The Portuguese dogfish (''Centroscymnus coelolepis'') or Portuguese shark, is a species of sleeper shark of the family Somniosidae. This globally distributed species has been reported down to a depth of , making it the deepest-living shark known. It inhabits lower continental slopes and abyssal plains, usually staying near the bottom. Stocky and dark brown in color, the Portuguese dogfish can be distinguished from similar-looking species (such as the kitefin shark, ''Dalatias licha'') by the small spines in front of its dorsal fins. Its

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...

s are also unusual, resembling the scales of a

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

. This species typically reaches in length; sharks in the Mediterranean Sea are much smaller and have distinct depth and food preferences. Relatively common, the Portuguese dogfish is an active hunter capable of tackling fast, large prey. It feeds mainly on

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s and fishes, though it also consumes invertebrates and

cetacean Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...

carrion Carrion () is the decaying flesh of dead animals, including human flesh. Overview Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters (or scavengers) include crows, vultures, c ...

. This shark has acute vision optimized for detecting the

bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

of its prey, as

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...

does not reach the depths at which it lives. The Portuguese dogfish is aplacental viviparous, with the young provisioned by yolk and perhaps uterine fluid. The females give birth to up to 29 young after a

gestation period In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it ...

of over one year. Valued for its liver oil and to a lesser extent meat, Portuguese dogfish are important to deepwater commercial fisheries operating off Portugal, the British Isles,

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, and

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. These fishing pressures and the low reproductive rate of this species have led the International Union for Conservation of Nature (IUCN) to assess it as Near Threatened.

Taxonomy

The first

scientific description A species description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species that have be ...

of the Portuguese dogfish was published by Portuguese zoologists

José Vicente Barbosa du Bocage José Vicente Barbosa du Bocage (2 May 1823 – 3 November 1907) was a Portuguese zoologist and politician. He was the curator of Zoology at the Museu Nacional de Lisboa in Lisbon. He published numerous works on mammals, birds, and fishes. In t ...

and Félix António de Brito Capello, in an 1864 issue of ''Proceedings of the General Meetings for Scientific Business of the Zoological Society of London''. They created the new genus ''Centroscymnus'' for this shark, and gave it the

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

''coelolepis'', derived from the Greek ' ("hollow") and ''lepis'' ("fish scale") and referring to the structure of the

s. The type specimen, caught off Portugal, has since been destroyed in a fire.

Distribution and habitat

One of the widest-ranging deepwater sharks, the Portuguese dogfish is patchily distributed around the world. In the western Atlantic, it occurs from the

Grand Banks of Newfoundland The Grand Banks of Newfoundland are a series of underwater plateaus south-east of the island of Newfoundland on the North American continental shelf. The Grand Banks are one of the world's richest fishing grounds, supporting Atlantic cod, swordf ...

to the U.S. state of Delaware. In the eastern Atlantic, it is found from Iceland to Sierra Leone, including the western Mediterranean Sea, the Azores and

Madeira ) , anthem = ( en, "Anthem of the Autonomous Region of Madeira") , song_type = Regional anthem , image_map=EU-Portugal_with_Madeira_circled.svg , map_alt=Location of Madeira , map_caption=Location of Madeira , subdivision_type=Sovereign st ...

, as well as from southern Namibia to western South Africa. In the Indian Ocean, this species has been caught off the Seychelles. In the Pacific, this shark occurs off

, New Zealand, and

from Cape Hawke, New South Wales, to Beachport, South Australia, including Tasmania. The deepest-living shark known, the Portuguese dogfish has been reported at depths of to from the lower continental slope to the abyssal plain, and is most common between and . This species is found deeper in the Mediterranean, seldom occurring above a depth of and being most common at . The deep Mediterranean has a relatively constant temperature of and a

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

of 38.4 ppt, whereas in the deep ocean the temperature is generally only and the salinity 34–35 ppt. The Portuguese dogfish is essentially

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

in nature, though young sharks can be found a considerable distance off the bottom. There is depth segregation by size and sex; pregnant females are found in shallower water, above , while juveniles are found deeper. There may be several separate populations in the Atlantic, and sharks in the Mediterranean and off Japan appear to be distinct as well.

