Thresher shark
Overview
Thresher sharks are large lamniform shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s of the family Alopiidae. Found in all temperate and tropical ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s of the world, the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 contains three species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 all within the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Alopias.
The genus and family name derive from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word alopex, meaning fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...

. Indeed the long-tailed thresher shark
Long-tailed thresher shark
The common thresher is the largest species of thresher shark, family Alopiidae, attaining a maximum known length of . Almost half of that length consists of the elongated upper lobe of its caudal fin...

, Alopias vulpinus, is named the fox shark by some authorities. The common name is derived from a distinctive thresher
Thresher
Thresher may refer to:*Threshing machine , a device that first separates the head of a stalk of grain from the straw, and then further separates the kernel from the rest of the head...

-like tail or caudal fin which can be as long as the body of the shark itself.
There are three extant thresher shark species, all in the genus Alopias.
Encyclopedia
Thresher sharks are large lamniform shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s of the family Alopiidae. Found in all temperate and tropical ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s of the world, the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 contains three species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 all within the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Alopias.

Taxonomy

The genus and family name derive from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word alopex, meaning fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...

. Indeed the long-tailed thresher shark
Long-tailed thresher shark
The common thresher is the largest species of thresher shark, family Alopiidae, attaining a maximum known length of . Almost half of that length consists of the elongated upper lobe of its caudal fin...

, Alopias vulpinus, is named the fox shark by some authorities. The common name is derived from a distinctive thresher
Thresher
Thresher may refer to:*Threshing machine , a device that first separates the head of a stalk of grain from the straw, and then further separates the kernel from the rest of the head...

-like tail or caudal fin which can be as long as the body of the shark itself.

Species

There are three extant thresher shark species, all in the genus Alopias. The possible existence of a hitherto unrecognized fourth species was revealed during the course of a 1995 allozyme
Allozyme
Variant forms of an enzyme that are coded by different alleles at the same locus are called allozymes. These are opposed to isozymes, which are enzymes that perform the same function, but which are coded by genes located at different loci....

 analysis by Blaise Eitner. This species is apparently found in the eastern Pacific off Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...

, and has before been misidentified as the bigeye thresher. Thus far, it is only known from muscle samples and no aspect of its morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 has been documented.
  • Alopias pelagicus Nakamura, 1935 (Pelagic thresher)
  • Alopias superciliosus R. T. Lowe
    Richard Thomas Lowe
    Richard Thomas Lowe was a British botanist, ichthyologist, malacologist, and clergyman. In 1825 he graduated from Christ's College, Cambridge and in the same year took holy orders. He became a clergyman in the Madeira Islands in 1832, where he was a part-time naturalist, extensively studying the...

    , 1841
    (Bigeye thresher)
  • Alopias vulpinus (Bonnaterre
    Pierre Joseph Bonnaterre
    Abbé Pierre Joseph Bonnaterre was a French naturalist who contributed sections on cetaceans, mammals, birds, reptiles, amphibians, and insects to the Tableau encyclopédique et méthodique...

    , 1788)
    (Common thresher)

Phylogeny and evolution

Based on cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...

 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s, Martin and Naylor (1997) concluded that the thresher sharks form a monophyletic sister group to the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 containing the families Cetorhinidae (basking shark) and Lamnidae
Lamnidae
Lamnidae is a family of sharks, commonly known as mackerel sharks or white sharks. They are large, fast-swimming sharks, found in oceans worldwide....

 (mackerel sharks). The megamouth shark
Megamouth shark
The megamouth shark, Megachasma pelagios, is an extremely rare species of deepwater shark. Since its discovery in 1976, only a few megamouth sharks have been seen, with 53 specimens known to have been caught or sighted as of 2011, including three recordings on film...

 (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
Leonard Compagno
Leonard Joseph Victor Compagno is an international authority on shark taxonomy and the author of many scientific papers and books on the subject, best known of which is his 1984 catalogue of shark species produced for the Food and Agriculture Organization of the United Nations.-Career:*Ph.D,...

 (1991) based on morphological characters, and Shimada (2005) based on dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...

, have both corroborated this interpretation.

Within the family, an analysis of allozyme variation by Eitner (1995) found that the common thresher is the most basal member, with a sister relationship to a group containing the unrecognized fourth Alopias species and a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 comprising the bigeye and pelagic threshers. However, the position of the undescribed fourth species was only based on a single synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 (derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...

 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
Pelagic fish
Pelagic fish live near the surface or in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs.The marine pelagic...

; they prefer the open ocean, venturing no deeper than 500 metres (1,640.4 ft). Common threshers tend to be more common in coastal waters over continental shelves
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

. 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. A thresher shark was seen on the live video feed from one of the ROVs monitoring BP's Macondo oil well blow out in the Gulf of Mexico. This is significantly deeper than the 500m previously thought to be their limit.

Anatomy and appearance

Named for and easily recognised by their exceptionally long, thresher
Thresher
Thresher may refer to:*Threshing machine , a device that first separates the head of a stalk of grain from the straw, and then further separates the kernel from the rest of the head...

-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. 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 6.1 metres (20 ft) and a weight of over 500 kilograms (1,102.3 lb). The Bigeye thresher
Bigeye thresher
The bigeye thresher is a species of thresher shark, family Alopiidae, found in temperate and tropical oceans worldwide. Like other thresher sharks, nearly half its total length consists of the elongated upper lobe of the tail fin. Its common name comes from its enormous eyes, which are placed in...

, Alopias superciliosus, is next in size, reaching a length of 4.9 m (16 ft); at just 3 m (10 ft), the Pelagic thresher
Pelagic thresher
The pelagic thresher is a species of thresher shark, family Alopiidae; this group of sharks are characterized by the greatly elongated upper lobes of their caudal fins. The pelagic thresher occurs in the tropical and subtropical waters of the Indian and Pacific Oceans, usually far from shore but...

, Alopias pelagicus, is the smallest.

Thresher sharks are fairly slender, with small dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

s 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. 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
Shoaling and schooling
In biology, any group of fish that stay together for social reasons are said to be shoaling , and if, in addition, the group is swimming in the same direction in a coordinated manner, they are said to be schooling . In common usage, the terms are sometimes used rather loosely...

 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. However, recently over ten Thresher sharks were reportedly spotted off the coast of a Long Island beach looking for food (1JUN11). It is known that thresher populations of the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 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 dolphin
Dolphin
Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s, this behaviour 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. 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.

Reproduction

No distinct breeding season is observed by thresher sharks. Fertilization and embryonic development occur internally; this ovoviviparous
Ovoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...

 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
Oophagy
Oophagy , literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek ᾠόν and classical Greek φᾱγεῖν ....

.

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.

Importance to humans

Like all large sharks, threshers have a low fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

 and are therefore highly vulnerable to overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....

. Other than for its meat, the sharks are hunted for their liver oil
Shark liver oil
Shark liver oil is obtained from sharks that are caught for food purposes and are living in cold, deep oceans. The liver oil from sharks has been used by fishermen for centuries as a folk remedy for general health...

, 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 a dubious 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.

Conservation

All three thresher shark species have been recently listed as vulnerable to extinction
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...

 by the World Conservation Union
World Conservation Union
The International Union for Conservation of Nature and Natural Resources is an international organization dedicated to finding "pragmatic solutions to our most pressing environment and development challenges." The organization publishes the IUCN Red List, compiling information from a network of...

(IUCN).

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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