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Daggernose shark

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Daggernose shark
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Carcharhiniformes
Family: Carcharhinidae
Genus: Isogomphodon
T. N. Gill,1862
Species:
I. oxyrhynchus
Binomial name
Isogomphodon oxyrhynchus
Range of the daggernose shark
Synonyms

Carcharias oxyrhynchusMüller & Henle, 1839

Thedaggernose shark(Isogomphodon oxyrhynchus) is a little-knownspeciesofrequiem shark,in thefamilyCarcharhinidae,and the only extant member of itsgenus.It inhabits shallowtropicalwaters off northeasternSouth America,fromTrinidadto northernBrazil,favoring muddy habitats such asmangroves,estuaries,and river mouths, though it is intolerant offresh water.A relatively small shark typically reaching 1.5 m (4.9 ft) in length, the daggernose shark is unmistakable for its elongated, flattened, and pointed snout, tiny eyes, and large paddle-shapedpectoral fins.

Daggernose sharks arepredatorsof smallschooling fishes.Its reproduction isviviparous,with females give birth to 2–8 pups every other year during therainy season;this species is capable of shifting the timing of its reproductive cycle by several months in response to the environment. Harmless to humans, the daggernose shark is caught for food and asbycatchinartisanalandcommercial fisheries.Limited in range and slow-reproducing, it has been assessed asCritically EndangeredbyInternational Union for Conservation of Naturein light of a steep population decline in recent years. The current population is believed to be extremely low, with no more than 250 individuals believed to be in existence as of 2023.[2]Indeed, it may even be in reproductive collapse, in which case it will likely become extinct in the near future.[3]

Taxonomy and phylogeny

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The illustration that accompanied Müller and Henle's original description.

The first scientific description of the daggernose shark, asCarcharias oxyrhynchus,was published by German biologistsJohannes Peter MüllerandFriedrich Gustav Jakob Henlein their 1839Systematische Beschreibung der Plagiostomen.They chose thespecific epithetoxyrhynchus,from the Greekoxys( "sharp" or "pointed" ) andrhynchos( "nose" ). In 1862, AmericanichthyologistTheodore Gillcoined the geniusIsogomphodonfor this species, from the Greekisos( "equal" ),gomphos( "nail" or "peg" ), andodous( "tooth" ). However,Isogomphodonwas subsequently relegated to being asynonymofCarcharhinus,until it was resurrected by shark authorityStewart Springerin 1950.[4]Regardless of its taxonomic validity,Isogomphodon,along with the generaNasolamiaandPrionace,is closely related toCarcharhinus.Anextinctrelative,I. acuarius,dates back to theMiddle Eocene(45Ma).[5]

Distribution and habitat

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The daggernose shark is found along the northeastern coast ofSouth America,offTrinidad,Guyana,Suriname,French Guiana,and northernBrazil.[4]It reportedly occurs as far south asValençain the central Brazilian state ofBahia,thoughfisherysurveys have not detected this species in the area and it is apparently unknown to local fishermen.[1]

An inhabitant of coastal waters at a depth of 4–40 m (13–131 ft), the daggernose shark prefers highly turbid waters and decreases in number with increasing water clarity. Females tend to be found at greater depths than males.[6]Its range encompasses a widecontinental shelfwith a humidtropicalclimate,extensivemangroves,and draining by numerous rivers including theAmazon.Watersalinityin this area ranges from 20 to 34 ppt, while thetidal amplitudecan measure up to 7 m (23 ft).[1]Daggernose sharks are most common over shallow muddy banks and inestuariesand river mouths. It seems to be intolerant of low salinity, moving inshore during thedry season(June to November) and offshore during therainy season(December to May).[7]This species is not known to make long-distance movements, though some local seasonal movements are possible.[1]

Description

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Ventral view of head.

As itscommon namesuggests, the daggernose shark has a very long, flattened snout with a pointed tip and an acutely triangular profile from above. The eyes are circular and minute in size, withnictitating membranes(a protective thirdeyelid). Thenostrilsare small, without prominent nasal skin flaps. There are short but deep furrows at the corners of the mouth on both jaws. The tooth rows number 49–60 and 49–56 in the upper and lower jaws respectively. Each tooth has a single narrow, upright cusp; the upper teeth are slightly broader and flatter than the lower teeth, with serrated rather than smooth edges.[4]

The body is robustly built, with large, broad, paddle-likepectoral finsthat originate under the fifthgill slit.The firstdorsal finoriginates over the posterior half of the pectoral fin bases. The second dorsal fin is about half as tall as the first and located over or slightly ahead of theanal fin.The anal fin is smaller than the second dorsal fin and has a deep notch in the rear margin. Thecaudal finhas a well-developed lower lobe and is preceded by a crescent-shaped notch on the upper side of thecaudal peduncle.The coloration is a plain gray above, sometimes with a brownish or yellowish cast, and lighter below.[4]Males attain a length of 1.4 m (4.6 ft) and females 1.6 m (5.2 ft).[1]There are unsubstantiated records of individuals 2.0–2.4 m (6.6–7.9 ft) long.[4]The maximum known weight is 13 kg (29 lb).[6]

