Hammerhead Sharks

By Jaime Enrique

Introduction to hammerhead sharks.

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil".

More on hammerhead sharks.

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna while the winghead shark is placed in its own genus, Eusphyra. Many not necessarily mutually exclusive functions have been proposed for the cephalofoil, including sensory reception, maneuvering, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in schools during the day, becoming solitary hunters at night. Some of these schools can be found near Malpelo Islandin Colombia, Cocos Island off Costa Rica, and near Molokai in Hawaii. Large schools are also seen in the waters off southern andeastern Africa.

Diet.

Hammerhead sharks are known to eat a large range of prey including fish, squid, octopus, crustaceans, and other sharks. Stingrays are a particular favorite. These sharks are often found swimming along the bottom of the ocean, stalking their prey. Their unique head is used as a weapon when hunting down prey. The hammerhead shark uses its head to pin down stingrays and eats the ray when the ray is weak and in shock.[11] The Great Hammerhead, tending to be larger and more aggressive than most hammerheads, occasionally engage in cannibalism, as they are known to eat other hammerhead sharks, including their own young.[13]

Taxonmy and Evolution.

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 thecarcharhinid sharks that evolved during the mid-Tertiary Period. According to DNA studies, the ancestor of the hammerheads probably lived in the Miocene epoch about 20 million years ago.[9]

Using mitochondrial DNA, a phylogenetic tree of the hammerhead sharks showed the winghead shark as its most basal member. As the winghead shark has proportionately the largest "hammer" of the hammerhead sharks, this suggests that the first ancestral hammerhead sharks also had large hammers.[10]

Reason for head shape.

The theory has been advanced that the hammer-like shape of the head may have evolved (at least in part) to enhance the animal's vision.[5] The positioning of the eyes, mounted on the sides of the shark's distinctive hammer head give the shark good 360-degree vision in the vertical plane, meaning they can see above and below them at all times.[6][7] The shape of the head was previously thought to help the shark find food, aiding in close-quarters maneuverability and allowing sharp turning movement without losing stability. However, it has been found that the unusual structure of its vertebrae was instrumental in making the turns correctly, more often than the shape of its head, though it would also shift and provide lift. From what is known about thewinghead shark, it would appear that the shape of the hammer-head has to do with an evolved sensory function. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, like a larger radio antenna, hammerheads can sweep for prey more effectively.[8]

Description.

The unknown species range from 0.9 to 6 m (3.0 to 19.7 ft) in length and weigh from 3 to 580 kg (6.6 to 1,278.7 lb).[2][3] They are usually light gray and have a greenish tint to them. Their bellies are white which allows them to blend into the ocean when viewed from the bottom and sneak up on their prey.[4] Their heads have lateral projections which give them a hammer-like shape.






Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become solitary hunters.

Reproduction.

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.[11]The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. Like other sharks, fertilization is internal with the male transferringsperm to the female through one of two intromittent organs called claspers. The developing embryos are at first sustained by a yolk sac. When the supply of yolk is exhausted, the depleted yolk sac transforms into a structure analogous to a mammalian placenta (called a "yolk sac placenta" or "pseudoplacenta"), through which the mother delivers sustenance until birth. Once the baby sharks are born, they are not taken care of by the parents in any way. There is usually a litter of 12 to 15 pups; except for the Great Hammerhead which births 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.[11]

In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, in which a female's ovum fuses with a polar body to form azygote without the need for a male. This was the first shark known to do this.[12]