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

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

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

 Chlamydoselachidae, with a wide but patchy distribution in the Atlantic
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...

 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. This uncommon species is found over the outer 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 upper continental slope, generally near the bottom though there is evidence of substantial upward movements. It has been caught as deep as 1570 m (5,150.9 ft), whereas in Suruga Bay
Suruga Bay
Suruga Bay is a bay on the Pacific coast of Honshū in Shizuoka Prefecture, Japan. It is situated north of an imaginary line joining Omaezaki Point and Irōzaki Point at the tip of the Izu Peninsula and surrounded by Honshū to the southwest and west and the Izu Peninsula to the east.-Geology:Suruga...

, 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...

 it is most common at depths of 50–200 m (164–656.2 ft). Exhibiting several "primitive" features, the frilled shark has often been termed a "living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...

". It reaches a length of 2 m (6.6 ft) and has a dark brown, eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-like body with the dorsal
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...

, pelvic, and anal fins placed far back. 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 the frilly or fringed appearance of the gill slit
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, of which there are six pairs with the first pair meeting across the throat.

Seldom observed, the frilled shark is speculated to capture its prey by bending its body and lunging forward like a snake. The long, extremely flexible jaws enable it to swallow large prey whole, while the many rows of small, needle-like teeth prevent escape. It feeds mainly on cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s, while also consuming bony fishes and other sharks. This species is aplacental viviparous: 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 emerge from their egg capsules inside the mother's 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...

, and are sustained to term primarily by yolk. 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:...

 may be as long as three and a half years, the longest of any vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

. Between 2 and 15 young are born at a time; there is no distinct 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...

. Frilled sharks are occasionally captured 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...

 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...

 but have little economic value. The International Union for Conservation of Nature (IUCN) has assessed it as Near Threatened
Near Threatened
Near Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...

, since given its very low reproductive rate even incidental catches may deplete its population. This shark, or a supposed giant relative, has been suggested as a source for reports of sea serpent
Sea serpent
A sea serpent or sea dragon is a type of sea monster either wholly or partly serpentine.Sightings of sea serpents have been reported for hundreds of years, and continue to be claimed today. Cryptozoologist Bruce Champagne identified more than 1,200 purported sea serpent sightings...

s.

Taxonomy and phylogeny

The frilled shark was first scientifically recognized by German ichthyologist Ludwig Döderlein, who visited Japan between 1879 and 1881 and brought two specimens to Vienna
Vienna
Vienna is the capital and largest city of the Republic of Austria and one of the nine states of Austria. Vienna is Austria's primary city, with a population of about 1.723 million , and is by far the largest city in Austria, as well as its cultural, economic, and political centre...

. However, his manuscript describing the species was lost, and so the first description of the frill shark became authored by American zoologist Samuel Garman, working from a 1.5 m (4.9 ft) long female caught from Sagami Bay
Sagami Bay
Sagami Bay , also known as the Sagami Gulf or Sagami Sea, lies south of Kanagawa Prefecture in Honshū, central Japan, contained within the scope of the Miura Peninsula, in Kanagawa, to the east, the Izu Peninsula, in Shizuoka Prefecture, to the west, and the Shōnan coastline to the north, while the...

 in Japan. His account, entitled "An Extraordinary Shark", was published in an 1884 volume of Proceedings of the Essex Institute. Garman placed the new species in its own genus and family, and gave it the name Chlamydoselachus anguineus 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;...

 chlamy ("frill") and selachus ("shark"), and 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...

 anguineus for "snake-like". Other 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...

s for this species include frill shark, lizard shark, scaffold shark, and silk shark.

