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 and other sharks, cephalopods, and crustaceans. It is found in warmer waters along coastlines and continental shelves.

The shape of the head seems to act as a wing, aiding in close-quarters maneuverability. It also seems to help in electrolocation by separating the receptors, allowing signals to be received as though in stereo. These sharks have been able to detect an electronic signal of one billionth of a volt. Hammerheads have proportionately small mouths and seem to do a lot of bottom-hunting. They are also known to school, sometimes in groups of over a hundred.

Of the eight species of hammerhead, three can be dangerous to humans—the scalloped, great and smooth hammerheads.

• Genus Sphyrna
  • Subgenus Sphyrna
    • Scalloped Hammerhead, S. (S.) lewini
    • Great Hammerhead , S. (S.) mokarran
    • Smooth Hammerhead , S. (S.) zygaena
    • Whitefin Hammerhead , S. (S.) couardi
  • Subgenus Mesozygaena
    • Scalloped Bonnethead , S. (M.) corona
  • Subgenus Platysqualus
    • Scoophead , S. (P.) media
    • Bonnethead or Shovelhead, S. (P.) tiburo
    • Golden Hammerhead (previously known as Smalleye Hammerhead, S. (P.) tudes

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, but the teeth of hammerheads resemble those of some Carcharhinids , making it difficult to be certain when the hammerheads first appeared. It is probable that the hammerheads evolved during the late Eocene, Oligocene or early Miocene.

External Links

• Animal Diversity Web Genus Sphyrna with species sub-pages
• Hammerhead Shark! web directory, with pictures
• Fishbase
• "Electroreception in juvenile scalloped hammerhead and sandbar sharks" by Stephen M. Kajiura and Kim N. Holland, The Journal of Experimental Biology (2002). Attempts to explain the "hammer" shape.
• MarineBio: Great hammerhead shark, Sphyrna mokarran