mordax), with Pacific hake (Merluccius productus), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), market squid? (Loligo opalescens), and pelagic red crab (Pleuroncodes planipes) also being important food items.