Hammerhead shark

Hammerhead sharks are any of the species of sharks belonging to the taxononic Genus Sphyrna. They are found in both shallow coastal lines and open deep ocean. The size of nine known species vary from 2~6 meter long. This shark is gray-brown above and have white belly. All species have their heads projected to each side which reminds of flattened hammer. This is the origin of the name "Hammerhead shark". Hammerhead sharks are not dangerous to human except three species: Mokarran (The Great Hammerhead sharks), Lewini (Scalloped Hammerhead sharks), and Zygaena (Smooth Hammerhead sharks). 

Anatomy
Hammerhead sharks are famous for their hammer looking like heads. They have their eyes at the margin of the rectangular head, a notch at the front center of the rectangular head. The front edge of the juveniles’ heads are slightly curved which becomes almost straight as they become adults. First dorsal fin of Hammerhead shark is extremely curved and pointed, second dorsal fin and pelvic fins are high and dented. The hammer looking like head is thought to be evolved to maximize the area of sensory organs such as the ampullae of Lorenzini (The special sensors in shark’s skin which allows the shark to detect chemical, physical and thermal changes, so it can find certain fishes that are buried in the bottom of ocean such as rays or crustaceans). The hammer shaped head also help the shark to hunt easily since it allows scanning significantly larger areas of the bottom than other shark species.

Reproduction
Unlike many other shark species, Hammerhead sharks are viviparous (species that have internal fertilization). They give birth to live young once a year and each litter contains 20 to 40 pups which are about 70 cm long when born. Hammerhead sharks mate very violently. The male will bite female until the female give permission to mate. The Hammerhead shark has internal fertilization which provides a safe environment for sperms to unite with eggs. The embryo develops in the placenta and gets the nutrition through an umbilical cord which is similar to how mammals grow up. The pregnancy period is 10 to 12 months. Once the pups come out from eggs, they are left alone to look after themselves. 

Ecology
Unlike other species, the Great Hammerhead sharks are unsociable species, they are rarely seen with other sharks. Scalloped Hammerhead sharks and Smooth Hammerhead sharks are often seen as small and large schools. The Hammerhead sharks swim in warm and relatively warm water along the coastlines. In summer, they migrate toward north areas. During winter, all Hammerhead sharks make migration to warmer water. Great Hammerhead sharks and schools of Scalloped Hammerhead sharks and Smooth Hammerhead sharks are not only fearsome to people but also to other sharks, since Hammerhead sharks eat other sharks. 

Diet
Hammerhead sharks are fierce predators with eyes that can see extremely large are of bottom areas of the sea and good sense of smell. Hammerhead sharks usually hunt for other sharks, little bony fishes (catfishes, toadfishes, boxfishes), squid, and crustaceans (shrimps, crabs), batoids (stingrays). They especially prefer stingrays. They hunt rays by using its hammer to stick the stingray down while it takes bite of the ray’s wings. Some Hammerhead sharks are known as cannibalistic. Mokarran (The Great Hammerhead sharks), Lewini (Scalloped Hammerhead sharks), and Zygaena (Smooth Hammerhead sharks) are three species in genus Sphyrna that are dangerous to humans. 

Related References

 * Wikipedia Hammerhead shark
 * Enchant Learning Hammerhead shark
 * Shark Information Hammerhead shark ecology
 * Marin Biology Hammerhead shark anatomy
 * Shark Diving Hammerhead shark diet