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| − | {{Taxobox | + | {{species id |
| − | | name = Great hammerhead | + | |common_name=Great Hammerhead |
| − | | image = Sphyrna mokarran at georgia.jpg | + | |latin_name=Sphyrna mokarran |
| − | | image_width = 240px | + | |image=Sphyrna mokarran at georgia.jpg |
| − | | status = EN
| + | |description= |
| − | | status_system = IUCN3.1
| + | The great hammerhead (Sphyrna mokarran) is the largest species of hammerhead shark, family Sphyrnidae, attaining a maximum length of 6.1 m (20 ft). The great hammerhead can be distinguished from other hammerheads by the shape of its "hammer" (called the "cephalofoil"), which is wide with an almost straight front margin, and by its tall, sickle-shaped first dorsal fin. A solitary, strong-swimming apex predator, the great hammerhead feeds on a wide variety of prey ranging from crustaceans and cephalopods, to bony fishes, to smaller sharks. Observations of this species in the wild suggest that the cephalofoil functions to immobilize stingrays, a favored prey. This species has a viviparous mode of reproduction, bearing litters of up to 55 pups every two years. |
| − | | status_ref = <ref name="iucn">{{IUCN2010 |assessors=Denham, J., Stevens, J., Simpfendorfer, C.A., Heupel, M.R., Cliff, G., Morgan, A., Graham, R., Ducrocq, M., Dulvy, N.D, Seisay, M., Asber, M., Valenti, S.V., Litvinov, F., Martins, P., Lemine Ould Sidi, M., Tous, P. and Bucal, D. |year=2007 |id=39386 |title=Sphyrna mokarran |version=2010.1 |downloaded=May 20, 2010}}</ref>
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| − | | regnum = [[Animal]]ia | + | It is found in tropical and warm temperate waters worldwide, inhabiting coastal areas and the continental shelf. |
| − | | phylum = [[Chordate|Chordata]]
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| − | | classis = [[Chondrichthyes]]
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| − | | subclassis = [[Elasmobranchii]]
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| − | | ordo = [[Carcharhiniformes]]
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| − | | familia = [[Sphyrnidae]]
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| − | | genus = ''[[Sphyrna]]''
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| − | | species = '''''S. mokarran'''''
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| − | | binomial = ''Sphyrna mokarran''
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| − | | binomial_authority = ([[Eduard Rüppell|Rüppell]], 1837)
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| − | | range_map = Sphyrna mokarran distribution map.svg
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| − | | range_map_width = 240px
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| − | | range_map_caption = Range of the great hammerhead
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| − | | synonyms = ''Zygaena dissimilis'' <small>Murray, 1887</small><br>''Sphyrna ligo'' <small>Fraser-Brunner, 1950</small>
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| − | The '''great hammerhead''' (''Sphyrna mokarran'') is the largest [[species]] of [[hammerhead shark]], family Sphyrnidae, attaining a maximum length of 6.1 m (20 ft). It is found in tropical and warm temperate waters worldwide, inhabiting coastal areas and the [[continental shelf]]. The great hammerhead can be distinguished from other hammerheads by the shape of its "hammer" (called the "cephalofoil"), which is wide with an almost straight front margin, and by its tall, sickle-shaped first [[dorsal fin]]. A solitary, strong-swimming [[apex predator]], the great hammerhead feeds on a wide variety of prey ranging from [[crustacean]]s and [[cephalopod]]s, to [[bony fish]]es, to smaller [[shark]]s. Observations of this species in the wild suggest that the cephalofoil functions to immobilize [[stingray]]s, a favored prey. This species has a [[viviparous]] mode of reproduction, bearing litters of up to 55 pups every two years.
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| − | Although potentially dangerous, the great hammerhead rarely [[shark attack|attacks]] humans. It sometimes behaves inquisitively toward divers and should be treated with respect. This shark is heavily fished for its large fins, which are extremely valuable on the Asian market as the main ingredient of [[shark fin soup]]. As a result, great hammerhead populations are declining substantially worldwide, and it has been assessed as [[Endangered]] by the [[International Union for Conservation of Nature]] (IUCN).
