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What To Look For:
Hammer-shaped head; the anterior margin of the head is broadly arched
with a medial indentation hence the name "scalloped"
hammerhead. The first dorsal fin of this species is moderately
falcate.
Color:
Grey brown to olive above, white below. Underside of pectoral fins
have dusky to black tips. Coloration darkens with age and large
individuals may be almost black.
Size:
Most sharks encountered by divers average 7 to 8 feet (2.1 to 2.4
mtrs) in length. Males mature at 4.5 to 5.4 ft (1.4 to 1.65 mtr) and
reach 9 ft (2.95 mtrs), females mature at 7 feet (2.12 mtrs). Maximum
length of this species is known to be at least 12 ft (3.7 mtrs), and
it is thought that a few individuals may reach a length of nearly 14
ft (4.2 mtrs).
Teeth:
Broad cusps and smooth to weakly serrated edges. The tooth formula is:
15-2-15 15/16-1-15/16.
Habitat:
Coastal-pelagic to semi-oceanic shark of warm-temperate and tropical
waters. This species is usually encountered over continental and
insular shelves and in deep water adjacent to them. The sharks have
been found from the intertidal and surface down to at least 900 ft
(275 mtr). Young sharks often come close inshore and enter enclosed
bays and estuaries. This species seems to prefer temperatures
averaging 75"F (24"C) or warmer, and is rare in water
cooler than 72"F (22"C).
Distribution:
Circumglobal in warm temperate and tropical seas.
Biology:
• Prey:
Scalloped hammerhead sharks feed on a wide variety of prey items.
Juveniles feed primarily on benthic bony fishes and crustaceans, while
squid are an important food source for adults. Fish prey includes
sardines and herring, anchovies, eels, milkfish, sea catfish,
silversides and halfbeaks, mullets, lizardfish, porgies, mojarras,
cardinal fishes, goatfish, grunts, damselfishes, parrotfishes, wrasses,
butterfly fishes, surgeonfish, gobies, flatfish, barracuda, bluefish,
jacks and Spanish mackerel, sharpnose sharks, blacktip reef sharks,
angelsharks, and stingrays. It also feeds on invertebrates
(particularly cephalopods), squid, octopus, cuttlefishes, as well as
shrimp, crabs, lobsters and isopods. In the lndo-West Pacific stomach
contents have included sea snakes.
• Reproduction:
Viviparous, with a yolk-sac placenta. Pups are born following a 12
month gestation. Litter sizes range from 15 to 31, and at birth each
pup is between 15 and 21.6 inches (38cm and 55 cm) in length. Newborns
are found close inshore but move to deeper water as they mature.
Behaviour:
• General:
This species forms large true schools at different stages of its
life-history. The sharks are highly mobile and in part migratory. Adult
males and females may segregate during certain phases of their
life-cycle. In some areas polarized schools of scalloped hammerheads of
mixed sexes (with females predominating) and sizes (from less than a
meter to adults over 3 m (9.8 ft) have been observed. These
congregations occur offshore over seamounts and near islands, and the
sharks show a range of behaviors including lateral tilting of the body
(perhaps to enhance the shark's view of divers when they approach 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 the 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 unclear; reproduction is
thought unlikely because of the presence of juveniles in the schools;
defense is unlikely because of the absence of possible predators; and
grouping to attain 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 if the sharks cluster
near food resources or even for social feeding, but 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.
Disposition:
Under baited conditions scalloped hammerheads may make close approaches
to divers but quickly lose interest and depart when they determine that
the divers are not the source of the food odors. Large schools of adult
scalloped hammerheads have been found to be rather timid and very
difficult to approach when SCUBA is used; and much of the work with them
is done by free divers.
• Danger To Humans:
Potentially dangerous because of their large size.
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