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| Thresher shark | |
|---|---|
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| Pelagic thresher (A. pelagicus) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Lamniformes |
| Family: | Alopiidae Bonaparte, 1838 |
| Genus: | Alopias Rafinesque, 1810 |
| Type species | |
| Squalus vulpinus Bonnaterre, 1788 | |
| Synonyms | |
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Thresher sharks are largemackerel sharks of the familyAlopiidae found in alltemperate andtropicaloceans of the world; thefamily contains three extantspecies, all within thegenusAlopias.
All three thresher shark species have been listed asvulnerable by theWorld Conservation Union since 2007 (IUCN).[2] All three are popularbig-gamesport fish,[3] and additionally they are huntedcommercially for theirmeat,livers (forshark liver oil),skin (forshagreen) andfins (for use indelicacies such asshark-fin soup).
Despite being activepredatory fish, thresher sharks do not appear to be athreat to humans.[4]
The genus and family name derive from theGreek wordἀλώπηξ,alṓpēx, meaningfox. As a result, thelong-tailed orcommon thresher shark,Alopias vulpinus, is also known as thefox shark.[5] The common name is derived from a distinctive,thresher-like tail orcaudal fin which can be as long as the body of the shark itself.
The three extant thresher shark species are all in the genusAlopias. The possible existence of a hitherto unrecognized fourth species was revealed during the course of a 1995allozyme analysis by Blaise Eitner. This species is apparently found in the eastern Pacific offBaja California, and has previously been misidentified as thebigeye thresher. So far, it is only known from muscle samples from one specimen, and no aspect of itsmorphology has been documented.[6]
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| Phylogeny of Alopiidae[6][7] |
Based oncytochromebgenes, Martin and Naylor (1997) concluded the thresher sharks form amonophyletic sister group to theclade containing the familiesCetorhinidae (basking shark) andLamnidae (mackerel sharks). Themegamouth shark (Megachasma pelagios) was placed as the next-closest relative to these taxa, though the phylogenetic position of that species has yet to be resolved with confidence.Cladistic analyses byCompagno (1991) based on morphological characters, and Shimada (2005) based ondentition, have both corroborated this interpretation.[7][8]
Within the family, an analysis of allozyme variation by Eitner (1995) found the common thresher is the mostbasal member, with a sister relationship to a group containing the unrecognized fourthAlopias species and aclade comprising the bigeye and pelagic threshers. However, the position of the undescribed fourth species was only based on a singlesynapomorphy (derived group-defining character) in one specimen, so some uncertainty in its placement remains.[6]
Although occasionally sighted in shallow, inshore waters, thresher sharks are primarilypelagic; they prefer the open ocean, characteristically preferring water 550 metres (1,800 ft) and less.[9] Common threshers tend to be more prevalent in coastal waters overcontinental shelves. Common thresher sharks are found along the continental shelves of North America and Asia of the North Pacific, but are rare in the Central and Western Pacific. In the warmer waters of the Central and Western Pacific, bigeye and pelagic thresher sharks are more common. A thresher shark was seen on the live video feed from one of the ROVs monitoring BP's Macondo oil well blowout in the Gulf of Mexico. This is significantly deeper than the 500 m (1,600 ft) previously thought to be their limit.A bigeye has also been found in the western Mediterranean, and so distribution may be wider than previously believed, or environmental factors may be forcing sharks to search for new territories.[10][11]
Named for their exceptionally long,thresher-likeheterocercal tail orcaudal fins (which can be as long as the total body length), thresher sharks are active predators; the tail is used as a weapon to stun prey.[12][13] The thresher shark has a short head and a cone-shaped nose. The mouth is generally small, and the teeth range in size from small to large.[14] By far the largest of the three species is thecommon thresher,Alopias vulpinus, which may reach a length of 6.1 metres (20 ft) and a mass of over 500 kilograms (1,100 lb). Thebigeye thresher,A. superciliosus, is next in size, reaching a length of 4.9 m (16 ft); at just 3 m (10 ft), thepelagic thresher,A. pelagicus, is the smallest.
Thresher sharks are fairly slender, with smalldorsal fins and large, recurvedpectoral fins. With the exception of the bigeye thresher, these sharks have relatively small eyes positioned to the forward of the head. Coloration ranges from brownish, bluish or purplish gray dorsally with lighter shades ventrally.[15]The three species can be roughly distinguished by the primary color of the dorsal surface of the body. Common threshers are dark green, bigeye threshers are brown and pelagic threshers are generally blue. Lighting conditions andwater clarity can affect how any one shark appears to an observer, but the color test is generally supported when other features are examined.
The thresher shark mainly feeds onschoolingpelagic fish such asbluefish, juveniletuna andmackerel, which they are known to follow into shallow waters, as well assquid andcuttlefish.[16]Crustaceans and occasionallyseabirds are also eaten. The thresher shark stuns its prey by using its elongated tail as a whipping weapon.
| External videos | |
|---|---|
 Stunning tail: Thresher sharks evolved to slap and kill their prey –NBC News | |
| Thresher Shark Stun Prey With Tail-Slap –Live Science |
Thresher sharks are solitary creatures that keep to themselves. It is known that thresher populations of theIndian Ocean are separated by depth and space according to sex. Some species however do occasionally hunt in a group of two or three contrary to their solitary nature. All species are noted for their highly migratory oroceanodromous habits. When hunting schooling fish, thresher sharks are known to "whip" the water.[15] The elongated tail is used to swat smaller fish, stunning them before feeding.[17] Thresher sharks are one of the few shark species known to jump fully out of the water, using their elongated tail to propel them out of the water, making turns likedolphins; this behavior is calledbreaching.
Two species of the thresher have been identified as having a modified circulatory system that acts as a counter-current heat exchanger, which allows them to retain metabolic heat.Mackerel sharks (family Lamnidae) have a similar homologous structure to this which is more extensively developed. This structure is a strip of red muscle along each of its flanks, which has a tight network of blood vessels that transfer metabolic heat inward towards the core of the shark, allowing it to maintain and regulate its body heat.
No distinct breeding season is observed by thresher sharks. Fertilization and embryonic development occur internally; thisovoviviparous or live-bearing mode of reproduction results in a small litter (usually two to four) of large well-developed pups, up to 150 cm (59 in) at birth in thintail threshers. The young fish exhaust their yolk sacs while still inside the mother, at which time they begin feasting on the mother's unfertilized eggs; this is known asoophagy.
Thresher sharks are slow to mature; males reach sexual maturity between seven and thirteen years of age and females between eight and fourteen years in bigeye threshers. They may live for 20 years or more.
In October 2013, the first picture of a thresher shark giving birth was taken off the coast of the Philippines.[18]
Thresher sharks are classified as prizedgame fish in the United States and South Africa.[citation needed] Common thresher sharks are the target of a popular recreational fishery off Baja, Mexico.
Because of their lowfecundity, thresher sharks are highly vulnerable tooverfishing.[19] All three thresher shark species have been listed asvulnerable to extinction by theWorld Conservation Union since 2007 (IUCN).[2]
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