Hammerhead shark

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Hammerhead sharks Fossil range: Middle Miocene to Present[1] PreЄ Є O S D C P T J K Pg N
Scalloped hammerhead, Sphyrna lewini
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes Subclass: Elasmobranchii Order: Carcharhiniformes Family: Sphyrnidae Gill, 1872 Genus: Sphyrna Rafinesque, 1810

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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; some authorities place the winghead shark 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.

Contents [hide] 1 Physical description 2 Taxonomy and evolution 3 Reproduction 4 Species 5 Relationship to humans 6 See also 7 References 8 External links

[edit] Physical description

The nine known species of hammerhead range from 0.9 to 6 m (3.0 to 20 ft) long. All the species have a projection on each side of the head that gives it a resemblance to a flattened hammer. The shark's eyes and nostrils are at the tips of the extensions.

The hammer shape of the head was thought to help sharks find food, aiding in close-quarters maneuverability and allowing the shark to turn sharply without losing stability. However, it was found that the unusual structure of its vertebrae allowed it to make the turns correctly, more than its head. But as the hammer would also shift and provide lift; hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively.^[2] These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer also allows the nostrils to be placed further apart, increasing its ability to detect chemical gradients and localize the source.

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.

Hammerheads are notably one of the few animals that acquire a tan from prolonged exposure to sunlight, a feature shared by pigs and humans. Tanning occurs when a hammerhead is in shallow waters or close to the surface for long periods.^{[citation needed]}