Several early authors believed the frilled shark to be a living representative of otherwise long-extinct groups of elasmobranchs (sharks, rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

, and their ancestors), based on its multi-pointed teeth, the articulation of its upper jaw directly to the cranium behind the eyes (called "amphistyly"), and its "notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

-like" spinal column with indistinct vertebrae. Garman proposed that it was allied with the "cladodonts", a now-obsolete taxonomic grouping containing forms that thrived during the Palaeozoic era, such as Cladoselache
Cladoselache
Cladoselache is a genus of extinct shark. It appeared in the Devonian period.This primitive shark grew to be up to long and roamed the oceans of North America. It is known to have been a fast moving and fairly agile predator due to its streamlined body and deep forked tail...

from the Devonian period (416–359 Ma). His contemporaries Theodore Gill
Theodore Gill
Theodore Nicholas Gill was an American ichthyologist, mammalogist, malacologist and librarian.Born and educated in New York City under private tutors, Gill early showed interest in natural history. He was associated with J...

 and Edward Drinker Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...

 suggested it was instead related to the hybodonts
Hybodontiformes
Hybodontiformes was an order of sharks, they were characterised by possessing 1-2 pairs of hooked protrusions on their heads.-External links:* http://www.palaeos.com/Vertebrates/Units/070Chondrichthyes/070.600.html#Hybodontiformes...

, which were the dominant sharks during the Mesozoic era. Cope went as far as to assign this species to the fossil genus Didymodus
Xenacanthus
Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end of the Triassic, 202 million years ago. Fossils of various species have been found worldwide....

.

More recent investigations have found that the similarities between the frilled shark and extinct groups may have been overstated or misinterpreted, and that this shark exhibits a number of skeletal and muscular traits that firmly place it with the neoselachians (modern sharks and rays), and more specifically with the cow sharks in the order Hexanchiformes
Hexanchiformes
Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....

 (though systematist Shigeru Shirai has proposed that it be placed in its own order, Chlamydoselachiformes). Nevertheless, the frilled shark belongs to one of the oldest still-extant shark lineages, dating back to at least the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

 (c. 95 Ma) and possibly to the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...

 (c. 150 Ma). Because of its ancient ancestry and "primitive" characteristics, it has been described as a "living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...

".

Distribution and habitat

Rather uncommon, the frilled shark has been recorded from a number of widely scattered locations in the Atlantic 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. In the eastern Atlantic, it occurs off northern Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, northern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 and western Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, from France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 to Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...

 including 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...

, and off Mauritania
Mauritania
Mauritania is a country in the Maghreb and West Africa. It is bordered by the Atlantic Ocean in the west, by Western Sahara in the north, by Algeria in the northeast, by Mali in the east and southeast, and by Senegal in the southwest...

. In the central Atlantic, it has been caught at several locations along the Mid-Atlantic Ridge
Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South...

, from north of the Azores
Azores
The Archipelago of the Azores is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about west from Lisbon and about east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the...

 to the Rio Grande Rise off 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...

, as well as over the Vavilov Ridge off West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...

. In the western Atlantic, it has been reported from off New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...

, Georgia
Georgia (U.S. state)
Georgia is a state located in the southeastern United States. It was established in 1732, the last of the original Thirteen Colonies. The state is named after King George II of Great Britain. Georgia was the fourth state to ratify the United States Constitution, on January 2, 1788...

, and Suriname
Suriname
Suriname , officially the Republic of Suriname , is a country in northern South America. It borders French Guiana to the east, Guyana to the west, Brazil to the south, and on the north by the Atlantic Ocean. Suriname was a former colony of the British and of the Dutch, and was previously known as...

. In the western Pacific, it is known from southeastern Honshu
Honshu
is the largest island of Japan. The nation's main island, it is south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Strait...

, 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...

, to 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...

, off New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

 and Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...

 in 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 around 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 the central and eastern Pacific, it has been found off 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...

, 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...

, and northern Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

. The frilled sharks off southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...

 were described as a different species, C. africana
Southern African frilled shark
The southern African frilled shark is a species of shark in the family Chlamydoselachidae, described in 2009. It is found in deep water from off southern Angola to southern Namibia. This species is difficult to distinguish from the better-known frilled shark The southern African frilled shark...

