The developing embryos are at first sustained by a yolk sac.
Like other sharks, fertilization is internal, with the male transferring sperm to the female through one of two intromittent organs called claspers. Documentation of hammerhead courtship is rare and has only been caught on film only a scant few times. The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. 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. By distributing the receptors over a wider area, like a larger radio antenna, hammerheads can sweep and scan for prey more effectively. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. From what is known about the winghead shark, the shape of the hammer-head apparently has to do with an evolved sensory function. 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. 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. 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. 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. The bonnethead shark ( Sphyrna tiburo) Cephalofoil 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. Using mitochondrial DNA, a phylogenetic tree of the hammerhead sharks showed the winghead shark as its most basal member. According to DNA studies, the ancestor of the hammerheads probably lived in the Miocene epoch about 20 million years ago. The hammerheads seem closely related to the carcharhinid sharks that evolved during the mid-Tertiary period. Since sharks do not have mineralized bones and rarely fossilize, their teeth alone are commonly found as fossils. The great hammerhead ( Sphyrna mokarran) Taxonomy and Evolution National Geographic explains that hammerheads can be found in warm tropical waters, but during the summer, many species of hammerheads participate in a mass migration to search for cooler waters. In the evening, like other sharks, they become solitary hunters.
They are also known to form schools during the day, sometimes in groups of over 100 or more. Hammerheads have disproportionately small mouths and seem to do a lot of hunting on or near the bottom. Their bellies are white which allows them to blend into the ocean when viewed from the bottom and sneak up on their prey. Their heads have lateral projections which give them a hammer-like shape. Hammerheads range in size from 0.9 to 6.0 m (3.0 to 19.7 ft) in length and weigh from 3 to 580 kg (6.6 to 1,278.7 lb). They are usually light gray in color and have a greenish tint. This family also contains the only known omnivorous shark the bonnethead, which eats aquatic vegetation in addition to fish, mollusks and invertebrates. Though many hammerheads are large and potentially dangerous, very few attacks are recorded for any species, save for the largest the great hammerhead ( Sphyrna mokarran), which has been known to attack humans on occasion. Additionally, large schools are also seen in the waters off southern and eastern Africa.
Some of these schools can be found near Malpelo Island in Colombia, Cocos Island off Costa Rica, and near Molokai in Hawaii. Unlike most sharks, several species of hammerheads usually swim in schools during the day, becoming solitary hunters at night. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Many not necessarily mutually exclusive functions have been proposed for the cephalofoils of these sharks, including sensory reception, manoeuvering, and prey manipulation. Named for their characteristic, laterally flattened heads formed into what is known as a cephalofoil, most hammerheads belong to the genus Sphyrna, while one, the winghead shark, belongs to the genus Eusphyra, of which it is the sole member. Extant Hammerhead sharks, or the Sphyrnidae, are a family of sharks in the order Carcharhiniformes.
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