Portuguese dogfish
Encyclopedia
The Portuguese dogfish or Portuguese shark (Centroscymnus coelolepis) is a species
of sleeper shark, family
Somniosidae
. This globally distributed species has been reported down to a depth of 3675 m (12,057.1 ft), making it the deepest-living shark known. It inhabits lower continental slopes and abyssal plain
s, 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 fin
s. Its dermal denticles are also unusual, resembling the scales
of a bony fish. This species typically reaches 0.9–1 m (3–3.3 ft) 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
s and fishes, though it also consumes invertebrate
s and cetacean carrion
. This shark has acute vision optimized for detecting the bioluminescence
of its prey, as sunlight
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
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
, and Australia
. 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
.
and Félix António de Brito Capello, in a 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 coelolepis, derived from the Greek koilos ("hollow") and lepidos ("fish scale") and referring to the structure of the dermal denticles. The type specimen, caught off Portugal
, has since been destroyed in a fire.
of Delaware
. In the eastern Atlantic, it is found from Iceland
to Sierra Leone
, including the western Mediterranean Sea
, the Azores
and Madeira
, 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 Japan
, New Zealand
, and Australia
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 150 m (492.1 ft) to 3675 m (12,057.1 ft) from the lower continental slope to the abyssal plain
, and is most common between 400 m (1,312.3 ft) and 2000 m (6,561.7 ft). This species is found deeper in the Mediterranean, seldom occurring above a depth of 1500 m (4,921.3 ft) and being most common at 2500–3000 m (8,202.1–9,842.5 ft). The deep Mediterranean has a relatively constant temperature of 13 °C (55.4 °F) and a salinity
of 38.4 ppt, whereas in the deep ocean the temperature is generally only 5 °C (41 °F) and the salinity 34–35 ppt. The Portuguese dogfish is essentially benthic 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 1200–1500 m (3,937–4,921.3 ft), 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.
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. The five pairs of gill slit
s 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 bases. The pectoral fins are medium-sized with a rounded margin. There is no anal fin. 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 dermal denticles 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 bony fishes. 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.
where little to no sunlight
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 0.072 m/s (0.236220472440945 ft/s), 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 monogenea
ns in the genus Erpocotyle, and the tapeworms Sphyriocephalus viridis, S. richardi, and Anthobothrium sp.
An active predator of mobile, relatively large organism
s, the Portuguese dogfish feeds mainly on cephalopod
s (including Mastigoteuthis
spp.) and bony fishes (including slickheads, orange roughy
, lantern fishes, and rattail
s). It has also been known to take other sharks and invertebrate
s (such as the medusa
Atolla wyvillei), 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 cell
s 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 bioluminescence
: the maximum absorption
of its visual pigments correspond to the wavelength
s of light emitted by favored prey, such as the squid
s Heteroteuthis dispar
, Histioteuthis
spp., Lycoteuthis lorigera
, and Taningia danae.
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 1500 m (4,921.3 ft). 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
Acanthephyra eximia, the most common decapod
crustacean
in their environment. Unlike in other regions, Mediterranean sharks seldom scavenge.
s take some time to mature; they are ovulated
into the uterus
at a diameter of 5–6 cm (2–2.4 in). Studies of females have found no traces of sperm
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 oocyte
s 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
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 120–130 g (4.2–4.6 oz). Recognizable sex organs develop by an embryonic length of 92 mm (3.6 in), and tissue differentiation is complete by a length of 150 mm (5.9 in). Body pigmentation appears when the embryo is 100–150 mm (3.9–5.9 in) long; the external gills regress at around the same time. An internal yolk sac develops when the embryo is 140 mm (5.5 in) long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is 233–300 mm (9.2–11.8 in) long the external yolk sac has been completely resorbed. Off Portugal, the young seem to be born in May and December following a gestation period
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 23–30 cm (9.1–11.8 in) in the Atlantic, and 30–35 cm (11.8–13.8 in) in the Pacific.
