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{{Taxobox
{{Taxobox
| color = pink
| color = pink
| name = Hammerhead shark
| name = Hammerhead sharks
| image = Hammerhai.jpg
| image = Hammerhai.jpg
| image_width = 250px
| image_width = 250px
Line 12: Line 12:
| genus_authority = [[Constantine Samuel Rafinesque-Schmaltz|Rafinesque]], 1810
| genus_authority = [[Constantine Samuel Rafinesque-Schmaltz|Rafinesque]], 1810
| subdivision_ranks = Species
| subdivision_ranks = Species
| subdivision =
| subdivision = See text.
''[[Scalloped hammerhead|Sphyrna (Sphyrna) lewini]]''<br />
''[[Great Hammerhead|Sphyrna (Sphyrna) mokarran]]''<br />
''[[Smooth hammerhead|Sphyrna (Sphyrna) zygaena]]''<br />
''[[Whitefin Hammerhead|Sphyrna (Sphyrna) couardi]],''<br />
''[[Scalloped Bonnethead|Sphyrna (Mesozygaena) corona]]''<br />
''[[Scoophead|Sphyrna (Platysqualus) media]]''<br />
''[[Bonnethead|Sphyrna (Platysqualus) tiburo]]''<br />
''[[Smalleye hammerhead|Sphyrna (Platysqualus) tudes]]''
}}
}}
{{Sharksportal}}
{{Sharksportal}}
The '''hammerhead shark''' ([[genus]] ''Sphyrna'') is a member of the family [[Sphyrnidae]]. The only other genus of Sphyrnidae, ''[[Eusphyra]]'', contains only one species, ''E. blochii'', the [[winghead shark]]).
'''Hammerhead sharks''' of the [[genus]] ''Sphyrna'' are members of the family [[Sphyrnidae]]. The only other genus of Sphyrnidae, ''[[Eusphyra]]'', contains only one species, ''[[Eusphyra blochii]]'', the [[winghead shark]].


The nine known species of hammerhead range from 2&ndash;6&nbsp;m long, and all species have projections on both sides of the head that give it a resemblance to a flattened [[hammer]]. The shark's eyes and nostrils are at the tips of the extensions.
The nine known species of hammerhead range from 2&ndash;6&nbsp;m long, and all species have a projection on each side of the head that give it a resemblance to a flattened [[hammer]]. The shark's eyes and nostrils are at the tips of the extensions.


They are aggressive [[predator]]s which eat [[fish]], [[batoidea|rays]], other [[shark]]s, [[cephalopod]]s, and [[crustacean]]s. They are found in warmer waters along coastlines and [[continental shelf|continental shelves]].
They are aggressive [[predator]]s which eat [[fish]], [[batoidea|rays]], other [[shark]]s, [[cephalopod]]s, and [[crustacean]]s. They are found in warmer waters along coastlines and [[continental shelves]].


The hammer shape of the head was once thought to act as a wing, aiding in close-quarters maneuverability and allowing the shark to execute sharp turns without loss of stability. But it was found that the special design of its vertebra allowed it to make the turns correctly, more than its head. But as a wing the hammer would also provide lift; hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have [[electrolocation]] sensory pores called [[ampullae of Lorenzini]]. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively<ref name="Hammer">{{cite web | author=R. Aidan Martin| title=If I Had a Hammer | work=Rodale's Scuba Diving August 1993| url=http://www.elasmo-research.org/education/topics/d_functions_of_hammer.htm | accessmonthday = March| accessyear=2006}}</ref>. These sharks have been able to detect an electronic signal of half a billionth of a volt. The hammer shaped head also gives these sharks larger nasal tract, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks.
The hammer shape of the head was once thought to act as a wing, aiding in close-quarters maneuverability and allowing the shark to execute sharp turns without loss of stability. But it was found that the special design of its vertebra allowed it to make the turns correctly, more than its head. But as a wing the hammer would also provide lift; hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have [[electrolocation]] sensory pores called [[ampullae of Lorenzini]]. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively<ref name="Hammer">{{cite web | author=R. Aidan Martin| title=If I Had a Hammer | work=Rodale's Scuba Diving August 1993| url=http://www.elasmo-research.org/education/topics/d_functions_of_hammer.htm | accessmonthday = March| accessyear=2006}}</ref>. These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer shaped head also gives these sharks larger nasal tracts, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks.


