Great hammerhead
Encyclopedia
The great hammerhead is the largest species
of hammerhead shark
, family Sphyrnidae, attaining a maximum length of 6.1 m (20 ft). It is found in tropical and warm temperate waters worldwide, inhabiting coastal areas and the continental shelf
. The great hammerhead can be distinguished from other hammerheads by the shape of its "hammer" (called the "cephalofoil"), which is wide with an almost straight front margin, and by its tall, sickle-shaped first dorsal fin
. A solitary, strong-swimming apex predator
, the great hammerhead feeds on a wide variety of prey ranging from crustacean
s and cephalopod
s, to bony fishes, to smaller shark
s. Observations of this species in the wild suggest that the cephalofoil functions to immobilize stingray
s, a favored prey. This species has a viviparous mode of reproduction, bearing litters of up to 55 pups every two years.
Although potentially dangerous, the great hammerhead rarely attacks
humans. It sometimes behaves inquisitively toward divers and should be treated with respect. This shark is heavily fished for its large fins, which are extremely valuable on the Asian market as the main ingredient of shark fin soup
. As a result, great hammerhead populations are declining substantially worldwide, and it has been assessed as Endangered by the International Union for Conservation of Nature (IUCN).
Eduard Rüppell
. The name was later changed to the current Sphyrna mokarran. However, for over 200 years the valid scientific name for the great hammerhead was thought to be Sphyrna tudes, which was coined in 1822 by Achille Valenciennes
. In 1950, Enrico Tortonese determined that the specimens illustrated by Valenciennes were in fact smalleye hammerhead
s, to which the name S. tudes then applied. As the next most senior synonym
, Sphyrna mokarran became the great hammerhead's valid name. The lectotype
for this species is a 2.5 m (8.2 ft) long male from the Red Sea
.
Phylogenetic analyses based on morphology
, isozyme
s, and mitochondrial DNA
show that the great hammerhead forms a clade
with the smooth hammerhead
(S. zygaena) and the scalloped hammerhead
(S. lewini). These studies also reveal a close relationship with the winghead shark
(Eusphyra blochii), and their position relative to the small hammerhead sharks (such as the bonnethead
, S. tiburo) suggest that the first hammerheads to evolve had large rather than small cephalofoils.
s of 40°N
and 37°S
. In the Atlantic Ocean
, it is found from North Carolina
to Uruguay
, including the Gulf of Mexico
and the Caribbean Sea
, and from Morocco
to Senegal
, and the Mediterranean Sea
. It is found all along the rim of the Indian Ocean
, and in the Pacific Ocean
from the Ryukyu Islands
to Australia
, New Caledonia
, and French Polynesia
, and from southern Baja California
to Peru
. It may occur off Gambia, Guinea
, Mauritania
, Sierra Leone
, and Western Sahara
, but this has not been confirmed. Great hammerheads may be found from inshore waters of less than 1 m (3.3 ft) deep, to a depth of 80 m (230 ft) offshore. They favor coral reef
s, but also inhabit continental shelves, island terraces, lagoons, and deep water near land. They are migratory
; populations off Florida
and in the South China Sea
have been documented moving closer to the poles in the summer.
The first dorsal fin is distinctive, being very tall and strongly falcate (sickle-shaped), and originates over the insertions of the pectoral fins. The second dorsal fin and anal fin are both relatively large, with deep notches in the rear margins. The pelvic fins are falcate with concave rear margins, in contrast to the straight-margined pelvic fins of the scalloped hammerhead. The skin is covered with closely placed dermal denticles. Each denticle is diamond-shaped, with 3–5 horizontal ridges leading to marginal teeth in smaller individuals, and 5–6 in larger ones. The great hammerhead is dark brown to light gray to olive above, fading to white on the underside. The fins are unmarked in adults, while the tip of the second dorsal fin may be dark in juveniles.
