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Portuguese dogfish

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Portuguese dogfish
A small, dark brown, heavy-bodied shark with large green eyes and small fins, lying on the ground next to a meterstick
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Squaliformes
Family: Somniosidae
Genus: Centroscymnus
Species:
C. coelolepis
Binomial name
Centroscymnus coelolepis
World map with blue shading in the northern Atlantic Ocean, western Mediterranean Sea, southern Indian Ocean, and off Japan
Range of the Portuguese dogfish
Synonyms

Centroscymnus macrops*Hu & Li, 1982
Scymnodon melasBigelow, Schroeder & Springer, 1953

* ambiguous synonym

ThePortuguese dogfish(Centroscymnus coelolepis) orPortuguese shark,is aspeciesofsleeper sharkof thefamilySomniosidae.This globally distributed species has been reported down to a depth of 3,675 m (12,057 ft), making it the deepest-living shark known. It inhabits lowercontinental slopesandabyssal plains,usually staying near the bottom. Stocky and dark brown in color, the Portuguese dogfish can be distinguished from similar-looking species (such as thekitefin shark,Dalatias licha) by the small spines in front of itsdorsal fins.Itsdermal denticlesare also unusual, resembling thescalesof abony fish.This species typically reaches 0.9–1 m (3.0–3.3 ft) in length; sharks in theMediterranean Seaare 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 oncephalopodsand fishes, though it also consumesinvertebratesandcetaceancarrion.This shark has acute vision optimized for detecting thebioluminescenceof its prey, assunlightdoes not reach the depths at which it lives. The Portuguese dogfish isaplacental viviparous,with the young provisioned byyolkand perhaps uterine fluid. The females give birth to up to 29 young after agestation periodof over one year. Valued for itsliver oiland to a lesser extent meat, Portuguese dogfish are important to deepwatercommercial fisheriesoperating offPortugal,theBritish Isles,Japan,andAustralia.These fishing pressures and the low reproductive rate of this species have led theInternational Union for Conservation of Nature(IUCN) to assess it asNear Threatened.

Taxonomy

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Black and white drawing of a dogfish from the side
Early illustration of a Portuguese dogfish.

The firstscientific descriptionof the Portuguese dogfish was published by Portuguese zoologistsJosé Vicente Barbosa du Bocageand Félix António de Brito Capello, in an 1864 issue ofProceedings of the General Meetings for Scientific Business of the Zoological Society of London.[2]They created the new genusCentroscymnusfor this shark, and gave it thespecific epithetcoelolepis,derived from the Greekkoilos( "hollow" ) andlepis( "fish scale" ) and referring to the structure of thedermal denticles.[3]Thetype specimen,caught offPortugal,has since been destroyed in a fire.[2]

Distribution and habitat

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One of the widest-ranging deepwater sharks, the Portuguese dogfish is patchily distributed around the world.[4]In the westernAtlantic,it occurs from theGrand Banks of Newfoundlandto theU.S. stateofDelaware.In the eastern Atlantic, it is found fromIcelandtoSierra Leone,including the westernMediterranean Sea,theAzoresandMadeira,as well as from southernNamibiato westernSouth Africa.[2]In theIndian Ocean,this species has been caught off theSeychelles.[5]In thePacific,this shark occurs offJapan,New Zealand,andAustraliafromCape Hawke,New South Wales,toBeachport,South Australia,includingTasmania.[1]

The deepest-living shark known,[6]the Portuguese dogfish has been reported at depths of 150 m (490 ft) to 3,675 m (12,057 ft) from the lowercontinental slopeto theabyssal plain,and is most common between 400 m (1,300 ft) and 2,000 m (6,600 ft).[1][7]This species is found deeper in the Mediterranean, seldom occurring above a depth of 1,500 m (4,900 ft) and being most common at 2,500–3,000 m (8,200–9,800 ft).[8]The deep Mediterranean has a relatively constant temperature of 13 °C (55 °F) and asalinityof 38.4 ppt, whereas in the deep ocean the temperature is generally only 5 °C (41 °F) and the salinity 34–35 ppt.[9]The Portuguese dogfish is essentiallybenthicin nature, though young sharks can be found a considerable distance off the bottom.[2][10]There is depth segregation by size and sex; pregnant females are found in shallower water, above 1,200–1,500 m (3,900–4,900 ft), while juveniles are found deeper.[1][8][11]There may be several separate populations in the Atlantic, and sharks in the Mediterranean and off Japan appear to be distinct as well.[12]