Description

The Portuguese dogfish typically reaches a length of for males and for females, though specimens up to long have been recorded. Sharks in the Mediterranean are smaller, growing no more than long. This species has a flattened, broadly rounded snout that is shorter than the mouth is wide. The nostrils are preceded by short flaps of skin. The eyes are large and oval in shape, positioned laterally on the head and equipped with a reflective tapetum lucidum that produces a yellow-green "eye shine". The mouth is wide and slightly arched, with moderately thick, smooth lips and short furrows at the corners extending onto both jaws. The upper teeth are slender and upright with a single cusp, numbering 43–68 rows. The lower teeth have a short, strongly angled cusp and number 29–41 rows; their bases interlock to form a continuous cutting surface.Burgess, G. and Bester, C
Biological Profiles: Portuguese Shark
Florida Museum of Natural History Ichthyology Department. Retrieved on October 17, 2009. The five pairs of gill slits are short and nearly vertical. The body of the Portuguese dogfish is thick and cylindrical except for the flattened belly. The two dorsal fins are small and of similar size and shape, each bearing a tiny grooved spine in front. The first dorsal fin originates well behind the pectoral fins, while the second dorsal originates over the middle of the

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

bases. The pectoral fins are medium-sized with a rounded margin. There is no

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

. The caudal fin has a short but well-developed lower lobe and a prominent ventral notch near the tip of the upper lobe. The very large

s change in shape with age: in juveniles, they are widely spaced and heart-shaped with an incomplete midline ridge and three posterior points, while in adults they are overlapping, roughly circular, smooth, and flattened with a round central concavity, superficially resembling the scales of

es. Young sharks are a uniform blue-black in color, while adults are brown-black; there are no prominent fin markings. In 1997, a partially albino individual, with a pale body but normal eyes, was caught in the northeastern Atlantic. This represented the first documented case of albinism in a deep-sea shark.

Biology and ecology

Living almost exclusively in the aphotic zone where little to no

penetrates, the Portuguese dogfish is relatively common and the dominant shark species in deeper waters. The large, squalene-rich liver of this shark allows it to maintain neutral buoyancy and hover with minimal effort; males contain more squalene in their livers than females. A tracking study in the Porcupine Seabight has found that the Portuguese dogfish has an average swimming speed of , and does not remain in any particular area for long. This species may be preyed upon by larger fishes and sharks. Known parasites of this species include monogeneans in the genus ''Erpocotyle'', and the tapeworms ''Sphyriocephalus viridis'', ''S. richardi'', and ''Anthobothrium'' sp. An active predator of mobile, relatively large organisms, the Portuguese dogfish feeds mainly on

s (including '' Mastigoteuthis'' spp.) and bony fishes (including

slickhead Slickheads, also known as nakedheads or smoothheads, are deep water fishes that belong to the family Alepocephalidae. They are most commonly found in the bathypelagic layer, which is approximately 3000m below the surface. They get their name from ...

s, orange roughy, lantern fishes, and rattails). It has also been known to take other sharks and invertebrates (such as the medusa ''

Atolla wyvillei ''Atolla wyvillei'', also known as the Atolla jellyfish or Coronate medusa, is a species of deep-sea crown jellyfish (Scyphozoa: Coronatae). It lives in oceans around the world. Like many species of mid-water animals, it is deep red in color. Thi ...

''), as well as scavenging from whale carcasses. The Portuguese dogfish has more acute vision than many other deepsea sharks: in addition to having a large pupil and lens, and a tapetum lucidum, its eyes also contain a high concentration of ganglion cells mostly concentrated in a horizontal streak that is densest at the center; these cells impart highly sensitive motion detection along the horizontal plane. The visual system of this species appears adapted for detecting

: the maximum

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

of its opsins correspond to the wavelengths of light emitted by favored prey, such as the

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...

s '' Heteroteuthis dispar'', ''

Histioteuthis ''Histioteuthis'' is a genus of squid in the family Histioteuthidae. It goes by the common 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 ...