, in 2009.

The frilled shark inhabits the outer 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 upper to middle continental slope, seeming to favor upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

s and other biologically productive
Productivity (ecology)
In ecology, productivity or production refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface per unit time, for instance grams per square metre per day. The mass unit may relate to dry matter or to the mass of carbon generated...

 areas. Though it has been caught from a depth of 1570 m (5,150.9 ft), it usually does not occur deeper than 1000 m (3,280.8 ft). In Suruga Bay
Suruga Bay
Suruga Bay is a bay on the Pacific coast of Honshū in Shizuoka Prefecture, Japan. It is situated north of an imaginary line joining Omaezaki Point and Irōzaki Point at the tip of the Izu Peninsula and surrounded by Honshū to the southwest and west and the Izu Peninsula to the east.-Geology:Suruga...

, it is most common at a depth of 50–200 m (164–656.2 ft), except from August to November when the temperature at the 100 m (328.1 ft) water layer exceeds 15 °C (59 °F) and the sharks shift into deeper water. On rare occasions, this species has been seen at the surface. The frilled shark is usually found close to the bottom, with one individual observed swimming over an area of small sand dunes. However, its diet suggests that it does make substantial forays upward into open water. This species may make vertical migrations
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...

, approaching the surface at night to feed. There is spatial segregation by size and reproductive condition.

Description

With its elongated, eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-like body and strange appearance, the frilled shark has long been likened to the mythical sea serpent
Sea serpent
A sea serpent or sea dragon is a type of sea monster either wholly or partly serpentine.Sightings of sea serpents have been reported for hundreds of years, and continue to be claimed today. Cryptozoologist Bruce Champagne identified more than 1,200 purported sea serpent sightings...

. The head is broad and flattened with a short, rounded snout. The nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...

s are vertical slits, separated into incurrent and excurrent openings by a leading flap of skin. The moderately large eyes are horizontally oval and lack nictitating membrane
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

s (protective third eyelids). The very long jaws are positioned terminally (at the end of the snout), as opposed to the underslung jaws of most sharks. The corners of the mouth are devoid of furrows or folds. The tooth rows are rather widely spaced, numbering 19–28 in the upper jaw and 21–29 in the lower jaw. The teeth number around 300 in all; each tooth is small, with three slender, needle-like cusps alternating with two cusplets. There are six pairs of long gill slit
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 with a "frilly" appearance created by the extended tips of the gill filaments, giving this shark its name. The first pair of gill slits meet across the throat, forming a "collar".

The pectoral fins are short and rounded. The single, small dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 is positioned far back on the body, about opposite the anal fin, and has a rounded margin. The pelvic and anal fins are large, broad, and rounded, and also positioned well back on the body. The caudal fin is very long and roughly triangular, without a lower lobe or a ventral notch on the upper lobe. There are a pair of thick skin folds of unknown function running along the belly, separated by a groove. The midsection is relatively longer in females than in males, with the pelvic fins pushed closer to the anal fin. The dermal denticles are small and shaped like the tip of a chisel
Chisel
A chisel is a tool with a characteristically shaped cutting edge of blade on its end, for carving or cutting a hard material such as wood, stone, or metal. The handle and blade of some types of chisel are made of metal or wood with a sharp edge in it.In use, the chisel is forced into the material...

, becoming enlarged and sharp on the dorsal margin of the caudal fin. This species is a uniform dark brown or gray. The frilled shark differs from its southern African relative, C. africana, in having more vertebrae (160–171 vs 147) and more turns in the 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
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

 (35–49 versus 26–28), as well as in various proportional measurements such as a longer head and shorter gill slits. The maximum known length is 1.7 m (5.6 ft) for males and 2 m (6.6 ft) for females.

