Pelagic thresher
Encyclopedia
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 Ocean
s, usually far from shore but occasionally entering coastal habitats. It is often confused with the common thresher (A. vulpinus), even in professional publications, but can be distinguished by the dark, rather than white, color over the bases of its pectoral fins. The smallest of the three thresher species, the pelagic thresher typically measures 3 m (10 ft) long.
The diet of the pelagic thresher consists mainly of small midwater fishes, which are stunned with whip-like strikes of their tails. Along with all other mackerel sharks
, the pelagic thresher exhibits ovoviviparity
and usually gives birth to litters of two. The developing embryo
s are oophagous
, feeding on unfertilized eggs produced by the mother. The young are born unusually large, up to 43% the length of the mother. Pelagic threshers are valued by commercial fisheries
for their meat, skin, liver oil, and fins, and are also pursued by sport fishers. The International Union for Conservation of Nature (IUCN) assessed this species as Vulnerable
in 2007.
, that Leonard Compagno
later concluded was probably of a common thresher. Several authors, including Gohar and Mazhar (1964, Red Sea), Kato, Springer and Wagner (1967, Eastern Pacific), Fourmanoir and Laboute (1976, New Caledonia), Johnson (1978, Tahiti), and Faughnan (1980, Hawaiian Islands) have published illustrations of "common threshers" that were in fact pelagic threshers.
An allozyme
analysis conducted by Blaise Eitner in 1995 showed that the closest relative of the pelagic thresher is the bigeye thresher
(A. superciliosus), with which it forms a clade
. The specific epithet pelagicus is from the Greek
pelagios, meaning "of the sea". Another common name
is the smalltooth thresher.
, with scattered records from South Africa
, the Red Sea
and the Arabian Sea
(off Somalia
, between Oman
and India
, and off Pakistan
), to China
, southeastern Japan
, northwestern Australia
, New Caledonia
, and Tahiti
, to the Hawaiian Islands
, the Gulf of California
, and the Galapagos Islands
. The North Pacific population shifts northward during warm El Nino years. Analysis of mitochondrial DNA
has shown that there is extensive gene flow
within the eastern and western Pacific pelagic thresher populations, but little flow between them.
The pelagic thresher primarily inhabits the open ocean
, occurring from the surface to a depth of at least 150 m (492 ft). However, it occasionally comes close to shore in regions with a narrow continental shelf
, and has been observed near coral reef
dropoffs or seamount
s in the Red Sea and the Sea of Cortez, and off Indonesia
and Micronesia
. It has also been known to enter large lagoon
s in the Tuamotu Islands.
The pelagic thresher is the smallest of the thresher sharks, averaging 3 m (10 ft) in length and 69.5 kg (153.3 lb) in weight, and usually not exceeding 3.3 m (10.8 ft) and 88.4 kg (194.9 lb). Males and females attain known maximum lengths of 3.5 m (11.5 ft) and 3.8 m (12.5 ft) respectively. A record of 5 m (16.4 ft) is dubious and may have resulted from confusion with other thresher species. This species has fusiform body (wide in the middle and tapered at the ends) and a very slender upper caudal fin lobe nearly as long as the rest of the shark. The pectoral fins are long and straight with broad, rounded tips. The first dorsal fin
is placed halfway between the pectoral and pelvic fins, and is of comparable size to the pelvic fins. The second dorsal and anal fins are tiny.
The head is narrow with a short conical snout and a distinctive "pinched" profile when viewed from below. The eyes are very large in juveniles and decrease in relative size with age. There are no furrows at the corners of the mouth. The teeth
are very small, numbering 21–22 rows on each side with a symphysial (central) row in the upper jaw and 21 on each side without a symphysial row in the lower jaw. There are 5–11 rows of posterior teeth. The teeth are smooth-edged, with oblique cusps and lateral cusplets on the outside margins. The body is covered with very small, smooth dermal denticles with flat crowns and cusps with parallel ridges. The coloration is an intense dark blue above and white below; the white does not extend to above the pectoral fins. The color rapidly fades to gray after death. The dark pigment above the pectoral fins, the rounded pectoral fin tips, and the absence of labial furrows separate this shark from the common thresher.
s. Known parasites of this species include the tapeworms Litobothrium amplifica, L. daileyi, and L. nickoli, which inhabit the shark's spiral valve
intestine, and copepod
s of the genus Echthrogaleus, which infest the skin. At Malapascua Island
in the Philippines
, pelagic threshers have been observed regularly visiting cleaning stations occupied by cleaner wrasse
s (Labroides dimidiatus and Thalassoma lunare), during which they exhibit characteristic behaviors to facilitate the cleaning interaction. These visits occur more frequently early in the morning, and may be why these normally oceanic sharks are sometimes encountered in shallow water.
