Bigeye thresher
Encyclopedia
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 keyhole-shaped sockets
that allow them to be rotated upward. This species can also be distinguished by a pair of deep grooves on the top of its head, from which its scientific name is derived.
The large eyes of the bigeye thresher are adapted
for hunting in low light conditions. It is one of the few sharks that conduct a diel vertical migration
, staying in deep water during the day and moving into surface waters at night to feed. To protect its sensitive brain
and eyes from the temperature changes accompanying these movements, the bigeye thresher has a vascular exchange system
called the rete mirabile
around those organ
s. This species feeds mainly on fish and squid, which are stunned via whip-like strikes of the long tail. Bigeye threshers are ovoviviparous, usually bearing litters of two pups. The embryo
s are oophagous
and feed on ova produced by the mother while inside the uterus
. This shark is caught by commercial fisheries
across its range
; the meat is not highly regarded but the skin, fins, and liver oil are valued. It has been assessed as Vulnerable
by the International Union for Conservation of Nature (IUCN).
was the first to scientifically describe the bigeye thresher, in papers published in Proceedings of the Royal Society of London (1840) and Transactions of the Zoological Society of London (1849, sometimes given as 1839). He based his description on a specimen caught off Madeira
in the eastern Atlantic Ocean
. However, Lowe's description was subsequently overlooked by researchers and this species was known by different names until the 1940s, when new specimens from Cuba
and Florida
prompted its original scientific name to be resurrected. The specific epithet superciliosus is from the Latin
super meaning "above", and ciliosus meaning "eyebrow", referring to the distinct lateral grooves above the eyes.
An allozyme
analysis conducted by Blaise Eitner in 1995 showed that the closest relative of the bigeye thresher is the pelagic thresher
(A. pelagicus), with which it forms a clade
. Fossil remains of the bigeye thresher dating to the Middle Miocene
(16.0–11.6 Ma) have been found in the Hokuriku region
of Japan
.
, it has been reported from New York
to Florida
, the Bahamas, Cuba
, Venezuela
, and southern Brazil
. In the eastern Atlantic, it is known from Portugal
, Madeira
, Senegal
, Guinea
to Sierra Leone
, Angola
, and the Mediterranean Sea
. In the western Indian Ocean
, it occurs off South Africa
, Madagascar
, and the Arabian Sea
. In the Pacific Ocean
, it is known from southern Japan
, Taiwan
, New Caledonia
, northwestern Australia
, and New Zealand
, and eastward to Hawaii
, southern California
, the Gulf of California
, and west of the Galapagos Islands
. Analysis of mitochondrial DNA
has shown that Atlantic and Indo-Pacific populations are somewhat genetically divergent
from each other.
Bigeye threshers are usually found over the continental shelf
and in the open sea
, though they are occasionally encountered in shallow coast
al waters. They occur in surface temperature
s of 16–25 °C (60.8–77 F), but have been tracked as far down as 723 m (2,372 ft), where the temperature is only 5 °C (41 °F). Little is known of their geographical movements; one individual has been documented moving from New York
to the Gulf of Mexico
, a straight-line distance of 2767 km (1,719.3 mi).
The eyes of the bigeye thresher can measure up to 10 cm (3.9 in) across in adults. Each eye is taller than wide, with a bulbous upper portion. The orbits extend onto the dorsal surface of the head, allowing the eyes to orient upwards. There are also a pair of distinctive lateral grooves that extend from above the eyes to over the gill slit
s, giving it a "helmet"-like appearance. The snout is moderately long and bulbous, and there are no labial furrows at the corners of the mouth. The teeth are moderately large with a single, narrow cusp. There are 19–24 teeth in the upper jaw and 20–24 teeth in the lower jaw; their shapes are similar in both jaws. There are large and small dermal denticles, with the smaller ones more numerous and interspersed amongst the larger ones. The smaller denticles taper to a point.
Up to half the body length is taken up by the long upper lobe of the caudal fin, which is broader than in other threshers. The large pectoral fins have a curved anterior margin and broad tips. The first dorsal fin
is placed further back than in the other thresher sharks, with the free rear tip located above or just before the pelvic fins. Its coloration is a deep, metallic violet to purplish brown above and creamy white below. This color rapidly fades to a dull gray after death. Most bigeye threshers are 3.3–4 m (10.8–13.1 ft) long and weigh 160 kg (352.7 lb). The largest known bigeye thresher measured 4.9 m (16.1 ft) long and weighed 364 kg (802.5 lb), and was caught near Tutukaka, New Zealand
, in February 1981.
