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Thresher shark
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Thresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.
genus and family name derive from the Greek word alopex, meaning fox. Indeed the long-tailed thresher shark, Alopias vulpinus, is named the fox shark by some authorities.
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Thresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.
Naming
The genus and family name derive from the Greek word alopex, meaning fox. Indeed the long-tailed thresher shark, Alopias vulpinus, is named the fox shark by some authorities. In some villages in the local islands of its habitat, it is known as quigano(kee-gan-oh), which directly translates to "long and mischievous".
Distribution and habitat
Although occasionally sighted in shallow, inshore waters, thresher sharks are primarily pelagic; they prefer the open ocean, venturing no deeper than . Common threshers tend to be more common in coastal waters over continental shelves. In the North Pacific, common thresher sharks are found along the continental shelves of North America and Asia. They are rare in the Central and Western Pacific. In the warmer waters of the Central & Western Pacific, bigeye and pelagic thresher sharks are more common.
Anatomy and appearance
Named for and easily recognised by their exceptionally long, thresher-like tail or caudal fins (which account for 1/3 of their total body length), thresher sharks are active predators; the tail is actually used as a weapon to stun prey. By far the largest of the three species is the Common thresher, Alopias vulpinus, which may reach a length of 20 ft and a weight of over 500 kg. The Bigeye thresher, Alopias superciliosus, is next in size, reaching a length of 4.9 m (16 ft); at just 3 m (10 ft), the Pelagic thresher, Alopias pelagicus, is the smallest.
Thresher sharks are fairly slender, with small dorsal fins and large, recurved pectoral fins. With the exception of the Bigeye thresher, these sharks have relatively small eyes. Coloration ranges from brownish, bluish or purplish gray dorsally with lighter shades ventrally.
The three species can be roughly distinguished by the main 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 and water clarity can affect how any one shark appears to an observer, but the color test is generally supported when other features are examined.
Diet
Pelagic schooling fish (such as bluefish, juvenile tuna, and mackerel), squid and cuttlefish are the primary food items of the thresher sharks. They are known to follow large schools of fish into shallow waters. Crustaceans and the odd seabird are also taken.
Behaviour
Thresher sharks are solitary creatures which keep to themselves. It is known that thresher populations of the Indian Ocean are separated by depth and space according to gender. All species are noted for their highly migratory or oceanodromous habits.
Thresher sharks are one of the few shark species known to jump fully out of the water making turns like dolphins, this behaviour is called breaching.
Reproduction
No distinct breeding season is observed by thresher sharks. Fertilization and embryonic development occur internally; this ovoviviparous or live-bearing mode of reproduction results in a small litter (usually 2 to 4) of large well-developed pups, up to 150 cm at birth in thintail threshers. The young fish exhaust their yolk sacs while still inside the mother, at which time they begin feeding on the mother's unfertilized eggs; this is known as oophagy.
Thresher sharks are slow to mature, males reaching sexual maturity between 7 and 13 years of age and females between 8 and 14 years in bigeye threshers. They may live for 20 years or more.
Conservation
All three thresher shark species have been recently listed as vulnerable to extinction by the World Conservation Union (IUCN).
Importance to humans
Like all large sharks, threshers are slow growing and are therefore threatened by commercial fisheries. Other than for its meat, the sharks are hunted for their liver oil, skin (for leather), and their fins, for use in shark-fin soup.
They do not appear to be a threat to humans, although some divers have been hit with the upper tail lobe. There is an unconfirmed account of a fisherman being decapitated by a tail swipe as the shark breached.
Thresher sharks are classified as prized gamefish in the United States and South Africa. Common thresher sharks are the target of a popular recreational fishery off Baja Mexico. Thresher sharks are managed in some areas for their value as both a recreational sport fish and commercial species.
Evolution
The Thresher Shark has traditionally been said to be closely related to other sharks such as the great white and the mackerel from the Lamnidae family. However recent research has identified them as having a closer ancestral connection with the ragged tooth sharks (odontaspididae), the megamouth (Megachasmidae), and the crocodile sharks (pseudocarchariidae) then with the Lamnidae sharks. Due to the similarity between the second dorsal fin, tail and anal fin of the thresher and megamouth sharks these species have been said to be very closely related and have not diverged much from their common ancestor.
Homologous features
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. The mackerel sharks (Lamnidae family) 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.
Species
See also
External links
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