'' spp., ''

Lycoteuthis lorigera ''Lycoteuthis lorigera'' is a species of squid in the genus '' Lycoteuthis''. They grow up to 8 cm in length. References Squid Molluscs described in 1875 {{squid-stub ...

'', and ''

Taningia danae ''Taningia danae'', the Dana octopus squid, is a species of squid in the family Octopoteuthidae. It is one of the largest known squid species, reaching a mantle length of and total length of . The largest known specimen, a mature female, weig ...

''. In the Mediterranean sea, the Portuguese dogfish is one of the most common deepwater sharks along with the blackmouth catshark (''Galeus melastomus'') and the velvet belly lantern shark (''Etmopterus spinax''), and the only shark abundant below a depth of . By inhabiting deeper water, Portuguese dogfish in the Mediterranean may reduce competition with the other two species. The smaller size of Mediterranean sharks relative to those from the rest of the world may be due to limited food availability and/or the warmer, saltier environment. Some 87% of the diet of Portuguese dogfish in the Mediterranean consists of cephalopods. Bony fishes are a secondary food source, while immature sharks favor the

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...

Acanthephyra eximia ''Acanthephyra'' is a genus of shrimp in the family Acanthephyridae, with species that live at depths from 0 to more than 5000 meters deep below the ocean surface. Species * '' Acanthephyra acanthitelsonis'' Spence Bate, 1888 * '' Acanthephyr ...

'', the most common decapod crustacean in their environment. Unlike in other regions, Mediterranean sharks seldom scavenge.

Life history

The Portuguese dogfish is aplacental viviparous, with the female retaining eggs internally until they hatch. The

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s are sustained by yolk, and possibly also by uterine fluid secreted by the mother. Figueiredo ''et al.'' (2008) reported that there are two breeding seasons per year off Portugal, from January to May and from August to December, with only a fraction of the population reproductively active at a time. However, previous accounts have described continuous reproduction with females in various stages of pregnancy present year-round. The ovarian follicles take some time to mature; they are ovulated into the uterus at a diameter of . Studies of females have found no traces of

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

inside their reproductive tracts, which suggests that fertilization occurs immediately following copulation, which may also trigger ovulation. The reproductive cycles of Portuguese dogfish in the Atlantic and Pacific are generally similar; sharks off Japan tend to produce larger numbers of smaller oocytes than elsewhere, while sharks off the British Isles exhibit a larger litter size and birth size (but smaller oocytes) than those off Portugal. There is a record of a hermaphroditic specimen with an

ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...

on its right side and a testis on its left. Early in development, the embryos are sexually undifferentiated, unpigmented, and possess filamentous external gills; the external yolk sac in this stage weighs . Recognizable sex organs develop by an embryonic length of , and tissue differentiation is complete by a length of . Body pigmentation appears when the embryo is long; the external gills regress at around the same time. An internal yolk sac develops when the embryo is long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is long the external yolk sac has been completely resorbed. Off Portugal, the young seem to be born in May and December following a

of over a year. As they near giving birth, the females undergo ovarian atresia (regression of the follicles), suggesting that they enter a resting period afterwards. The litter size ranges from 1 to 29 (typically 12), and is not correlated with female size. Parturition may occur in a yet-unknown nursery area, as newborns are rarely ever caught. The length at birth has been reported as in the Atlantic, and in the Pacific.Daley, R., J. Stevens and K. Graham. (2002). Catch analysis and productivity of the deepwater dogfish resource in southern Australia. FRDC Final Report, 1998/108. Canberra: Fisheries Research and Development Corporation. Aside from the distinctive Mediterranean population, Portuguese dogfish attain sexual maturity at similar sizes around the world: males and females mature at and respectively off the Iberian Peninsula, and respectively west of the British Isles, and respectively in

Suruga Bay Suruga Bay (駿河湾, ''Suruga-wan'') is a bay on the Pacific coast of Honshū in Shizuoka Prefecture, Japan. It is situated north of a straight line from Omaezaki Point to Irōzaki Point at the tip of the Izu Peninsula and surrounded by Honshū ...

