Hammerhead Shark

gliding along the sea bed]] Because sharks do not have mineralized bones and rarely fossilize, only their teeth are commonly found as fossils. Their closest relatives are the requiem sharks (Carcharinidae). Based on a recent combined molecular and paleontological study, the ancestor of the hammerheads probably lived in the Cretaceous or Paleocene. Definitive fossils of the family date back to the Eocene. Prior DNA studies had estimated the family to have arisen in the Miocene, about 20 million years ago. Using mitochondrial DNA, a phylogenetic tree of the hammerhead sharks showed the winghead shark as sister to the rest of the hammerhead sharks. As the winghead shark has proportionately the largest "hammer" of the hammerhead sharks, this suggests that the first ancestral hammerhead sharks also had large hammers.

The hammer-like shape of the head may have evolved at least in part to enhance the animal's vision. The positioning of the eyes, mounted on the sides of the shark's distinctive hammer head, allows 360° of vision in the vertical plane, meaning the animals can see above and below them at all times. They also have an increased binocular vision and depth of visual field as a result of the cephalofoil. The shape of the head was previously thought to help the shark find food, aiding in close-quarters maneuverability, and allowing sharp turning movement without losing stability. The unusual structure of its vertebrae, though, has been found to be instrumental in making the turns correctly, more often than the shape of its head, though it would also shift and provide lift. From what is known about the winghead shark, the shape of the hammerhead apparently has to do with an evolved sensory function. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. The pores on the shark's head lead to sensory tubes, which detect electric fields generated by other living creatures. By distributing the receptors over a wider area, like a larger radio antenna, hammerheads can sweep for prey more effectively.

Reproduction occurs only once a year for hammerhead sharks, and usually occurs with the male shark biting the female shark violently until she agrees to mate with him. The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. Like other sharks, fertilization is internal, with the male transferring sperm to the female through one of two intromittent organs called 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 analogous to a mammalian placenta (called a "yolk sac placenta" or "pseudoplacenta"), through which the mother delivers sustenance until birth. Once the baby sharks are born, they are not taken care of by the parents in any way. Usually, a litter consists of 12 to 15 pups, except for the great hammerhead, which gives birth to litters of 20 to 40 pups. These baby sharks huddle together and swim toward warmer water until they are old and large enough to survive on their own.

There are ten distinct species of Hammerhead sharks in the wild: {| class="wikitable" ! !Species !Common names !IUCN Red List status !Population trend !References |- | |Eusphyra blochii |Winghead shark | Critically endangered |Decreasing | |- | |Sphyrna alleni |Shovelbill shark | Not Evaluated |Unknown | |- | |Sphyrna corona |Scalloped bonnethead | Critically Endangered |Decreasing | |- | |Sphyrna gilberti |Carolina hammerhead | Data Deficient |Unknown | |- | |Sphyrna lewini |Scalloped hammerhead | Critically Endangered |Decreasing | |- | |Sphyrna media |Scoophead | Critically Endangered |Decreasing | |- | |Sphyrna mokarran |Great hammerhead | Critically Endangered |Decreasing | |- | |Sphyrna tiburo |Bonnethead | Endangered |Decreasing | |- | |Sphyrna tudes |Smalleye hammerhead | Critically Endangered |Decreasing | |- | |Sphyrna zygaena |Smooth hammerhead | Vulnerable |Decreasing | |}

, from theIllustrated Guide to the Products of Oki Province]] shark along a street in Mogadishu, Somalia]] According to the International Shark Attack File, humans have been subjects of 17 documented, unprovoked attacks by hammerhead sharks within the genus Sphyrna since AD 1580. No human fatalities have been recorded. Most hammerhead shark species are too small to inflict serious damage to humans. Shark fins are prized as a delicacy in certain countries in Asia (such as China and Japan), and overfishing is putting many hammerhead sharks at risk of extinction. Fishermen who harvest the animals typically cut off the fins and toss the remainder of the fish, which is often still alive, back into the sea. This practice, known as finning, is lethal to the shark.

The relatively small bonnethead is regular at public aquariums, as it has proven easier to keep in captivity than the larger hammerhead species, and it has been bred at a handful of facilities. Nevertheless, at up to in length and with highly specialized requirements, very few private aquarists have the experience and resources necessary to maintain a bonnethead in captivity. The larger hammerhead species can reach more than twice that size and are considered difficult, even compared to most other similar-sized sharks (such as Carcharhinus species, lemon shark, and sand tiger shark) regularly kept by public aquariums. As a consequence, relatively few public aquaria have kept them for long periods. In 2014, fewer than 15 public aquaria in the world kept scalloped hammerheads. Smooth hammerheads have also been kept in the past.

Humans are the main threat to hammerhead sharks. Although they are not usually the primary target, hammerhead sharks are caught in fisheries all over the world. Tropical fisheries are the most common place for hammerheads to be caught because of their preference to reside in warm waters. In March 2013, three endangered, commercially valuable sharks, the hammerheads, the oceanic whitetip, and porbeagle, were added to Appendix II of CITES, bringing shark fishing and commerce of these species under licensing and regulation.

Among Torres Strait Islanders, the hammerhead shark, known as the beizam, is a common family totem and often represented in cultural artefacts such as the elaborate headdresses worn for ceremonial dances, known as dhari (dari). They are associated with law and order. Renowned artist Ken Thaiday Snr is known for his representations of beizam in his sculptural dari and other works. In native Hawaiian culture, sharks are considered to be gods of the sea, protectors of humans, and cleaners of excessive ocean life. Some of these sharks are believed to be family members who died and have been reincarnated into shark form, but others are considered man-eaters, also known as niuhi. These sharks include great white sharks, tiger sharks, and bull sharks. The hammerhead shark, also known as mano kihikihi, is not considered a man-eater or niuhi; it is considered to be one of the most respected sharks of the ocean, an aumakua. Many Hawaiian families believe that they have an aumakua watching over them and protecting them from the niuhi. The hammerhead shark 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. Hammerhead sharks rarely pass through the waters of Maui, but many Maui natives believe that their swimming by is a sign that the gods are watching over the families, and the oceans are clean and balanced.