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| − | ==Taxonomy and phylogeny==
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| − | The great hammerhead was first described as ''Zygaena mokarran'' in 1837 by the German [[naturalist]] [[Eduard Rüppell]]. The name was later changed to the current ''Sphyrna mokarran''.<ref name="bester"/> However, for many years the valid scientific name for the great hammerhead was thought to be ''Sphyrna tudes'', which was coined in 1822 by [[Achille Valenciennes]]. In 1950, Enrico Tortonese determined that the specimens illustrated by Valenciennes were in fact [[smalleye hammerhead]]s, to which the name ''S. tudes'' then applied. As the next most senior [[synonym (taxonomy)|synonym]], ''Sphyrna mokarran'' became the great hammerhead's valid name. The [[lectotype]] for this species is a {{convert|2.5|m|ft|abbr=on}} long male from the [[Red Sea]].<ref name="compagno"/>
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| − | {{cladogram|title=|align=left|caption=Phylogenetic relationships of hammerhead sharks, based on morphology, isozymes, and mitochondrial DNA.<ref name="cavalcanti"/>
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| − | |clades={{clade|style=font-size:80%;line-height:80%
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| − | |1={{clade|
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| − | |1=''[[Eusphyra blochii]]''
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| − | |2={{clade|
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| − | |1={{clade|
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| − | |1='''''Sphyrna mokarran'''''
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| − | |2={{clade|
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| − | |1=''[[Sphyrna zygaena]]''
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| − | |2=''[[Sphyrna lewini]]''
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| − | }}
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| − | }}
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| − | |2={{clade|
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| − | |1=''[[Sphyrna tiburo]]''
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| − | |2={{clade|
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| − | |1=''[[Sphyrna tudes]]''
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| − | |2={{clade|
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| − | |1=''[[Sphyrna corona]]''
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| − | |2=''[[Sphyrna media]]''
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| − | }}
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| − | }}
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| − | }}
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| − | }}
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| − | }}
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| − | }}
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| − | [[Phylogenetic]] analyses based on [[morphology (biology)|morphology]], [[isozyme]]s, and [[mitochondrial DNA]] show that the great hammerhead forms a [[clade]] with the [[smooth hammerhead]] (''S. zygaena'') and the [[scalloped hammerhead]] (''S. lewini''). These studies also reveal a close relationship with the [[winghead shark]] (''Eusphyra blochii''), and their position relative to the small hammerhead sharks (such as the [[bonnethead]], ''S. tiburo'') suggest that the first hammerheads to evolve had large rather than small cephalofoils.<ref name="cavalcanti">{{cite journal |author=Cavalcanti, M.J. |title=A Phylogenetic Supertree of the Hammerhead Sharks (Carcharhiniformes: Sphyrnidae) |journal=Zoological Studies |volume=46 |issue=1 |pages=6–11 |date=2007}}</ref>
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| − | ==Distribution and habitat==
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| − | The great hammerhead inhabits tropical waters around the world, between the [[latitude]]s of [[40th parallel north|40°N]] and [[37th parallel south|37°S]]. In the [[Atlantic Ocean]], it is found from [[North Carolina]] to [[Uruguay]], including the [[Gulf of Mexico]] and the [[Caribbean Sea]], and from [[Morocco]] to [[Senegal]], and the [[Mediterranean Sea]]. It is found all along the rim of the [[Indian Ocean]], and in the [[Pacific Ocean]] from the [[Ryukyu Islands]] to [[Australia]], [[New Caledonia]], and [[French Polynesia]], and from southern [[Baja California]] to [[Peru]].<ref name="bester">Bester, Cathleen. [http://www.flmnh.ufl.edu/fish/Gallery/Descript/GreatHammerhead/GHammerhead.html Biological Profiles: Great Hammerhead]. ''Florida Museum of Natural History Ichthyology Department''. Retrieved on October 18, 2008.</ref> It may occur off [[Gambia]], [[Guinea]], [[Mauritania]], [[Sierra Leone]], and [[Western Sahara]], but this has not been confirmed.<ref name="iucn"/> Great hammerheads may be found from inshore waters of less than 1 m (3.3 ft) deep, to a depth of 80 m (230 ft) offshore. They favor [[coral reef]]s, but also inhabit continental shelves, island terraces, lagoons, and deep water near land. They are [[fish migration|migratory]]; populations off [[Florida]] and in the [[South China Sea]] have been documented moving closer to the poles in the summer.<ref name="compagno">{{cite book |author=Compagno, L.J.V. |date=1984 |title=Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date |place=Rome |publisher=Food and Agricultural Organization |isbn=92-5-101384-5 |pages=548–549}}</ref>
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| − | ==Description==
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| − | [[File:Great hammerhead nmfs.jpg|thumb|upright=1.2|The great hammerhead has a distinctive tall, sickle-shaped dorsal fin.]]