Little information is available on the feeding ecology of the pelagic thresher. Its very slender tail and fine dentition suggest an exclusive diet of small, pelagic prey. Analysis of stomach contents reveal that pelagic threshers feed mainly on barracudina
s, lightfishes
, and escolars, all inhabitants of the mesopelagic zone. There is therefore little competition between the pelagic thresher and other large oceanic piscivores such as billfish
es, tuna
s, and dolphinfishes, which tend to feed near the surface. As in other threshers, pelagic threshers may swim in circles to drive schooling
prey into a compact mass, before striking them sharply with the upper lobe of its tail to stun them. Because of this behavior, pelagic threshers are often hooked on longlines by their tails.
, the pelagic thresher is ovoviviparous and gives birth to two pups at a time (rarely just one), one per uterus
. There is no defined breeding season
and most adult females are pregnant throughout the year; the gestation period
is uncertain but has been suggested to be less than one year as in the common thresher. The developing embryo
s are sustained by a yolk sac
until they are 12 cm (4.7 in) long, after which they are oophagous
and feed on egg capsules produced by the mother. Each capsule measures about 55 mm (2.2 in) long and 12 mm (0.5 in) across, and contains 20–30 ova. Early stage embryos have specialized teeth for opening the capsules, while later-stage embryos have their teeth hidden and swallow the capsules whole, their teeth not becoming functional again until just after birth. There is no evidence of sibling cannibalism as in the sand tiger shark (Carcharias taurus). Young pelagic threshers are born at an unusually large size, up to 1.6 m (5.2 ft) long or 43% the length of the mother, which likely reduces predation on the newborns.
The growth rate of pelagic threshers slows with age: 9 cm/year for ages 0–1, 8 cm/year for ages 2–3, 6 cm/year for ages 5–6, 4 cm/year for ages 7–10, 3 cm/year for ages 10–12, and 2 cm/year for ages 13 and greater. Females reach maturity at 2.8–2.9 m (9.2–9.5 ft) long and 8–9 years old, while males mature at 2.7–2.8 m (8.9–9.2 ft) long and 7–8 years old. The oldest confirmed ages for females and males are 16 and 14 years respectively. Extrapolating the growth curves to the largest known individuals suggests that females may have a lifespan exceeding 28 years, and males 17 years. A single female produces about 40 young over her entire life.
like the common thresher. The pelagic thresher lacks a rete mirabile
, a blood vessel
countercurrent exchange
system that prevents metabolic heat
from being dissipated into the water, inside its trunk. Furthermore, its aerobic red muscles, responsible for generating heat in the common thresher, are positioned in two lateral strips just beneath the skin rather than at the core of the body. There is a rete system present around the pelagic thresher's brain and eyes, albeit less developed than in the bigeye thresher, which may serve to buffer those organs against temperature changes.
The pelagic thresher has never been implicated in an attack on humans; it has small jaws and teeth for its size and tends to flee from divers. This shark is taken by commercial fisheries in the central Pacific and western Indian Oceans, as well as off California and Mexico. Abundant off northeastern Taiwan
, it comprises over 12% (c. 3,100 fish, 220 metric tons) of the annual Taiwanese shark landings. The meat is sold for human consumption, the skin is made into leather
, and the fins used for shark fin soup
in Asia. The squalene oil
in the liver
of the pelagic thresher can comprise 10% of its weight, and is used in the manufacture of cosmetics
, health food
s, and high-grade machine oil.
Though rarely caught, pelagic threshers are also valued by sport fishers and are listed as game fish
by the International Game Fish Association
. The largest overall records are from New Zealand
, while the light tackle
records are from California
. Pelagic threshers are frequently taken as bycatch
on longlines and in driftnets meant for other species such as tuna, and also rarely in gillnet
s and anti-shark nets
. All three thresher shark species were assessed as Vulnerable
by the IUCN in 2007, as their low fecundity
cannot sustain high levels of exploitation.
Thresher shark
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.-Taxonomy:...
, 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
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...
and Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
s, usually far from shore but occasionally entering coastal habitats. It is often confused with the common thresher (A. vulpinus), even in professional publications, but can be distinguished by the dark, rather than white, color over the bases of its pectoral fins. The smallest of the three thresher species, the pelagic thresher typically measures 3 m (10 ft) long.