Biology and ecology

Highly specialized for life in the deep sea, the frilled shark has a reduced, poorly calcified
Calcification
Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

 skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 and an enormous 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...

 filled with low-density lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

s, allowing it to maintain its position in the water column with little effort. It is one of the few sharks with an "open" lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

, in which the mechanoreceptive
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

 hair cell
Hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...

s are positioned in grooves that are directly exposed to the surrounding seawater. This configuration is thought to be basal in sharks and may enhance its sensitivity to the minute movements of its prey. Many frilled sharks are found with the tips of their tails missing, probably from predatory attacks by other shark species. Parasites identified from this shark include a tapeworm in the genus Monorygma, the fluke
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...

 Distomum veliporum, and the nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

 Metaleptus rabuka.

Feeding

The long jaws of the frilled shark are highly distensible with an extremely wide gape, allowing it to swallow whole prey over one-half its size. However, the length and articulation of its jaws means it cannot deliver as strong a bite as more conventionally built sharks. Most captured individuals are found with no or barely identifiable stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 contents, suggesting a fast digestion
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....

 rate and/or long intervals between feedings. This species preys upon cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s, bony fishes, and smaller sharks. One 1.6 m (5.2 ft) long individual, caught off Chōshi
Choshi, Chiba
is a city located on the Pacific Ocean coast of Chiba Prefecture, Japan. It is the easternmost city in the Greater Tokyo Area. The easternmost point in the Greater Tokyo Area is Cape Inubō....

, was found to have swallowed a 590 g (1.3 lb) Japanese catshark
Japanese catshark
The Japanese catshark, Apristurus japonicus, is a cat shark of the family Scyliorhinidae found in the northwest Pacific from off Chiba Prefecture, Honshū, Japan between latitudes 36° N and 34° N. Its length is up to 71 cm, and it can be found at depths of up to 2,000 m....

 (Apristurus japonicus). 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...

 comprise some 60% of the diet of sharks in Suruga Bay; this includes not only slow-moving, deep-dwelling types such as Chiroteuthis
Chiroteuthis
Chiroteuthis is a genus of chiroteuthid squid, comprising two subgenera. The hectocotylus is absent from all members of the genus; instead, a penis extending from the mantle opening is utilised. The genus is characterised by enlarged, lidded photophores present at the end of the tentacular club...

and Histioteuthis
Histioteuthis
Histioteuthis is a genus of squid and the only member of the Histioteuthidae family. It goes by the English name cock-eyed squid, because in all species the right eye is normal-sized, round, blue and sunken; whereas the left eye is at least twice the diameter of the right eye, tubular,...

, but also relatively large, powerful swimmers of the open ocean such as Onychoteuthis
Onychoteuthis
Onychoteuthis is a genus of squid in the family Onychoteuthidae. The type species is the Common Clubhook Squid...

, Sthenoteuthis, and Todarodes.

How the ostensibly weak-swimming frilled shark captures active, fast-moving squid is a matter of speculation. One possibility is that it takes advantage of injured squid, or those that are exhausted and dying after spawning
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

. Alternatively, it may surprise its prey by curving its body and, bracing itself with its posteriorly positioned fins, launching a quick strike forward in the manner of a snake. It may also be able to close its gill slits and create negative pressure
Negative pressure
Negative pressure may refer to:*negative pressure as opposed to positive pressure*stretched liquid*vacuum*negative gauge pressure, a way of expressing pressure measurements below atmospheric pressure*suction*transpirational pull...

 to suck prey into its mouth. The many small, sharp, recurved teeth of the frilled shark are functionally similar to squid jigs and could easily snag the body or tentacles of a squid, particularly as they are rotated outwards when the jaws are protruded. Observations of captive frilled sharks swimming with their mouths open suggest that the small teeth, light against the dark mouth, may even fool squid into attacking and entangling themselves.

Life history

The frilled shark is aplacental viviparous; 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 mainly nourished by yolk, though the difference in weight between the egg and the newborn indicates that the mother also provides additional nutrition via unknown means. Adult females have two functional ovaries
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 and one functional 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...