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| − | The streamlined body of the great hammerhead with the expanded cephalofoil is typical of the hammerhead sharks. Adult great hammerheads can be distinguished from the scalloped hammerhead and the smooth hammerhead by the shape of the cephalofoil, which has a nearly straight front margin (as opposed to arched), with prominent medial and lateral indentations. The width of the cephalofoil is 23–27% of the body length. The teeth are triangular and strongly serrated, becoming more oblique towards the corners of the mouth. There are 17 tooth rows on either side of the upper jaw with 2–3 teeth at the symphysis (the midline of the jaw), and 16–17 teeth on either side of the lower jaw and 1–3 at the symphysis.<ref name="bester"/>
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| − | The first dorsal fin is distinctive, being very tall and strongly falcate (sickle-shaped), and originates over the insertions of the [[pectoral fin]]s. The second dorsal fin and [[anal fin]] are both relatively large, with deep notches in the rear margins. The [[pelvic fin]]s are falcate with concave rear margins, in contrast to the straight-margined pelvic fins of the scalloped hammerhead. The skin is covered with closely placed [[dermal denticle]]s. Each denticle is diamond-shaped, with 3–5 horizontal ridges leading to marginal teeth in smaller individuals, and 5–6 in larger ones. The great hammerhead is dark brown to light gray to olive above, fading to white on the underside. The fins are unmarked in adults, while the tip of the second dorsal fin may be dark in juveniles.<ref name="bester"/><ref name="compagno"/>
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| − | The average great hammerhead measures up to 3.5 m (11.5 ft) long and weighs over 230 kg (500 lb). A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was 6.1 m (20 ft).<ref name="bester"/><ref name="compagno"/> The heaviest known great hammerhead is a 4.4 m (14.4 ft) long, 580 kg (1,280 lb) female caught off [[Boca Grande, Florida]] in 2006. The weight of the female was due to her being pregnant with 55 near-natal pups.<ref name="record">{{cite news |first= |last= |authorlink= |coauthors= |title=Record Hammerhead Pregnant With 55 Pups |url=http://dsc.discovery.com/news/2006/07/24/hammerhead_ani.html?category=earth&guid=20060724100030 |work=Discovery News |publisher= |agency=[[Associated Press]] |date=July 1, 2006 |accessdate=October 18, 2008}}</ref>
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| − | ==Biology and ecology==
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| − | The great hammerhead is a solitary, nomadic predator that tends to be given a wide berth by other reef sharks. If confronted, they may respond with an [[agonistic display]]: dropping their pectoral fins and swimming in a stiff or jerky fashion.<ref>{{cite journal |author=Martin, R.A. |title=A review of shark agonistic displays: comparison of display features and implications for shark–human interactions |journal=Marine and Freshwater Behaviour and Physiology |date=Mar. 2007 |volume=40 |issue=1 |pages=3–34 |doi=10.1080/10236240601154872}}</ref> Juveniles are preyed upon by larger sharks such as [[bull shark]]s (''Carcharhinus leucas''), while adults have no major predators.<ref name="bester"/> [[Yellow jack]]s (''Carangoides bartholomaei'') have been seen rubbing themselves against the hammerhead's flanks, possibly to rid themselves of [[parasite]]s.<ref name="elasmodiver"/> Schools of [[pilot fish]] (''Naucrates ductor'') sometimes accompany the great hammerhead.<ref name="deitsch2"/> The great hammerhead is parasitized by several species of [[copepod]]s, including ''Alebion carchariae'', ''A. elegans'', ''Nesippus orientalis'', ''N. crypturus'', ''Eudactylina pollex'', ''Kroyeria gemursa'', and ''Nemesis atlantica''.<ref name="bester"/>
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| − | ===Feeding===
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| − | [[File:Sphyrna mokarran camden.jpg|thumb|upright=1.2|left|The great hammerhead uses its "hammer" to pin down stingrays.]]
