Thresher sharks are large
lamniform
The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the grea ...
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s of the family Alopiidae found in all temperate and tropical
oceans of the world; the
family contains three extant
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, all within the
genus ''Alopias''.
All three thresher shark species have been listed as
vulnerable to extinction by the
World Conservation Union
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natur ...
since 2007 (IUCN).
All are popular sport fish. In addition, they are hunted for their meat, livers (for
shark liver oil
Shark liver oil is an oil obtained from the livers of sharks. It has been used for centuries as a folk remedy to promote the healing of wounds and as a remedy for respiratory tract and digestive system problems. ), skin (for leather), and fins for use in
shark-fin soup
Shark fin soup is a traditional soup or stewed dish served in parts of China, Taiwan, and Southeast Asia. The shark fins provide texture, while the taste comes from the other soup ingredients. It is commonly served at special occasions such as ...
.
They do not appear to be a threat to humans.
Taxonomy
The genus and family name derive from the
Greek word , , meaning
fox. As a result, the long-tailed or
common thresher shark, ''Alopias vulpinus'', is also known as the fox shark. The common name is derived from a distinctive,
thresher-like tail or
caudal fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
which can be as long as the body of the shark itself.
Species
The three extant thresher shark species are all in the genus ''Alopias''. The possible existence of a hitherto unrecognized fourth species was revealed during the course of a 1995
allozyme analysis by Blaise Eitner. This species is apparently found in the eastern Pacific off
Baja California, and has previously been misidentified as the
bigeye thresher. So far, it is only known from muscle samples from one specimen, and no aspect of its
morphology has been documented.
Phylogeny and evolution
Based on
cytochrome ''b'' gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s, Martin and Naylor (1997) concluded the thresher sharks form a
monophyletic sister group to the
clade containing the families
Cetorhinidae
Cetorhinidae is a family of filter feeding mackerel sharks, whose members are commonly known as basking sharks. It includes the extant basking shark, ''Cetorhinus'', as well as two extinct genera, '' Caucasochasma'' and ''Keasius
''Keasius'' ...
(basking shark) and
Lamnidae
The Lamnidae are the family of mackerel sharks known as white sharks. They are large, fast-swimming predatory fish found in oceans worldwide, though prefer environments with colder water. The name of the family is formed from the Greek word ''l ...
(mackerel sharks). The
megamouth 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 by
Compagno (1991) based on morphological characters, and Shimada (2005) based on
dentition, have both corroborated this interpretation.
Within the family, an analysis of allozyme variation by Eitner (1995) found the common thresher is the most
basal member, with a sister relationship to a group containing the unrecognized fourth ''Alopias'' species and a
clade comprising the bigeye and pelagic threshers. However, the position of the undescribed fourth species was only based on a single
synapomorphy (
derived
Derive may refer to:
*Derive (computer algebra system), a commercial system made by Texas Instruments
* ''Dérive'' (magazine), an Austrian science magazine on urbanism
*Dérive, a psychogeographical concept
See also
*
*Derivation (disambiguation ...
group-defining character) in one specimen, so some uncertainty in its placement remains.
Distribution and habitat
Although occasionally sighted in shallow, inshore waters, thresher sharks are primarily
pelagic; they prefer the open ocean, characteristically preferring water and less. Common threshers tend to be more prevalent in coastal waters over
continental 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 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.
Anatomy and appearance
Named for their exceptionally long,
thresher-like
heterocercal
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...
tail or ''
caudal fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
s'' (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.
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. By far the largest of the three species is the
common thresher, ''Alopias vulpinus'', which may reach a length of and a mass of over . The
bigeye thresher, ''A. superciliosus'', is next in size, reaching a length of 4.9 m (16 ft); at just 3 m (10 ft), the
pelagic thresher, ''A. pelagicus'', is the smallest.
Thresher sharks are fairly slender, with small
dorsal fins and large, recurved
pectoral fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
s. 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.
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 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
The thresher shark mainly feeds on
pelagic schooling fish such as bluefish, juvenile tuna, and mackerel, which they are known to follow into shallow waters, as well as squid and cuttlefish.
Crustaceans and occasionally seabirds are also taken. The thresher shark stuns its prey by using its elongated tail as a weapon.
Behavior
Thresher sharks are solitary creatures that keep to themselves. It is known that thresher populations of the
Indian 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 or
oceanodromous habits. When hunting schooling fish, thresher sharks are known to "whip" the water.
The elongated tail is used to swat smaller fish, stunning them before feeding. Sometimes the thresher shark will slice the fish in half before eating. 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 like
dolphins; this behavior is called
breaching.
Endothermy
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 shark
The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the gre ...
s (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.
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 two to four) of large well-developed pups, up to 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 as
oophagy.
Thresher sharks are slow to mature; males reach sexual maturity between seven and 13 years of age and females between eight and 14 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.
Fisheries
Thresher sharks are classified as prized
game fish in the United States and South Africa. Common thresher sharks are the target of a popular recreational fishery off Baja, Mexico.
Status
Because of their low
fecundity, thresher sharks are highly vulnerable to
overfishing. All three thresher shark species have been listed as
vulnerable to extinction by the
World Conservation Union
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natur ...
since 2007 (IUCN).
See also
*
Shark meat
Shark meat is a seafood consisting of the flesh of sharks. Several sharks are fished for human consumption, such as porbeagles, shortfin mako shark, requiem shark, and thresher shark, among others. Shark meat is popular in Asia, where it is oft ...
References
*
*
External links
FishBase entry on AlopiidaeThresher shark videoon Check123
{{Taxonbar, from=Q627902
Commercial fish
Cosmopolitan fish
Ovoviviparous fish
Sport fish
Extant Ypresian first appearances
Fish described in 1810
Taxa named by Constantine Samuel Rafinesque