Great hammerhead (Sphyrna mokarran)

Also known as: great hammerhead shark, horned shark, squat-headed hammerhead shark
  
French: Grand Requin-marteau, Marieau Millet, Poisson Pantouflier, Sorosena
Spanish: Cornuda, El Tiburon, Guardia Civil, Pez Martillo, Tiburon
KingdomAnimalia
PhylumChordata
ClassChondrichthyes
OrderCarcharhiniiformes
FamilySphyrnidae
GenusSphyrna (1)
SizeMaximum length: 610 cm (2)
Length at birth: 56 – 70 cm (2)
Average weight: 230 kg (3)
Maximum weight: 500 kg (2)

The great hammerhead is classified as Endangered (EN) on the IUCN Red List (1).

The massive and well-known great hammerhead (Sphyrna mokarran) has the most distinctive hammer of its genus. It is particularly wide and virtually straight along the front edge, with just a small notch in the centre. The eyes are at either end of the hammer and the mouth is positioned on the underside in line with the trail edge of the hammer. The first dorsal fin is particularly tall, sickle-shaped and has a pointed tip. The second dorsal fin is also tall with a concave rear edge. The body is dark grey above fading to light grey below. The teeth are triangular and strongly serrated. Juveniles are similar but have a more curved hammer (3).

The great hammerhead is found around the world in warm temperate and tropical waters, including the Atlantic, Pacific and Indian Oceans, and smaller seas such as the Mediterranean, the Black Sea, and the Arabian Gulf (1) (2). It migrates poleward during the summer in search of cooler waters (3).

Preferring warm temperate and tropical waters, the great hammerhead is usually found close inshore, particularly around reefs, but may also found some distance offshore to depths of 300 metres (2).

The great hammerhead is particularly keen on feeding on stingrays, despite the danger of their barbs. The hammerhead pins down each ray with its hammer and then bites chunks from the wings until the ray is immobilised (4). It also feeds on groupers, sea catfish, small, bony fish, crabs, squid, other sharks and lobsters (2). It can be cannibalistic although it is not known what triggers this behaviour. Feeding mainly at dusk, the great hammerhead uses an electro-sensory system to locate prey; sensing the weak electric field produced by all living organisms. Larger sharks may prey on juvenile great hammerheads but adults have no natural predators (3).

Mating has rarely been observed in this species, but unlike other sharks it is known to mate at the surface (3). Males use extensions of the pelvic fins called ‘claspers’ to transfer sperm to the female (5), resulting in a pregnancy lasting 11 months (3). Young are born live in the spring or summer (3).

The function of the hammer is much discussed and a great many theories have been put forward as to its purpose. Amongst these theories, the most popular are that it helps the great hammerhead to scan larger areas of the ocean floor for food, and that it maximises the area of the sensory organs (known as the ampullae of Lorenzini) that can detect chemical, physical and thermal changes in the water, as well as electric fields (5).

The great hammerhead is targeted both directly and also taken as bycatch (1). It is prized for its fins for shark fin soup, as well as for its liver oil for vitamins, its skin for leather, and its meat for fishmeal (2). Catastrophic losses have occurred in the eastern Atlantic where directed fishing is unregulated, while in the Gulf of Mexico and north-west Atlantic, this species has suffered serious declines as a result of high mortality when caught accidently by commercial fisheries. Illegal fishing of this species for its valuable fins is also increasing in Australian waters. Like most other sharks, the great hammerhead's slow growth and low number of offspring makes it highly vulnerable to overexploitation (1).

The great hammerhead is in urgent need of coordinated conservation efforts involving the management of directed fisheries and those that take this species as bycatch. Fortunately, the increasing recognition of the detrimental effects of shark finning has led to the implementation of finning bans by fishing states in the U.S.A., Australia and the European Union. Nevertheless, improved enforcement of legislation is required to prevent ongoing illegal finning activities. Bycatch limits for sharks in the South African longline fishery are also helping to conserve this imperilled species (1).

For further information on the conservation of sharks and rays: 

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1. IUCN Red List (August, 2009)
    http://www.iucnredlist.org
  2. FishBase (August, 2009)
    http://www.fishbase.org
  3. Florida Museum of Natural History (August, 2009)
    http://www.flmnh.ufl.edu/fish/Gallery/Descript/GreatHammerhead/GHammerhead.html
  4. Carwardine, M. and Watterson, K. (2002) The Shark Watcher’s Handbook. BBC Worldwide, London.
  5. MarineBio.org - Great Hammerhead Shark (August, 2009)
    http://www.marinebio.com/species.asp?id=87