Hammerhead shark

From Freepedia

Hammerhead shark

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	Carcharhiniformes
Family:	Sphyrnidae
Genus:	Sphyrna Rafinesque, 1810

Species

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 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 half a 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 100.

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.

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 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 theories for the odd head shape of hammerheads. Some scientists suggest that the shape evolved over many generations. Others suggest that it was a mutation that helped the hammerhead and therefore provided an advantage. Because the hammerhead has sensors along its entire head, one theory for why the hammerhead has a hammer-shaped head is to provide a greater area for sensors to scan the bottom of the seafloor for fish, crustaceans, or other food items to eat.

Image:Bonnethead.jpg