Biology of the Pelagic Thresher
|Field Marks||Upper caudal lobe long and curving, nearly as long as rest of shark; no labial furrows at mouth corners; lateral teeth with distinct basal cusplets; eyes large, but not expanded onto top of head; no deep groove behind eyes; pectoral fins with nearly straight leading edge and broad tips; first dorsal fin base closer to pectoral fin free rear tips than pelvic fin origins; white color of abdomen not extending over pectoral fin bases as conspicuous patch. Color grey above shading to paler grey on flanks, with silvery luster.|
|Size||Pups are about 38 in (96 cm) at birth; average length is about 10 ft (3 m); largest on record is 11 ft (3.3 m) long. Smallest species of thresher shark.|
|Range||Wide-ranging in the tropical to subtropical Indo-Pacific.|
|Habitat||Primarily epipelagic (oceanic near the surface), but sometimes caught near shore; known depth range from the surface to at least 500 ft (150 m); sometimes observed near coral reef drop offs or seamounts in the Red Sea, off Indonesia, Micronesia, and in the Sea of Cortez; in Tuamotu Islands, it sometimes enters large lagoons (where it has been misidentified as the Common Thresher, A. vulpinus). Often leaps out of the water (one was observed to leap five times in succession)|
|Feeding||No data available; presumably preys on small fishes and squids, which it stuns with its tail before feeding.|
|Reproduction||Ovoviviparous, developing embryos oophagous; number of pups 2; no data on gestation period, pupping season or nursery grounds.|
|Age & Growth||Males adolescent at a length of about 6.3 ft (1.9 m) and mature by 9 ft (2.75 m), females mature at a length of about 8.7-10.8 ft (2.6-3.3 m); no data on age at maturity or longevity of either sex.|
|Danger to Humans||Minimal; usually shy and difficult for divers to approach, but free-swimming individuals have been photographed underwater.|
|Utilization||Mainly exploited by the Russian longline fishery in the northwestern Indian Ocean; also fished in the central Pacific and probably elsewhere; caught incidentally in oceanic longline and driftnet fisheries; utilized for meat (for human consumption), liver oil for vitamin extraction, hides for leather, and fins for shark-fin soup.|
|Remarks||Few reliable records known, partly due to confusion with the Common Thresher (Alopias vulpinus).|
Collectively, the thresher sharks (family Alopiidae) are among the most easily recognizable of sharks. But distinguishing the individual species from one another is not such an easy matter. The Pelagic Thresher Shark is the smallest member of its family and the least known. Indeed, this species is perhaps best known for being confused with other threshers.
Part of this identity crisis stems from the fact that the original description of this species is somewhat muddled. The Pelagic Thresher was originally described by Japanese ichthyologist Hiroshi Nakamura in a paper entitled "On the Two Species of the Thresher Shark from Formosan Waters", published in August 1935. Nakamura described Alopias pelagicus based on three large specimens, ranging from 9.35 to 10.83 feet (2.85 to 3.30 metres) in length, one of which he illustrated. (The other species Nakamura described and named in this paper is Alopias profundus, which is now recognized to be a junior synonym of the Bigeye Thresher Shark, Alopias superciliosus, originally described by Lowe in 1839 from a specimen taken off Madeira) From Nakamura's presented morphometrics (proportional measurements), illustrations of a lateral (side) view, and - especially - illustrations of the distinctive teeth with lateral cusplets, it is clear that pelagicus is indeed a valid species. But the story does not end there.
Nakamura also included a separate description and illustration of at least one fetus (the one illustrated being about 38 inches or 97 centimetres long) under the name Alopias pelagicus. He did not, however, designate type material (a representative specimen, deposited in the permanent collections of a museum, there to serve as the ultimate standard for comparison), nor did he indicate whether the illustrated fetus was removed from one of the three large specimens or was obtained separately. Unfortunately, according to shark systematist Leonard J.V. Compagno, the illustrated fetus may be of a Common Thresher (Alopias vulpinus). Compagno bases his identification of the fetus as vulpinus based on its small eyes, broad head with a strongly convex dorsal profile, short snout, the presence of labial furrows, and falcate pectoral fins. Thus, Nakamura's original description and illustrations of the Pelagic Thresher may be based - at least in part - on the Common Thresher.
From such ambiguous beginnings, it is hardly surprising that the Pelagic Thresher has been regularly mistaken for the Common Thresher. For example, in their classic study of north-western Red Sea sharks, H.A.F. Gohar and F.M. Mazhar illustrate the teeth, lateral view and head of a shark they identify as "Alopias vulpinus", but which is clearly A. pelagicus. In their elegantly concise and useful 1964 Field Guide to Eastern Pacific and Hawaiian Sharks, Susumu Kato, Stewart Springer, and Mary H. Wagner describe the differences between pelagicus and vulpinus accurately, but illustrate their species account of the Common Thresher with a beautifully rendered Pelagic Thresher. Likewise, in his 1980 popular book, The National Shark-O-Pedia, shark fisherman-cum-author Victor R. Faughnan misidentified clear photographs of Pelagic Threshers from Hawaiian waters as being Common Threshers and includes a muddled discussion of thresher shark taxonomy, only part of which can be blamed on Nakamura's original blunder.
But sometimes, such misidentifications reveal more than the need for caution in identifying thresher shark species. In his otherwise excellent 1977 book, Sharks of Polynesia, Richard H. Johnson (who did much pioneering work with Donald R. Nelson on the behavioral ecology of reef sharks in French Polynesia, including the first study of the now-famous agonistic display in the Grey Reef Shark [Carcharhinus amblyrhynchos]) misidentifies a Pelagic Thresher as a Common Thresher. (The two can be most readily distinguished by pigmentation pattern: in the Common Thresher, the white belly color extends over the pectoral fin bases, while in the Pelagic Thresher it does not.) What makes this identification interesting is not that Johnson originally 'got it wrong' (as we have seen, that' s easy to do — even for the most competent of field biologists), but the habitat in which some Pelagic Threshers are encountered. According to Johnson, the species depicted in the accompanying photographs — which we now know to be the so-called Pelagic Thresher — is occasionally found inside the larger coral lagoons in the Tuomotu Islands!
It is not known whether Pelagic Threshers actually feed within these coral lagoons. For the diving naturalist or photographer, such an encounter must rank as the ultimate in underwater serendipity. The Pelagic Thresher is notoriously shy and difficult for divers to approach. I have only seen two underwater photographs of thresher sharks - captured by two different photographers at undisclosed but presumably separate locations - and both of them Pelagic Threshers, as it happens. But, due to the murkiness of the water (a technical challenge with which a photographer's skill can only do so much), neither image is particularly sharp. In the crystalline waters inside a coral lagoon, however, it should be relatively easy for a diver to obtain a clear view or capture a good, crisp underwater photograph.
Such a photograph, if properly identified, would almost certainly become widely used in popular shark books and fish identification guides. And that would go a long way toward making it less likely that a Pelagic Thresher will be confused with other threshers. For only then can we hope to fill in some of the missing details of the behavior, ecology, and life history of this oft-misidentified shark.