Common thresher
Common thresher | |
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Scientific classification![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Chondrichthyes |
Subclass: | Elasmobranchii |
Subdivision: | Selachimorpha |
Order: | Lamniformes |
Family: | Alopiidae |
Genus: | Alopias |
Species: | A. vulpinus
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Binomial name | |
Alopias vulpinus (Bonnaterre,1788)
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Confirmed (dark blue) and suspected (light blue) range of the common thresher[3] | |
Synonyms | |
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Thecommon thresher(Alopias vulpinus), also known asAtlantic thresher,is the largestspeciesofthresher shark,familyAlopiidae, reaching some 6 m (20 ft) in length. About half of its length consists of the elongated upper lobe of itscaudal fin.With a streamlined body, short pointed snout, and modestly sized eyes, the common thresher resembles (and has often been confused with) thepelagic thresher(A. pelagicus). It can be distinguished from the latter species by the white of its belly extending in a band over the bases of itspectoral fins.The common thresher is distributed worldwide intropicalandtemperatewaters, though it prefers cooler temperatures. It can be found both close to shore and in the open ocean, from the surface to a depth of 550 m (1,800 ft). It is seasonallymigratoryand spends summers at lowerlatitudes.
The long tail of the common thresher, the source of many fanciful tales through history, is used in a whip-like fashion to deliver incapacitating blows to its prey.[citation needed]This species feeds mainly on smallschoolingforage fishessuch asherringsandanchovies.It is a fast, strong swimmer that has been known to leap clear of the water, and possessesphysiological adaptationsthat allow it to maintain an internal body temperature warmer than that of the surrounding sea water. The common thresher has anaplacental viviparousmode of reproduction, withoophagousembryosthat feed on undeveloped eggsovulatedby their mother. Females typically give birth to four pups at a time, following agestation periodof nine months.
Despite its size, the common thresher is minimally dangerous to humans due to its relatively small teeth and timid disposition. It is highly valued bycommercial fishersfor its meat,fins,hide, andliver oil;large numbers are taken bylonglineandgillnetfisheries throughout itsrange.This shark is also esteemed byrecreational anglersfor 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 offCaliforniain the 1980s. With commercial exploitation increasing in many parts of the world, theInternational Union for Conservation of Naturehas assessed this species asvulnerable.
Taxonomy and phylogeny
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Alopias_vulpinus.jpg/220px-Alopias_vulpinus.jpg)
The firstscientific descriptionof the common thresher, asSqualus vulpinus,was written by FrenchnaturalistPierre Joseph Bonnaterrein the 1788Tableau encyclopédique et méthodique des trois règnes de la nature.[5]In 1810,Constantine Samuel RafinesquedescribedAlopias macrourusfrom a thresher shark caught offSicily.Later authors recognized the genusAlopiasas valid, whilesynonymizingA. macrouruswithS. vulpinus,thus the common thresher's scientific name becameAlopias vulpinus.[3]
Thespecific epithetvulpinusis derived from theLatinvulpesmeaning "fox",and in some older literature the species name was given incorrectly asAlopias vulpes.[6]"Fox shark" is the earliest known English name for this species and is rooted inclassical antiquity,from a belief that it was especially cunning. In the mid-19th century, the name "fox" was mostly superseded by "thresher", referencing the shark's flail-like use of its tail. This species is often known simply as thresher shark or thresher;Henry BigelowandWilliam Schroederintroduced the name "common thresher" in 1945 to differentiate it from thebigeye thresher(A. superciliosus).[7]It is also known by many othercommon names,including Atlantic thresher, grayfish, green thresher, long-tailed shark, sea ape, sea fox, slasher, swiveltail, thintail thresher, thrasher shark, and whiptail shark.[3][8]
