hammerhead shark

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.

Physical description

The nine known species range from 0.9 to 6 m (3.0 to 20 ft) long. All the species have a projection of their face on all sides of the head that gives it a resemblance to a flattened hammer.

The hammer-like 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 often than its head. The hammer would also shift and provide lift. It was determined recently that the development of vision led to the hammer-like shape. The positioning of the eyes allow for 360 degree vision of the shark. ^[2] 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.^[3] 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]}

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.^[4]

Hammerhead sharks are extremely aggressive.

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.^[5]

Species

School of scalloped hammerheads, Wolf Island, Galapagos Islands

• 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.^[6]

Relationship to humans

A hammerhead shark at Atlantis Paradise Island.

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.

See also

For a topical guide to this subject, see Outline of sharks.

• List of prehistoric cartilaginous fish

References

1. ^ Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Chondrichthyes entry)". Bulletins of American Paleontology 364: 560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=575&rank=class. Retrieved 01/09/08. 
2. ^ http://news.bbc.co.uk/earth/hi/earth_news/newsid_8376000/8376740.stm
3. ^ R. Aidan Martin. "If I Had a Hammer". Rodale's Scuba Diving August 1993. http://www.elasmo-research.org/education/topics/d_functions_of_hammer.htm. Retrieved March 2006. 
4. ^ R. Aidan Martin. "Origin and Evolution of the 'Hammer'". www.elasmo-research.org. http://www.elasmo-research.org/education/topics/d_hh_origin.htm. Retrieved January 2005. 
5. ^ Chapman, DD; Shivji, MS; Louis, E; Sommer, J; Fletcher, H; Prodöhl, PA (2007-08-22). "Virgin birth in a hammerhead shark". Biology Letters 3 (4). 
6. ^ "Scientist Finds 'Genetically Distinct' Shark". PhysOrg.com. http://www.physorg.com/news68994294.html. Retrieved June 2006. 

External links

• Animal Diversity Web Genus Sphyrna with species sub-pages
• Hammerhead Shark! web directory, with pictures
• Hammerhead Shark Facts fact sheet, general info, shark diving info
• Scaloped Hammerhead Shark Fact Sheet Scalloped Hammerhead Shark Fact Sheet
• Hammerhead Photos and Profile
• "Electroreception in juvenile scalloped hammerhead and sandbar sharks" by Stephen M. Kajiura and Kim N. Holland, The Journal of Experimental Biology (2002). Attempts to explain the "hammer" shape.
• MarineBio: Great hammerhead shark, Sphyrna mokarran
• BBC News announcement of new Hammerhead species
• BBC Earth News announcement on why Hammerhead species have unusual head shape