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| − | An active predator with a varied diet, known prey of the great hammerhead include [[invertebrate]]s such as crabs, lobsters, squid, and octopus, [[bony fish]]es such as [[tarpon]], [[sardines]], [[Ariidae|sea catfish]]es, [[Batrachoididae|toadfish]], [[Sparidae|porgies]], [[grunt (fish)|grunt]]s, [[Carangidae|jacks]], [[Sciaenidae|croaker]]s, [[grouper]]s, [[flatfish]]es, [[boxfish]]es, and [[porcupine fish]]es, and smaller sharks such as [[smoothhound]]s.<ref name="compagno"/> At [[Rangiroa Atoll]], great hammerheads prey opportunistically on [[grey reef shark]]s (''Carcharhinus amblyrhynchos'') that have exhausted themselves pursuing mates.<ref>{{cite book |title=The Fragile Edge: Diving and Other Adventures in the South Pacific |author=Whitty, J. |publisher=Houghton Mifflin Harcourt |date=2007 |isbn=0-618-19716-8 |pages=9}}</ref> The species is known to be cannibalistic.<ref name="bester"/>
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| − | The favorite prey of the great hammerhead are [[Batoidea|rays]] and [[skate (fish)|skate]]s, especially [[stingray]]s. The venomous spines of stingrays are frequently found lodged inside its mouth and do not seem to bother the shark as one specimen caught off Florida had 96 spines in and around its mouth. Great hammerheads primarily hunt at dawn or dusk, swinging their heads in broad angles over the sea floor so as to pick up the electrical signatures of stingrays buried in the sand, via numerous [[ampullae of Lorenzini]] located on the underside of the cephalofoil. The cephalofoil also serves as a [[hydrofoil]] that allows the shark to quickly turn around and strike at a ray once detected.<ref name="hammerschlag">Hammerschlag, Rick. [http://www.elasmo-research.org/education/ecology/sandy-gt_hammerhead.htm Sandy Plains: Great Hammerhead Shark]. ''ReefQuest Centre for Shark Research''. Retrieved on October 18, 2008.</ref> Off Florida, large hammerheads are often the first to reach newly baited sharklines, suggesting a particularly keen sense of [[Olfaction|smell]].<ref name="compagno"/>
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| − | Another function of the cephalofoil is suggested by an observation of a great hammerhead attacking a [[southern stingray]] (''Dasyatis americana'') in the [[Bahamas]]: the shark first knocked the ray to the sea bottom with a powerful blow from above, and then pinned it with its head while pivoting to take a large bite from each side of the ray's pectoral fin disc. This effectively crippled the stingray, which was then picked up in the jaws and sawed apart with rapid shakes of the head.<ref name="strong et al.">{{cite journal |author=Strong, W.R., Snelson, Jr., F.F., and Gruber, S.H. |title=Hammerhead Shark Predation on Stingrays: An Observation of Prey Handling by ''Sphyrna mokarran'' |journal=Copeia |volume=1990 |issue=3 |pages=836–840 |date=Sep. 19, 1990 |doi=10.2307/1446449 |publisher=American Society of Ichthyologists and Herpetologists |jstor=1446449}}</ref> A great hammerhead has also been seen attacking a [[spotted eagle ray]] (''Aetobatus narinari'') in open water by taking a massive bite out of one of its pectoral fins. The ray thus incapacitated, the shark once again used its head to pin it to the bottom and pivoted to take the ray in its jaws head-first. These observations suggest that the great hammerhead seeks to disable rays with the first bite, a strategy similar to that of the [[great white shark]] (''Carcharodon carcharias''), and that its cephalofoil is an [[adaptation]] for prey handling.<ref name="chapman and gruber">{{cite journal |author=Chapman, D.D. and Gruber, S.H. |title=A further observation of the prey-handling behavior of the great hammerhead shark, ''Sphyrna mokarran'': predation upon the spotted eagle ray, ''Aetobatus narinari'' |journal=Bulletin of Marine Science |volume=70 |issue=3 |pages=947–952 |date=May 2002}}</ref>