Morphologicalandallozymeanalyses have agreed that the common thresher isbasalto thecladeformed by the bigeye thresher and thepelagic thresher(A. pelagicus).[3][6]The closest relative of this species within the family may be a fourth, unrecognized thresher shark species offBaja California,reported from allozyme evidence by Blaise Eitner in 1995.[9]However, the existence of this fourth species has yet to be confirmed by other sources.[1][3]
Description
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/6/69/Pacifica_thresher_shark.jpg/300px-Pacifica_thresher_shark.jpg)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Alopias_vulpinus_teeth.jpg/220px-Alopias_vulpinus_teeth.jpg)
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 lacknictitating membranes.The small mouth is arched and, unlike in other thresher sharks, has furrows at the corners. The species has 32-53 upper and 25-50 lowertooth rows;the teeth are small, triangular, and smooth-edged, lacking lateral cusplets. The five pairs ofgill slitsare short, with the fourth and fifth pairs located over thepectoral finbases.[3][6]
The long, falcate (sickle-shaped) pectoral fins taper to narrowly pointed tips. The firstdorsal finis tall and positioned slightly closer to the pectoral fins than thepelvic fins.The pelvic fins are almost as large as the first dorsal fin and bear long, thinclaspersin males. The second dorsal andanal finsare tiny, with the former positioned ahead of the latter. Crescent-shaped notches occur on thecaudal peduncleat the upper and lower origins of thecaudal 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.[3][7]
The skin is covered by small, overlappingdermal 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. A white spot may be seen at the tips of the pectoral fins.[3][7]The common thresher is the largest thresher shark species, commonly reaching 5 m (16 ft) long and 230 kg (510 lb) in weight.[10]The confirmed length record for this shark is 5.7 m (19 ft), while the maximum possible length may be 6.1–6.5 m (20–21 ft).[3]The heaviest individual on record is a 4.8 m (16 ft) female that weighed 510 kg (1,120 lb).[11]Exceptional specimens may possibly weigh up to 900 kg (2,000 lb), though such claims are not verified yet.[12]
Distribution
[edit]The range of the common thresher encompassestropicaland cold-temperatewaters worldwide. In the westernAtlantic,it is found fromNewfoundlandto theGulf of Mexico,[13]though it is rare north ofNew England,and fromVenezuelatoArgentina.In the eastern Atlantic, it has been reported from theNorth Seaand theBritish IslestoGhana(includingMadeira,theAzores,and theMediterraneanandBlack Seas), as well as fromAngolatoSouth Africa.In theIndo-Pacific,this species is known fromTanzaniatoIndiaand theMaldives,Japan,andKoreato southeasternChina,Sumatra,easternAustralia,andNew Zealand;it also occurs around a number of Pacific islands includingNew Caledonia,theSociety Islands,Tabuaeran,and theHawaiian Islands.In the eastern Pacific, it has been recorded fromBritish ColumbiatoChile,including theGulf of California.[1][3]
The common thresher ismigratory,moving to higherlatitudesfollowing warm-water masses. In the eastern Pacific, males travel further than females, reaching as far asVancouver Islandin the late summer and early fall. Juveniles tend to remain in warmnursery areas.[6]In New Zealand waters juveniles can be found over the inner shelf around the North Island and around the upper South Island.[14]Separate populations with different life history characteristics apparently exist in the eastern Pacific and western Indian Ocean and possibly elsewhere; this species is not known to make transoceanic movements.[15]In the northwestern Indian Ocean, males and females segregate by location and depth during the pupping season from January to May.[10]Analysis ofmitochondrial DNAhas revealed substantial regional genetic variation within common threshers in all three oceans. This could support the idea that sharks from different areas, despite being highly mobile, rarely interbreed.[16]
Habitat