Aside from the distinctive Mediterranean population, Portuguese dogfish attain sexual maturity
at similar sizes around the world: males and females mature at 90–101 cm (35.4–39.8 in) and 85–115 cm (33.5–45.3 in) respectively off the Iberian Peninsula
, 86 cm (33.9 in) and 102 cm (40.2 in) respectively west of the British Isles, 70 cm (27.6 in) and 95–100 cm (37.4–39.4 in) respectively in Suruga Bay
, Japan, and 82–90 cm (32.3–35.4 in) and 99–110 cm (39–43.3 in) respectively off southeastern Australia. In the Mediterranean, males mature at around 53 cm (20.9 in) long.
, using hook-and-line, gillnet
s, and trawls. It is mainly valued for its liver
, which contains 22–49% squalene
by weight and is processed for vitamin
s. 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 quota
s following the relaxation of seafood
mercury
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
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 shark
s (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
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 artisanal
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.
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 sleeper shark, 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...
Somniosidae
Somniosidae
Somniosidae is a family of sharks in the order Squaliformes, commonly known as sleeper sharks.-Genera:* Centroscymnus Barbosa du Bocage & Brito Capello, 1864...
. This globally distributed species has been reported down to a depth of 3675 m (12,057.1 ft), making it the deepest-living shark known. It inhabits lower continental slopes and abyssal plain
Abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest...
s, 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
Kitefin shark
The kitefin shark or seal shark is a species of dogfish shark in the family Dalatiidae, and the only species in its genus. It is found sporadically around the world, usually close to the sea floor at depths of . With a sizable oil-filled liver to maintain neutral buoyancy, this shark is able to...
, Dalatias licha) by the small spines in front of its dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s. Its dermal denticles are also unusual, resembling the scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
of a bony fish. This species typically reaches 0.9–1 m (3–3.3 ft) in length; sharks in 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...
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
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 and fishes, though it also consumes invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and cetacean carrion
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...
. This shark has acute vision optimized for detecting the bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
of its prey, as sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
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
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:...
of over one year. Valued for its 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...
and to a lesser extent meat, Portuguese dogfish are important to deepwater commercial fisheries operating off 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...
, the British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
, 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...
, and 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...
. 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
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...
.
Taxonomy
The first scientific description of the Portuguese dogfish was published by Portuguese zoologists José Vicente Barbosa du BocageJosé Vicente Barbosa du Bocage
José Vicente Barbosa du Bocage was a Portuguese zoologist and politician. He was the curator of Zoology at the Museum of Natural History in Lisbon. He published numerous works on mammals, birds, and fishes. In the 1880s he became the Minister of the Navy and later the Minister for Foreign Affairs...
and Félix António de Brito Capello, in a 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 coelolepis, derived from the Greek koilos ("hollow") and lepidos ("fish scale") and referring to the structure of the dermal denticles. The type specimen, caught off 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...
, 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 to the U.S. stateU.S. state
A U.S. state is any one of the 50 federated states of the United States of America that share sovereignty with the federal government. Because of this shared sovereignty, an American is a citizen both of the federal entity and of his or her state of domicile. Four states use the official title of...
of Delaware
Delaware
Delaware is a U.S. state located on the Atlantic Coast in the Mid-Atlantic region of the United States. It is bordered to the south and west by Maryland, and to the north by Pennsylvania...
. In the eastern Atlantic, it is found from Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
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...
, including the western 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...
, 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...
and 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...
, as well as from southern Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
to western 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...
. In the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
, this species has been caught off the Seychelles
Seychelles
Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
. In the Pacific, this shark occurs off 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...
, 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 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...
from Cape Hawke
Cape Hawke
Cape Hawke is a coastal headland in Australia on the New South Wales coast, just south of Forster/Tuncurry and within the Booti Booti National Park....
, 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...
, to Beachport
Beachport, South Australia
Beachport is a small coastal town 379 kilometres south-east of Adelaide, South Australia, located in the Wattle Range Council. Known for its 772 metre long jetty , Beachport is located on the northern end of Rivoli Bay...
, South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, including 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...
.
The deepest-living shark known, the Portuguese dogfish has been reported at depths of 150 m (492.1 ft) to 3675 m (12,057.1 ft) from the lower continental slope to the abyssal plain
Abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest...