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| − | ===Life history===
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| − | As with other hammerhead sharks, great hammerheads are [[viviparity|viviparous]]: once the developing young use up their supply of [[yolk]], the [[yolk sac]] is transformed into a structure analogous to a mammalian [[placenta]]. Unlike most other sharks, which mate on or near the sea bottom, great hammerheads have been observed mating near the surface. In one account from the Bahamas, a mating pair ascended while swimming around each other, mating when they reached the surface.<ref name="bester"/> Females breed once every two years, giving birth from late spring to summer in the Northern Hemisphere and from December to January in Australian waters.<ref name="iucn"/> The [[gestation period]] is 11 months.<ref name="bester"/> The litter size ranges from 6–55 pups, with 20–40 being typical.<ref name="record"/> The young measure 50–70 cm (20–28 in) at birth; males reach [[sexual maturation|maturity]] at 2.3–2.8 m (7.5–8.9 ft) long and 51 kg (113 lbs) and the females at 2.5–3.0 m (8.2–9.8 ft) and 41 kg (90 lbs). The young differ from the adults in having a rounded frontal margin on the head.<ref name="iucn"/><ref name="bester"/> The typical [[life expectancy|lifespan]] of this species is 20–30 years;<ref name="bester"/> the record Boca Grande female was estimated to be 40–50 years old.<ref name="record"/>
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| − | ==Human interactions==
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| − | [[File:Sphyrna mokarran fishing.jpg|thumb|A great hammerhead caught by a sport fisherman. Human exploitation now threatens the survival of this species.]]
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| − | With its large size and cutting teeth, the great hammerhead is certainly capable of inflicting fatal injuries to a human and caution should be exercised around them. This species has a (possibly undeserved) reputation for aggression and being the most dangerous of the hammerhead sharks.<ref name="deitsch1">{{cite book |author=Stafford-Deitsch, J. |title=Sharks of Florida, the Bahamas, the Caribbean and the Gulf of Mexico |publisher=Trident Press Ltd |date=2000 |isbn=1-900724-45-6 |pages=90–91}}</ref><ref>{{cite book |title=Surfing Australia |author=Thornley, M., Dante, V., Wilson, P. and Bartholomew, W. |edition=second |publisher=Tuttle Publishing |date=2003 |isbn=962-593-774-9 |pages=264}}</ref> Divers underwater have reported that great hammerheads tend to be shy or unreactive to humans.<ref name="elasmodiver">[http://www.elasmodiver.com/great%20hammerhead.htm Great Hammerhead]. ''Elasmodiver.com''. Retrieved on October 18, 2008.</ref><ref name="hammerschlag"/> However, there are reports of great hammerheads approaching divers closely and even charging them when they first enter the water.<ref name="deitsch2">{{cite book |author=Stafford-Deitsch, J. |title=Red Sea Sharks |publisher=Trident Press Ltd |date=1999 |isbn=1-900724-28-6 |pages=92–93}}</ref><ref name="deitsch1"/> As of 2009, the [[International Shark Attack File]] lists 34 attacks, 17 of them unprovoked and 1 fatal, attributable to hammerhead sharks of the genus ''Sphyrna''. Due to the difficulty in identifying the species involved, it is uncertain how many were caused by great hammerheads. This shark has been confirmed to be responsible for only one (provoked) attack.<ref>[http://www.flmnh.ufl.edu/fish/sharks/statistics/species2.htm ISAF Statistics on Attacking Species of Shark]. ''International Shark Attack File, Florida Museum of Natural History, University of Florida.'' Retrieved on April 24, 2009.</ref>
