There are eight different species of Hammerhead shark. All the species have the remarkable projections on both sides of the head, which probably is they reason why these sharks can detect electronic signals of no more than half a billionth of a volt. The head is probably used during electrolocation. By separating the receptors, the Hammerhead shark can receive signals in stereo. The oddly shaped head also seem to act as a wing that the Hammerhead shark uses for close-quarters maneuverability. The head looks somewhat like a flattened hammer, which is the reason behind the name of the Hammerhead shark. The nostrils and eyes are located at the tip of the extensions. All Hammerhead shark species have proportionately small mouths. The size of the eight different Hammerhead sharks varies between 2 and 6 metres. The largest Hammerhead species, the Great Hammerhead shark, will typically weigh around 230 kg (500 pounds) but can reach a weight of 450 kg (1,000 pounds). Three Hammerhead species can be dangerous to humans: the Great Hammerhead shark, the Scalloped Hammerhead shark and the Smooth Hammerhead sharks.
The strangely shaped head of the Hammerhead shark has given raise to a lot of speculation. There are two main theories regarding its development. Some scientists believe that the shape evolved gradually over numerous generations, while others suggest that it was a sudden mutation that rapidly proved to be an advantage. The entire head is equipped with sensors, and might be hammer-shaped in order to provide a larger area for the sensors. A larger sensor area would give the Hammerhead shark an advantage when it scans the bottom of the sea for food.
The Hammerhead sharks are probably related to the Carcharhinid sharks which evolved during the mid-tertiary period. It is somewhat difficult to investigate the evolutionary process of sharks, since the bones of sharks aren’t mineralized and seldom fossilize. There teeth are however often found as fossils. The teeth of the Hammerhead sharks are very similar to fossilized teeth from the Carcharhinid sharks. The Hammerhead sharks most likely developed during the late Eocene, Oligocene or early Miocene. It is hard to establish the exact era since it is difficult to know which teeth belong to Hammerhead sharks and which belong to Carcharhinid sharks.
All the Hammerhead shark species belong to the order of Carcharhiniformes, which means that they are ground sharks with one anal fin and two dorsal ones. They fins are spineless and the mouth is located behind the eyes. All Hammerhead sharks have five gill slits and a spiral intestinal valve. The spiral shape means that the food will pass through the intestines at a remarkably slow pace. This makes it impossible for Hammerhead sharks to eat often, and their growth rate is therefore very slow. They save energy by not swimming around a lot. The liver of a Hammerhead shark is filled with oil that has a lower density than water. This provides enough buoyancy for the Hammerhead to float in the water rather than swim. They often look very gracious when they float around, slowly scanning the bottom for food.
All Hammerhead sharks belong to the genus Sphyrna.
Four of the Hammerhead sharks belong to the subgenus which is also called Sphyrna:
Great Hammerhead, S. (S.) mokarran
Scalloped Hammerhead, S. (S.) lewini
Smooth Hammerhead, S. (S.) zygaena
Whitefin Hammerhead, S. (S.) couardi
The subgenus Platysqualus contains three species:
Scoophead, S. (P.) media
Bonnethead or Shovelhead, S. (P.) tiburo
Golden Hammerhead (previously known as Smalleye Hammerhead), S. (P.) tudes
The subgenus Mesozygaena includes only one species:
Scalloped Bonnethead, S. (M.) corona
Hammerhead sharks are found in warm tropical and subtropical waters along continental shelves and coastlines all over the globe. Hammerhead sharks are often seen in the upper part of the so called mesopelagic zone; the drop-off adjacent to the continental shelf. They dive down to 80 metres (260 feet). The Great Hammerhead shark migrates to cooler waters during the summer season. All Hammerhead sharks spend a lot of time hunting at the bottom. They are capable predators and eat primarily fish, rays, crustaceans, cephalopods and other sharks. They are particularly found of rays and will use the “hammer” to pin the ray down while eating the wings bite by bite.
Hammerhead sharks can sometimes form schools that contain up to 500 individuals, which is very unusual for sharks. They only school during the day and always spend the nights separated from each other. There seems to be an established order of dominance in every school, where the “rank” of each individual Hammerhead shark is determined by size, age and sex. We still do not know why the Hammerhead sharks choose to live in schools during the day. What we do know is that the largest Hammerhead sharks are seldom seen in schools, it is instead the smaller and medium sized ones that seem to like the company of other Hammerheads. This can indicate that the schools are formed as a way of protection against larger predatory fish, such as larger sharks. Hammerhead sharks use nine known ways of communicating with each other. One of the more stunning ways of communication is the violent shaking of the head from side to side performed by large female Hammerhead sharks in the centre of large schools. The vigorous movement radiates pulses in the water, and makes smaller female Hammerhead sharks swim to the outskirts of the school. It is most likely a part of the mating behaviour. The large female Hammerhead shark wants to attract the attention of suitable males.
The Hammerhead shark reproduces once a year. During the courting period, the male Hammerhead shark will bite the female shark quite violently until she agrees to mate. The fertilization takes place inside the female's body. Even though the Hammerhead shark is a fish, the development of the embryo is very similar to that of a mammal. The embryo develops inside the female shark, and receives nutrients via an umbilical cord. After a gestation period of 10-12 months most Hammerhead shark species gives birth to 12 to 15 pups. The pups are developed enough to survive on their own, and they are not guarded or cared for in any way by the parents. The Great Hammerhead shark gives birth to a larger litter, typically 20-40 pups, that are around 70 cm (27 inches) at birth.
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