Description

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Side view of a dark brown shark
The Portuguese dogfish can be distinguished from similar species by its tiny dorsal fin spines.
Dorsal and ventral view of the Portuguese dogfish's head, showing the broad, flattened snout and wide mouth.

The Portuguese dogfish typically reaches a length of 0.9 m (3.0 ft) for males and 1.0 m (3.3 ft) for females, though specimens up to 1.2 m (3.9 ft) long have been recorded.[13]Sharks in the Mediterranean are smaller, growing no more than 65 cm (26 in) long.[4]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.[2]The eyes are large and oval in shape, positioned laterally on the head and equipped with a reflectivetapetum lucidumthat produces a yellow-green "eye shine".[14]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.[13]The five pairs ofgill slitsare short and nearly vertical.[15]

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 thepectoral fins,while the second dorsal originates over the middle of thepelvic finbases. The pectoral fins are medium-sized with a rounded margin. There is noanal fin.Thecaudal finhas a short but well-developed lower lobe and a prominent ventral notch near the tip of the upper lobe.[2]The very largedermal denticleschange 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 thescalesofbony fishes.[16]Young sharks are a uniform blue-black in color, while adults are brown-black; there are no prominent fin markings.[13]In 1997, a partiallyalbinoindividual, 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.[17]

Biology and ecology

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Living almost exclusively in theaphotic zonewhere little to nosunlightpenetrates, the Portuguese dogfish is relatively common and the dominant shark species in deeper waters.[11][14]The large,squalene-richliverof this shark allows it to maintainneutral buoyancyand hover with minimal effort; males contain more squalene in their livers than females.[18]A tracking study in thePorcupine Seabighthas 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.[19]This species may be preyed upon by larger fishes and sharks.[13]Knownparasitesof this species includemonogeneansin the genusErpocotyle,[20]and thetapewormsSphyriocephalus viridis,[21]S. richardi,andAnthobothriumsp.[22]

An active predator of mobile, relatively largeorganisms,the Portuguese dogfish feeds mainly oncephalopods(includingMastigoteuthisspp.) and bony fishes (includingslickheads,orange roughy,lantern fishes,andrattails). It has also been known to take other sharks andinvertebrates(such as themedusaAtolla wyvillei), as well asscavengingfromwhalecarcasses.[1][23]The Portuguese dogfish has more acute vision than many other deepsea sharks: in addition to having a largepupilandlens,and a tapetum lucidum, its eyes also contain a high concentration ofganglion cellsmostly 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 appearsadaptedfor detectingbioluminescence:the maximumabsorptionof itsopsinscorrespond to thewavelengthsof light emitted by favored prey, such as thesquidsHeteroteuthis dispar,Histioteuthisspp.,Lycoteuthis lorigera,andTaningia danae.[14]

In the Mediterranean sea, the Portuguese dogfish is one of the most common deepwater sharks along with theblackmouth catshark(Galeus melastomus) and thevelvet belly lantern shark(Etmopterus spinax), and the only shark abundant below a depth of 1,500 m (4,900 ft).[8]By inhabiting deeper water, Portuguese dogfish in the Mediterranean may reduce competition with the other two species.[4]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 theshrimpAcanthephyra eximia,the most commondecapodcrustaceanin their environment.[9]Unlike in other regions, Mediterranean sharks seldom scavenge.[8]

