FLMNH Ichthyology Department: Great Hammerhead

GREAT HAMMERHEAD
Order - Carcharhiniformes
Family - Sphyrnidae
Genus - Sphyrna
Species - mokarran

Taxonomy

The great hammerhead was originally described as Zygaena mokarran by German naturalist Eduard Rüppell in 1837, however he changed this name to the currently valid Sphyrna mokarran later that same year. 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 include Sphyrna tudes Valenciennes 1822, Zygaena dissimilis Murray 1887 and Sphyrna ligo Fraser-Brunner 1950.

There are approximately 10 related species of hammerheads throughout tropical and temperate regions including the bonnethead (Sphyrna tiburo), scalloped hammerhead (Sphyrna lewini), and smooth hammerhead (Sphyrna zygaena).

Common Names

English language common names include great hammerhead, great hammerhead shark, and squat-headed hammerhead shark. Other common names are abu garn (Arabic), akran (Arabic), cação-martelo (Portuguese), cachona (Spanish), cachona grande (Spanish), cambeva (Portuguese), cawar (Somali), cornuda de ley (Spanish), cornuda gigante (Spanish), glowomlot olbrzymi (Polish), grand requin marteau (French), grande squalo martello (Italian), großer hammerhai (German), grote hamerhaai (Dutch), hira-shumokuzame (Japanese), isovasarahai (Finnish), jarjur (Arabic), martelo (Portuguese), martillo (Spanish), megalozygena (Greek), nami-shumokuzame (Japanese), papa mbingusi (Swahili), peix martelo (Portuguese), : pesce martello maggiore (Italian), pez martillo (Spanish), requin marteau (French), tollo cruz (Spanish), yu parang (Malay), and yu tukul (Malay).

Geographical Distribution

Circumtropical in distribution, the great hammerhead is found in coastal warm temperate and tropical waters within 40°N - 37°S latitude. In the western Atlantic Ocean, it ranges from North Carolina (US) south to Uruguay, including the Gulf of Mexico and Caribbean regions, while in the eastern Atlantic Ocean, this species ranges from Morocco to Senegal, including the Mediterranean Sea. Distribution of the great hammerhead includes the Indian Ocean and the Indo-Pacific region from Ryukyu Island to New Caledonia and French Polynesia. The eastern Pacific range is from southern Baja, California (US) through Mexico, south to Peru. The great hammerhead is considered a highly migratory species within Annex I of the 1982 Convention on the Law of the Sea.

World distribution map for the great hammerhead

Habitat

This large coastal/semi-oceanic shark is found far offshore to depths of 300 m as well as in shallow coastal areas such as over continental shelves and lagoons. The great hammerhead migrates seasonally, moving poleward to cooler waters during the summer months.

Biology

Great hammerhead illustration
Bigelow and Schroeder (1948) FNWA

· Distinctive Features

The great hammerhead is a very large shark with the characteristic hammer-shaped head from which it gets its common name. The font margin of the head is nearly straight with a shallow notch in the center in adult great hammerheads, distinguishing it from the smooth hammerhead and scalloped hammerhead. The first dorsal fin is very tall with a pointed tip and strongly falcate in shape while the second dorsal is also high with a strongly concave rear margin. The origin of the first dorsal fin is opposite or slightly behind the pectoral fin axil with the free rear tip falling short to above the origin of the pelvic fins. The rear margins of the pelvic fins are concave and falcate in shape, not seen in scalloped hammerhead (S. lewini). The posterior edge of the anal fin is deeply notched.

Great hammerhead differences between the A. Juvenile and B. Adult head morphology
Bigelow and Schroeder (1948) FNWA

Juvenile morphology includes the front margin of the head being gently curved in contrast to the nearly straight margin in adult specimens.

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 scalloped hammerhead (S. lewini) is distinguished as a smaller species with a rounded anterior margin and notch on the head. The smooth hammerhead (S. zygaena) has a broad, flat unnotched head. The smaller 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.

· Coloration

The dorsal side of the great hammerhead is dark brown to light grey or even olive in color fading to white on the underside. The fins lack markings in adults while the apex of the second dorsal fin may appear dusky in juveniles.