The average great hammerhead measures up to 3.5 m (11.5 ft) long and weighs over 230 kg (500 lb). A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was 6.1 m (20 ft). The heaviest known great hammerhead is a 4.4 m (14.4 ft) long, 580 kg (1,280 lb) female caught off Boca Grande, Florida
in 2006. The weight of the female was due to her being pregnant with 55 near-natal pups.
s (Carcharhinus leucas), while adults have no major predators. Yellow jack
s (Carangoides bartholomaei) have been seen rubbing themselves against the hammerhead's flanks, possibly to rid themselves of parasites. Schools of pilot fish
(Naucrates ductor) sometimes accompany the great hammerhead. The great hammerhead is parasitized by several species of copepod
s, including Alebion carchariae, A. elegans, Nesippus orientalis, N. crypturus, Eudactylina pollex, Kroyeria gemursa, and Nemesis atlantica.
s such as crabs, lobsters, squid, and octopus, bony fishes such as tarpon
, sardines, sea catfish
es, toadfish, porgies
, grunts, jacks
, croaker
s, grouper
s, flatfish
es, boxfish
es, and porcupine fishes, and smaller sharks such as smoothhounds. At Rangiroa Atoll, great hammerheads prey opportunistically on grey reef sharks (Carcharhinus amblyrhynchos) that have exhausted themselves pursuing mates. The species is known to be cannibalistic.
The favorite prey of the great hammerhead are rays
and skates, especially stingray
s. The venomous spines of stingrays are frequently found lodged inside its mouth and do not seem to bother the shark; one specimen caught off Florida had 96 spines in and around its mouth. Great hammerheads primarily hunt at dawn or dusk, swinging their heads in broad angles over the sea floor so as to pick up the electrical signatures of stingrays buried in the sand, via numerous ampullae of Lorenzini
located on the underside of the cephalofoil. The cephalofoil also serves as a hydrofoil
that allows the shark to quickly turn around and strike at a ray once detected. Off Florida, large hammerheads are often the first to reach newly baited sharklines, suggesting a particularly keen sense of smell
.
Another function of the cephalofoil is suggested by an observation of a great hammerhead attacking a southern stingray
(Dasyatis americana) in the Bahamas: the shark first knocked the ray to the sea bottom with a powerful blow from above, and then pinned it with its head while pivoting to take a large bite from each side of the ray's pectoral fin disc. This effectively crippled the stingray, which was then picked up in the jaws and sawed apart with rapid shakes of the head. A great hammerhead has also been seen attacking a spotted eagle ray
(Aetobatus narinari) in open water by taking a massive bite out of one of its pectoral fins. The ray thus incapacitated, the shark once again used its head to pin it to the bottom and pivoted to take the ray in its jaws head-first. These observations suggest that the great hammerhead seeks to disable rays with the first bite, a strategy similar to that of the great white shark
(Carcharodon carcharias), and that its cephalofoil is an adaptation
for prey handling.
is transformed into a structure analogous to a mammalian placenta
. Unlike most other sharks, which mate on or near the sea bottom, great hammerheads have been observed mating near the surface. In one account from the Bahamas, a mating pair ascended while swimming around each other, mating when they reached the surface. Females breed once every two years, giving birth from late spring to summer in the Northern Hemisphere and from December to January in Australian waters. The gestation period
is 11 months. The litter size ranges from 6–55 pups, with 20–40 being typical. The young measure 50–70 cm (20–28 in) at birth; males reach maturity at 2.3–2.7 m (7.5–8.9 ft) long and 51 kg (113 lbs) and the females at 2.5–3.0 m (8.2–9.8 ft) and 41 kg (90 lbs). The young differ from the adults in having a rounded frontal margin on the head. The typical lifespan
of this species is 20–30 years; the record Boca Grande female was estimated to be 40–50 years old.
lists 34 attacks, 17 of them unprovoked and 1 fatal, attributable to hammerhead sharks of the genus Sphyrna. Due to the difficulty in identifying the species involved, it is uncertain how many were caused by great hammerheads. This shark has been confirmed to be responsible for only one (provoked) attack.
