Great hammerhead

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. 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. ==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 (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 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 (the midline of the jaw), and 16–17 teeth on either side of the lower jaw and one to three at the symphysis. Exceptionally large individuals may possibly reach in weight, though this has not been verified. File:Sphyrna mokarran jaws.jpg|Jaws File:Sphyrna mokarran upper teeth.jpg|Upper teeth File:Sphyrna mokarran lower teeth.jpg|Lower teeth ==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 (Carcharhinus leucas), while adults have no major predators except for killer whales, which hunt hammerheads of any age. The species is known to be cannibalistic. 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. 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. Adult great hammerheads have been observed hunting in coral reef flats as shallow as ~. Rolled swimming and energy conservation A 2016 study of specimens logged with accelerometers showed that great hammerhead sharks reduce drag and lessen energy expenditure by swimming on their side in a posture termed "rolled swimming". The shark uses its very large dorsal fin to help achieve lift, a habit that had previously been noted in captive specimens, and may spend up to 90% of its time in this swimming orientation. The technique is thought to save about 10% in drag and consequently movement costs. Life history As with other hammerhead sharks, great hammerheads are viviparous; once the developing young use up their supply of yolk, the yolk sac 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 six to 55 pups, with 20–40 being typical. The young measure at birth; males reach maturity at long and and the females at and . 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. ==Human interactions==

With its large size and cutting teeth, the great hammerhead could seriously injure a human, so 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 nonreactive toward humans. The great hammerhead is regularly caught both commercially and recreationally in the tropics, using longlines, fixed bottom nets, hook-and-line, and trawls. 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 vitamins, and their carcasses for fishmeal. In Australia's other northern jurisdictions (the Northern Territory and Western Australia) recreational targeting and catching of great hammerheads is allowed. ==Conservation status==

The great hammerhead is extremely vulnerable to overfishing 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 critically endangered on the IUCN Red List. It is endangered in the north-western Atlantic and the Gulf of Mexico, where though it is not a 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 whether these represent localized or widespread depletions is uncertain. 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 fishing, reflecting the rising value of this shark's fins. ==References==