, spending daytime in deeper water between 300–500 m (984.3–1,640.4 ft), beneath the thermocline
where the temperature ranges from 6 to 12 °C (42.8 to 53.6 F), and ascending above it to water less than 100 m (328.1 ft) deep during nighttime. This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents.
Bigeye threshers are likely preyed upon by larger sharks and marine mammal
s. Known parasites of the bigeye thresher include the copepod
Pagina tunica, and the tapeworm Litobothrium janovyi. Sea lamprey
s (Petromyzon marinus) have been seen attached near the cloaca
of this shark. The bigeye thresher appears to be an ecological competitor of the blue shark
(Prionace glauca), and the numbers of the two species are negatively correlated such as that only one of two occurs in any given location.
The bigeye thresher has larger teeth than other threshers and feeds on a wider variety of prey. Known food items include schooling
forage fish
such as mackerel
and herring
, benthic fishes such as hake
and whiting
, larger pelagic fish
es such as lancetfish
and small billfish
, squid
such as lycoteuthids
and ommastrephids
, and possibly crab megalopae. They likely use their long tails to stun their prey prior to capture, as they are often found hooked by their tails on longlines and with the bait fish in their stomachs. The shape of their eye sockets give them binocular vision
in an upward direction to better aim their strikes. In the Mediterranean, they are strongly associated with schools of frigate mackerel (Auxis rochei), suggesting that these sharks follow concentrations of their prey from place to place.
s, bigeye threshers are ovoviviparous and bear litters of two pups, one in each uterus
. Very rarely, the litter size may be one, three, or four. There is no defined breeding season
and most adult females are pregnant throughout the year without any apparent lag time between pregnancies. The gestation period
is unknown. The developing fetus
es are initially nourished by a yolk sac
, and later on exhibit oophagy
, in which they consume infertile eggs produced by their mother (and possibly also uterine fluid). There is no evidence of sibling cannibalism as in the sand tiger shark (Carcharias taurus). Unborn embryos are similar in appearance to adults, with proportionally larger heads and eyes. They are covered with a thin layer of epithelium
that prevents the uterine wall from being abraded by the embryo's sharp dermal denticles; this has not been observed in the young of other thresher sharks. The young measure 1.35–1.4 m (4.4–4.6 ft) long at birth. Males mature at a length around 2.7–2.9 m (8.9–9.5 ft) and at an age of 9–10 years, while females mature at a length around 3.3–3.6 m (10.8–11.8 ft) and at an age of 12–14 years. The maximum lifespan of this species is believed to be 19 years for a male and 20 years for a female. A female bigeye thresher is estimated to produce only 20 young over her entire life.
like the common thresher (A. vulpinus). In a 1971 study, Carey et al. probed the swimming muscles of two bigeye threshers with a thermister needle and reported finding a temperature elevation of 1.8 and 4.3°C (3.8 and 9.1°F) compared to the ambient environment. However, an anatomical examination conducted by Sepulveda et al. in 2005 found that though the bigeye thresher possesses the aerobic red muscles responsible for generating heat in the common thresher, these muscles are arranged in two strips along the flanks just beneath the skin, as opposed to near the core of the body. There is also no blood vessel countercurrent exchange
system (the rete mirabile
) in the trunk to limit the loss of metabolic heat
to the water. Based on these differences, the authors questioned earlier measurements and concluded it was unlikely that the bigeye thresher maintains an elevated body temperature. The bigeye thresher does possess a highly developed rete system around its brain
and eyes. This is thought to function in buffering those sensitive organ
s against temperature changes during the shark's daily migrations up and down the water column, which can be as much as 15–16°C (27–29°F).
The bigeye thresher shark is rarely encountered by divers underwater and poses no danger. This species is or was taken by longline fisheries operated by many countries, including the United States, Japan, Spain, Brazil, Uruguay, and Mexico, and constitutes about 10% of the pelagic shark catch. The bigeye thresher comprises 20% of the longline catch off Cuba, where it is attracted at night using cyalume sticks (chemical lights). It is also significant to Taiwanese fisheries, which land about 220 metric tons annually. The meat is marketed fresh, smoked, or dried and salted
, though it is not highly regarded due to its mushy texture. The skin is used to make leather
products, the liver oil
for vitamin
s, and the fins for shark fin soup
.