Great hammerhead
© Wes Pratt

·Dentition

The teeth of this hammerhead are triangular and strongly serrated, but increasingly oblique toward the corners of the mouth. There are 17 teeth on either side of the 2-3 teeth at the symphysis in the upper jaw and 16 or 17 teeth on either side of the 1-3 teeth at the symphysis on the lower jaw.

Teeth from the great hammerhead, A. Fifth upper tooth, B. Twelfth upper tooth, C. Fifth lower tooth, D. Eleventh lowe tooth
Bigelow and Schroeder (1948) FNWA

·Denticles

The skin is covered by dermal denticles that are closely spaced, overlapping along the front and lateral margins. Each blade is diamond-shaped and smooth along the base. There are 3-5 ridges on each blade on small specimens and as many as 5 or 6 in larger individuals. The teeth along the posterior margin are short with the median tooth slightly longer than the other teeth.

Dermal denticles from the great hammerhead
as adapted from Bigelow and Schroeder (1948) FNWA

·Size, Age, and Growth

As the largest of the hammerheads, the great hammerhead averages over 500 pounds (230 kg). The world record great hammerhead was caught off Sarasota, Florida (US) weighing 991 pounds (450 kg). The largest reported length of a great hammerhead is 20 feet (6.1 m). Expected life span of this species is approximately 20-30 years of age.

In waters off Australia, males reach maturity at a length of 7.4 feet (2.25 m) corresponding to a weight of 113 pounds (51 kg) and females are mature at a total length of 6.9 feet (2.10 m) corresponding to a weight of 90 pounds (41 kg) (source: Stevens and Lyle 1989).

The great hammerhead is the largest of all hammerheads
© George Burgess

· Food Habits

Great hammerheads are active predators, preying upon a wide variety of marine organisms, from invertebrates to bony fishes and sharks. A favorite prey item is the stingray, which is consumed along with the tail spine! Invertebrate prey include crabs, squid, octopus, and lobsters while commonly consumed bony fish are groupers, catfishes, jacks, grunts, and flatfishes. Great hammerheads have also been reported as cannibalistic, eating individuals of their own species. It feeds primarily at dusk along the seafloor as well as near the surface using its complex electro-sensory system to located prey.

The great hammerhead feeds on a variety of marine organisms including stingrays
© George Ryschkewitsch

· Reproduction

As common with all hammerheads, this species is viviparous with nutrition provided through a yolk-sac placenta. Following a gestation period of approximately 11 months, birth occurs during the spring or summer in the Northern Hemisphere. The resulting litters range in size from 6 to 42 young with the pups measuring between 60 and 70 cm total length. The head shape of a young pup is more rounded than that of an adult, changing as it reaches maturity.

In contrast to most other species of sharks that reportedly mate at or near the bottom, the great hammerhead has been observed mating near the surface of the water. In one event reported from the Bahamas, mating sharks ascended, swimming slowly around each other until mating at the surface.

· Predators

Larger sharks will prey on juvenile and subadult great hammerheads, with no major predators of the adults of this species.

Bull sharks feed on juvenile sharks including hammerheads
© Klaus Jost

· Parasites

Copepods parasitize many species of shark with those found on great hammerheads including Alebion carchariae, A. elegans, Nesippus orientalis, N. crypturus, Eudactylina pollex, Kroyeria gemursa, and Nemesis atlantica.

Importance to Humans

Fished both commercially and recreationally, great hammerheads are highly valued for their fins while the meat is rarely consumed by humans. They are also provide liver oil used in vitamins, hides for leather, and carcasses ground into fishmeal. Although not often a targeted species, the great hammerhead is regularly caught in tropical regions with longlines, bottom nets, hook-and-line, and trawls.

Recreationally fished with surface bait, the great hammerhead provides a good fight and exciting sport to fishers.

Great hammerhead swimming at the surface during migration
courtesy NOAA

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. However, few if any attacks can be directly linked with this species due to the difficulty distinguishing species of hammerheads involved in attacks. Due to its large size and variety of prey, this shark should be treated with respect and caution.

Conservation

In the US, hammerheads (with the exception of the bonnethead which is a small coastal species) are grouped with large coastal species, a group that biologist consider to be one of the most vulnerable to overfishing. Although not a targeted species, this hammerhead is 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, the great hammerhead is considered to be "Endangered" throughout its range 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.

Prepared by:
Cathleen Bester

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