The great hammerhead is regularly caught both commercially
and recreationally
in the tropics, using longlines, fixed bottom nets, hook-and-line, and trawl
s. Though the meat is rarely consumed, their fins are becoming increasing valuable due to the Asian demand for shark fin soup
. In addition, their skin used for leather
, their liver oil
for vitamin
s, and their carcasses for fishmeal. The great hammerhead is also taken unintentionally as bycatch
and suffers very high mortality, over 90% for fisheries in the northwest Atlantic and the Gulf of Mexico. Entanglement in shark net
s around Australian and South African beaches are another source of mortality.
due to its low overall abundance and long generation time. Assessment of its conservation status is difficult as few fisheries separate the great hammerhead from other hammerheads in their reported catches. This species is listed as globally Endangered on the IUCN Red List
. It is Endangered in the northwestern Atlantic and the Gulf of Mexico, where though it is a non-targeted species, populations have dropped 50% since the 1990s due to bycatch. It is also Endangered in the southwestern Indian Ocean, where large numbers of longline vessels operate illegally along the coasts for hammerheads and the giant guitarfish
(Rhynchobatus djiddensis). The great hammerhead catch rate in Indian Ocean has declined 73% from 1978 to 2003, though it is yet undetermined whether these represent localized or widespread depletion. The great hammerhead is Critically Endangered
along the western coast of Africa, where stocks have collapsed with an estimated 80% decline in the past 25 years. The West African Sub-Regional Fishing Commission (SRFC) has recognized the great hammerhead as one of the four most threatened species in the region, though fishing continues unmonitored and unregulated. Off northern Australia
, this species was assessed as Data Deficient
but at "high risk". Concern has arisen there over a substantial increase in illegal, unreported, and unregulated (IUU) fishing, reflecting the raising value of this shark's fins.
No conservation measures specifically protecting the great hammerhead have been enacted. It is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea, though no management schemes have yet been implemented under this agreement. The banning of shark finning
by countries and supranational entities such as United States, Australia, and the European Union, and international regulatory bodies such as the International Commission for the Conservation of Atlantic Tunas
(ICCAT), should reduce fishing pressure on the great hammerhead.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of hammerhead shark
Hammerhead shark
The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna while the...
, family Sphyrnidae, attaining a maximum length of 6.1 m (20 ft). It is found in tropical and warm temperate waters worldwide, inhabiting coastal areas and the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
. The great hammerhead can be distinguished from other hammerheads by the shape of its "hammer" (called the "cephalofoil"), which is wide with an almost straight front margin, and by its tall, sickle-shaped first dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
. A solitary, strong-swimming apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
, the great hammerhead feeds on a wide variety of prey ranging from crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s and cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, to bony fishes, to smaller shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s. Observations of this species in the wild suggest that the cephalofoil functions to immobilize stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...
s, a favored prey. This species has a viviparous mode of reproduction, bearing litters of up to 55 pups every two years.
Although potentially dangerous, the great hammerhead rarely attacks
Shark attack
A shark attack is an attack on a human by a shark. Every year around 60 shark attacks are reported worldwide, although death is quite unusual. Despite the relative rarity of shark attacks, the fear of sharks is a common phenomenon, having been fueled by the occasional instances of serial attacks,...
humans. It sometimes behaves inquisitively toward divers and should be treated with respect. This shark is heavily fished for its large fins, which are extremely valuable on the Asian market as the main ingredient of shark fin soup
Shark fin soup
Shark fin soup is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture. The shark fins provide texture while the taste comes from the other soup ingredients.There is controversy over the practice of shark...
. As a result, great hammerhead populations are declining substantially worldwide, and it has been assessed as Endangered by the International Union for Conservation of Nature (IUCN).
Taxonomy and phylogeny
The great hammerhead was first described as Zygaena mokarran in 1837 by the German naturalistNaturalist
Naturalist may refer to:* Practitioner of natural history* Conservationist* Advocate of naturalism * Naturalist , autobiography-See also:* The American Naturalist, periodical* Naturalism...
