Order: Carcharhiniformes
Family: Sphyrnidae
Genus: Sphyrna
Species: lewini
Taxonomy:
The scalloped hammerhead was originally described as Zygaena lewini by Griffith and
Smith in 1834. This shark was later renamed Sphyrna lewini (Griffith and Smith, 1834), which remains the current valid name. The name Sphyrna
translates from Greek to the English language "hammer", referring to the hammer-shaped head of this species. Synonyms used in past scientific
literature to refer to the scalloped hammerhead include Cestracion leeuwenii (Day 1865), Zygaena erythraea (Klunzinger 1871), Cestracion oceanica
(Garman 1913), and Sphyrna diplana (Springer 1941).
There are approximately 10 related species of hammerheads throughout tropical and temperate regions including the bonnethead (Sphyrna tiburo),
great hammerhead (Sphyrna mokarran), and smooth hammerhead (Sphyrna zygaena).
Common Names:
Common names in the English language include scalloped hammerhead, bronze hammerhead
shark, hammerhead, hammerhead shark, kidney-headed shark, scalloped hammerhead shark, and southern hammerhead shark. Other common names
are abul-garn (Arabic), aka-shumokuzame (Japanese), cação-cornudo (Portugese), cachona (Spanish), chadayan sravu (Malayalam), cornuda
(Spanish), geschulpte hamerhaai (Dutch), Glowomlot tropikalny (Polish), jarjur (Arabic), jerong tenggiri (Malayan), kalhigandu miyaru
(Maldivian), kampavasarahai (Finnish), krusan (Bikol), ktenozygena (Greek), mano kihikihi (Hawaiian), morfillo (Spanish), peixe-martelo
(Portuguese), pez martillo (Spanish), requin marteau (French), skulprand-hamerkop (Afrikaans), tiburón martillo (Spanish), and yu palang (Malay).
Geographical Distribution:
The scalloped hammerhead is circumglobal, residing in coastal warm temperate and tropical
seas. In the western Atlantic Ocean, this shark is found from New Jersey (US) south to Brazil including the Gulf of Mexico and Caribbean Sea; and in
the eastern Atlantic from the Mediterranean Sea to Namibia. Distribution in the Indo Pacific includes from South Africa and the Red Sea, throughout
the Indian Ocean, and from Japan to New Caledonia, Hawaii, and Tahiti. Off the coasts of southern California to Ecuador and perhaps south to Peru
are locations where the scalloped hammerhead is found in the eastern Pacific Ocean. In Australia, this hammerhead may be found off the northwestern
Western Australia coast.
World distribution map for the scalloped hammerhead
Habitat:
As a coastal pelagic semi-oceanic species, this shark occurs over continental and
insular shelves as well as adjacent to deeper water. It has been observed close inshore and even entering estuarine habitats as well as
offshore to depths of 275m. Scalloped hammerheads spend most of the day closer inshore, moving offshore in search of prey at night.
Adults occur singly, in pairs, and in small schools while young scalloped hammerhead sharks live in large schools. In some locations,
schools of small hammerheads have been observed migrating toward the poles during the summer months while permanent resident populations
exist in other areas including the East China Sea. It is thought that male and female scalloped hammerheads may segregate during certain
times of their life history. In the Gulf of California (US), aggregations of predominantly females ranging from immature to adult have
been observed around seamounts and islands displaying a wide range of behaviors. These behaviors have ranged from headshaking, corkscrew
swimming, and knocking into other hammerheads with their snouts. Although the function of such schooling behaviors is unknown, it is
suspected that some of these behaviors may be displays of aggression or courtship.
Biology:
Scalloped hammerhead, courtesy NOAA
Distinctive Features: The scalloped hammerhead is distinguished from other hammerheads by an indentation
located centrally on the front margin of the broadly arched head. The head is expanded laterally, resembling a hammer, hence
the common name "hammerhead". Two more indentations flank the main central indentation, giving this hammerhead a "scalloped" appearance.
The mouth is broadly arched and the rear margin of the head is slightly swept backward. The body of this shark is fusiform and moderately
slender with a large first dorsal fin and low second dorsal and pelvic fins. The first dorsal is mildy falcate with its origin over or
slightly behind the insertion point of the pectoral fins and the rear tip in front of the origins of the pelvic fin. The pelvic fin has
a straight posterior margin while the anal fin is deeply notched on the posterior margin. The second dorsal fin has a posterior margin
that is approximately twice the height of the fin, with the free rear tip nearly reaching the origin of the upper caudal lobe.
Comparison of hammerhead sharks: A. smooth hammerhead, B. scalloped hammerhead, C. great hammerhead, D. bonnethead
© George Burgess
Within the hammerhead family, several species are differentiated from each other by variations within the cephalophoil. The great
hammerhead (S. mokarran) is distinguished by the T-shaped head that has an almost straight front edge as well as a notch in the center.
The smooth hammerhead (S. zygaena) has a broad, flat unnotched head. The bonnethead (S. tiburo) is much easier to identify with a
shovel-shaped head. Another distinguishing characteristic of the great hammerhead is the curved rear margins on the pelvic fins while
the scalloped hammerhead has straight posterior edges.
The smooth hammerhead's back is smooth, lacking a mid-dorsal ridge. The moderately tall first dorsal fin has a rounded apex and is falcate
in shape with a free rear tip in front of the origin of the pelvic fins. The origin of this first dorsal is located over the pectoral fin
insertions. The low second dorsal fin is shorter than the anal fin, with the free rear tip not extending to the precaudal pit. Pelvic fins
are not falcate with straight of slightly concave posterior margins. The pectoral fins have only slightly falcate posterior margins. The
anal fin has a deeply notched posterior margin.