The diet of the pelagic thresher consists mainly of small midwater fishes, which are stunned with whip-like strikes of their tails. Along with all other mackerel sharks
Lamniformes
Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...
, the pelagic thresher exhibits ovoviviparity
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...
and usually gives birth to litters of two. The developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s are oophagous
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 ....
, feeding on unfertilized eggs produced by the mother. The young are born unusually large, up to 43% the length of the mother. Pelagic threshers are valued by commercial fisheries
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
for their meat, skin, liver oil, and fins, and are also pursued by sport fishers. The International Union for Conservation of Nature (IUCN) assessed this species as Vulnerable
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:...
in 2007.
Taxonomy and phylogeny
The pelagic thresher was originally described by Japanese ichthyologist Hiroshi Nakamura on the basis of three large specimens, none of which were kept as a type specimen. He illustrated one of the three specimens in his paper, "On the two species of the thresher shark from Formosan waters", published in August 1935. Nakamura also separately illustrated and described a fetusFetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
, that Leonard 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,...
later concluded was probably of a common thresher. Several authors, including Gohar and Mazhar (1964, Red Sea), Kato, Springer and Wagner (1967, Eastern Pacific), Fourmanoir and Laboute (1976, New Caledonia), Johnson (1978, Tahiti), and Faughnan (1980, Hawaiian Islands) have published illustrations of "common threshers" that were in fact pelagic threshers.
An 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 conducted by Blaise Eitner in 1995 showed that the closest relative of the pelagic thresher is 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...
(A. superciliosus), with which it forms 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...
. The specific epithet pelagicus is 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;...
pelagios, meaning "of the sea". Another common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
is the smalltooth thresher.
Distribution and habitat
Due to confusion with the common thresher, the distribution of the pelagic thresher may be wider than is currently known. It ranges extensively in the Indo-PacificIndo-Pacific
The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...
, with scattered records from South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, the Red Sea
Red Sea
The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
and the Arabian Sea
Arabian Sea
The Arabian Sea is a region of the Indian Ocean bounded on the east by India, on the north by Pakistan and Iran, on the west by the Arabian Peninsula, on the south, approximately, by a line between Cape Guardafui in northeastern Somalia and Kanyakumari in India...
(off Somalia
Somalia
Somalia , officially the Somali Republic and formerly known as the Somali Democratic Republic under Socialist rule, is a country located in the Horn of Africa. Since the outbreak of the Somali Civil War in 1991 there has been no central government control over most of the country's territory...
, between Oman
Oman
Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...
and India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, and off Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
), to China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, southeastern Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, northwestern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Caledonia
New Caledonia
New Caledonia is a special collectivity of France located in the southwest Pacific Ocean, east of Australia and about from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Belep archipelago, the Isle of...
, and Tahiti
Tahiti
Tahiti is the largest island in the Windward group of French Polynesia, located in the archipelago of the Society Islands in the southern Pacific Ocean. It is the economic, cultural and political centre of French Polynesia. The island was formed from volcanic activity and is high and mountainous...
, to the Hawaiian Islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
, the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
, and the Galapagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
. The North Pacific population shifts northward during warm El Nino years. Analysis of mitochondrial DNA
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
has shown that there is extensive gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
within the eastern and western Pacific pelagic thresher populations, but little flow between them.
The pelagic thresher primarily inhabits the open ocean
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
, occurring from the surface to a depth of at least 150 m (492 ft). However, it occasionally comes close to shore in regions with a narrow continental shelf
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,...
, and has been observed near coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
dropoffs or seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
s in the Red Sea and the Sea of Cortez, and off Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
and Micronesia
Micronesia
Micronesia is a subregion of Oceania, comprising thousands of small islands in the western Pacific Ocean. It is distinct from Melanesia to the south, and Polynesia to the east. The Philippines lie to the west, and Indonesia to the southwest....
. It has also been known to enter large lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
s in the Tuamotu Islands.
Description
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...
is placed halfway between the pectoral and pelvic fins, and is of comparable size to the pelvic fins. The second dorsal and anal fins are tiny.
