The Portuguese Dogfish, also known as the Portuguese shark (Centroscymnus coelolepis) is a species of Sleeper shark of the family Somniosidae. It is globally distributed, and reported down to a depth of 12,057 feet. This makes the Portuguese Dogfish Shark the deepest living shark known currently.
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 by the small spines in front of its dorsal fins. Its dermal denticles are also unusual, resembling the scales of a bony fish.
Family: Somniosidae – Sleeper sharks
Genus: Centroscymnus
Species: coelolepis
Phylum– Chordata
Class– Chondrichthyles
Subclass– Elasmobranchii
Common Name– Dogfish Sharks
Family– Somniosidae
Common Name– Sleeper Sharks
Genus– Centroscymnus
Species– coelolepis
Status: IUCN Red List NEAR THREATENED
Average Size and Length: The Portuguese Dogfish typically reaches a length of 3.0 feet for males and 1.0 3.3 feet for females. Specimens up to 3.9 feet long have been recorded. The Mediterranean population grow much smaller, only reaching 26 inches.
Teeth and Jaw: The mouth of the Portuguese Dogfish 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.
Head: The Portuguese Dogfish has a flattened, broadly rounded snout that is shorter than the mouth is wide. The nostrils have short skin flaps. The eyes are large and oval in shape, positioned laterally on the head and equipped with a reflective tapetum lucidum that produces that yellow-green eye shine typical of deep-sea sharks and other deep-sea creatures.
Denticles: The dermal denticles are very large, and they 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.
Tail: The caudal fin has a short but well-developed lower lobe and a prominent ventral notch near the tip of the upper lobe.
Demographic, Distribution, Habitat, Environment and Range: The Portuguese Dogfish is intermittently distributed around the world and has one of the widest ranges. In the western Atlantic, it can be found from the Grand Banks of Newfoundland 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, as well as from southern Namibia to western South Africa. In the Indian Ocean, it has been caught off the Seychelles. In the Pacific, it can be found off Japan, New Zealand, and Australia from Cape Hawke, New South Wales, to Beachport, South Australia, including Tasmania.
The Portuguese Dogfish is the deepest living known shark. Reaching a maximum known depth of 12,057 feet. It has been reported from 490 feet to 12,057 feet on the lower part of the continental slope down to the abyssal plain. It is commonly found between 1,300 and 6,600 feet.
In the Mediterranean Sea, it is smaller, but is found commonly at deeper depths between 8,200-9,800 feet. They rarely come above 4,900 feet. The deep Mediterranean has a relatively constant temperature of 55 °F and a salinity of 38.4 ppt. In the deep ocean the temperature is generally only 41 °F and the salinity 34–35 ppt.
he Portuguese Dogfish is fundamentally 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 3,900–4,900 feet, while juveniles are found at deeper depths. There may be several separate populations in the Atlantic, and sharks in the Mediterranean and off Japan appear to be distinct. Living almost exclusively in the aphotic zone where little to no sunlight penetrates, the Portuguese dogfish is relatively common and the dominant shark species in deeper waters.
Diet: The Portuguese dogfish is an active and mobile predator of somewhat large organisms. They mainly feed on cephalopods and bony fishes. Some fish include slickheads, orange roughy, lantern fishes, and rattails. The Portuguese dogfish has also been known to consume other sharks and invertebrates as well as scavenging from whale carcasses. The Mediterranean population doesn’t scavenge.
The Portuguese Dogfish is also a known prey of larger sharks.
Aesthetic Identification: The body of the Portuguese Dogfish is thick and cylindrical except for the flattened belly. It has 5 pairs of short gill slits that are just about vertical. Young sharks are 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 (Deynat, P.P. (September 30, 2003). “Partial albinism in the Portuguese dogfish Centroscymnus coelolepis (Elasmobranchii, Somniosidae)”). 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.
Biology and Reproduction: The known parasites of the Portuguese Dogfish include monogeneans in the genus Erpocotyle, and the tapeworms Sphyriocephalus viridis, S. richardi, and Anthobothrium sp.
The Portuguese Dogfish is ovoviviparous, or aplacental viviparous. 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. 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 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 on its right side and a testis on its left (Veríssimo, A.; L. Gordo & I. Figueiredo (2003). “Reproductive biology and embryonic development of Centroscymnus coelolepis in Portuguese mainland waters“. ICES Journal of Marine Science.).
Early in development, the embryos are sexually undifferentiated, un-pigmented, and possess filamentous external gills; the external yolk sac in this stage weighs 4.2–4.6 ounces. Recognizable sex organs develop by an embryonic length of 3.6 inches, and tissue differentiation is complete by a length of 5.9 inches. Body pigmentation appears when the embryo is 3.9–5.9 inches long; the external gills regress at around the same time. An internal yolk sac develops when the embryo is 5.5 inches long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is 9.2–11.8 inches long the external yolk sac has been completely resorbed (Veríssimo, A.; L. Gordo & I. Figueiredo (2003). “Reproductive biology and embryonic development of Centroscymnus coelolepis in Portuguese mainland waters“. ICES Journal of Marine Science.).
Off Portugal, the young seem to be born in May and December following a gestation period of over a year. The litter size ranges from 1 to 29 (but 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 9.1–11.8 inches in the Atlantic, and 12–14 inches in the Pacific.
With exception of the Mediterranean population (males mature around 21 inches long), Portuguese Dogfish attain sexual maturity at similar sizes around the world: males and females mature at 35–40 inches and 33–45 inches off the Iberian Peninsula, 34 inches and 40 inches west of the British Isles, 28 inches and 37–39 inches in Suruga Bay, Japan, and 32–35 inches and 39–43 inches off southeastern Australia.
The large, squalene-rich liver of the Portuguese Dogfish shark allows it to maintain neutral buoyancy and hover with minimal effort; males contain more squalene in their livers than females.
Behavioral Traits, Sensing and Intelligence: The Portuguese Dogfish is extremely active and mobile.
The Portuguese Dogfish has extremely acute vision, even for deep-sea sharks overall. 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 bioluminescence: the maximum absorption of its visual pigments corresponds to the wavelengths of light emitted by favored prey, such as the squids Heteroteuthis dispar, Histioteuthis spp., Lycoteuthis lorigera, and Taningia danae (Bozzano, A. (December 2004). “Retinal specialisations in the dogfish Centroscymnus coelolepis from the Mediterranean deep-sea“. Scientia Marina.).
Speed: A tracking study in the Porcupine Seabight has found that the Portuguese dogfish has an average swimming speed of 0.072 m/s (0.24 ft/s), and does not remain in any particular area for long (Bagley, P.M.; A. Smith & I.G. Priede (August 1994). “Tracking movements of deep demersal fishes in the Porcupine Seabight, north-east Atlantic Ocean“. Journal of the Marine Biological Association of the United Kingdom.).
They are extremely active and mobile.
Portuguese Dogfish Shark Future and Conservation: The Portuguese Dogfish is commercially fished using hook-and-line, gillnets, 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.
One important fishery for the Portuguese dogfish is 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 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 (Stevens, J. & Correia, J.P.S. (SSG Australia & Oceania Regional Workshop, March 2003).
Portuguese Dogfish Shark Recorded Attacks on Humans: No threat to humans. It is a deep-sea shark and a small shark.