A few facts about thresher sharks…
Taxonomy
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. The common name is derived from a distinctive thresher-like tail or caudal fin which can be as long as the body of the shark itself.
Species
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.
- Alopias pelagicus – Pelagic thresher
- Alopias superciliosus – Bigeye thresher
- Alopias vulpinus – Common thresher
Distribution and habitat
Although occasionally sighted in shallow, inshore waters, thresher sharks are primarily pelagic; they prefer the open ocean, venturing no deeper than 500 metres (1,600 ft). 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 can be as long as the 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 6.1 metres (20 ft) and a weight of over 500 kilograms (1,100 lb). 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 positioned to the forward of the head. Colouration ranges from brownish, bluish or purplish gray dorsally with lighter shades ventrally. The three species can be roughly distinguished by the main colour 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 colour test is generally supported when other features, such as fin alignment, 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. When hunting schooling fish, thresher sharks are known to “slap” the water, herding and stunning prey. The elongated tail is used to swat smaller fish, stunning them before feeding. 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. Fertilisation 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. 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).
Where can they be seen ?
Monad Shoal near Malapascua is a sunken island at 18-24m whose sides drop off to 230m. The thresher sharks live and hunt in this deep water for most of the day, but in the early morning, before it gets too light, they come up to the Shoal, attracted by its cleaning stations. Here they have a symbiotic (mutually beneficial) relationship with the small fish called cleaning wrasse which eat dead skin and bacteria from the shark’s body, its gills, and even inside its mouth.