Hammerheads have proportionately 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 solo hunters.
Hammerheads have proportionately 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 solo hunters.


==Reproduction==
==Reproduction==
Reproduction in the hammerhead shark occurs once a year and each litter contains 12&ndash;15 pups. Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization which creates a safe environment for the [[sperm]] to unite with the [[ova|egg]]. The embryo develops within the female inside a [[placenta]] and is fed through an [[umbilical cord]], much like in mammals. The gestation period is 10&ndash;12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves. A world-record 1,280 pound pregnant female hammerhead shark was caught off [[Boca Grande, Florida]] on May 23, 2006. The shark was carrying 55 pups, which suggests scientists had previously underestimated number of babies per gestation.<ref>[http://www.wftv.com/news/9451628/detail.html WFTV News]</ref>
Reproduction in the hammerhead shark occurs once a year and each litter contains 12 to 15 pups. Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization which creates a safe environment for the [[sperm]] to unite with the [[ova|egg]]. The embryo develops within the female inside a [[placenta]] and is fed through an [[umbilical cord]], similar to mammals. The gestation period is 10 to 12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves. A world-record 1,280 pound pregnant female hammerhead shark was caught off [[Boca Grande, Florida]] on May 23, 2006. The shark was carrying 55 pups, which suggests scientists had previously underestimated the number of pups per gestation.<ref>[http://www.wftv.com/news/9451628/detail.html WFTV News]</ref>


==Species==
==Species==
Line 41: Line 33:
*'''Genus ''Sphyrna'''''
*'''Genus ''Sphyrna'''''
**Subgenus ''Sphyrna''
**Subgenus ''Sphyrna''
***[[Scalloped hammerhead]], ''S. (S.) lewini''
***[[Scalloped hammerhead]], ''[[Sphyrna lewini|Sphyrna (Sphyrna) lewini]]'' <small>(Griffith & Smith, 1834)</small>
****"Cryptic scalloped hammerhead" -- Scalloped hammerheads turn out to be divided into two separate species, which have not been officially reclassified with separate names.
****"Cryptic scalloped hammerhead" -- Scalloped hammerheads turn out to be divided into two separate species, which have not been officially reclassified with separate names.
***[[Great hammerhead]], ''S. (S.) mokarran''
***[[Great hammerhead]], ''[[Sphyrna mokarran|Sphyrna (Sphyrna) mokarran]]'' <small>(Rüppell, 1837)</small>
***[[Smooth hammerhead]], ''S. (S.) zygaena''
***[[Smooth hammerhead]], ''[[Sphyrna zygaena|Sphyrna (Sphyrna) zygaena]]'' <small>(Linnaeus, 1758)</small>
***[[Whitefin hammerhead]], ''S. (S.) couardi''
***[[Whitefin hammerhead]], ''[[Sphyrna couardi|Sphyrna (Sphyrna) couardi]]'' <small>Cadenat, 1951</small>
**Subgenus ''Mesozygaena''
**Subgenus ''Mesozygaena''
***[[Scalloped bonnethead]], ''S. (M.) corona''
***[[Scalloped bonnethead]], ''[[Sphyrna corona|Sphyrna (Mesozygaena) corona]]'' <small>Springer, 1940</small>
**Subgenus ''Platysqualus''
**Subgenus ''Platysqualus''
***[[Scoophead]], ''S. (P.) media''
***[[Scoophead]], ''[[Sphyrna media|Sphyrna (Platysqualus) media]]'' <small>Springer, 1940</small>
***[[Bonnethead]] or Shovelhead, ''S. (P.) tiburo''
***[[Bonnethead]] or [[shovelhead]], ''[[Sphyrna tiburo|Sphyrna (Platysqualus) tiburo]]'' <small>(Linnaeus, 1758)</small>
***[[Smalleye hammerhead]] or golden hammerhead, ''S. (P.) tudes''
***[[Smalleye hammerhead]], ''[[Sphyrna tudes|Sphyrna (Platysqualus) tudes]]'' <small>(Valenciennes, 1822)</small>