, and is most common between 400 m (1,312.3 ft) and 2000 m (6,561.7 ft). This species is found deeper in the Mediterranean, seldom occurring above a depth of 1500 m (4,921.3 ft) and being most common at 2500–3000 m (8,202.1–9,842.5 ft). The deep Mediterranean has a relatively constant temperature of 13 °C (55.4 °F) and a salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
of 38.4 ppt, whereas in the deep ocean the temperature is generally only 5 °C (41 °F) and the salinity 34–35 ppt. The Portuguese dogfish is essentially benthic 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 1200–1500 m (3,937–4,921.3 ft), 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 0.9 m (3 ft) for males and 1 m (3.3 ft) for females, though specimens up to 1.2 m (3.9 ft) long have been recorded. Sharks in the Mediterranean are smaller, growing no more than 65 cm (25.6 in) 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 lucidumTapetum lucidum
The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....
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. The five pairs of 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 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 bases. The pectoral fins are medium-sized with a rounded margin. There is no anal fin. 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 dermal denticles 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
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
of bony fishes. 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
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
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 zoneAphotic zone
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone...
where little to no sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
penetrates, the Portuguese dogfish is relatively common and the dominant shark species in deeper waters. The large, squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
-rich liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
of this shark allows it to maintain neutral buoyancy
Neutral buoyancy
Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink...
and hover with minimal effort; males contain more squalene in their livers than females. A tracking study in the Porcupine Seabight
Porcupine Seabight
The Porcupine Seabight is a deep-water basin in the Porcupine Bank in the northeastern Atlantic Ocean. It lies southwest of Ireland and is approximately 180 nautical miles by 100 nautical miles . The feature is shaped roughly like an amphitheatre with the deepest point lying to the southwest...
has found that the Portuguese dogfish has an average swimming speed of 0.072 m/s (0.236220472440945 ft/s), 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 monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
ns in the genus Erpocotyle, and the tapeworms Sphyriocephalus viridis, S. richardi, and Anthobothrium sp.
An active predator of mobile, relatively large organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s, the Portuguese dogfish 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 (including Mastigoteuthis
Mastigoteuthis
Mastigoteuthis is a genus of whip-lash squid containing approximately 12 species. Some teuthologists consider Idioteuthis synonymous with this taxon.-Species:*Genus Mastigoteuthis**Mastigoteuthis agassizii...
spp.) and bony fishes (including slickheads, orange roughy
Orange roughy
The orange roughy, red roughy, or deep sea perch, Hoplostethus atlanticus, is a relatively large deep-sea fish belonging to the slimehead family . The Marine Conservation Society has categorized orange roughy as vulnerable to exploitation...
, lantern fishes, and rattail
Rattail
Grenadiers or rattails are generally large, brown to black gadiform marine fish of the family Macrouridae...
s). It has also been known to take other sharks and invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s (such as the medusa
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
Atolla wyvillei), as well as scavenging from whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
carcasses. The Portuguese dogfish has more acute vision than many other deepsea sharks: in addition to having a large pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
and lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
, and a tapetum lucidum, its eyes also contain a high concentration of ganglion cell
Ganglion cell
A retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
s 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
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 detecting bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
: the maximum absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of its visual pigments correspond to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of light emitted by favored prey, such as the 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...
s Heteroteuthis dispar
Heteroteuthis dispar
Heteroteuthis dispar, also known as the odd bobtail, is a small deep water squid found in the North Atlantic Ocean and the Mediterranean Sea.-Taxonomy:...
, 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,...
spp., Lycoteuthis lorigera
Lycoteuthis lorigera
Lycoteuthis lorigera is a species of squid in the genus Lycoteuthis. They grow up to 8 cm in length....
, and Taningia danae.
In the Mediterranean sea, the Portuguese dogfish is one of the most common deepwater sharks along with the blackmouth catshark
Blackmouth catshark
The blackmouth catshark is a species of catshark, family Scyliorhinidae, common in the northeastern Atlantic Ocean from Iceland to Senegal, including the Mediterranean Sea. It is typically found over the continental slope at depths of , on or near muddy bottoms. The youngest sharks generally...
(Galeus melastomus) and the velvet belly lantern shark
Velvet belly lantern shark
The velvet belly lantern shark is a species of dogfish shark in the family Etmopteridae. One of the most common deepwater sharks in the northeastern Atlantic Ocean, the velvet belly is found from Iceland and Norway to Gabon and South Africa at a depth of...
