Pelagic thresher

The pelagic thresher was originally described by Japanese ichthyologist Hiroshi Nakamura on the basis of three large specimens, none of which was designated a holotype. He illustrated one of the three specimens in his paper, "On the two species of the thresher shark from Formosan waters", published in August 1935. Nakamura also separately illustrated and described a fetus, that Leonard Compagno later concluded was probably of a common thresher. Several authors, including Gohar and Mazhar (1964, Red Sea), Kato, Springer and Wagner (1967, Eastern Pacific), Fourmanoir and Laboute (1976, New Caledonia), Johnson (1978, Tahiti), and Faughnan (1980, Hawaiian Islands) have published illustrations of "common threshers" that were in fact pelagic threshers. The specific epithet pelagicus is from the Greek pelagios, meaning "of the sea". Another common name is the smalltooth thresher. ==Distribution and habitat==

Due to confusion with the common thresher, the distribution of the pelagic thresher may be wider than is currently known. It ranges extensively in the Indo-Pacific, with scattered records from South Africa, the Red Sea, and the Arabian Sea (off Somalia, between Oman and India, and off Pakistan), to China, southeastern Japan, northwestern Australia, New Caledonia, and Tahiti, to the Hawaiian Islands, California, and the Galapagos Islands. The North Pacific population shifts northward during warm El Nino years. Analysis of mitochondrial DNA has shown extensive gene flow within the eastern and western Pacific pelagic thresher populations, but little flow between them. The pelagic thresher primarily inhabits the open ocean, occurring from the surface to a depth of at least . 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 Gulf of California, and off Indonesia and Micronesia. It has also been known to enter large lagoons in the Tuamotu Islands. ==Description==

The pelagic thresher is the smallest of the thresher sharks, typically in length and in weight, and usually not exceeding and . A record of is dubious and may have resulted from confusion with other thresher species. This species has a 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. No furrows occur 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. Five to 11 rows of posterior teeth are present. 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. Alopias pelagicus head.jpg|Pelagic threshers can be identified by the color above their pectoral fins Alopias pelagicus teeth.jpg|Jaws Alopias pelagicus teeth2.jpg|Teeth Alopias pelagicus teeth3.jpg|Teeth ==Biology and ecology==

The pelagic thresher is an active, strong swimmer and has been known to leap clear of the water (five times in a row on one documented occasion). Predators of the pelagic thresher include larger fishes (including other sharks) and toothed whales. Known parasites of this species include the tapeworms Litobothrium amplifica, L. daileyi, and L. nickoli, which inhabit the shark's spiral valve intestine, Feeding 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. As in other threshers, pelagic threshers may swim in circles to drive schooling prey into a compact mass, before striking them sharply with the upper lobe of their tails to stun them. Because of this behavior, pelagic threshers are often hooked on longlines by their tails. Early-stage embryos have specialized teeth for opening the capsules, while later-stage embryos have their teeth hidden and swallow the capsules whole, their teeth not becoming functional again until just after birth. No evidence of sibling cannibalism has been found as in the sand tiger shark (Carcharias taurus). Young pelagic threshers are born unusually large, up to 1.6 m (5.2 ft) long or 43% the length of the mother, which likely reduces predation on the newborns. A rete system is present around the pelagic thresher's brain and eyes, albeit less developed than in the bigeye thresher, which may serve to buffer those organs against temperature changes. ==Human interactions==

The pelagic thresher has never been implicated in an attack on humans; it has small jaws and teeth for its size and tends to flee from divers. Though rarely caught, pelagic threshers are also valued by sport fishers and are listed as game fish by the International Game Fish Association. The largest overall records are from New Zealand, while the light tackle records are from California. Pelagic threshers are frequently taken as bycatch on longlines and in driftnets meant for other species such as tuna, and also rarely in gillnets and antishark nets. ==References==