Hammerhead shark

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{{Taxobox | color = pink | name = Hammerhead shark | image = Hammerheadshark.jpg | image_width = 250px | regnum = Animalia | phylum = Chordata | classis = Chondrichthyes | ordo = Carcharhiniformes | familia = Sphyrnidae | genus = Sphyrna | genus_authority = Rafinesque, 1810 | subdivision_ranks = Species | subdivision = Sphyrna (Sphyrna) lewini
Sphyrna (Sphyrna) mokarran
Sphyrna (Sphyrna) zygaena
Sphyrna (Sphyrna) couardi,
Sphyrna (Mesozygaena) corona
Sphyrna (Platysqualus) media
Sphyrna (Platysqualus) tiburo
Sphyrna (Platysqualus) tudes }}The hammerhead shark (genus Sphyrna) is a member of the family Sphyrnidae. The only other genus of Sphyrnidae, Eusphyra, contains only one species, E. blochii, the winghead shark).

The eight species of hammerhead range from 2–6 m long, and all species have projections on both sides of the head that give it a resemblance to a flattened hammer. The shark's eyes and nostrils are at the tips of the extensions.

It is an aggressive predator that eats fish, rays, other sharks, cephalopods, and crustaceans. It is found in warmer waters along coastlines and continental shelves.

The hammer shape of the head was thought to act as a wing, aiding in close-quarters maneuverability, allowing sharks to execute sharp turns without loss of stability. Instead it was found that the vertebra allowed it to make the turns correctly. It also seems to help in electrolocation by separating the receptors, thus giving hammerheads a wider area of search. These sharks have been able to detect an electronic signal of half a billionth of a volt. The hammer shaped head also gives these sharks larger nasal tract, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks. Hammerheads have proportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become lonely hunters.

Reproduction in the hammerhead shark occurs once a year and each litter contains 12–15 pups. Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization which creates a safe environment for the sperm to unite with the egg. The embryo develops within the female inside a placenta and is fed through an umbilical cord, much like in mammals. The gestation period is 10–12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves.

Of the eight species of hammerhead, three (3) can be dangerous to humans: the scalloped, great, and smooth hammerheads.

Since sharks do not have mineralized bones and rarely fossilize, it is their teeth alone that are commonly found as fossils. The hammerheads seem closely related to the carcharhinid sharks that evolved during the mid-Tertiary Period. Because the teeth of hammerheads resemble those of some carcharhinids, it has been difficult to determine when hammerheads first appeared. It is probable that the hammerheads evolved during the late Eocene, Oligocene or early Miocene.

There are two schools of thought for the odd head shape of the hammerheads. One states that it is because of the greater area for sensors to scan the bottom of the seafloor for food and the other that it provides lift (Hammerheads are one of the most negatively buoyant of sharks) and added maneuverability. Both of the theories are probably correct <ref name="Hammer">Template:Cite web</ref>. Geneticist Andrew Martin used DNA to study all hammer head species and he concluded that the first hammer appeared on the Winghead shark which have the largest hammer and the rest of the hammerhead sharks evolved one at a time from the original Winghead shark each with a smaller hammer <ref name="Mutation">Template:Cite web</ref>.

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References

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  • Kajiura, Stephen; Forni, Jesica; Summers, Adam; "Maneuvering In Carcharhinid and Sphyrnid Sharks: The Role and Non-Role of the Hammerhead Shark Cephalofoil" (2001)
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External links

et:Vasarhai fr:Requin-marteau it:Sphyrnidae nl:Hamerhaaien ja:シュモクザメ pl:Młotowate pt:Tubarão-martelo zh:双髻鲨科

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