Announcements in June, 2006 reported the discovery of a possible new species of hammerhead off the shores of [[South Carolina]]. The 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 two different species, not that a new species has been sighted, in the normal way. The discovery that scalloped hammerheads are two species is purely a result of genetic testing, not identification of physical differences. <ref name="New">{{cite web | title=Scientist Finds 'Genetically Distinct' Shark | work=www.physorg.com | url=http://www.physorg.com/news68994294.html |accessmonthday = June| accessyear=2006 }}</ref>
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. <ref name="New">{{cite web | title=Scientist Finds 'Genetically Distinct' Shark | work=www.physorg.com | url=http://www.physorg.com/news68994294.html |accessmonthday = June| accessyear=2006 }}</ref>


==Evolution==
==Evolution==
Since sharks do not have mineralized bones and rarely [[fossil]]ize, it is their teeth alone that are commonly found as fossils. The hammerheads seem closely related to the [[carcharhinus|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]].
Since sharks do not have mineralized bones and rarely [[fossil]]ize, it is their teeth alone that are commonly found as fossils. The hammerheads seem closely related to the [[carcharhinus|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]].


Geneticist Andrew Martin used [[DNA]] to study all hammer head species and he concluded that the first hammer appeared on the [[winghead shark]], which has the largest hammer, and the rest of the hammerhead sharks evolved one at a time from the original winghead shark each with a smaller hammer <ref name="Mutation">{{cite web | author=R. Aidan Martin| title=Origin and Evolution of the 'Hammer' | work=www.elasmo-research.org | url=http://www.elasmo-research.org/education/topics/d_hh_origin.htm |accessmonthday=January | accessyear=2005 }}</ref>.
Geneticist Andrew Martin used [[DNA]] to study all hammerhead species and he concluded that the first hammer appeared on the [[winghead shark]], which has the largest hammer, and the rest of the hammerhead sharks evolved one at a time from the original winghead shark each with a smaller hammer <ref name="Mutation">{{cite web | author=R. Aidan Martin| title=Origin and Evolution of the 'Hammer' | work=www.elasmo-research.org | url=http://www.elasmo-research.org/education/topics/d_hh_origin.htm |accessmonthday=January | accessyear=2005 }}</ref>.
==References==
==References==
<div class="references-small">
<div class="references-small">

Revision as of 17:18, 7 September 2006

Hammerhead sharks
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Chondrichthyes
Order:
Carcharhiniformes
Family:
Sphyrnidae
Genus:
Sphyrna

Species

See text.

Template:Sharksportal Hammerhead sharks of the genus Sphyrna are members of the family Sphyrnidae. The only other genus of Sphyrnidae, Eusphyra, contains only one species, Eusphyra blochii, the winghead shark.

The nine known species of hammerhead range from 2–6 m long, and all species have a projection on each side of the head that give it a resemblance to a flattened hammer. The shark's eyes and nostrils are at the tips of the extensions.

They are aggressive predators which eat fish, rays, other sharks, cephalopods, and crustaceans. They are found in warmer waters along coastlines and continental shelves.

The hammer shape of the head was once thought to act as a wing, aiding in close-quarters maneuverability and allowing the shark to execute sharp turns without loss of stability. But it was found that the special design of its vertebra allowed it to make the turns correctly, more than its head. But as a wing the hammer would also provide lift; hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have electrolocation sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively[1]. These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer shaped head also gives these sharks larger nasal tracts, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks.

Hammerheads have proportionately 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 solo hunters.

Reproduction

Reproduction in the hammerhead shark occurs once a year and each litter contains 12 to 15 pups. Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization which creates a safe environment for the sperm to unite with the egg. The embryo develops within the female inside a placenta and is fed through an umbilical cord, similar to mammals. The gestation period is 10 to 12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves. A world-record 1,280 pound pregnant female hammerhead shark was caught off Boca Grande, Florida on May 23, 2006. The shark was carrying 55 pups, which suggests scientists had previously underestimated the number of pups per gestation.[2]

Species

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

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. [3]

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.

Geneticist Andrew Martin used DNA to study all hammerhead species and he concluded that the first hammer appeared on the winghead shark, which has the largest hammer, and the rest of the hammerhead sharks evolved one at a time from the original winghead shark each with a smaller hammer [4].

References

  1. ^ R. Aidan Martin. "If I Had a Hammer". Rodale's Scuba Diving August 1993. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. ^ WFTV News
  3. ^ "Scientist Finds 'Genetically Distinct' Shark". www.physorg.com. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  4. ^ R. Aidan Martin. "Origin and Evolution of the 'Hammer'". www.elasmo-research.org. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
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