Hammerhead Sharks
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Sphyrna (Sphyrna) lewini |
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).
The eight species of hammerhead range from 2–6 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.
It is an aggressive predator that eats fish, rays and other sharks, cephalopods, and crustaceans. It is found in warmer waters along coastlines and continental shelves.
The shape of the head seems to act as a wing, aiding in close-quarters maneuverability. It also seems to help in electrolocation by separating the receptors, allowing signals to be received as though in stereo. These sharks have been able to detect an electronic signal of half a billionth of a volt. Hammerheads have proportionately small mouths and seem to do a lot of bottom-hunting. They are also known to school, sometimes in groups of over a hundred.
Reproduction in the hammerhead shark occurs once a year and each litter contains 12-15 pups. The courtship involved in mating is very violent affair in hammerhead sharks. The male will bite the female until she gives in which is when mating occurs. 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, much like in mammals. The gestation period is 10-12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves.
Of the eight species of hammerhead, three can be dangerous to humans—the scalloped, great and smooth hammerheads.
- Genus Sphyrna
- Subgenus Sphyrna
- Scalloped Hammerhead, S. (S.) lewini
- Great Hammerhead, S. (S.) mokarran
- Smooth Hammerhead, S. (S.) zygaena
- Whitefin Hammerhead, S. (S.) couardi
- Subgenus Mesozygaena
- Scalloped Bonnethead, S. (M.) corona
- Subgenus Platysqualus
- Scoophead, S. (P.) media
- Bonnethead or Shovelhead, S. (P.) tiburo
- Golden Hammerhead (previously known as Smalleye Hammerhead, S. (P.) tudes
- Subgenus Sphyrna
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, but the teeth of hammerheads resemble those of some Carcharhinids, making it difficult to be certain when the hammerheads first appeared. It is probable that the hammerheads evolved during the late Eocene, Oligocene or early Miocene.
Bonnethead.jpg
External Links
- Animal Diversity Web Genus Sphyrna (http://animaldiversity.ummz.umich.edu/site/accounts/classification/Sphyrna.html#Sphyrna) with species sub-pages
- Hammerhead Shark! (http://www.isidore-of-seville.com/hammerhead/) web directory, with pictures
- Fishbase (http://www.fishbase.org/NomenClature/ScientificNameSearchList.cfm?Crit1_FieldName=SYNONYMS.SynGenus&Crit1_FieldType=CHAR&Crit1_Operator=EQUAL&Crit1_Value=Sphyrna&Crit2_FieldName=SYNONYMS.SynSpecies&Crit2_FieldType=CHAR&Crit2_Operator=contains&Crit2_Value=&group=summary&backstep=-2)
- "Electroreception in juvenile scalloped hammerhead and sandbar sharks" (http://jeb.biologists.org/cgi/content/full/205/23/3609) 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 (http://marinebio.org/species.asp?id=87)
| Shark articles | |
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| Angel | Basking | Blacktip Reef | Blue | Bull | Carpet | Cat | Cookiecutter | Freshwater | Frilled | Goblin | Gray Reef | Grey Nurse | Great White | Hammerhead | Mako | Megamouth | Nurse | Oceanic Whitetip | Porbeagle | Requiem | River | Sand | Sandbar | Saw | Silky | Sleeper | Smooth dogfish | Thresher | Tiger | Whale (shark) | Whitetip reef | Wobbegong | Zebra / Leopard | |
| Extinct shark species | |
| Megalodon | Cladoselache | Squalicorax
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