[edit]Common threshers are inhabitants of bothcontinentalwaters and theopen ocean.They tend to be most abundant in proximity to land, particularly thejuveniles,which frequent near-coastal habitats such as bays.[3]The species has been described as "coastal–oceanic", mostly occurring within 30 km (20 mi) of the coast with considerably lower populations beyond this limit.[17]Most individuals are encountered near the surface, but this species has been recorded to at least a depth of 550 m (1,800 ft).[18]Among eight individuals tagged and tracked for 22–49 hours off southern California, all spent the majority of their time within 40 m (130 ft) of the water surface, but periodically dived much deeper, in five individuals to depths of around 100 m (330 ft) or more.[17]A study from the tropicalMarshall Islandsindicated that common threshers mainly spend the day at depths of about 160–240 m (520–790 ft) where the temperature is 18–20 °C (64–68 °F).[19]Common threshers appear to prefer water temperatures between 16 and 21 °C (61 and 70 °F), but at least occasionally occur down to around 9 °C (48 °F).[17]
Biology and ecology
[edit]Common threshers are active, strong swimmers, with infrequent reports of them leaping completely out of the water.[20]Like the fast-swimming sharks of the familyLamnidae,the common thresher has a strip ofaerobicredmusclealong its flank that is able to contract powerfully and efficiently for long periods of time.[21]In addition, they have slow-oxidativemuscles centrally located within their bodies and ablood vesselcountercurrent exchangesystem called therete mirabile( "wonderful net" ), allowing them to generate and retainbody heat.The temperature inside the red muscles of a common thresher averages 2 °C (3.6 °F) above that of the ambientseawater,though significant individual variation is seen.[22]Unlike the pelagic and bigeye threshers, the common thresher lacks anorbitalrete mirabile to protect its eyes and brain from temperature changes.[23]
Immature common threshers fallpreyto largersharks.Aside from observations ofkiller whalesfeeding on common threshers offNew Zealand,[24]adults have no known natural predators.Parasitesdocumented from the common thresher include theprotozoanGiardia intestinalis,[25]thetrematodesCampula oblonga(not usual host)[26]andParonatrema vaginicola,[27]thetapewormsAcanthobothrium coronatum,[28]Anthobothrium laciniatum,[29]Crossobothrium angustum,[30]Hepatoxylon trichiuri,Molicola uncinatus,[31]Paraorygmatobothrium exiguum,[32]P. filiforme,[33]andSphyriocephalus tergetinus,[34]and thecopepodsDinemoura discrepans,Echthrogaleus denticulatus,[35]Gangliopus pyriformis,[36]Kroeyerina benzorum,[37]Nemesis aggregatus,N. robusta,N. tiburo,[38]Nesippus orientalis,[39]andPandarus smithii.[35]
Feeding
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Alopias_vulpinus_tail_noaa.jpg/220px-Alopias_vulpinus_tail_noaa.jpg)
The long upper tailfin lobe of the common thresher is used to strike and incapacitate prey.[10]
Some 97% of the common thresher's diet is composed ofbony fishes,mostly smallschoolingforage fishsuch assardine,anchovy,mackerel,hake,bluefish,herring,needlefish,andlanternfish.[40]Before striking, the sharks compact schools of prey by swimming around them and splashing the water with their tails, often in pairs or small groups. Threshers are also known to take large, solitary fishes such aslancetfish,as well assquidand otherpelagicinvertebrates.[18]Off California, common threshers feed mostly on thenorthern anchovy(Engraulis mordax), withPacific hake(Merluccius productus),Pacific sardine(Sardinops sagax),Pacific mackerel(Scomber japonicus),market squid(Loligo opalescens), andpelagic red crab(Pleuroncodes planipes) also being important food items. The sharks concentrate on a few prey species during cold-water years, but become less discriminating during less productive, warmerEl Niñoperiods.[41]
Numerous accounts have been given of common threshers using the long upper lobes of their tail fins to stun prey, and they are often snagged onlonglinesby their tails after presumably striking at the bait. In July 1914, shark-watcher Russell J. Coles reported seeing a thresher shark use its tail to flip prey fish into its mouth, and that one fish that missed was thrown a "considerable distance". On April 14, 1923, notedoceanographerW.E. Allen observed a 2 m (6.6 ft) thresher shark pursuing aCalifornia smelt(Atherinopsis californiensis) off a pier at theScripps Institution of Oceanography.The shark overtook the small fish and swung its tail above the water like a "coachwhip"with" confusing speed ", severely injuring its target. In the winter of 1865,IrishichthyologistHarry Blake-Knox claimed to have seen a thresher shark inDublin Bayuse its tail to strike a woundedloon(probably agreat northern diver,Gavia immer), which it then swallowed. Blake-Knox's account was subsequently disputed by other authorities, who asserted that the thresher's tail is not rigid or muscular enough to effect such a blow.[10]