, on the right. Unsurprisingly, there is no defined breeding season for either sex, as this shark inhabits depths at which there is little to no seasonal influence. A possible mating aggregation of 15 male and 19 female sharks has been recorded over a 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...

 on the Mid-Atlantic Ridge. The litter size ranges from 2 to 15, with an average of 6. Females ovulate
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

 eggs into the uterus about once every two weeks; vitellogenesis
Vitellogenesis
Vitellogenesis is the process of yolk formation via nutrients being deposited in the oocyte, or female germ cell involved in reproduction. It starts when the fat body stimulates the release of juvenile hormones and produces vitellogenin protein. It occurs in all animal groups lower than the mammals...

 (yolk formation) and the growth of new ovarian eggs halt during pregnancy, apparently due to insufficient space inside the body cavity
Body cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...

.

Newly ovulated eggs and early-stage embryos are enclosed in a thin, ellipsoid, golden-brown capsule. When the embryo is 3 cm (1.2 in) long, the head is pointed when seen from above or below, the jaws are barely developed, the external gills have begun to appear, and all the fins are present. The egg capsule is shed when the embryo grows to 6–8 cm (2.4–3.1 in) long, and is expelled from the female's body; at this time the embryo's external gills are fully developed. The size of the 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...

 remains mostly constant until around an embryonic length of 40 cm (15.7 in), whereupon it begins to shrink, mostly or completely disappearing by an embryonic length of 50 cm (19.7 in). The embryonic growth rate averages 1.4 cm (0.551181102362205 in) per month, and therefore the entire 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:...

 may last three and a half years, far longer than any other vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

. Newborn sharks measure 40–60 cm (15.7–23.6 in) long; males attain sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...

 at 1–1.2 m (3.3–3.9 ft) long, and females at 1.3–1.5 m (4.3–4.9 ft) long.

Human interactions

The frilled shark has seldom been encountered alive, and thus poses no danger to humans (though scientists have accidentally cut themselves examining its teeth). On August 27, 2004, the first observation of this species in its natural habitat was made by the ROV Johnson-Sea-Link II, on the Blake Plateau off the southeastern United States. On January 21, 2007, a Japanese fisherman discovered a 1.6 m (5.2 ft) long female alive at the surface, perhaps there because of illness or weakness from the warm water. It was brought to Awashima Marine Park in Shizuoka
Shizuoka, Shizuoka
is the capital city of Shizuoka Prefecture, Japan, and the prefecture's second-largest city in terms of both population and area. It became one of Japan's 19 "designated cities" in 2005.-Geography:...

, where it died after a few hours. Garman, and numerous authors since, have advanced the frilled shark as an explanation for sea serpent sightings. Because of the shark's modest size, some cryptozoologists have posited the existence of a giant relative, particularly as larger Chlamydoselachus species are known from the fossil record.

Small numbers of frill sharks are caught incidentally
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...

 by various deepwater 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...

 around the world, using trawls, 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 longlines. In particular, it is regularly taken in Suruga Bay in bottom gillnets meant for sea breams and gnomefish
Gnomefish
The gnomefishes are a small family Scombropidae of marine fish, consisting of three species in the genus Scombrops.They have two dorsal fins and are notable for scales covering the soft parts of the dorsal and anal fins. The eyes are large....

es, and in midwater trawls meant for the shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...

 Sergia lucens. Japanese fishers regard it as a nuisance, as it damages the nets
Fishing net
A fishing net or fishnet is a net that is used for fishing. Fishing nets are meshes usually formed by knotting a relatively thin thread. Modern nets are usually made of artificial polyamides like nylon, although nets of organic polyamides such as wool or silk thread were common until recently and...

. This shark is sometimes sold for meat or processed into fishmeal, but is not economically significant. Because of its very low reproductive rate and the continuing expansion of 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...

 into its habitat, the International Union for Conservation of Nature (IUCN) has listed it as Near Threatened
Near Threatened
Near Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...

.

External links

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