In the waters of the United States
, this species is considered a nuisance bycatch
of longlines, gillnet
s, and trawls
. It is also occasionally caught in shark net
s around beaches in South Africa
. Along with the other thresher species, the bigeye thresher is listed as a game fish
by the International Game Fish Association
(IGFA), and is pursued by recreational anglers off the United States, South Africa, and New Zealand. The bigeye thresher is highly susceptible to over-exploitation due to its low lifetime fecundity
. All three thesher shark species were assessed as Vulnerable
by the International Union for Conservation of Nature (IUCN) in 2007.
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, 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
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...
comes from its enormous eyes, which are placed in keyhole-shaped sockets
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
that allow them to be rotated upward. This species can also be distinguished by a pair of deep grooves on the top of its head, from which its scientific name is derived.
The large eyes of the bigeye thresher are adapted
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
for hunting in low light conditions. It is one of the few sharks that conduct a diel vertical migration
Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...
, staying in deep water during the day and moving into surface waters at night to feed. To protect its sensitive brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and eyes from the temperature changes accompanying these movements, the bigeye thresher has a vascular exchange system
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...
called the 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...
around those organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s. This species feeds mainly on fish and squid, which are stunned via whip-like strikes of the long tail. Bigeye threshers are ovoviviparous, usually bearing litters of two pups. The 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 ....
and feed on ova produced by the mother while inside the 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...
. This shark is caught 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...
across its range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...
; the meat is not highly regarded but the skin, fins, and liver oil are valued. It has been 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 International Union for Conservation of Nature (IUCN).
Taxonomy and phylogeny
British biologist Richard Thomas LoweRichard 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...
was the first to scientifically describe the bigeye thresher, in papers published in Proceedings of the Royal Society of London (1840) and Transactions of the Zoological Society of London (1849, sometimes given as 1839). He based his description on a specimen caught off Madeira
Madeira
Madeira is a Portuguese archipelago that lies between and , just under 400 km north of Tenerife, Canary Islands, in the north Atlantic Ocean and an outermost region of the European Union...
in the eastern Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. However, Lowe's description was subsequently overlooked by researchers and this species was known by different names until the 1940s, when new specimens from Cuba
Cuba
The Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...
and Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
prompted its original scientific name to be resurrected. The specific epithet superciliosus is from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
super meaning "above", and ciliosus meaning "eyebrow", referring to the distinct lateral grooves above the eyes.
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 bigeye thresher is 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...
(A. pelagicus), 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...
. Fossil remains of the bigeye thresher dating to the Middle Miocene
Middle Miocene
The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene....
(16.0–11.6 Ma) have been found in the Hokuriku region
Hokuriku region
The is located in the northwestern part of Honshū, the main island of Japan. It lies along the Sea of Japan within the Chūbu region. It is almost equivalent to Koshi Province and Hokurikudō area in pre-modern Japan....
of 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...
.
Distribution and habitat
The bigeye thresher has a virtually circumtropical distribution. In the western Atlantic OceanAtlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, it has been reported from New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
to Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
, the Bahamas, Cuba
Cuba
The Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...
, Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
, and southern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. In the eastern Atlantic, it is known from Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
, Madeira
Madeira
Madeira is a Portuguese archipelago that lies between and , just under 400 km north of Tenerife, Canary Islands, in the north Atlantic Ocean and an outermost region of the European Union...
, Senegal
Senegal
Senegal , officially the Republic of Senegal , is a country in western Africa. It owes its name to the Sénégal River that borders it to the east and north...
, Guinea
Guinea
Guinea , officially the Republic of Guinea , is a country in West Africa. Formerly known as French Guinea , it is today sometimes called Guinea-Conakry to distinguish it from its neighbour Guinea-Bissau. Guinea is divided into eight administrative regions and subdivided into thirty-three prefectures...
to Sierra Leone
Sierra Leone
Sierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea to the north and east, Liberia to the southeast, and the Atlantic Ocean to the west and southwest. Sierra Leone covers a total area of and has an estimated population between 5.4 and 6.4...
, Angola
Angola
Angola, officially the Republic of Angola , is a country in south-central Africa bordered by Namibia on the south, the Democratic Republic of the Congo on the north, and Zambia on the east; its west coast is on the Atlantic Ocean with Luanda as its capital city...
