The hammerhead sharks are a group of sharks so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a “hammer” shape.
There are 10 species of hammerhead shark.
Hammerheads are found worldwide in warmer waters along coastlines and continental shelves.
The lifespan of hammerhead sharks is estimated to be 25 to 35 years.
Hammerheads vary in size.
The great hammerhead (Sphyrna mokarran) is the largest species of hammerhead shark. The average great hammerhead measures up to 3.5 m (11 ft) long and weighs over 230 kg (510 lb). A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was 6.1 m (20 ft). The heaviest known great hammerhead is a 4.4 m (14 ft) long, 580 kg (1,280 lb) female caught off Boca Grande, Florida in 2006.
The scalloped bonnethead (Sphyrna corona) is the smallest species of hammerhead shark, measures up to 92 centimeters (36 inches) long. It is a rare, little-known species of hammerhead shark.
Hammerheads are usually gray-brown to olive-green on top. Their bellies are white which allows them to blend into the ocean when viewed from the bottom and sneak up on their prey.
Many, but not necessarily mutually exclusive functions have been proposed for the hammer-like shape of the head, including sensory reception, vision, manoeuvering, and prey manipulation.
Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini, which enable them to detect electric fields, including the weak electrical impulses generated by prey, which helps them to hunt. One theory is that the wide expanse of head allows for a broader spread of this sensory organ. By distributing the receptors over a wider area, like a larger radio antenna, hammerheads can sweep for prey more effectively.
Also the hammer-like shape of the head may have evolved (at least in part) to enhance the animal’s vision. The positioning of the eyes, mounted on the sides of the shark’s distinctive hammer head, gives the shark good 360° vision in the vertical plane, meaning they can see above and below them at all times.
They have disproportionately small mouths and seem to do a lot of bottom-hunting.
Hammerhead sharks are carnivorous and eat a variety of fish, although stingrays are the preferred prey. Hammerhead sharks take advantage of their wide, flat heads to pin down stingrays and other bottom dwelling fish before delivering a fatal bite. They will also squid, octopus, crustaceans and other sharks.
Like many other species of shark the hammerhead shark is a solitary hunter during the night, but during the daytime hammerhead sharks are known to form schools of up to 100 hammerhead shark individuals.
It is not known why they group, but apparently, the practice provides them protection against larger predators.
During the summer, hammerheads participate in a mass migration to search for cooler waters.
Believe it or not, the hammerhead has the ability to sport a nice tan! They are one of rare marine animals that can get nice tan. This happens because hammerheads are often cruising in shallow water or near the surface for extended periods of time.
Hammerhead sharks can swim up to 25 km/h (15 mph).
The hammerhead shark is well known for its ability to make very sudden and sharp turns. Not only does the hammer at as an organ of balance, but its body seems to be specifically designed to twist and bend.
Reproduction occurs only once a year for hammerhead sharks, and usually occurs with the male shark biting the female shark violently until she agrees to mate with him.
All hammerhead sharks are viviparous – a female gives birth to live young that grow inside the female shark, similar to humans.
Once the baby sharks are born, they are not taken care of by the parents in any way. Usually, a litter consists of 12 to 15 pups, except for the great hammerhead, which gives birth to litters of 20 to 40 pups. These baby sharks huddle together and swim toward warmer water until they are old enough and large enough to survive on their own.
An international team of scientists surprised the world by reporting that female hammerhead sharks can reproduce without males through parthenogenesis, or “virgin birth”. It was previously believed that sharks reproduced only sexually. Researchers from Ireland and the United States performed genetic tests on a baby hammerhead born in an Omaha, Nebraska aquarium in 2001. The three occupants of the tank were all females who were captured as babies and had never been introduced to a male in captivity, which is what prompted the curiosity. The genetic tests proved that there was no “DNA of male origin” in the baby hammerhead.
The great and the scalloped hammerheads are listed on the World Conservation Union’s (IUCN) 2008 Red List as endangered, whereas the smalleye hammerhead is listed as vulnerable. The status given to these sharks is as a result of overfishing and demand for their fins, an expensive delicacy.
Of the 10 species of hammerhead, only three are known to be particularly dangerous to humans: the scalloped, great, and smooth hammerheads. But, attacks to humans are extremely rare.
Since sharks do not have mineralized bones and rarely fossilize, their teeth alone are commonly found as fossils. According to DNA studies, the ancestor of the hammerheads probably lived in the Miocene epoch about 20 million years ago.