Eduard Rüppell
Eduard Rüppell
Wilhelm Peter Eduard Simon Rüppell was a German naturalist and explorer. Rüppell is occasionally transliterated to "Rueppell" for the English alphabet....
. The name was later changed to the current Sphyrna mokarran. However, for over 200 years the valid scientific name for the great hammerhead was thought to be Sphyrna tudes, which was coined in 1822 by Achille Valenciennes
Achille Valenciennes
Achille Valenciennes was a French zoologist.Valenciennes was born in Paris, and studied under Georges Cuvier. Valenciennes' study of parasitic worms in humans made an important contribution to the study of parasitology...
. In 1950, Enrico Tortonese determined that the specimens illustrated by Valenciennes were in fact smalleye hammerhead
Smalleye hammerhead
The smalleye hammerhead or golden hammerhead is a small species of hammerhead shark, family Sphyrnidae, common in the shallow coastal waters of the western Atlantic Ocean from Venezuela to Uruguay. It favors muddy habitats with poor visibility, reflected by its relatively small eyes...
s, to which the name S. tudes then applied. As the next most senior synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
, Sphyrna mokarran became the great hammerhead's valid name. The lectotype
Lectotype
In botanical nomenclature and zoological nomenclature, a lectotype is a kind of name-bearing type. When a species was originally described on the basis of a name-bearing type consisting of multiple specimens, one of those may be designated as the lectotype...
for this species is a 2.5 m (8.2 ft) long male from the Red Sea
Red Sea
The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
.
Phylogenetic analyses based on morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
, isozyme
Isozyme
Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes...
s, and mitochondrial DNA
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
show that the great hammerhead forms a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
with the smooth hammerhead
Smooth hammerhead
The smooth hammerhead is a species of hammerhead shark, family Sphyrnidae, so named because of the distinctive shape of the head, which is flattened and laterally extended into a hammer shape , without an indentation in the middle of the front margin...
(S. zygaena) and the scalloped hammerhead
Scalloped hammerhead
The scalloped hammerhead is a species of hammerhead shark, family Sphyrnidae. Originally Zygaena lewini, it was later moved to its current name. The Greek word sphyrna translates into "hammer" in English, referring to the shape of this shark's head.This shark is also known as the bronze,...
(S. lewini). These studies also reveal a close relationship with the winghead shark
Winghead shark
The winghead shark is a species of hammerhead shark, family Sphyrnidae, named for the remarkably wide lobes on its head. It is the sole species in the genus Eusphyra...
(Eusphyra blochii), and their position relative to the small hammerhead sharks (such as the bonnethead
Bonnethead
The bonnethead shark or shovelhead, Sphyrna tiburo, is a member of the hammerhead shark genus Sphyrna. The Greek word sphyrna translates as hammer, referring to the shape of this shark's head - tiburo is the Taino word for shark....
, S. tiburo) suggest that the first hammerheads to evolve had large rather than small cephalofoils.
Distribution and habitat
The great hammerhead inhabits tropical waters around the world, between the latitudeLatitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s of 40°N
40th parallel north
The 40th parallel north is a circle of latitude that is 40 degrees north of the Earth's equatorial plane. It crosses Europe, the Mediterranean Sea, Asia, the Pacific Ocean, North America, and the Atlantic Ocean....
and 37°S
37th parallel south
The 37th parallel south is a circle of latitude that is 37 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, the Indian Ocean, Australasia, the Pacific Ocean and South America....
. In the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, it is found from North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
to Uruguay
Uruguay
Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...
, including the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
and the Caribbean Sea
Caribbean Sea
The Caribbean Sea is a sea of the Atlantic Ocean located in the tropics of the Western hemisphere. It is bounded by Mexico and Central America to the west and southwest, to the north by the Greater Antilles, and to the east by the Lesser Antilles....
, and from Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
to Senegal
Senegal
Senegal , officially the Republic of Senegal , is a country in western Africa. It owes its name to the Sénégal River that borders it to the east and north...