(Etmopterus spinax), and the only shark abundant below a depth of 1500 m (4,921.3 ft). 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
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...
Acanthephyra eximia, the most common decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
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 embryos 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 follicleOvarian follicle
Ovarian follicles are the basic units of female reproductive biology, each of which is composed of roughly spherical aggregations of cells found in the ovary. They contain a single oocyte . These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single...
s take some time to mature; they are ovulated
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...
into 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...
at a diameter of 5–6 cm (2–2.4 in). Studies of females have found no traces of sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
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 oocyte
Oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
s 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
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...
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
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...
in this stage weighs 120–130 g (4.2–4.6 oz). Recognizable sex organs develop by an embryonic length of 92 mm (3.6 in), and tissue differentiation is complete by a length of 150 mm (5.9 in). Body pigmentation appears when the embryo is 100–150 mm (3.9–5.9 in) long; the external gills regress at around the same time. An internal yolk sac develops when the embryo is 140 mm (5.5 in) long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is 233–300 mm (9.2–11.8 in) long the external yolk sac has been completely resorbed. Off Portugal, the young seem to be born in May and December following a 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:...
of over a year. As they near giving birth, the females undergo ovarian atresia
Ovarian follicle atresia
Ovarian follicle atresia is the periodic process in which immature ovarian follicles degenerate and are subsequently re-absorbed during the follicular phase of the menstrual cycle. Typically around 20 follicles mature each month and only a single follicle is ovulated. The rest undergo 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 23–30 cm (9.1–11.8 in) in the Atlantic, and 30–35 cm (11.8–13.8 in) in the Pacific.
Aside from the distinctive Mediterranean population, Portuguese dogfish 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 similar sizes around the world: males and females mature at 90–101 cm (35.4–39.8 in) and 85–115 cm (33.5–45.3 in) respectively off the Iberian Peninsula
Iberian Peninsula
The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...
, 86 cm (33.9 in) and 102 cm (40.2 in) respectively west of the British Isles, 70 cm (27.6 in) and 95–100 cm (37.4–39.4 in) respectively 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, and 82–90 cm (32.3–35.4 in) and 99–110 cm (39–43.3 in) respectively off southeastern Australia. In the Mediterranean, males mature at around 53 cm (20.9 in) long.
Human interactions
The Portuguese dogfish is too small and occurs too deep to pose a danger to humans. This species has long been commercially fishedCommercial 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...
, using hook-and-line, 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. It is mainly valued for its 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...
, which contains 22–49% squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
by weight and is processed 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. 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, but declined soon after from over-exploitation
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
. 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 quota
Individual fishing quota
Individual fishing quotas also known as "individual transferable quotas" are one kind of catch share, a means by which many governments regulate fishing. The regulator sets a species-specific total allowable catch , typically by weight and for a given time period. A dedicated portion of the TAC,...
s following the relaxation of seafood
Seafood
Seafood is any form of marine life regarded as food by humans. Seafoods include fish, molluscs , crustaceans , echinoderms . Edible sea plants, such as some seaweeds and microalgae, are also seafood, and are widely eaten around the world, especially in Asia...
mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
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
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 the black scabbardfish
Black scabbardfish
The black scabbardfish, Aphanopus carbo, is a bathypelagic cutlassfish of the family Trichiuridae found in the Atlantic Ocean between latitudes 69° N and 27° N at depths of between 180 and 1,700 m...
(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 shark
Leafscale gulper shark
The leafscale gulper shark, Centrophorus squamosus, is a dogfish of the family Centrophoridae.-Physical characteristics:The leafscale gulper shark has no anal fin, two dorsal fins with spines, the first dorsal being relatively low and long, large eyes, and rough leaf-like denticles...
s (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
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...
, Irish
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...
, and Scottish
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...
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 artisanal
Artisan fishing
Artisan fishing is a term used to describe small scale low-technology commercial or subsistence fishing practices. The term particularly applies to coastal or island ethnic groups using traditional techniques such as rod and tackle, arrows and harpoons, throw nets and drag nets, and traditional...
Portuguese fishery and the commercial French fishery. The International Union for Conservation of Nature (IUCN) has assessed this species 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...
, because of its commercial value and low reproductive productivity.