Biology and ecology

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The dominant shark species within the daggernose shark's range are thesmalltail shark(Carcharhinus porosus) and thebonnethead(Sphyrna tiburo).[1]Its elongated snout and tiny eyes are likely consequences of living in murky sediment-laden waters, reflecting anadaptiveemphasis onelectroreceptionand otherrostralsenses rather than vision. The snout bears a superficial similarity to thegoblin shark(Mitsukurina owstoni), someApristuruscatsharks,and thelong-nosed chimaeras,all found in thedeep sea.[6]With long jaws and numerous small teeth, the daggernose shark is well-suited for capturing the smallschooling fishesthat comprise most of its diet. Known prey taken includeherring,anchovies,catfish,andcroakers.[4]

The daggernose shark isviviparous;once the developingembryosexhaust their supply ofyolk,the depletedyolk sacis converted into aplacentalconnection through which the mother delivers sustenance. Females give birth to litters of 2–8 pups every other year, following a year-longgestation period.There is no correlation between female size and number of offspring.[6]Matingand parturition take place over a roughly six-month period from the beginning to the end of the rainy season. However, the daggernose shark is capable of shifting the timing of its reproductive cycle by at least four months, possibly in response to varying environmental conditions.[8]Females move into shallow coastal nurseries to give birth; one important nursery exists off Brazilian state ofMaranhão.[6]

Newly born daggernose sharks measure 38–43 cm (15–17 in) in length.[6]Malesmatureat 103 cm (41 in) long, corresponding to an age of 5–6 years, while females mature at 115 cm (45 in) long, corresponding to an age of 6–7 years. Thelifespanof males has been measured at up to 7 years, and females 12 years; extrapolating from growth rates suggest that the largest known males and females may be 12 and 20 years old respectively.[9]

Human interactions

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The daggernose shark poses little danger to humans due to its small size and teeth.[4]This shark is caught in small numbers bysubsistence fishersin Trinidad, Guyana, Suriname, and French Guiana.[6]It also comprises about one-tenth of the catch of a northern Brazilfloating gillnetfishery targetingSerra Spanish mackerel(Scomberomorus brasiliensis) andAcoupa weakfish(Cynoscion acoupa), which operates in estuaries during the dry season. This shark is often found in markets, but is not highly regarded as a food fish. TheInternational Union for Conservation of Nature(IUCN) has assessed this species asCritically Endangered,as it has a limited distribution and is highly susceptible tooverfishingdue to its low reproductive rate. The daggernose shark has declined over 90% over the past decade off Brazil, and similar declines have likely also occurred elsewhere in its range as fishing pressure in the region continues to grow more intense. The IUCN has urgently recommended the implementation of conservation schemes and the expansion of fishery monitoring for this species.[1]Evidence also points to this species being on the verge of or already in reproductive collapse.[3]

See also

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References

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  1. ^abcdefgPollom, R.; Charvet, P.; Faria, V.; Herman, K.; Lasso-Alcalá, O.; Marcante, F.; Nunes, J.; Rincon, G.; Kyne, P.M. (2020)."Isogomphodon oxyrhynchus".IUCN Red List of Threatened Species.2020:e.T60218A3094144.doi:10.2305/IUCN.UK.2020-3.RLTS.T60218A3094144.en.Retrieved19 November2021.
  2. ^Lessa, Rosangela, et al. “Close to Extinction? The Collapse of the Endemic Daggernose Shark (Isogomphodon Oxyrhynchus) off Brazil.” Global Ecology and Conservation, vol. 7, 2016, pp. 70–81., doi:10.1016/j.gecco.2016.04.003.
  3. ^ab"The Daggernose Shark is Near Extinction".
  4. ^abcdefgCompagno, L.J.V. (1984).Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date.Rome: Food and Agricultural Organization. pp. 510–511.ISBN92-5-101384-5.
  5. ^Carrier, J.C.; J.A. Musick & M.R. Heithaus (2004).Biology of Sharks and Their Relatives.CRC Press. pp. 51–52.ISBN0-8493-1514-X.
  6. ^abcdefgFowler, S.L.; R.D. Cavanagh; M. Camhi; G.H. Burgess; G.M. Cailliet; S.V. Fordham; C.A. Simpfendorfer & J.A. Musick (2005).Sharks, Rays and Chimaeras: The Status of the Chondrichthyan Fishes.International Union for Conservation of Nature and Natural Resources. pp. 306–308.ISBN2-8317-0700-5.
  7. ^Compagno, L.J.V.; M. Dando & S. Fowler (2005).Sharks of the World.Princeton University Press. p. 313.ISBN978-0-691-12072-0.
  8. ^Lessa, R.; F.M.Santana & P. Duarte-Neto (December 2006). "A critical appraisal of marginal increment analysis for assessing temporal periodicity in band formation among tropical sharks".Environmental Biology of Fishes.77(3–4): 309–315.Bibcode:2006EnvBF..77..309L.doi:10.1007/s10641-006-9111-7.S2CID23735874.
  9. ^Lessa, R.; F.M. Santana; V. Batista & Z. Almeida (2000). "Age and growth of the daggernose shark,Isogomphodon oxyrhynchus,from northern Brazil ".Marine and Freshwater Research.51(4): 339–347.doi:10.1071/MF99125.