, and the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
. It is found all along the rim of the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
, and in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
from the Ryukyu Islands
Ryukyu Islands
The , also known as the , is a chain of islands in the western Pacific, on the eastern limit of the East China Sea and to the southwest of the island of Kyushu in Japan. From about 1829 until the mid 20th century, they were alternately called Luchu, Loochoo, or Lewchew, akin to the Mandarin...
to Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Caledonia
New Caledonia
New Caledonia is a special collectivity of France located in the southwest Pacific Ocean, east of Australia and about from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Belep archipelago, the Isle of...
, and French Polynesia
French Polynesia
French Polynesia is an overseas country of the French Republic . It is made up of several groups of Polynesian islands, the most famous island being Tahiti in the Society Islands group, which is also the most populous island and the seat of the capital of the territory...
, and from southern Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
to Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
. It may occur off Gambia, Guinea
Guinea
Guinea , officially the Republic of Guinea , is a country in West Africa. Formerly known as French Guinea , it is today sometimes called Guinea-Conakry to distinguish it from its neighbour Guinea-Bissau. Guinea is divided into eight administrative regions and subdivided into thirty-three prefectures...
, Mauritania
Mauritania
Mauritania is a country in the Maghreb and West Africa. It is bordered by the Atlantic Ocean in the west, by Western Sahara in the north, by Algeria in the northeast, by Mali in the east and southeast, and by Senegal in the southwest...
, Sierra Leone
Sierra Leone
Sierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea to the north and east, Liberia to the southeast, and the Atlantic Ocean to the west and southwest. Sierra Leone covers a total area of and has an estimated population between 5.4 and 6.4...
, and Western Sahara
Western Sahara
Western Sahara is a disputed territory in North Africa, bordered by Morocco to the north, Algeria to the northeast, Mauritania to the east and south, and the Atlantic Ocean to the west. Its surface area amounts to . It is one of the most sparsely populated territories in the world, mainly...
, but this has not been confirmed. Great hammerheads may be found from inshore waters of less than 1 m (3.3 ft) deep, to a depth of 80 m (230 ft) offshore. They favor coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s, but also inhabit continental shelves, island terraces, lagoons, and deep water near land. They are migratory
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
; populations off Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
and in the South China Sea
South China Sea
The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...
have been documented moving closer to the poles in the summer.
Description
The streamlined body of the great hammerhead with the expanded cephalofoil is typical of the hammerhead sharks. Adult great hammerheads can be distinguished from the scalloped hammerhead and the smooth hammerhead by the shape of the cephalofoil, which has a nearly straight front margin (as opposed to arched), with prominent medial and lateral indentations. The width of the cephalofoil is 23–27% of the body length. The teeth are triangular and strongly serrated, becoming more oblique towards the corners of the mouth. There are 17 tooth rows on either side of the upper jaw with 2–3 teeth at the symphysis (the midline of the jaw), and 16–17 teeth on either side of the lower jaw and 1–3 at the symphysis.The first dorsal fin is distinctive, being very tall and strongly falcate (sickle-shaped), and originates over the insertions of the pectoral fins. The second dorsal fin and anal fin are both relatively large, with deep notches in the rear margins. The pelvic fins are falcate with concave rear margins, in contrast to the straight-margined pelvic fins of the scalloped hammerhead. The skin is covered with closely placed dermal denticles. Each denticle is diamond-shaped, with 3–5 horizontal ridges leading to marginal teeth in smaller individuals, and 5–6 in larger ones. The great hammerhead is dark brown to light gray to olive above, fading to white on the underside. The fins are unmarked in adults, while the tip of the second dorsal fin may be dark in juveniles.
The average great hammerhead measures up to 3.5 m (11.5 ft) long and weighs over 230 kg (500 lb). A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was 6.1 m (20 ft). The heaviest known great hammerhead is a 4.4 m (14.4 ft) long, 580 kg (1,280 lb) female caught off Boca Grande, Florida
Boca Grande, Florida
Boca Grande is a small residential community on Gasparilla Island, in southwest Florida. Gasparilla Island is a part of both Charlotte and Lee Counties, while the actual village of Boca Grande, which is home to many seasonal and some year-round residents, is entirely in the Lee County portion of...
in 2006. The weight of the female was due to her being pregnant with 55 near-natal pups.