Life history
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/78/Alopias_vulpinus_pups.jpg/220px-Alopias_vulpinus_pups.jpg)
Like other mackerel sharks, common threshers areaplacental viviparous.They give birth to litters of two to four (rarely six) pups in the eastern Pacific, and three to seven pups in the eastern Atlantic.[15]They are believed to reproduce throughout their range; one knownnursery areais theSouthern California Bight.Breedingoccurs in the summer, usually July or August, andparturitionoccurs from March to June following agestation periodof nine months. The developingembryosareoophagous,feeding oneggsovulatedby the mother.[6]The teeth of small embryos are peg-like and nonfunctional, being covered by a sheath of soft tissues. As the embryos mature, their series of teeth become progressively more like those of adults in shape, though they remain depressed and hidden until shortly before birth.[42]
Newborn pups usually measure 114–160 cm (3.74–5.25 ft) long and weigh 5–6 kg (11–13 lb), depending on the size of the mother. The juveniles grow about 50 cm (1.6 ft) a year, while adults grow about 10 cm (0.33 ft) a year.[18]The size at maturation appears to vary between populations. In the eastern North Pacific, males mature at 3.3 m (11 ft) and five years old, and females around 2.6–4.5 m (8.5–14.8 ft) and seven years old. They are known to live to at least 15 years and their maximum lifespan has been estimated to be 45–50 years.[6][15]
Relationship with humans
[edit]While any large shark is capable of inflicting injury and thus merits respect, the common thresher poses little danger to humans. Most divers report that they are shy and difficult to approach under water. TheInternational Shark Attack Filelists a single provoked attack by the thresher shark and four attacks onboats,which were probably incidental from individuals fighting capture.[citation needed]
Commercial fishing
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/Alopias_vulpinus_hooked2.jpg/220px-Alopias_vulpinus_hooked2.jpg)
The common thresher is widely caught by offshorelonglineand pelagicgillnetfisheries,especially in the northwestern Indian Ocean, the western, central, and eastern Pacific, and the North Atlantic. Participating countries include theformer USSR,Japan,Taiwan,Spain,theUnited States,Brazil,Uruguay,andMexico.The meat is highly prized for human consumption cooked,dried and salted,orsmoked.In addition, their skin is made intoleather,theirliver oilis processed forvitamins,and their fins are used forshark fin soup.TheUnited NationsFood and Agriculture Organizationreported a worldwide common thresher take of 411 metric tons in 2006.[15]
In the United States, a drift gillnet fishery for the common thresher developed in southern California in 1977, beginning with 10 vessels experimenting with a larger-sized mesh. Within two years, the fleet had increased to 40 vessels, and the fishery peaked in 1982 when 228 vessels landed 1,091 metric tons. The common thresher population rapidly collapsed fromoverfishing,with landings decreasing to less than 300 metric tons a year by the late 1980s and larger size classes disappearing from the population.[15][43]Common threshers are still taken commercially in the United States, with about 85% coming from the Pacific and 15% from the Atlantic. The largest catches remain from the California-Oregon gillnet fishery, which had shifted its focus to the more valuableswordfish(Xiphias gladius), but still takes threshers asbycatch.Small numbers of Pacific threshers are also taken byharpoons,small-mesh driftnets, and longlines. In the Atlantic, threshers are primarily taken on longlines meant for swordfish andtuna.[44][45]
Recreational fishing