, and the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
. In the western 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...
, it occurs off 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...
, Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, 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...
. In the 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...
, it is known from southern 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...
, Taiwan
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...
, 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...
, 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...
, and 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...
, and eastward to Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
, southern 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...
, 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 west of 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...
. 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 Atlantic and Indo-Pacific populations are somewhat genetically divergent
Genetic divergence
Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes through time, often after the populations have become reproductively isolated for some period of time...
from each other.
Bigeye threshers are usually found over the 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 in the open sea
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...
, though they are occasionally encountered in shallow coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...
al waters. They occur in surface temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s of 16–25 °C (60.8–77 F), but have been tracked as far down as 723 m (2,372 ft), where the temperature is only 5 °C (41 °F). Little is known of their geographical movements; one individual has been documented moving from New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
to the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
, a straight-line distance of 2767 km (1,719.3 mi).
Description
Gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s, giving it a "helmet"-like appearance. The snout is moderately long and bulbous, and there are no labial furrows at the corners of the mouth. The teeth are moderately large with a single, narrow cusp. There are 19–24 teeth in the upper jaw and 20–24 teeth in the lower jaw; their shapes are similar in both jaws. There are large and small dermal denticles, with the smaller ones more numerous and interspersed amongst the larger ones. The smaller denticles taper to a point.
Up to half the body length is taken up by the long upper lobe of the caudal fin, which is broader than in other threshers. The large pectoral fins have a curved anterior margin and broad tips. The first 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...
is placed further back than in the other thresher sharks, with the free rear tip located above or just before the pelvic fins. Its coloration is a deep, metallic violet to purplish brown above and creamy white below. This color rapidly fades to a dull gray after death. Most bigeye threshers are 3.3–4 m (10.8–13.1 ft) long and weigh 160 kg (352.7 lb). The largest known bigeye thresher measured 4.9 m (16.1 ft) long and weighed 364 kg (802.5 lb), and was caught near Tutukaka, 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...
, in February 1981.
Biology and ecology
The size and upward orientation of the bigeye thresher's eyes are adapted to search for the silhouettes of prey in dim light. This species is one of a handful of shark species that conducts a diel vertical migrationDiel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...
, spending daytime in deeper water between 300–500 m (984.3–1,640.4 ft), beneath the thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
where the temperature ranges from 6 to 12 °C (42.8 to 53.6 F), and ascending above it to water less than 100 m (328.1 ft) deep during nighttime. This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents.
Bigeye threshers are likely preyed upon by larger sharks and marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s. Known parasites of the bigeye thresher include the 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,...
Pagina tunica, and the tapeworm Litobothrium janovyi. Sea lamprey
Sea lamprey
The sea lamprey is a parasitic lamprey found on the Atlantic coasts of Europe and North America, in the western Mediterranean Sea, and in the Great Lakes. It is brown, gray, or black on its back and white or gray on the underside and can grow up to 90 cm long. Sea lampreys prey on a wide...
s (Petromyzon marinus) have been seen attached near the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
of this shark. The bigeye thresher appears to be an ecological competitor of the blue shark
Blue shark
The blue shark is a species of requiem shark, family Carcharhinidae, that inhabits deep waters in the world's temperate and tropical oceans. Preferring cooler waters, blue sharks migrate long distances, for example from New England to South America. Although generally lethargic, they can move very...
(Prionace glauca), and the numbers of the two species are negatively correlated such as that only one of two occurs in any given location.
Feeding
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...
forage fish
Forage fish
Forage fish, also called prey fish or bait fish, are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding...
such as mackerel
Mackerel
Mackerel is a common name applied to a number of different species of fish, mostly, but not exclusively, from the family Scombridae. They may be found in all tropical and temperate seas. Most live offshore in the oceanic environment but a few, like the Spanish mackerel , enter bays and can be...
and herring
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...
, benthic fishes such as hake
Hake
The term hake refers to fish in either of:* family Phycidae of the northern oceans* family Merlucciidae of the southern oceans-Hake fish:...
and whiting
Whiting
Whiting is the name of several species of fish:*Merlangius merlangus, the original species to receive the name; a common food fish of the cod family found in the northeastern Atlantic Ocean around Europe and the Mediterranean regions...
, larger pelagic fish
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...
es such as lancetfish
Lancetfish
Lancetfishes are large oceanic predatory fishes in the genus Alepisaurus , the only living genus in the family Alepisauridae....
and small 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...
, squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
such as lycoteuthids
Lycoteuthidae
Lycoteuthidae is a family of squid comprising four known genera in two subfamilies. They are characterised by a lack of hooks and by photophores present on the viscera, eyeballs and tentacles.-Species:Subfamily Lampadioteuthinae...
and ommastrephids
Ommastrephidae
Ommastrephidae is a family of squid. It contains 3 subfamilies, 11 genera, and over 20 species.-External links:***...
, and possibly crab megalopae. They likely use their long tails to stun their prey prior to capture, as they are often found hooked by their tails on longlines and with the bait fish in their stomachs. The shape of their eye sockets give them binocular vision
Binocular vision
Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...
in an upward direction to better aim their strikes. In the Mediterranean, they are strongly associated with schools of frigate mackerel (Auxis rochei), suggesting that these sharks follow concentrations of their prey from place to place.
Life history
Like other mackerel sharkLamniformes
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...
s, bigeye threshers are ovoviviparous and bear litters of two pups, one in each 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...
. Very rarely, the litter size may be one, three, or four. 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 without any apparent lag time between pregnancies. 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 unknown. The developing fetus
Fetus
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...
es are initially nourished 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...
, and later on exhibit 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 ....
, in which they consume infertile eggs produced by their mother (and possibly also uterine fluid). There is no evidence of sibling cannibalism as in the sand tiger shark (Carcharias taurus). Unborn embryos are similar in appearance to adults, with proportionally larger heads and eyes. They are covered with a thin layer of epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that prevents the uterine wall from being abraded by the embryo's sharp dermal denticles; this has not been observed in the young of other thresher sharks. The young measure 1.35–1.4 m (4.4–4.6 ft) long at birth. Males mature at a length around 2.7–2.9 m (8.9–9.5 ft) and at an age of 9–10 years, while females mature at a length around 3.3–3.6 m (10.8–11.8 ft) and at an age of 12–14 years. The maximum lifespan of this species is believed to be 19 years for a male and 20 years for a female. A female bigeye thresher is estimated to produce only 20 young over her entire life.
Thermoregulation
There is conflicting evidence on whether the bigeye thresher is 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 (A. vulpinus). In a 1971 study, Carey et al. probed the swimming muscles of two bigeye threshers with a thermister needle and reported finding a temperature elevation of 1.8 and 4.3°C (3.8 and 9.1°F) compared to the ambient environment. However, an anatomical examination conducted by Sepulveda et al. in 2005 found that though the bigeye thresher possesses the aerobic red muscles responsible for generating heat in the common thresher, these muscles are arranged in two strips along the flanks just beneath the skin, as opposed to near the core of the body. There is also no blood vessel 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 (the 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...
) in the trunk to limit the loss of 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...
to the water. Based on these differences, the authors questioned earlier measurements and concluded it was unlikely that the bigeye thresher maintains an elevated body temperature. The bigeye thresher does possess a highly developed rete system around its brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and eyes. This is thought to function in buffering those sensitive organ
Organ (music)
The organ , is a keyboard instrument of one or more divisions, each played with its own keyboard operated either with the hands or with the feet. The organ is a relatively old musical instrument in the Western musical tradition, dating from the time of Ctesibius of Alexandria who is credited with...
s against temperature changes during the shark's daily migrations up and down the water column, which can be as much as 15–16°C (27–29°F).
Human interactions
Salting (food)
Salting is the preservation of food with dry edible salt. It is related to pickling . It is one of the oldest methods of preserving food, and two historically significant salt-cured foods are dried and salted cod and salt-cured meat.Salting is used because most bacteria, fungi and other potentially...
, though it is not highly regarded due to its mushy texture. The skin is used to make 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:...
products, the 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...
for vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...
s, and the fins 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 the waters of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, this species is considered a nuisance 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...
of longlines, 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 trawls
Trawling
Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net that is used for trawling is called a trawl....
. It is also occasionally caught in shark net
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...
s around beaches in 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...
. Along with the other thresher species, the bigeye thresher is listed as a 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...
(IGFA), and is pursued by recreational anglers off the United States, South Africa, and New Zealand. The bigeye thresher is highly susceptible to over-exploitation due to its low lifetime 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...
. All three thesher 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 International Union for Conservation of Nature (IUCN) in 2007.