Biology and ecology
The great hammerhead is a solitary, nomadic predator that tends to be given a wide berth by other reef sharks. If confronted, they may respond with an agonistic display: dropping their pectoral fins and swimming in a stiff or jerky fashion. Juveniles are preyed upon by larger sharks such as bull sharkBull shark
The bull shark, Carcharhinus leucas, also known as Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers...
s (Carcharhinus leucas), while adults have no major predators. Yellow jack
Yellow jack
The yellow jack, Carangoides bartholomaei , is a species of offshore marine fish in the jack family, Carangidae. It is one of only two representatives of its genus present in the Atlantic Ocean, inhabiting waters off the east coast of the Americas from Massachusetts in the north to Brazil in the...
s (Carangoides bartholomaei) have been seen rubbing themselves against the hammerhead's flanks, possibly to rid themselves of parasites. Schools of pilot fish
Pilot fish
The pilot fish is a carnivorous commensal fish in the family Carangidae. It is widely distributed and lives in warm or tropical open seas.- Description :...
(Naucrates ductor) sometimes accompany the great hammerhead. The great hammerhead is parasitized by several species of copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, including Alebion carchariae, A. elegans, Nesippus orientalis, N. crypturus, Eudactylina pollex, Kroyeria gemursa, and Nemesis atlantica.
Feeding
An active predator with a varied diet, known prey of the great hammerhead include invertebrateInvertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s such as crabs, lobsters, squid, and octopus, bony fishes such as tarpon
Tarpon
Tarpons are large fish of the genus Megalops. There are two species of Megalops, one native to the Atlantic, and the other to the Indo-Pacific oceans.They are the only members of the family Megalopidae.- Species and habitats :...
, sardines, sea catfish
Ariidae
The Ariidae or ariid catfish are a family of catfish that mainly live in marine waters with many freshwater and brackish water species. They are found worldwide in tropical to warm temperate zones.-Taxonomy:...
es, toadfish, porgies
Sparidae
The Sparidae is a family of fish, included in the order Perciformes. The fish of the family are commonly called sea breams and porgies . The sheepshead, scup, and red sea bream are species in this family. They are found in shallow temperate and tropical waters and are bottom-dwelling carnivores....
, grunts, jacks
Carangidae
Carangidae is a family of fish which includes the jacks, pompanos, jack mackerels, and scads.They are marine fish found in the Atlantic, Indian and Pacific Oceans...
, croaker
Sciaenidae
Sciaenidae is a family of fish commonly called drums, croakers, or hardheads for the repetitive throbbing or drumming sounds they make...
s, grouper
Grouper
Groupers are fish of any of a number of genera in the subfamily Epinephelinae of the family Serranidae, in the order Perciformes.Not all serranids are called groupers; the family also includes the sea basses. The common name grouper is usually given to fish in one of two large genera: Epinephelus...
s, flatfish
Flatfish
The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...
es, boxfish
Boxfish
Ostraciidae is a family of squared, bony fish belonging to the order Tetraodontiformes, closely related to the pufferfishes and filefishes. Fish in the family are known variously as boxfishes, cofferfishes, cowfishes and trunkfishes...
es, and porcupine fishes, and smaller sharks such as smoothhounds. At Rangiroa Atoll, great hammerheads prey opportunistically on grey reef sharks (Carcharhinus amblyrhynchos) that have exhausted themselves pursuing mates. The species is known to be cannibalistic.
The favorite prey of the great hammerhead are rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...
and skates, especially stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...
s. The venomous spines of stingrays are frequently found lodged inside its mouth and do not seem to bother the shark; one specimen caught off Florida had 96 spines in and around its mouth. Great hammerheads primarily hunt at dawn or dusk, swinging their heads in broad angles over the sea floor so as to pick up the electrical signatures of stingrays buried in the sand, via numerous ampullae of Lorenzini
Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...
located on the underside of the cephalofoil. The cephalofoil also serves as a hydrofoil
Hydrofoil
A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...
that allows the shark to quickly turn around and strike at a ray once detected. Off Florida, large hammerheads are often the first to reach newly baited sharklines, suggesting a particularly keen sense of smell
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
.
