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Characteristics, distribution and ecology
Taxonomische classification
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Author: (Linnaeus, 1758)

Field Marks:
An easily recognized large hammerhead with a broad, narrow-bladed head, anterior margin of head broadly arched in adults and without a median indentation at any stage, teeth with very broad cusps and smooth to weakly serrated edge, moderately falcate first dorsal fin with free rear tip in front of pelvic origins, low second dorsal fin with weakly concave posterior margin and long inner margin about twice fin height, nonfalcate pelvic fins, and a deeply notched posterior anal margin.

Diagnostic Features:
Expanded prebranchial head hammer- or axe-shaped and very wide but longitudinally short, its width 26 to 29% of total length (mostly above 26%); 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 with prominent lateral indentations, but no medial indentation; posterior margins of head wide, angled posterolaterally, and generally broader than mouth width; well-developed prenarial grooves present anteromedial to nostrils; preoral snout about 1/5 to less than 1/3 of head width; rear ends of eyes slightly behind upper symphysis of mouth; mouth rather broadly arched; anterior teeth with moderately long, very stout cusps, and smooth or weakly serrated edges, posterior teeth mostly cuspidate and not keeled and molariform. First dorsal moderately falcate, its origin over pectoral insertions, its free rear tip well anterior to pelvic origins; second dorsal fin low, less than anal height, with a shallowly concave posterior margin; its inner margin long, about twice fin height, and ending well in front of upper caudal origin; pelvic fins not falcate, with straight or slightly concave posterior margins; anal fin slightly larger than second dorsal fin and rather long, base 4.3 to 5.7% of total length; its origin slightly ahead of second dorsal origin, its posterior margin deeply notched. Total vertebral centra 193 to 206. A large hammerhead to over 3 m. Colour dark olive or dark grey-brown above, white below, undersides of pectoral fin tips dusky.

Geographical Distribution:
Amphitemperate and tropical. Western Atlantic: Nova Scotia to Florida and Virgin Islands; southern Brazil to southern Argentina. Eastern North Atlantic: Mediterranean and British Isles to Senegal, Cape Verde Islands, Guinea, and Ivory Coast. Western Indian Ocean: South Africa and southern Mozambique, India and Sri Lanka. Western Pacific: Viet Nam (Gulf of Tonkin) to southern Japan and southern Siberia; Australia (New South Wales, Western Australia), New Zealand. Central Pacific: Hawaiian Islands. Eastern Pacific: Northern California to Gulf of California, Panama, Galapagos Islands, Ecuador to Chile.

Habitat and Biology:
An active, common, coastal-pelagic and semi-oceanic hammerhead, found close inshore and in shallow water over the continental and insular shelves to offshore, at depths from the surface down to at least 20 m and probably much more. In the East China Sea, this hammerhead apparently occurs at or near the surface, while S. mokarran and S. Iewini range into deeper water. This is apparently the hammerhead most tolerant of temperate waters, and has been thought to be only amphitemperate in its distribution; however, it definitely occurs in the tropics in places such as the Gulf of Mannar off southern India and Sri Lanka and off southern Mozambique, but its tropical range is spottily known at present due to probable confusion with the more abundant S. lewini. In some localities, such as off the eastern Cape of South Africa, it may occur in enormous migrating schools of young sharks 1.5 m or less long.

Viviparous, with a yolk-sac placenta; number of fetuses 29 to 37 per litter.

Feeds on a variety of bony fishes, including herring and menhaden, sea catfishes, sea bass, spanish mackerel, and porgies, and also small sharks, skates, stingrays, shrimp, crabs, barnacles, and squid and other cephalopods. Small sharks, skates and stingrays are especially favoured, and sharks are readily scavanged from nets and hooks. This species is regarded as being dangerous to people, though of the several attacks by large hammerheads only a few can be tentatively attributed to this species due to their occurrence in temperate waters. Off southern California, hammerheads apparently of this species have stolen catches from sportsfishermen and divers.

Size:
Maximum about 370 to 400 cm, adults maturing at about 210 to 240 cm, adult males to at least 256 cm, adult females at least 304 cm; size at birth 50 to 61 cm.

Interest to Fisheries:
A common to abundant species caught with pelagic longlines, handlines, and even pelagic and bottom trawls. It is utilized fresh, dried salted, and possibly smoked for human consumption; hides are processed for leather; liver oil is extracted for vitamins; fins are processed into shark fin soup base; and carcasses utilized for fishmeal.

Type material:
Holotype: None. Type Locality: "Habitat in Europa, America".

Smooth hammerhead (Sphyrna zygaena)