Coloration: Coloration of the scalloped hammerhead is brownish-gray to bronze or olive on the top of body with a pale
yellow or white underside. Juvenile scalloped hammerheads have dark pectoral, lower caudal and second dorsal fin tips while adults have
dusky pectoral fin tips with no other distinctive markings.
Scalloped hammerhead dentition,
A. Twelfth upper tooth, B. Fourth upper tooth, C. Eleventh lower tooth, D. Third lower tooth
modified from Bigelow & Schroeder (1948) FWNA
Dentition: The teeth are small with smooth or slightly serrated cusps on large bases. The upper jaw contains teeth that are narrow
and triangular with the first three nearly symmetrical and erect and the others increasingly oblique towards the corners of the mouth. Progressing
towards the corners, the teeth become nearly straight along the inner margins and more deeply notched along the outer margins. The lower teeth
are more erect and slender than the upper teeth.
Dermal denticles from a scalloped hammerhead modified from Bigelow & Schroeder (1948) FWNA
Denticles: Dermal denticles are partially overlapping, exposing the skin. Each blade is thin and moderately arched with 3 sharp
ridges in small individuals and 4 or 5 on large sharks. These ridges run about half the length of each blade. The axial marginal
tooth is longest with short, slender pedicels.
Size, Age, and Growth: In the northwestern Atlantic Ocean, males reach maturity at lengths of 5.9 feet total length (1.8 m)
corresponding to a weight of approximately 64 pounds (29 kg) while females mature at 8.2 feet total length (2.5 m) corresponding to a weight
of approximately 177 pounds (80 kg). Maximum total length ranges from 12.1-14.1 feet (3.7-4.3 m) with females growing larger than males while
maximum recorded weight is 336 pounds (152.4 kg). Life span of the scalloped hammerhead is thought to be over 30 years..
Food Habits: Scalloped hammerheads feed primarily on teleost fishes and a variety of invertebrates as well as other sharks
and ray. Common prey sardines, herring, anchovies, conger eels, silversides, halfbeaks, mullet, barracuda, Spanish mackerel, jacks, grunts,
parrotfishes, and goatfish as well as smaller elasmobranchs such as blacktip reef sharks, angelsharks and stingrays. Stingray spines, up to
50 or more, are often found in the mouth and digestive systems of hammerhead sharks. Invertebrates consumed by the scalloped hammerhead
include squid, octopus, shrimp, crabs, and lobsters.
Reproduction: As with all hammerheads, the scalloped hammerhead is viviparous with the eggs hatching inside the body and nourishment
provided by a yolksac placenta. This placenta also transports oxygen to the embryo and removes wastes. Following a 9-10 month gestation period,
scalloped hammerheads move inshore to shallow waters to give birth during the summer months. Large litters are born, ranging from 12-38 pups.
Pups measure approximately 15-18 inches (38-45 cm) in length at birth in waters off of North Amercia. Kaneohe Bay, Oahu, Hawaii (US) is a nursery
ground for scalloped hammerhead sharks where much research has been done looking at the biological and ecological aspects of these sharks.
Predators: Larger sharks will prey on small or injured scalloped hammerheads, while there are no major predators of the adults of this species.
Parasites: Scalloped hammerheads often visit cleaning stations, allowing cleaner wrasses to pick parasites from their skin and from inside
their mouths. External leeches (Stilarobdella macrotheca) and copepods (Alebion carchariae, A. elegans, Nesippus crypturus, Kroyerina scotterum) often
parasitize scalloped hammerheads.
Importance to Humans:
Scalloped hammerhead being landed ©Commercial Shark Fishery Observer Program/FLMNH
The scalloped hammerhead is fished both as a gamefish and commercially. It is readily accessible to inshore fishers as well as offshore commercial
operations. This shark can be caught on longlines, bottom nets and trawls. Although the flesh is sold fresh, dried, smoked, and frozen, this
species is also highly regarded for its fins and hides. The remainder of the shark is used for vitamins and fishmeal. Scalloped hammerhead pups
reside in shallow coastal nursery areas making them quite vulnerable to fishing pressures.
Danger to Humans Hammerheads are considered potentially dangerous sharks. According to the International
Shark Attack File, there have been 21 unprovoked attacks with 2 resulting in fatalities for all species of the genus Sphyrna. Scalloped hammerheads
have been reported to display threat postures when closely approached by divers on some occasions while other times they show no aggressive behaviors.
In Hawaii, some families regarded this shark as 'aumakua', a guardian spirit. They would feed rather than hunt whitetip reef sharks.
Conservation: In the US, hammerhead sharks are grouped with large coastal species, a group that
biologist consider to be most vulnerable to overfishing. Along with being a targeted species, this hammerhead is also taken by gillnet and
longline and as bycatch in driftnet fisheries. Mortality is likely to be significant although little data is available on populations and fishing
impact. Different species of hammerheads are sometimes difficult to identify in high seas fisheries where observers are often not present, resulting
in insufficient bycatch data.
Currently, this species is considered to be "Near Threatened" throughout its range with the exception of Australia where it is assessed as
"Least Concern" by the World Conservation Union (IUCN). The IUCN is a global union of states, governmental agencies, and non-governmental
organizations in a partnership that assesses the conservation status of species. Off the coast of northern Australia, where the fishery is
well-managed, the scalloped hammerhead is abundant.