Another function of the cephalofoil is suggested by an observation of a great hammerhead attacking a southern stingray
Southern stingray
The southern stingray, Dasyatis americana, is a stingray of the family Dasyatidae found in tropical and subtropical waters of the Western Atlantic Ocean from New Jersey to Brazil. It has a flat, diamond-shaped disc, with a mud brown, olive, and grey dorsal surface and white underbelly...
(Dasyatis americana) in the Bahamas: the shark first knocked the ray to the sea bottom with a powerful blow from above, and then pinned it with its head while pivoting to take a large bite from each side of the ray's pectoral fin disc. This effectively crippled the stingray, which was then picked up in the jaws and sawed apart with rapid shakes of the head. A great hammerhead has also been seen attacking a spotted eagle ray
Spotted eagle ray
The spotted eagle ray is a cartilaginous fish of the eagle ray family, Myliobatidae. It can be found globally in tropical regions, including the Gulf of Mexico, Hawaii, off the coast of West Africa, the Indian Ocean, Oceania, and on both coasts of the Americas at depths down to about . The rays...
(Aetobatus narinari) in open water by taking a massive bite out of one of its pectoral fins. The ray thus incapacitated, the shark once again used its head to pin it to the bottom and pivoted to take the ray in its jaws head-first. These observations suggest that the great hammerhead seeks to disable rays with the first bite, a strategy similar to that of the great white shark
Great white shark
The great white shark, scientific name Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached...
(Carcharodon carcharias), and that its cephalofoil is an adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
for prey handling.
Life history
As with other hammerhead sharks, great hammerheads are viviparous: once the developing young use up their supply of yolk, the yolk sacYolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
is transformed into a structure analogous to a mammalian placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. Unlike most other sharks, which mate on or near the sea bottom, great hammerheads have been observed mating near the surface. In one account from the Bahamas, a mating pair ascended while swimming around each other, mating when they reached the surface. Females breed once every two years, giving birth from late spring to summer in the Northern Hemisphere and from December to January in Australian waters. The gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
is 11 months. The litter size ranges from 6–55 pups, with 20–40 being typical. The young measure 50–70 cm (20–28 in) at birth; males reach maturity at 2.3–2.7 m (7.5–8.9 ft) long and 51 kg (113 lbs) and the females at 2.5–3.0 m (8.2–9.8 ft) and 41 kg (90 lbs). The young differ from the adults in having a rounded frontal margin on the head. The typical lifespan
Life expectancy
Life expectancy is the expected number of years of life remaining at a given age. It is denoted by ex, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience...
of this species is 20–30 years; the record Boca Grande female was estimated to be 40–50 years old.
Human interactions
With its large size and cutting teeth, the great hammerhead is certainly capable of inflicting fatal injuries to a human and caution should be exercised around them. This species has a (possibly undeserved) reputation for aggression and being the most dangerous of the hammerhead sharks. Divers underwater have reported that great hammerheads tend to be shy or unreactive to humans. However, there are reports of great hammerheads approaching divers closely and even charging them when they first enter the water. As of 2009, the International Shark Attack FileInternational Shark Attack File
The International Shark Attack File is a global database of shark attacks. It began as an attempt to catalogue shark attacks on servicemen during World War II. The Office of Naval Research funded it from 1958 until 1968. During that time a panel of shark experts developed a standard system for...
lists 34 attacks, 17 of them unprovoked and 1 fatal, attributable to hammerhead sharks of the genus Sphyrna. Due to the difficulty in identifying the species involved, it is uncertain how many were caused by great hammerheads. This shark has been confirmed to be responsible for only one (provoked) attack.
