Thresher Shark
Scientific Name: Alopias Vulpinus
The thresher shark is characterized by its large upper caudal fin. This tail fin may often be 50 percent of the total length of the shark. The maximum size recorded for this species is 6.1 meters, however they generally are between 2 to 5 meters in length. Threshers are fairly slender and have a short snout and relatively small eyes (except the bigeyed thresher, whose eyes may measure 10 cm in diameter) placed forward on the head. The second dorsal fin is much smaller than the first. They have a gray, brownish, bluish, or purplish upper body and whitish belly. The thresher is a strong swimmer and can leap clear of the water, one of the few sharks known to breech. The jaws are small with small, curved sharp teeth and without basal cusps or serrations. The thresher is elusive and not much is known about its habits. It is considered harmless and it is a nocturnal and solitary creature. It's also called blue thresher, green thresher, longtail shark, swiveltail, fox shark, thintail thresher, or sea fox.
Encounters
The thresher often swims at the surface and sometimes at depths of 350 meters or more. They swim in coastal waters, as well as far off shore. The young are usually found inshore in shallow water. They are considered a pelagic or open ocean shark. The thresher shark is a summer visitor to the Canadian Atlantic region. The thresher ranges through all warm and temperate areas of the worlds oceans. Its northernmost range in the western Atlantic is eastern Newfoundland and it ranges all the way down the Atlantic to the West Indies and northern South America.
Diet
The thresher shark eats schooling fish, such as herring and mackerel and cephalopods, such as squid and cuttlefish. Crustaceans and seabirds are also eaten. The large caudal fin is used to slap the surface of the water forcing fish to form tighter schools; the tail can then be used as a whip to stun or kill the prey.
Reproduction
No distinct breeding season is observed by thresher sharks. The thresher is ovoviviparous, with the eggs being hatched inside the female, and 2 to 6 live pups being delivered at a size of 1.5 meters long. During development the young may cannibalize their siblings within the uterine chamber. The young fish exhaust their yolk sacs while still inside the mothers and then begin feeding on the unfertilized eggs. This behavior is known as oophagy. Males reach sexual maturity at 7-13 years and females at 8-14 years, a very slow maturing process. They may live for twenty years or more.
Attacks
Thresher sharks are considered harmless, however some divers have been struck by their whip-like tail.
Conservation
Since all large sharks are slow growing, threshers are therefore threatened by commercial fisheries. They are caught for their meat and liver oil, skin and fins. The fins are used in shark-fin soup. They also become entangled in nets. Thresher shark abundance in U.S. Atlantic waters has apparently decreased by about 67%.
Distinguishing Characteristics
- Extremely large caudal fin that may be up to 50 percent of the body length
- Relatively large eyes
- Relatively small teeth and jaw