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| − | The great hammerhead is regularly caught both [[commercial fishing|commercially]] and [[recreational fishing|recreationally]] in the tropics, using [[longline fishing|longlines]], fixed bottom nets, hook-and-line, and [[trawling|trawl]]s. Though the meat is rarely consumed, their fins are becoming increasing valuable due to the Asian demand for [[shark fin soup]].<ref name="iucn"/> In addition, their skin used for [[leather]], their [[shark liver oil|liver oil]] for [[vitamin]]s, and their carcasses for [[fishmeal]].<ref name="bester"/> The great hammerhead is also taken unintentionally as [[bycatch]] and suffers very high mortality, over 90% for fisheries in the northwest Atlantic and the Gulf of Mexico. Entanglement in [[shark net]]s around Australian and South African beaches is another source of mortality.<ref name="iucn"/>
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| − | ===Conservation status===
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| − | The great hammerhead is extremely vulnerable to [[overfishing]] due to its low overall abundance and long [[generation time]]. Assessment of its conservation status is difficult as few fisheries separate the great hammerhead from other hammerheads in their reported catches. This species is listed as globally [[Endangered]] on the [[IUCN Red List]]. It is Endangered in the northwestern Atlantic and the Gulf of Mexico, where though it is a non-targeted species, populations have dropped 50% since the 1990s due to bycatch. It is also Endangered in the southwestern Indian Ocean, where large numbers of longline vessels operate illegally along the coasts for hammerheads and the [[giant guitarfish]] (''Rhynchobatus djiddensis''). The great hammerhead catch rate in Indian Ocean has declined 73% from 1978 to 2003, though it is yet undetermined whether these represent localized or widespread depletion. The great hammerhead is [[Critically Endangered]] along the western coast of Africa, where stocks have collapsed with an estimated 80% decline in the past 25 years. The West African Sub-Regional Fishing Commission (SRFC) has recognized the great hammerhead as one of the four most threatened species in the region, though fishing continues unmonitored and unregulated. Off northern [[Australia]], this species was assessed as [[Data Deficient]] but at "high risk". Concern has arisen there over a substantial increase in illegal, unreported, and unregulated (IUU) fishing, reflecting the raising value of this shark's fins.<ref name="iucn"/>
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| − | No conservation measures specifically protecting the great hammerhead have been enacted. It is listed on Annex I, Highly Migratory Species, of the [[UN Convention on the Law of the Sea]], though no management schemes have yet been implemented under this agreement. The banning of [[shark finning]] by countries and supranational entities such as United States, Australia, and the European Union, and international regulatory bodies such as the [[International Commission for the Conservation of Atlantic Tunas]] (ICCAT), should reduce fishing pressure on the great hammerhead.<ref name="iucn"/>
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| − | ==References==
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| − | {{reflist|colwidth=30em}}
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| − | ==External links==
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| − | * [http://shark-references.com/species/view/Sphyrna-mokarran Species Description of Sphyrna mokarran at www.shark-references.com]
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| − | {{Carcharhiniformes}}
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| − | [[Category:Sphyrnidae]]
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| − | {{good article}}
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| − | {{Link GA|de}}
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