Life history

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The Portuguese dogfish isaplacental viviparous,with the female retaining eggs internally until they hatch. Theembryosare sustained byyolk,and possibly also by uterine fluid secreted by the mother.[12]Figueiredoet 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.[12][24]Theovarian folliclestake some time to mature; they areovulatedinto theuterusat a diameter of 5–6 cm (2.0–2.4 in).[13]Studies of females have found no traces ofsperminside their reproductive tracts, which suggests thatfertilizationoccurs immediately followingcopulation,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 smalleroocytesthan 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 ahermaphroditicspecimen with anovaryon its right side and atestison its left.[12]

Early in development, the embryos are sexually undifferentiated, unpigmented, and possess filamentous external gills; the externalyolk sacin 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.[12]Off Portugal, the young seem to be born in May and December following agestation periodof over a year. As they near giving birth, the females undergoovarian atresia(regression of the follicles), suggesting that they enter a resting period afterwards.[24]The litter size ranges from 1 to 29 (typically 12), and is not correlated with female size.[1][12]Parturition may occur in a yet-unknown nursery area, as newborns are rarely ever caught.[12]The length at birth has been reported as 23–30 cm (9.1–11.8 in) in the Atlantic,[12][25]and 30–35 cm (12–14 in) in the Pacific.[26][27]

Aside from the distinctive Mediterranean population, Portuguese dogfish attainsexual maturityat similar sizes around the world: males and females mature at 90–101 cm (35–40 in) and 85–115 cm (33–45 in) respectively off theIberian Peninsula,[12][24][25]86 cm (34 in) and 102 cm (40 in) respectively west of the British Isles,[28]70 cm (28 in) and 95–100 cm (37–39 in) respectively inSuruga Bay,Japan,[26]and 82–90 cm (32–35 in) and 99–110 cm (39–43 in) respectively off southeastern Australia.[27][29]In the Mediterranean, males mature at around 53 cm (21 in) long.[30]

Human interactions

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Black and white photo of a row of small sharks lying side by side on the deck of a fishing boat
The Portuguese dogfish is an important component of several deepwater fisheries.

The Portuguese dogfish is too small and occurs too deep to pose a danger to humans.[13]This species has long beencommercially fished,using hook-and-line,gillnets,andtrawls.It is mainly valued for itsliver,which contains 22–49%squaleneby weight and is processed forvitamins.The meat may also be sold fresh or dried and salted for human consumption, or converted intofishmeal.[1][7]An important fishery for the Portuguese dogfish exists in Suruga Bay for liver oil; catches peaked duringWorld War II,but declined soon after fromover-exploitation.[1]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 commercialquotasfollowing the relaxation ofseafoodmercuryregulations. Shark landings in this fishery are affected by a prohibition on landing livers without the rest of the carcass.[1]

Until recently, Portugal was the only European country to utilize the Portuguese dogfish. An importantbycatchof theblack 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.[1]Around 1990, Frenchbottom trawlersbegan to fish for Portuguese dogfish andleafscale gulper sharks(Centrophorus squamosus) west of the British Isles for meat and livers; these two species are together referred to assiki.Thesikicatch peaked at 3,284 tons in 1996 before declining to 1,939 tons in 1999. The French have since been joined byNorwegian,Irish,andScottishlongliners and trawlers, making the Portuguese dogfish a significant component of deepwater fisheries in the northwest Atlantic.[10][11]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 theless massifiedPortuguese fishery and the commercial French fishery.[12]TheInternational Union for Conservation of Nature(IUCN) has assessed this species asNear Threatened,because of its commercial value and low reproductive productivity.[1]

In June 2018 the New ZealandDepartment of Conservationclassified the Portuguese dogfish as "Not Threatened" with the qualifier "Data Poor" under theNew Zealand Threat Classification System.[31]

