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Alopias pelagicus

The pelagic thresher (Alopias pelagicus) is a species of thresher shark, family Alopiidae; this group of sharks is characterized by the greatly elongated upper lobes of their caudal fins. The pelagic thresher occurs in the tropical and subtropical waters of the Indian and Pacific Oceans, usually far from shore, but occasionally entering coastal habitats. It is often confused with the common thresher (A. vulpinus), even in professional publications, but can be distinguished by the dark, rather than white, color over the bases of its pectoral fins. The smallest of the three thresher species, the pelagic thresher typically measures 3 m (10 ft) long. The diet of the pelagic thresher consists mainly of small midwater fishes, which are stunned with whip-like strikes of its tail. Along with all other mackerel sharks, the pelagic thresher exhibits ovoviviparity and usually gives birth to litters of two. The developing embryos are oophagous, feeding on unfertilized eggs produced by the mother. The young are born unusually large, up to 43% the length of the mother. 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 assessed this species as vulnerable in 2007. The pelagic thresher was originally described by Japanese ichthyologist Hiroshi Nakamura on the basis of three large specimens, none of which was designated a type specimen. 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. An allozyme analysis conducted by Blaise Eitner in 1995 showed that the closest relative of the pelagic thresher is the bigeye thresher (A. superciliosus), with which it forms a clade. The specific epithet pelagicus is from the Greek pelagios, meaning 'of the sea'. Another common name is the smalltooth thresher. 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, the Gulf of 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 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 Gulf of California, and off Indonesia and Micronesia. It has also been known to enter large lagoons in the Tuamotu Islands. The pelagic thresher is the smallest of the thresher sharks, typically 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 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.

[ "Pelagic zone", "Fishing", "Carcharhinus", "Family Alopiidae" ]
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