, Japan, and and respectively off southeastern Australia.Irvine, S.B. (2004). Age, growth and reproduction of deepwater dogfishes from southeastern Australia. PhD Thesis, Deakin University. In the Mediterranean, males mature at around long.Cló, S., M. Dalú, R. Danovaro and M. Vacchi (2002). Segregation of the Mediterranean population of ''Centroscymnus coelolepis'' (Chondrichthyes: Squalidae): a description and survey. NAFO SCR Doc. 02/83

Human interactions

The Portuguese dogfish is too small and occurs too deep to pose a danger to humans. This species has long been commercially fished, using hook-and-line,

gillnet Gillnetting is a fishing method that uses gillnets: vertical panels of netting that hang from a line with regularly spaced floaters that hold the line on the surface of the water. The floats are sometimes called "corks" and the line with corks is ...

s, and trawls. It is mainly valued for its liver, which contains 22–49% squalene by weight and is processed for vitamins. The meat may also be sold fresh or dried and salted for human consumption, or converted into fishmeal. An important fishery for the Portuguese dogfish exists in Suruga Bay for liver oil; catches peaked during World War II, but declined soon after from over-exploitation. In the past few years, catches by the South East Trawl Fishery off Australia have been increasing, as fishers have been seeking out species not covered by commercial quotas following the relaxation of

seafood Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters and mussels, and cephalopods such as octopus an ...

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

regulations. Shark landings in this fishery are affected by a prohibition on landing livers without the rest of the carcass. Until recently, Portugal was the only European country to utilize the Portuguese dogfish. An important

bycatch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...

of the black scabbardfish (''Aphanops carbo'') longline fishery, between 300 and 900 tons of this shark were landed annually from 1986 to 1999. Its per-weight value has been increasing since 1986, and thus exploitation is likely to continue. Around 1990, French bottom trawlers began to fish for Portuguese dogfish and leafscale gulper sharks (''Centrophorus squamosus'') west of the British Isles for meat and livers; these two species are together referred to as ''siki''. The ''siki'' catch peaked at 3,284 tons in 1996 before declining to 1,939 tons in 1999. The French have since been joined by Norwegian, Irish, and

Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish ide ...

longliners and trawlers, making the Portuguese dogfish a significant component of deepwater fisheries in the northwest Atlantic. While stocks off Portugal seem to be stable for now, stocks off the British Isles have diminished substantially in recent years; this may reflect the disparity between the less massified Portuguese fishery and the commercial French fishery. The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened, because of its commercial value and low reproductive productivity. In June 2018 the New Zealand

Department of Conservation An environmental ministry is a national or subnational government agency politically responsible for the environment and/or natural resources. Various other names are commonly used to identify such agencies, such as Ministry of the Environment, ...

classified the Portuguese dogfish as "Not Threatened" with the qualifier "Data Poor" under the

New Zealand Threat Classification System The New Zealand Threat Classification System is used by the Department of Conservation to assess conservation priorities of species in New Zealand. The system was developed because the IUCN Red List, a similar conservation status system, had some ...

References

{{DEFAULTSORT:dogfush, Portuguese

Portuguese dogfish The Portuguese dogfish (''Centroscymnus coelolepis'') or Portuguese shark, is a species of sleeper shark of the family Somniosidae. This globally distributed species has been reported down to a depth of , making it the deepest-living shark known. ...

Fauna of Portugal Fish of the East Atlantic Fish of the Eastern United States