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Characteristics, distribution and ecology
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Author: (Griffith and Smith, 1834)

Field Marks:
A large hammerhead with a broad, narrowbladed head, anterior margin of head very broadly arched in adults and with a prominent median indentation, teeth with moderately broad cusps and smooth to weakly serrated edges, moderately falcate first dorsal fin with origin over or behind pectoral insertions and free rear tip in front of pelvic origins, low second dorsal fin with weakly concave posterior margin, long posterior margin about twice fin height, and free rear tip nearly or quite reaching upper caudal origin, non-falcate pelvic fins, a deeply notched posterior anal margin, and dusky or black-tipped pectoral fins.

Diagnostic Features:
Expanded prebranchial head hammer- or axe-shaped and very wide but longitudinally short, its width 24 to 30% of total length (mostly above 26%); distance from tip of snout to rear insertions of posterior margins of expanded blades less than halfof head width; anterior margin of head very broadly arched, with prominent medial and lateral indentations; 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 1/3 of head width; rear ends of eyes slightly anterior to upper symphysis of mouth; mouth rather broadly arched; anterior teeth with moderately long, stout to slender cusps, smooth or weakly serrated, posterior teeth mostly cuspidate and frot keeled and molariform. First dorsal moderately falcate, its origin above or slightly behind pectoral insertions, its free rear tip well anterior to pelvic origins; second dorsal fin low, less than anal height, with a shallow concave posterior margin; its inner margin long, about twice the fin height, and ending almost opposite upper caudal origin; pelvic fins not falcate, with straight or slightly concave posterior margins; anal fin larger than second dorsal fin and rather long, base 4.3 to 6.4% of total length; its origin well ahead of second dorsal origin, its posterior margin shallowly concave to nearly straight. Total vertebral centra 174 to 209. A large hammerhead, to over 3 m. Colour grey-brown above, white below, with dusky to black pectoral fin tips.

Geographical Distribution:
Essentially circumglobal in coastal warm temperate and tropical seas. Western Atlantic: New Jerscy to Brazil, including Gulf of Mexico and Caribbean. Eastern Atlantic from ? Mediterranean and Senegal to Zaire. Indo-West Pacific: South Africa and Red Sea to Pakistan, India, Burma, Thailand, Indonesia, China (including TaiwanIsland), Japan, The Philippines, Australia (Queensland, Western Australia), New Caledonia. Central Pacific: Hawaii and Tahiti. Eastern Pacific: Southern California and Gulf of California to Panama, Ecuador and ? northern Peru.

Habitat and Biology:
Probably the most abundant hammmerhead, a coastal-pelagic, semioceanic warm-temperate and tropical species occurring over continental and insular shelves and in deep water adjacent to them, often approaching close inshore and entering enclosed bays and estuaries. Ranges from the intertidal and surface down to at least 275 m depth. Young sharks primarily occur close inshore. Forms large true schools at different stages of its life-history, though solitary individuals of both young and adults also occur.

This species is apparently highly mobile and in part migratory, and forms huge schools of small migrating individuals that move poleward in the summer in certain areas such as off Natal, South Africa. Elsewhere, as in the East China Sea, it may not migrate and is thought to form large resident populations. Adults males and females may segregate during certain phases of their life-cycle. Off southern Baja California, in the Gulf of California, polarized schools of scalloped hammerheads of mixed sexes with females predominating and sizes from immatures of slightly less than a metre to adults over 3 m have been intensely observed underwater by A. Peter Klimley and Donald R. Nelson. These congregate offshore over seamounts and near islands, and show a considerable range of behaviours including lateral tilting of the body (possibly to enhance the shark's view ofdivers when approached from above and behind them); accelerated swimming variants with headshaking, thrusting the midsection while swimming rightside up or upside down, and corkscrew swimming with rotation around their longitudinal axes; hitting other hammerheads with their snouts; jaw opening; and clasper flexion. Some of these displays may involve aggression or courtship. Many females bear apparent courtship scars, but a smaller proportion of males have them too. The function of these schools is uncertain: reproduction is thought unlikely because of the presence of juveniles in the schools; defence unlikely because of the absence of possible predators on the hammerheads; and grouping for attaining a swimming advantage in the strong currents that are common in these places is also unlikely because the sharks school when currents are absent. Feeding advantages may occur for the sharks to cluster near food resources or even for social feeding, but so far this is hypothetical because the sharks have never been seen to feed in the daytime when observations can be made, though they may do so at night. Sharks have been tracked and may wander off from the schooling area.

Viviparous, with a yolk-sac placenta; number of young in a litter 15 to 31. Off Hawaii, adults move inshore in Kaneohe Bay, Oahu to drop young and mate. The smallest young are found close inshore in the bay but these move into deeper water as they grow, to eventually depart for open water.

The scalloped hammerhead takes a wide variety of fish prey, but also invertebrates (especially cephalopods). Food items include sardines and herring, anchovies, ten-pounders (Elopidae), conger eels, milkfish, sea catfish, silversides, halfbeaks, mullet, lizardfish, barracuda, bluefish, spanish mackeral, jacks, porgies, mojarras, cardinal fishes, goatfish, grunts, damselfishes, parrotfishes, wrasses, butterfly fishes, surgeonfish, gobies, flatfish, sharpnose sharks (Rhizoprionodon), blacktip reef sharks, angelsharks, stingrays, squid, octopi, cuttlefishes, sea snails, shrimp, mantis shrimp, crabs, lobsters and isopods.

The scalloped hammerhead is probably dangerous to people but this is uncertain because until recently large hammerheads, particularly this species and S. zygaena, have been regularly confused with one another, and so several unprovoked and provoked attacks on swimmers and divers as well as a few boat attacks can only be attributed to 'hammerheads'. Under baited conditions these hammerheads have made close approaches to divers but quickly lost interest and departed when they apparently determined that the divers were not the source of the food odour. In ongoing studies on the social behaviour of these sharks off seamounts in the Gulf of California A. Peter Klimley and Donald R. Nelson (pers. comm.) have found large schools of adult scalloped hammerheads to be rather timid and very difficult to approach when they used SCUBA, so that much of their work must be done by free-diving on the sharks to measure, sex, tag, track with sonic tags, photographs, and record their activities. These sharks are probably less dangerous than the smaller but more aggressive grey reef shark (Carcharhinus amblyrhynchos), and much less than the bull, tiger and great white sharks.

Size:
Maximum about 370 to 420 cm, males maturing at 140 to 165 cm and reching at least 295 cm, females maturing at about 212 cm and reaching at least 309 cm; size at birth 42 to 55 cm.

Interest to Fisheries:
This is probably the commonest hammerhead in the tropics and is readily available in abundance to ishore artisanal and small commercial fisheries as well as offshore operations; it is caught with pelagic longlines, fixed bottom longlines, fixed bottom nets, and even bottom and pelagic trawls; the young are easily caught on light longline gear. The meat is utilized fresh, fresh-frozen, dried salted and smoked for human consumption; the fins are used to prepare shark-fin soup base; the hides are prepared into leather; the oil used for vitamins; and carcasses for fishmeal.

Type material:
Holotype: Unknown. Type Locality: South coast of New Holland (Australia).

Scalloped hammerhead (Sphyrna lewini)