Marine Species Identification Portal : Great hammerhead

Author: (Rüppell, 1837)

Field Marks:
An easily recognized large hammerhead with anterior margin of head nearly straight in adults and with a median indentation, strongly serrate teeth, strongly falcate first dorsal fin with rear tip in front of pelvic origins, high second dorsal fin with strongly concave posterior margin and short inner margin, falcate pelvic finsx and a deeply notched posterior anal margin.

Diagnostic Features:
Expanded prebranchial head hammer- or axe-shaped and very wide but longitudinally short, its width 23 to 27% of total length (mostly above 23%); distance from tip of snout to rear insertions of posterior margins of expanded blades less than half of head width; anterior margin of head very broadly arched in young but nearly straight in adults, with prominent medial and lateral indentations; posterior margins of head long, angled posterolaterally in young but transverse in adults, and about as broad as mouth width; prenarial grooves absent or hardly developed; preoral snout less than 1/3 of head width; rear ends of eyes anterior to upper symphysis of mouth; mouth rather broadly arched; anterior teeth with moderately long stout cusps, strongly serrated edges, posterior teeth mostly cuspidate and not keeled and molariform. First dorsal strongly falcate, its origin over or slightly behind pectoral insertions, its free rear tip well anterior to pelvic origins; second dorsal fin high, about equal to anal height, with a strongly concave posterior margin; its inner margin short, about equal to fin height, and ending well in front of upper caudal origin; pelvic finsstrongly falcate, with strongly concave posterior margins; anal fin about as large as or larger than second dorsal fin and moderately long, its base 5.6 to 7.3% of total length, its origin well ahead of second dorsal origin, its posterior margin deeply notched. Total vertebral centra 197 to 212. A large to gigantic hammerhead to 3 to 5.5+ m. Colour grey-brown above, light below, without fin markings.

Geographical Distribution:
Circumtropical. Western Atlantic: North Carolina to Brazil, including Gulf of Mexico and Caribbean. Eastern North Atlantic: Morocco, Senegal, ?Canary Island, ?Gambia, ? Guinea; Mediterranean. Indo-West Pacific: South Africa and Red Sea to India, Thailand, China, Taiwan Island, Riu Kyu Islands, Australia (Northern Territory, Queensland, New South Wales), New Caledonia, French Polynesia. Eastern Pacific: Southern Baja California and Gulf of California to Panama, Ecuador and northern Peru.

Habitat and Biology:
A coastalpelagic and semi-oceanic tropical hammerhead occuring close inshore and well offshore, over the continental shelves, island terraces, and in passes and lagoons of coral atolls, as well as over deep water near land; depths range from near the surface and in water about a metre deep to over 80 rh. The great hammerhead often favours continental and insular coral reefs. It apparently is nomadic and migratory, with some populations moving poleward in the summer, as off Florida and in the South China Sea.

Viviparous, with a yolk-sac placenta; number of young 13 to 42. Sex ratios of fetuses are approximately 1:1. The gestation period may be at least 7 months. Birth occurs in late spring or summer in the Northern Hemisphere.

The great hammerhead takes a variety of prey, but seems especially to favour stingrays and other batoids, groupers and sea catfishes. Its diet includes tarpon, sardines, sea catfishes, toadfish, porgies, grunts, jacks, croakers, groupers and other serranids, tongue-soles, boxfishes, porcupine fishes, smooth-hounds (Mustelus) and other sharks, guitarfish, skates, stingrays, cownosed rays, crabs and squid. This species seems not to be bothered by the poisonous spines of its stingray and catfish prey, and is sometimes found with stings imbedded in its buccal cavity (one had about fifty stings in its mouth, throat and tongue). This and other large hammerheads were the first to reach newly baited sharklines in the Florida shark fishery, indicating a particularly keen olfactory sense.

This species is thought to be dangerous to people, though few if any attacks can be definitely attributed to it because of the apparent difficulty of distinguishing the large hammerhead species involved in attacks. In unbaited situations these hammerheads have approached divers but behaved unaggressively. The size and rather broad food spectrum of the great hammerhead, plus the considerable number of attacks attributed to hammerheads in general, make it a shark to be treated with respect and caution.

Size:
Maximum 550 to 610+ cm, but most adults of either sex not above 366 cm; a small percentage of the population (mainly or entirely females?) attain a size much greater than the adult average; males maturing at about 234 to 269 cm and reaching at least 341 cm; females maturing at about 250 to 300 cm and reaching 482 to 549+ cm; size at birth 50 to 70 cm.

Interest to Fisheries:
Although less abundant than S. Iewini, this species is regularly caught in the tropics, with longlines, fixed bottom nets, hook-and-line, and possibly with pelagic and bottom trawls. This species is utilized for its meat, fresh, fresh-frozen, dried salted and smoked for human consumption; for hides, processed into leather; for fins used for shark-fin soup base; for liver oil, processed for vitamins; and carcasses for fishmeal.

Typematerial:
Holotype: According to Klausewitz (1960:293) there is a lectotype, Naturmuseums Senckenberg SMB 3590, 2515 mm stuffed adult or adolescent male. Type Locality: Massaua, Red Sea.

Great hammerhead (Sphyrna mokarran)