References

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  1. ^abcdefghijkStevens, J. & Correia, J.P.S. (SSG Australia & Oceania Regional Workshop, March 2003) (2003)."Centroscymnus coelolepis".The IUCN Red List of Threatened Species.2003:e.T41747A10552910.doi:10.2305/IUCN.UK.2003.RLTS.T41747A10552910.en.{{cite journal}}:CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  2. ^abcdefCompagno, L.J.V. (1984).Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date.Rome: Food and Agricultural Organization. p. 55–56.ISBN978-92-5-101384-7.
  3. ^Lineaweaver, T.H. & R.H. Backus (1970).The Natural History of Sharks.Lippincott. p.239.
  4. ^abcTosti, L.; R. Danovaro; A. Dell'anno; I. Olivotto; S. Bompadre; S. Clo & O. Carnevali (August 2006). "Vitellogenesis in the deep-sea sharkCentroscymnus coelolepis".Chemistry and Ecology.22(4): 335–345.doi:10.1080/02757540600812016.S2CID86109497.
  5. ^Baranes, A. (2003). "Sharks from the Amirantes Islands, Seychelles, with a description of two new species of squaloids from the deep sea".Israel Journal of Zoology.49(1): 33–65.doi:10.1560/N4KU-AV5L-0VFE-83DL.
  6. ^Priede I.G.; R. Froese; D.M. Bailey; O.A. Bergstad; M.A. Collins; J.E. Dyb; C. Henriques; E.G. Jones & N. King (2006)."The absence of sharks from abyssal regions of the world's oceans".Proceedings of the Royal Society B.273(1592): 1435–1441.doi:10.1098/rspb.2005.3461.PMC1560292.PMID16777734.
  7. ^abFroese, Rainer; Pauly, Daniel (eds.) (2009)."Centroscymnus coelolepis"inFishBase.November 2009 version.
  8. ^abcdSion, L.; A. Bozzano; G. D'Onghia; F. Capezzuto & M. Panza (December 2004). "Chondrichthyes species in deep waters of the Mediterranean Sea".Scientia Marina.68 (Supplement 3): 153–162.doi:10.3989/scimar.2004.68s3153.hdl:10261/5645.
  9. ^abCarrasson, M.; C. Stefanescu & J.E. Cartes (1992)."Diets and bathymetric distributions of two bathyal sharks of the Catalan deep sea (western Mediterranean)".Marine Ecology Progress Series.82(1): 21–30.Bibcode:1992MEPS...82...21C.doi:10.3354/meps082021.
  10. ^abClarke, M.W., L. Borges and R.A. Officer (April 2005)."Comparisons of trawl and longline catches of deepwater elasmobranchs west and north of Ireland".Journal of Northwest Atlantic Fishery Science.35:429–442.doi:10.2960/J.v35.m516.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  11. ^abcClarke, M.W.; P.L. Connolly & J.J. Bracken (2001). "Aspects of reproduction of the deep water sharksCentroscymnus coelolepisandCentrophorus squamosusfrom west of Ireland and Scotland ".Journal of the Marine Biological Association of the United Kingdom.81(6): 1019–1029.doi:10.1017/S0025315401005008.S2CID83052597.
  12. ^abcdefghijVeríssimo, A.; L. Gordo & I. Figueiredo (2003)."Reproductive biology and embryonic development ofCentroscymnus coelolepisin Portuguese mainland waters ".ICES Journal of Marine Science.60(6): 1335–1341.doi:10.1016/S1054-3139(03)00146-2.
  13. ^abcdefBurgess, G. and Bester, C.Biological Profiles: Portuguese Shark.Florida Museum of Natural History Ichthyology Department. Retrieved on October 17, 2009.
  14. ^abcBozzano, A. (December 2004)."Retinal specialisations in the dogfishCentroscymnus coelolepisfrom the Mediterranean deep-sea ".Scientia Marina.68(S3): 185–195.doi:10.3989/scimar.2004.68s3185.
  15. ^Yano, K. & S. Tanaka (1983). "Portuguese shark,Centroscymnus coelolepisfrom Japan, with notes onC. owstoni".Japanese Journal of Ichthyology.30(3): 208–216.
  16. ^Taniuchi, T. & J.A.F. Garrick (1986). "A new species ofScymnodalatiasfrom the southern oceans, and comments on other squaliform sharks ".Japanese Journal of Ichthyology.33(2): 119–134.doi:10.1007/BF02905840.S2CID198492003.
