Featured Elasmobranch – Scalloped Hammerhead Shark

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Featured Elasmobranch – Scalloped Hammerhead Shark

Scalloped Hammerhead Shark

SPHYRNA LEWINI
(GRIFFITH & SMITH 1936)

FAMILY SPHRYNIDAE (HAMMERHEAD
SHARKS)

Identification: The scalloped hammerhead,
like other members of the highly-derived family Sphyrnidae, possesses
a distinctive dorso-ventrally flattened head. The species can be distinguished
from other hammerheads that occur within its range by the characteristic
scalloped outline of the head and the straight posterior edges of the
pectoral fins [see the Featured Elasmobranch archives for the great hammerhead (July 2008), bonnethead (June 2008), and smooth hammerhead
(November 2005)
for comparison]. Color is uniformly grayish brown to
olive dorsally fading to light grey or white ventrally. Dark blotches
are present on the pectoral, second dorsal, and caudal fin tips of juveniles;
this coloration fades with age.

Size: Females grow larger and mature later
than males and may reach total lengths of up to 4 m (13.1 ft). Males
reach reproductive maturity between 1.4 and 1.65 m (4.6 and 5.4 ft),
with females maturing between 2 and 2.5 m (6.6 and 8.2 ft). At birth,
pups measure between 38 and 56 cm (1.2 and 1.8 ft).

Distribution: In the eastern North Pacific
this species occurs from southern California, where it is rare, to the
Gulf of California where it is very common. Elsewhere, the species is
found circumglobally in warm-temperate to tropical waters, typically
in waters warmer than 22oC (72oF).

Habitat: Neonates and juveniles tend to inhabit
shallow inshore areas, preferring bays and other such protected ‘nursery’
areas. Adults are coastal-pelagic and are found in waters over the continental
and insular shelves to depths of up to 275 m (902 ft). A highly migratory
species, scalloped hammerheads form size and sex segregated schools
at certain periods during their life history. In the Gulf of California,
immense schools comprised predominantly of females have been observed
near seamounts. Although the significance of these aggregations remains
unknown, they presumably play a role in mating.

Biology: Scalloped hammerheads may live up
to 35 years, reaching reproductive maturity between 10 and 15 years
of age. The species is yolk sac viviparous and gives birth to large
litters of 13 to 31 young in inshore waters during the summer after
a gestation period of 9 to 10 months. Juveniles grow rapidly in their
first few years of life until they attain sizes large enough to minimize
their risk of predation by larger sharks and other predators. The species’
diet primarily consists of teleosts (sardine, herring, anchovy, mullet,
flatfishes), with cephalopods (octopuses and squids) and crustaceans
(crab and lobster) taken with less frequency. Larger individuals also
prey on other elasmobranchs (sharks and stingrays), sometimes consuming
young of their own species.

General interest:
In 2006, it was announced that a new ‘cryptic’ species of
scalloped hammerhead had been discovered in the western north Atlantic.
Visually identical, the two species can only be distinguished through
genetic testing. Although their ranges overlap, the two species do not
interbreed. The finding has important fisheries management implications
since the new species appears to be less abundant and highly localized
along the South Carolina coast. Research must be undertaken to gather
life history and distribution information for this new species to ensure
that both stocks are managed responsibly.

By Megan Winton
Pacific Shark Research Center
Moss Landing Marine Laboratories
8272 Moss Landing Road
Moss Landing, CA 95039
mwinton@mlml.calstate.edu

 

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