The head is narrow with a short conical snout and a distinctive "pinched" profile when viewed from below. The eyes are very large in juveniles and decrease in relative size with age. There are no furrows at the corners of the mouth. The teeth
Shark tooth
A shark tooth is one of the numerous teeth of a shark. Sharks continually shed their teeth, and some Carcharhiniformes shed approximately 35,000 teeth in a lifetime. In some geological formations, shark's teeth are a common fossil...
are very small, numbering 21–22 rows on each side with a symphysial (central) row in the upper jaw and 21 on each side without a symphysial row in the lower jaw. There are 5–11 rows of posterior teeth. The teeth are smooth-edged, with oblique cusps and lateral cusplets on the outside margins. The body is covered with very small, smooth dermal denticles with flat crowns and cusps with parallel ridges. The coloration is an intense dark blue above and white below; the white does not extend to above the pectoral fins. The color rapidly fades to gray after death. The dark pigment above the pectoral fins, the rounded pectoral fin tips, and the absence of labial furrows separate this shark from the common thresher.
Biology and ecology
The pelagic thresher is an active, strong swimmer and has been known to leap clear of the water (five times in a row on one documented occasion). Predators of the pelagic thresher include larger fishes (including other sharks) and toothed whaleToothed whale
The toothed whales form a suborder of the cetaceans, including sperm whales, beaked whales, dolphins, and others. As the name suggests, the suborder is characterized by the presence of teeth rather than the baleen of other whales.-Anatomy:Toothed whales have a single blowhole on the top of the head...
s. Known parasites of this species include the tapeworms Litobothrium amplifica, L. daileyi, and L. nickoli, which inhabit the shark's spiral valve
Spiral valve
A spiral valve is the lower portion of the intestine of some sharks, rays, skates and bichirs. A modification of the ileum, the spiral valve is internally twisted or coiled to increase the surface area of the intestine, to increase nutrient absorption....
intestine, and copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s of the genus Echthrogaleus, which infest the skin. At Malapascua Island
Malapascua Island
Malapascua Island is a Filipino island situated in the Visayan Sea, located across a shallow strait from the northernmost tip of Cebu Island. Administratively, it is part of the insular barangay of Logon, Daanbantayan Municipality, Cebu Province, the Philippines...
in the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, pelagic threshers have been observed regularly visiting cleaning stations occupied by cleaner wrasse
Cleaner fish
Cleaner fish are fish that provide a service to other fish species by removing dead skin and ectoparasites. This is an example of mutualism, an ecological interaction that benefits both parties involved. A wide variety of fishes have been observed to display cleaning behaviors including wrasses,...
s (Labroides dimidiatus and Thalassoma lunare), during which they exhibit characteristic behaviors to facilitate the cleaning interaction. These visits occur more frequently early in the morning, and may be why these normally oceanic sharks are sometimes encountered in shallow water.
Feeding
Barracudina
Barracudinas are about 50 species of marine fishes of the family Paralepididae, found almost worldwide in deep waters.They are elongated, slender fish with large eyes, and a pointed snout containing fang-like teeth...
s, lightfishes
Phosichthyidae
Lightfishes are small stomiiform fishes in the family PhosichthyidaeThey are very small fishes found in oceans throughout the world: most species grow no longer than 10 cm, while those in the genus Vinciguerria only reach 4 cm or so....
, and escolars, all inhabitants of the mesopelagic zone. There is therefore little competition between the pelagic thresher and other large oceanic piscivores such as billfish
Billfish
The term billfish is applied to a number of different large, predatory fish characterised by their large size and their long, sword-like bill. Billfish include the sailfish and marlin, which make up the family Istiophoridae, and the swordfish, sole member of the family Xiphiidae...
es, tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...
s, and dolphinfishes, which tend to feed near the surface. As in other threshers, pelagic threshers may swim in circles to drive 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...
prey into a compact mass, before striking them sharply with the upper lobe of its tail to stun them. Because of this behavior, pelagic threshers are often hooked on longlines by their tails.
Life history
Like the rest of the mackerel sharksLamniformes
Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...
, the pelagic thresher is ovoviviparous and gives birth to two pups at a time (rarely just one), one per uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
. There is no defined breeding season
Breeding season
The breeding season is the most suitable season, usually with favourable conditions and abundant food and water, for breeding among some wild animals and birds . Species with a breeding season have naturally evolved to have sexual intercourse during a certain time of year in order to achieve the...
and most adult females are pregnant throughout the year; the gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
is uncertain but has been suggested to be less than one year as in the common thresher. The developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s are sustained by a yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
until they are 12 cm (4.7 in) long, after which they are oophagous
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 ....
and feed on egg capsules produced by the mother. Each capsule measures about 55 mm (2.2 in) long and 12 mm (0.5 in) across, and contains 20–30 ova. Early stage embryos have specialized teeth for opening the capsules, while later-stage embryos have their teeth hidden and swallow the capsules whole, their teeth not becoming functional again until just after birth. There is no evidence of sibling cannibalism as in the sand tiger shark (Carcharias taurus). Young pelagic threshers are born at an unusually large size, up to 1.6 m (5.2 ft) long or 43% the length of the mother, which likely reduces predation on the newborns.
