These hammerheads are unique because of the scalloped shape of the front edge of their cephalophoil, their ‘hammer head’. They are grayish copper, with a classic shark shape, and a slightly longer first dorsal fin. These open-water sharks have been observed in many parts of the world and in many habitats looking for food, usually fish, invertebrates, and smaller sharks and rays. Because they can grow up to 12 to 14 feet long, and their unpredictable response to humans, they are considered dangerous and to be avoided.
Order – Carcharhiniformes
Family – Sphyrnidae
Genus – Sphyrna
Species – lewini
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).
Importance to Humans
The scalloped hammerhead is fished both as a game fish 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 two 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 the US, hammerhead sharks are grouped with large coastal species, a group that biologist consider to be most vulnerable to over fishing. 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.
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.
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.
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.
1. Head broadly arched and hammer-shaped and marked by a prominent indentation at midline (“scalloped”)
2. Pelvic fins with straight rear margins
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 mildly 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.
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 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.
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 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.
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.
As with all hammerheads, the scalloped hammerhead is viviparous with the eggs hatching inside the body and nourishment provided by a yolk-sac 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 America. 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.
Larger sharks will prey on small or injured scalloped hammerheads, while there are no major predators of the adults of this species.
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.
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).
Prepared by: Cathleen Bester