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Hammerhead sharks (Sphyrna)Hammerhead sharks (Sphyrna) belong to the family Sphyrnidae.
Hammerheads occur worldwide at depths of 0-4,050 m in warmer waters along coastlines and continental shelves. They usually swim in schools by day and become solitary hunters at night.
Hammerheads range from 0.9-6 m (3-20 ft) long and weigh from 3-580 kg (6.6-1,300 lb). While several hammerhead species are quite large, these are more slender and streamlined compared to other large sharks, increasing their speed and manoeuverability in the water. They are usually light gray with a greenish tint. Their bellies are white which lets them be close to the bottom of the ocean and blend in to sneak up on prey. Their heads have lateral projections which giving them a hammer-like shape or "cephalofoil". This shape may have evolved to enhance vision and other sensory reception, manoeuvering and prey manipulation.  The eyes are mounted on the sides of the hammer head and provide good 360 degree vision in the vertical plane, so the sharks can see above and below at all times. The shape of the head was thought to help the shark find food, aiding in close-quarters manoeuverability and allowing sharp turning movement without losing stability. It was found that the unusual structure of its vertebrae was instrumental in making the turns correctly, more often than the shape of its head, though it would also shift and provide lift. Hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively.
Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They form schools by day, sometimes in groups of over 100. In the evening, they become solitary hunters.
Reproduction occurs once a year. The male usually bites the female violently until she agrees to mate with him. The shark is viviparous; females give birth to live young. Fertilization is internal with the male transferring sperm to the female through one of two intromittent organs (claspers). The developing embryos are at first sustained by a yolk sac. When the supply of yolk is exhausted, the depleted yolk sac transforms into a structure similar to a mammalian placenta and called a "yolk sac placenta" or "pseudoplacenta", through which the mother delivers sustenance until birth. Once the baby sharks are born, the parents do not take care of them. There is usually a litter of 12-15 pups, but the great hammerhead has litters of 20-40 pups. The baby sharks huddle together and swim toward warmer water and stay together until they are older and big enough to be on their own.In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, where a female's ovum fuses with a polar body to form a zygote without needing a male. This was the first shark known to do this.
Hammerheads have a varied diet including sharks and other fish, squid, octopus and crustaceans. Stingrays are a favourite. The sharks often swim along the bottom of the ocean, stalking their prey. The head is used as a weapon when hunting down prey. The shark uses its head to pin down stingrays and eats the ray when the ray is weak and in shock. The great hammerhead tends to be more aggressive and can eat other hammerhead sharks and eat their own young.
As do not have mineralized bones and rarely fossilize, only their teeth are commonly found as fossils. The hammerheads seem closely related to the carcharhinid sharks, which have similar teeth. Using mitochondrial DNA, Andrew Martin constructed a phylogenetic tree of hammerhead sharks, which suggests that the first ancestral hammerhead sharks had large hammers.
Three species of hammerhead can be dangerous to humans: the scalloped, great and smooth hammerheads. The great and scalloped hammerhead are listed on the World Conservation Union's (IUCN) 2008 Red List as endangered, while the smalleye hammerhead is listed as vulnerable. This status is due to over-fishing and demand for their fins, an expensive delicacy. Fishermen typically cut off the fins and often toss the rest of the fish back into the sea.
In Native Hawaiian culture, sharks are considered to be gods of the sea, also caled aumakua, protectors of humans, and cleaners of excessive ocean life. Some sharks are believed to be family members who died and have been reincarnated into shark form. Some sharks are considered man-eaters (niuhi). The hammerhead shark or mano kihikihi is not considered a man-eater or niuhi; but is considered to be one of the most respected sharks of the ocean, an aumakua. Many Hawaiian families believe they have an aumakua watching over them and protecting them from the niuhi. The hammerhead is thought to be the birth animal of some children. Hawaiian children who are born with the hammerhead shark as an animal sign are believed to be warriors and are meant to sail the oceans. It is very rare for hammerheads to pass through the waters of Maui, but many Maui natives believe that when the sharks pass by, it is a sign that the gods are watching over the families, and the oceans are clean and balanced.