Hammerhead shark

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.

Physical description

The nine known species of hammerhead range from 0.9 to 6 m (3.0 to 19.7 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. 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.

Taxonomy 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 the carcharhinid sharks that evolved during the mid-Tertiary Period. Because the teeth of hammerheads resemble those of some carcharhinids, it has been difficult to determine when hammerheads first appeared. It is probable that the hammerheads evolved during the late Eocene, Oligocene or early Miocene.

Using mitochondrial DNA, Andrew Martin constructed a phylogenetic tree of the hammerhead sharks that 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.

Reproduction

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 transferring sperm 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.

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 a zygote without the need for a male. This was the first shark known to do this.

Species

Genus Sphyrna
Subgenus Sphyrna
Scalloped hammerhead, Sphyrna (Sphyrna) lewini (Griffith & Smith, 1834)
"Cryptic scalloped hammerhead" - Scalloped hammerheads are two separate species, which have not yet been officially reclassified with separate names.
Great hammerhead, Sphyrna (Sphyrna) mokarran (Rüppell, 1837)
Smooth hammerhead, Sphyrna (Sphyrna) zygaena (Linnaeus, 1758)
Whitefin hammerhead, Sphyrna (Sphyrna) couardi Cadenat, 1951
Subgenus Mesozygaena
Scalloped bonnethead, Sphyrna (Mesozygaena) corona Springer, 1940
Winghead shark Sphyrna (Mesozygaena) sp. listed on elasmo-research's list
Subgenus Platysqualus
Scoophead, Sphyrna (Platysqualus) media Springer, 1940
Bonnethead or shovelhead, Sphyrna (Platysqualus) tiburo (Linnaeus, 1758)
Smalleye hammerhead, Sphyrna (Platysqualus) tudes (Valenciennes, 1822)

Announcements in June, 2006 reported the discovery of a possible new species of hammerhead off the shores of South Carolina. The possible new species is referred to simply as a cryptic species until it receives an official designation. This is prolonged, in part, because the discovery is really that the "scalloped hammerhead" is possibly two different species, not that a new species has been sighted, in the normal way. The discovery that scalloped hammerheads are possibly two species is purely a result of genetic testing, not identification of physical differences.

Relationship to humans

Of the nine known species of hammerhead, three can be dangerous to humans: the scalloped, great, and smooth hammerheads.

The great and the scalloped hammerhead 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 over-fishing and demand for their fins, an expensive delicacy. Among others, scientists expressed their concern about the plight of the scalloped hammerhead at the American Association for the Advancement of Science annual meeting in Boston. The young swim mostly in shallow waters along shores all over the world to avoid predators.

Translation

The phrase "Hammerhead shark" occurs as such in the following languages: English, Simple English.

Translation(s) in other languages: Arabic: القرش المطرقة, Catalan: Tauró martell, Czech: Žralok kladivoun, Danish: Hammerhaj, German: Hammerhaie, Navajo: Łóóʼ hashkéhé bitsiiʼ dikʼání, Estonian: Vasarhai, Spanish: Sphyrnidae, French: Sphyrnidae, Indonesian: Hiu martil, Italian: Sphyrnidae, Lithuanian: Kūjarykliai, Hungarian: Pörölycápafélék, Dutch: Hamerhaaien, Japanese: シュモクザメ, Norwegian (Bokmål): Hammerhaier, Norwegian (Nynorsk): Hammarhai, Polish: Młotowate, Portuguese: Tubarão-martelo, Russian: Молотоголовые акулы, Finnish: Vasarahait, Swedish: Hammarhajar, Chinese: 双髻鲨.