The growth rate of pelagic threshers slows with age: 9 cm/year for ages 0–1, 8 cm/year for ages 2–3, 6 cm/year for ages 5–6, 4 cm/year for ages 7–10, 3 cm/year for ages 10–12, and 2 cm/year for ages 13 and greater. Females reach maturity at 2.8–2.9 m (9.2–9.5 ft) long and 8–9 years old, while males mature at 2.7–2.8 m (8.9–9.2 ft) long and 7–8 years old. The oldest confirmed ages for females and males are 16 and 14 years respectively. Extrapolating the growth curves to the largest known individuals suggests that females may have a lifespan exceeding 28 years, and males 17 years. A single female produces about 40 young over her entire life.
Thermoregulation
Based on an anatomical examination, Sepulveda et al. (2005) concluded that the pelagic thresher is unlikely to be warm-bodiedWarm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
like the common thresher. The pelagic thresher lacks a rete mirabile
Rete mirabile
A rete mirabile is a complex of arteries and veins lying very close to each other, found in some vertebrates. The rete mirabile utilizes countercurrent blood flow within the net...
, a blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
countercurrent exchange
Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
system that prevents metabolic heat
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
from being dissipated into the water, inside its trunk. Furthermore, its aerobic red muscles, responsible for generating heat in the common thresher, are positioned in two lateral strips just beneath the skin rather than at the core of the body. There is a rete system present around the pelagic thresher's brain and eyes, albeit less developed than in the bigeye thresher, which may serve to buffer those organs against temperature changes.
Human interactions
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
, it comprises over 12% (c. 3,100 fish, 220 metric tons) of the annual Taiwanese shark landings. The meat is sold for human consumption, the skin is made into leather
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
, and the fins used for shark fin soup
Shark fin soup
Shark fin soup is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture. The shark fins provide texture while the taste comes from the other soup ingredients.There is controversy over the practice of shark...
in Asia. The squalene 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...
in the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
of the pelagic thresher can comprise 10% of its weight, and is used in the manufacture of cosmetics
Cosmetics
Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...
, health food
Health food
The term health food is generally used to describe foods that are considered to be beneficial to health, beyond a normal healthy diet required for human nutrition. However, the term is not precisely defined by national regulatory agencies such as the U.S...
s, and high-grade machine oil.
Though rarely caught, pelagic threshers are also valued by sport fishers and are listed as game fish
Game fish
Game fish are fish pursued for sport by recreational anglers. They can be freshwater or marine fish. Game fish can be eaten after being caught, though increasingly anglers practise catch and release to improve fish populations. Some game fish are also targeted commercially, particularly...
by the International Game Fish Association
International Game Fish Association
The International Game Fish Association is the leading authority on angling pursuits and the keeper of the most current World Record fishing catches by fish categories. Fishermen who are sport fishers are careful to follow their stringent rules for fair play and line requirements in order to...
. The largest overall records are from New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, while the light tackle
Fishing tackle
Fishing tackle, is a general term that refers to the equipment used by fishermen when fishing. Almost any equipment or gear used for fishing can be called fishing tackle. Some examples are hooks, lines, sinkers, floats, rods, reels, baits, lures, spears, nets, gaffs, traps, waders and tackle...
records are from California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. Pelagic threshers are frequently taken as bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
on longlines and in driftnets meant for other species such as tuna, and also rarely in gillnet
Gillnet
Gillnetting is a common fishing method used by commercial and artisanal fishermen of all the oceans and in some freshwater and estuary areas. The gillnet also is used by fisheries scientists to monitor fish populations. Because gillnets can be so effective their use is closely monitored and...
s and anti-shark nets
Shark net
A shark net is a submerged net placed around beaches to reduce shark attacks on swimmers.Shark nets do not offer complete protection but work on the principle of "fewer sharks, fewer attacks". They reduce occurrence via shark mortality. Reducing the local shark populations is believed to reduce the...
. All three thresher shark species were assessed as Vulnerable
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 IUCN in 2007, as their 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...
cannot sustain high levels of exploitation.