[edit]Common threshers are well regarded bysports fishersas one of the strongest fighting sharks alongside theshortfin mako shark(Isurus oxyrhinchus), and are ranked asgame fishby theInternational Game Fish Association.They are pursued by anglers usingrod and reeloff California, South Africa, and elsewhere. Frank Mundus has called thresher sharks "exceedingly stubborn" and "pound for pound, a harder fish to whip" than the mako.[10]Fishing for the common thresher is similar to that for the mako; the recommended equipment is a 24 kg (53 lb) rod and a big-game reel holding at least 365 m (400 yd) of 24 kg (53 lb) line. The ideal method istrollingwith baitfish, either deep or allowing it to drift.[46][47]
Conservation
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/Alopias_vulpinus_swfsc.jpg/250px-Alopias_vulpinus_swfsc.jpg)
All three thresher shark species were reassessed fromData DeficienttoVulnerableby theInternational Union for Conservation of Naturein 2007.[1]The rapid collapse of the Californian subpopulation (over 50% within three generations) prompted concerns regarding the species' susceptibility to overfishing in other areas, where fishery data are seldom reported and aspects of life history and population structure are little known.[1]In addition to continued fishing pressure, common threshers are also taken as bycatch in other gear such asbottom trawlsandfish traps,and are considered a nuisance by mackerel fishers, as they become entangled in the nets.[15][18]
The United States manages common thresher fisheries by regulations such as commercial quotas and trip limits, and recreational minimum sizes and retention limits.Shark finningis illegal under U.S. federal law. The Atlantic common thresher fishery is regulated by theNational Marine Fisheries ServiceHighly Migratory Species Management Division through the 2006 Consolidated Atlantic Highly Migratory Species Fishery Management Plan (FMP), and the Pacific common thresher fishery is regulated by the Pacific Fishery Management Council through the FMP for U.S. West Coast Fisheries for Highly Migratory Species.[44][45]In the 1990s, after the depletion of common thresher stocks by the California gillnet fishery, the fleet was limited to 70 boats and restrictions were placed on season, operation range, and landings. Some evidence shows the California subpopulation is recovering, and the potential population growth rate has been estimated to be 4–7% per year.[43]
In New Zealand, theDepartment of Conservationhas classified the common thresher shark as "Not Threatened" under theNew Zealand Threat Classification System.[48]
Historical perceptions
[edit]TheGreekphilosopherAristotle(384–322BCE) wrote some of the earliest observations about the common thresher. In hisHistoria Animalia,he claimed that hooked threshers had a propensity for freeing themselves by biting throughfishing lines,and that they protected their young by swallowing them. These "clever" behaviors, which have not been borne out by science, led the ancient Greeks to call italopex(meaning "fox" ), on which its modern scientific name is based.[10]
An oft-repeated myth about the common thresher is that they cooperate withswordfishto attackwhales.In one version of events, the thresher shark circles the whale and distracts it by beating the sea to a froth with its tail, thereby allowing the swordfish to impale it in a vulnerable spot with itsrostrum.In an alternate account, the swordfish positions itself beneath the whale, while the thresher leaps out of the water and lands on top of the whale, hammering it onto the swordfish's rostrum. Yet other authors describe the thresher "cutting huge gashes" in the side of the whale with its tail. Neither threshers nor swordfish, however, are known to feed on whales or indeed possess thedentitionto do so. The story may have arisen from mariners mistaking the tall dorsal fins ofkiller whales,which do attack largecetaceans,for thresher shark tails. Swordfish bills have also been found embedded inblueandfin whales(likely accidents due to the fast-moving fish'sinertia), and thresher sharks do exhibit some of the aforementioned behaviors independent of whales.[10][49]
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External links
[edit]![](https://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Alopias vulpinus,ThresheratFishBase
- Biological Profiles: Thresher SharkatFlorida Museum of Natural History Ichthyology Department
- Biology of the Common Thresher (Alopias vulpinus)atReefQuest Centre for Shark Research
- Species Fact Sheets:Alopias vulpinusatFAO Fisheries and Agriculture Department