Great hammerhead
The great hammerhead is the largest species of hammerhead shark, belonging to the family Sphyrnidae, attaining an average length of and reaching a maximum length of. 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", which is wide with an almost straight front margin, its tall, sickle-shaped first dorsal fin, and its strongly falcated pelvic fins. A solitary, strong-swimming apex predator, the great hammerhead feeds on a wide variety of prey ranging from crustaceans and cephalopods, to bony fish, to smaller sharks. Observations of this species in the wild suggest that the cephalofoil functions to immobilize stingrays, a favored prey. This species has a viviparous mode of reproduction, bearing litters of up to 50 pups every two years.
Although potentially dangerous, the great hammerhead rarely attacks humans and can sometimes behave inquisitively toward divers. This shark is heavily fished for its large fins, which are valuable on the Chinese 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 critically endangered by the International Union for Conservation of Nature as of 2019.
Taxonomy and phylogeny
The great hammerhead was first described as Zygaena mokarran in 1837 by German naturalist Eduard Rüppell. The species name is the common name in Arabic مقرن and means "horned". The name was later changed to the current Sphyrna mokarran. For many years, though, 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 hammerheads, 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-long male from the Red Sea.Older studies based on morphology have generally placed the great hammerhead as one of the more derived members of its family, reflecting the traditional view that cephalofoil size gradually increased over the course of hammerhead shark evolution, but this view has been refuted by phylogenetic analyses using nuclear and mitochondrial DNA, which found that the great hammerhead and the smooth hammerhead form a clade that is basal to all other Sphyrna species. These results also show that the first hammerheads to evolve had large rather than small cephalofoils.
The earliest fossil teeth of the great hammerhead are known from the Early Miocene of Cuba. They are also known from the Middle Miocene to Pliocene of Florida, US, and the Late Miocene of Panama and Brunei.
Distribution and habitat
The great hammerhead inhabits tropical waters around the world, between the latitudes 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, Western Sahara, and occasionally seen near Hawaii but this has not been confirmed. Great hammerheads may be found from inshore waters less than deep, to a depth of offshore. They favor coral reefs, 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.Description
The streamlined body of the great hammerhead with the expanded cephalofoil is typical of the hammerhead sharks. Adults can be distinguished from the scalloped hammerhead and the smooth hammerhead by the shape of the cephalofoil, which has a nearly straight front margin, 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 toward the corners of the mouth. Seventeen tooth rows are on either side of the upper jaw, with two or three teeth at the symphysis, and 16-17 teeth on either side of the lower jaw and one to three at the symphysis.The first dorsal fin is distinctive, being very tall and strongly 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 sickle-shaped 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 three to five horizontal ridges leading to marginal teeth in smaller individuals, and five or six 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 typical minimum great hammerhead measures long and weighs over, while the maximum measures long and weighs. A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was. The heaviest known great hammerhead is a female, long and in weight caught off Boca Grande, Florida, in 2006. The weight of the female was due to her being pregnant with 55 neonatal pups. Exceptionally large individuals may possibly reach in weight, though this has not been verified.
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, it may respond with an agonistic display, dropping its pectoral fins and swimming in a stiff or jerky fashion. Juveniles are preyed upon by larger sharks such as bull sharks, while adults have no major predators except for killer whales, which hunt hammerheads of any age. Yellow jacks have been seen rubbing themselves against the hammerhead's flanks, possibly to rid themselves of parasites. Schools of pilot fish sometimes accompany the great hammerhead. The great hammerhead is parasitized by several species of copepods, including Alebion carchariae, A. elegans, Nesippus orientalis, N. crypturus, Eudactylina pollex, Kroyeria gemursa, and Nemesis atlantica.Feeding
The great hammerhead shark is an active predator with a varied diet. Known prey of the great hammerhead include invertebrates such as crabs, lobsters, squid, and octopus; bony fishes such as tarpon, sardines, sea catfishes, toadfish, porgies, grunts, jacks, croakers, groupers, flatfishes, boxfishes, and porcupine fishes; and smaller sharks such as smoothhounds. At Rangiroa Atoll, great hammerheads prey opportunistically on grey reef sharks that have exhausted themselves pursuing mates. The species is known to be cannibalistic. In the Gatun Formation of Panama, fossil teeth belonging to great hammerheads suggest the shark once preyed on juvenile Otodus megalodon, alongside the extinct snaggletooth shark Hemipristis serra, with which it was once contemporary.Great hammerheads are apex predators among sharks, and are specialists at feeding on other sharks, rays, and skates, especially stingrays. The venomous spines of stingrays are frequently found lodged inside its mouth and do not seem to bother the shark, as 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 electroreceptory organs 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 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 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, and that its cephalofoil is an adaptation for prey handling. Adult great hammerheads have been observed hunting in coral reef flats as shallow as ~.