  17. ^Deynat, P.P. (September 30, 2003). "Partial albinism in the Portuguese dogfishCentroscymnus coelolepis(Elasmobranchii, Somniosidae) ".Cybium.27(3): 233–236.
  18. ^Hernandez-Perez, M.; R.M. Rabanal Gallego & M.J. Gonzalez Carlos (2002). "Sex difference in liver-oil concentration in the deep-sea shark,Centroscymnus coelolepis".Marine and Freshwater Research.53(5): 883–886.doi:10.1071/MF01035.
  19. ^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.74(3): 473–480.doi:10.1017/S0025315400047603.S2CID84555258.
  20. ^Pascoe, P.L. (1987). "Monogenean parasites of deep-sea fishes from the Rockall Trough (N.E. Atlantic) including a new species".Journal of the Marine Biological Association of the United Kingdom.67(3): 603–622.doi:10.1017/S0025315400027326.S2CID84642290.
  21. ^Bussieras, J. (1970)."Nouvelles observations sur les cestodes tétrarhynques de la collection du Musée Océanographique de Monaco. I.Sphryiocephalus albertiGuiart, 1935 ".Annales de Parasitologie Humaine et Comparée.45:5–12.doi:10.1051/parasite/1970451005.
  22. ^Guitart, J. (1935). "Cestodes parasites provenant des campagnes scientifiquesde S.A.S. le Prince Albert ler de Monaco (1886–1913)".Résultats des Campagnes Scientifiques Accomplies Sur Son Yacht Par Albert Ier Prince Souverain de Monaco Publiés Sous Sa Direction Avec le Concours de M. Jules Richard.91:1–100.
  23. ^Mauchline, J. & J.D.M. Gordon (1983). "Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean".Marine Biology.75(2–3): 269–278.doi:10.1007/BF00406012.S2CID84676692.
  24. ^abcFigueiredo, I.; T. Moura; A. Neves & L.S. Gordo (July 2008). "Reproductive strategy of leafscale gulper sharkCentrophorus squamosusand the Portuguese dogfishCentroscymnus coelolepison the Portuguese continental slope ".Journal of Fish Biology.73(1): 206–225.doi:10.1111/j.1095-8649.2008.01927.x.
  25. ^abBañón, R.; C. Piñeiro & M. Casas (2006). "Biological aspects of deep-water sharksCentroscymnus coelolepisandCentrophorus squamosusin Galician waters (north-western Spain) ".Journal of the Marine Biological Association of the United Kingdom.86(4): 843–846.doi:10.1017/S0025315406013774.S2CID86283124.
  26. ^abYano, K. & S. Tanaka (1988)."Size at maturity, reproductive cycle, fecundity and depth segregation of the deep sea squaloid sharksCentroscymnus owstoniandC. coelolepisin Suruga Bay, Japan ".Bulletin of the Japanese Society of Scientific Fisheries.54(2): 167–174.doi:10.2331/suisan.54.167.
  27. ^abDaley, 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.
  28. ^Girard, M. & M.H. Du Buit (1999). "Reproductive biology of two deep-water sharks from the British Isles,Centroscymnus coelolepisandCentrophorus squamosus(Chondrichthyes: Squalidae) ".Journal of the Marine Biological Association of the United Kingdom.79(5): 923–931.doi:10.1017/S002531549800109X.S2CID84868639.
  29. ^Irvine, S.B. (2004). Age, growth and reproduction of deepwater dogfishes from southeastern Australia. PhD Thesis, Deakin University.
  30. ^Cló, S., M. Dalú, R. Danovaro and M. Vacchi (2002). Segregation of the Mediterranean population ofCentroscymnus coelolepis(Chondrichthyes: Squalidae): a description and survey. NAFO SCR Doc. 02/83
  31. ^Duffy, Clinton A. J.; Francis, Malcolm; Dunn, M. R.; Finucci, Brit; Ford, Richard; Hitchmough, Rod; Rolfe, Jeremy (2018).Conservation status of New Zealand chondrichthyans (chimaeras, sharks and rays), 2016(PDF).Wellington, New Zealand: Department of Conservation. p. 9.ISBN9781988514628.OCLC1042901090.

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