The Pelagic Shark (Alopias pelagicus) is a species of thresher shark, family Alopiidae. Pelagic threshers are valued by commercial fisheries for their meat, skin, liver oil, and fins, and are also pursued by sport fishers. The International Union for Conservation of Nature (IUCN) assessed this species as Vulnerable in 2007.
The pelagic thresher is the smallest of the thresher sharks, averaging 3 m (10 ft) in length and 69.5 kg (153.3 lb) in weight, and usually not exceeding 3.3 m (10.8 ft) and 88.4 kg (194.9 lb). Males and females attain known maximum lengths of 3.5 m (11.5 ft) and 3.8 m (12.5 ft) respectively. A record of 5 m (16.4 ft) is dubious and may have resulted from confusion with other thresher species. This species has fusiform body (wide in the middle and tapered at the ends) and a very slender upper caudal fin lobe nearly as long as the rest of the shark. The pectoral fins are long and straight with broad, rounded tips. The first dorsal fin is placed halfway between the pectoral and pelvic fins, and is of comparable size to the pelvic fins. The second dorsal and anal fins are tiny.
The head is narrow with a short conical snout and a distinctive "pinched" profile when viewed from below. The eyes are very large in juveniles and decrease in relative size with age. There are no furrows at the corners of the mouth. The teeth are very small, numbering 21–22 rows on each side with a symphysial (central) row in the upper jaw and 21 on each side without a symphysial row in the lower jaw. There are 5–11 rows of posterior teeth. The teeth are smooth-edged, with oblique cusps and lateral cusplets on the outside margins. The body is covered with very small, smooth dermal denticles with flat crowns and cusps with parallel ridges. The coloration is an intense dark blue above and white below; the white does not extend to above the pectoral fins. The color rapidly fades to gray after death. The dark pigment above the pectoral fins, the rounded pectoral fin tips, and the absence of labial furrows separate this shark from the common thresher.
The pelagic thresher primarily inhabits the open ocean, occurring from the surface to a depth of at least 150 m (492 ft). However, it occasionally comes close to shore in regions with a narrow continental shelf, and has been observed near coral reef dropoffs or seamounts in the Red Sea and the Sea of Cortez, and off Indonesia and Micronesia. It has also been known to enter large lagoons in the Tuamotu Islands.
Little information is available on the feeding ecology of the pelagic thresher. Its very slender tail and fine dentition suggest an exclusive diet of small, pelagic prey. Analysis of stomach contents reveal that pelagic threshers feed mainly on barracudinas, lightfishes, and escolars, all inhabitants of the mesopelagic zone.