The great hammerhead is regularly caught both commercially
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
and recreationally
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
in the tropics, using longlines, fixed bottom nets, hook-and-line, and trawl
Trawling
Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net that is used for trawling is called a trawl....
s. Though the meat is rarely consumed, their fins are becoming increasing valuable due to the Asian demand for shark fin soup
Shark fin soup
Shark fin soup is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture. The shark fins provide texture while the taste comes from the other soup ingredients.There is controversy over the practice of shark...
. In addition, their skin used for leather
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
, their liver oil
Shark liver oil
Shark liver oil is obtained from sharks that are caught for food purposes and are living in cold, deep oceans. The liver oil from sharks has been used by fishermen for centuries as a folk remedy for general health...
for vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...
s, and their carcasses for fishmeal. The great hammerhead is also taken unintentionally as bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
and suffers very high mortality, over 90% for fisheries in the northwest Atlantic and the Gulf of Mexico. Entanglement in shark net
Shark net
A shark net is a submerged net placed around beaches to reduce shark attacks on swimmers.Shark nets do not offer complete protection but work on the principle of "fewer sharks, fewer attacks". They reduce occurrence via shark mortality. Reducing the local shark populations is believed to reduce the...
s around Australian and South African beaches are another source of mortality.
Conservation status
The great hammerhead is extremely vulnerable to overfishingOverfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
due to its low overall abundance and long generation time. Assessment of its conservation status is difficult as few fisheries separate the great hammerhead from other hammerheads in their reported catches. This species is listed as globally Endangered on the IUCN Red List
IUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
. It is Endangered in the northwestern Atlantic and the Gulf of Mexico, where though it is a non-targeted species, populations have dropped 50% since the 1990s due to bycatch. It is also Endangered in the southwestern Indian Ocean, where large numbers of longline vessels operate illegally along the coasts for hammerheads and the giant guitarfish
Giant guitarfish
The giant guitarfish, Rhynchobatus djiddensis, is a species of guitarfish in the family Rhynchobatidae.-Taxonomy and range:The giant guitarfish was previously believed to range throughout a large part of the Indo-Pacific, but recent evidence has shown that it, as traditionally defined, actually was...
(Rhynchobatus djiddensis). The great hammerhead catch rate in Indian Ocean has declined 73% from 1978 to 2003, though it is yet undetermined whether these represent localized or widespread depletion. The great hammerhead is Critically Endangered
Critically Endangered
Critically Endangered is the highest risk category assigned by the IUCN Red List for wild species. Critically Endangered means that a species' numbers have decreased, or will decrease, by 80% within three generations....
along the western coast of Africa, where stocks have collapsed with an estimated 80% decline in the past 25 years. The West African Sub-Regional Fishing Commission (SRFC) has recognized the great hammerhead as one of the four most threatened species in the region, though fishing continues unmonitored and unregulated. Off northern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, this species was assessed as Data Deficient
Data Deficient
Data Deficient is a category applied by the IUCN, other agencies, and individuals to a species when the available information is not sufficient for a proper assessment of conservation status to be made...
but at "high risk". Concern has arisen there over a substantial increase in illegal, unreported, and unregulated (IUU) fishing, reflecting the raising value of this shark's fins.
No conservation measures specifically protecting the great hammerhead have been enacted. It is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea, though no management schemes have yet been implemented under this agreement. The banning of shark finning
Shark finning
Shark finning refers to the removal and retention of shark fins and the discarding of the rest of the fish. Shark finning takes place at sea so the fishers only have to transport the fins.Shark finning is widespread, and largely unmanaged and unmonitored...
by countries and supranational entities such as United States, Australia, and the European Union, and international regulatory bodies such as the International Commission for the Conservation of Atlantic Tunas
International Commission for the Conservation of Atlantic Tunas
The International Commission for the Conservation of Atlantic Tunas is an intergovernmental organisation responsible for the management and conservation of tunas and tuna-like species in the Atlantic Ocean and adjacent seas. The organization was established in 1969, at a conference in Rio de...
(ICCAT), should reduce fishing pressure on the great hammerhead.