- Hammerhead shark
name = Hammerhead shark
fossil_range = Middle
Mioceneto Present [cite journal
last = Sepkoski
first = Jack
title = A compendium of fossil marine animal genera (Chondrichthyes entry)
journal = Bulletins of American Paleontology
volume = 364
pages = p.560
date = 2002
url = http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=575&rank=class
accessdate = 01/09/08 ]
status = EN | status_system = IUCN3.1
image_width = 250px
phylum = Chordata
familia = Sphyrnidae
genus = "Sphyrna"
genus_authority = Rafinesque, 1810
subdivision_ranks = Species
subdivision = See text.
Hammerhead sharks of the
genus"Sphyrna" are members of the family Sphyrnidiae. The only other genus of Sphyrnidae, "Eusphyra", contains only one species, " Eusphyra blochii", the winghead shark. The hammerheads are aggressive predators, eating fish, rays, cephalopods, and crustaceans. They are found in warmer waters along coastlines and continental shelves.
The nine known
species of hammerhead range from 0.9 to 6 m long (3 to 20 feet). 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 once thought to help sharks get food, aiding in close-quarters maneuverability and allowing the shark to turn sharply without losing stability. However, it was found that the special design of its
vertebrae allowed it to make the turns correctly, more than its head. But as a wingthe hammer would also 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.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] 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 nasals, increasing the chance of finding a particle in the water by at least 10 times as against the ability of other 'classical' sharks.
Wider spacing between sensory organs better enables an organism to detect
gradients and therefore the location of a gradient source such as food or a mate. The peculiar head of this shark can be thought of as analogous to the antennae of an insect.
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. hammer heads have a hammer shaped head.
Reproduction in the hammerhead shark occurs once a year with each litter containing 20 to 40 pups. Hammerhead shark mating courtship is a 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
placentaand 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. Young hammerheads are often born headfirst, with the tip of their hammerhead folded backward to make them more streamlined for birth. A world-record 1,280 pound (580 kg) pregnant female was caught off Boca Grande, Floridaon May 23, 2006. The shark was carrying 55 pups, which suggests scientists had previously underestimated the number of pups per gestation.
In May 2007 scientists discovered that hammerhead sharks can reproduce asexually through a rare method known as
parthenogenesis, (a direct development without the need of a sperm, similar to how social insects can reproduce). At first the announcement was considered skeptically, because a female shark can store sperm inside her for months, even years, but it was confirmed through DNAtesting that the pup lacked any paternal DNA. This is the first documented case of any shark doing this. [ cite journal | last=Chapman | first=DD | coauthors=Shivji, MS; Louis, E; Sommer, J; Fletcher, H; Prodöhl, PA | title=Virgin birth in a hammerhead shark | volume=3 | issue=4 | date=2007-08-22 | journal= Biology Letters]
Of the nine known species of hammerhead, three can be dangerous to humans: the scalloped, great, and smooth hammerheads.
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
Scalloped bonnethead, "Sphyrna (Mesozygaena) corona" Springer, 1940
Winghead sharkSphyrna (Mesozygaena) sp. listed on elasmo-research's list
Scoophead, "Sphyrna (Platysqualus) media" Springer, 1940
Bonnetheador 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 speciesuntil 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.cite web | title=Scientist Finds 'Genetically Distinct' Shark | work= PhysOrg.com| url=http://www.physorg.com/news68994294.html |accessmonthday = June| accessyear=2006 ]
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- TertiaryPeriod. 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, Oligoceneor early Miocene.
Geneticist Andrew Martin used
DNAto study all of the 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.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 ]
The scalloped hammerhead is listed on the World Conservation Union's (IUCN) 2008 Red List as "endangered”. The status given to the fish is as a result of over-fishing and demand for its fins, an expensive delicacy. Scientists expressed their concern about the plight of this unique creature 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.
The great hammerhead is currently listed as endangered.
* [http://www.diveaday.tv/content/view/60/37/ Video of divers with hammerhead sharks in the
* [http://animaldiversity.ummz.umich.edu/site/accounts/classification/Sphyrna.html#Sphyrna Animal Diversity Web Genus "Sphyrna"] with species sub-pages
* [http://www.isidore-of-seville.com/hammerhead/ Hammerhead Shark!] web directory, with pictures
* [http://www.sharkdiving.us/hammerheadshark.html Hammerhead Shark Facts] fact sheet, general info, shark diving info
* [http://www.sharkinfo.ch/SI2_00e/slewini.html Scaloped Hammerhead Shark Fact Sheet] Scalloped Hammerhead Shark Fact Sheet
* [http://www.sharkinformation.org/sharkprofiles/hammerhead-shark/ Hammerhead Photos and Profile]
* [http://jeb.biologists.org/cgi/content/full/205/23/3609 "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.
* [http://marinebio.org/species.asp?id=87 MarineBio: Great hammerhead shark, "Sphyrna mokarran"]
* [http://news.bbc.co.uk/2/hi/science/nature/5065510.stm BBC News announcement of new Hammerhead species]
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Look at other dictionaries:
hammerhead shark — paprastasis kūjaryklis statusas T sritis zoologija | vardynas taksono rangas rūšis atitikmenys: lot. Sphyrna zygaena angl. balance fish; common hammerhead; hammerhead shark; round headed hammerhead shark; smooth hammerhead rus. молот рыба;… … Žuvų pavadinimų žodynas
hammerhead shark — Any of the swift, powerful sharks in the family Sphyrnidae, having a broad, flattened, hammer or spade shaped head, with the eyes and nostrils at the ends of the sidewise projections. Widely distributed in all oceans, in warm and temperate waters … Universalium
hammerhead shark — noun medium sized live bearing shark with eyes at either end of a flattened hammer shaped head; worldwide in warm waters; can be dangerous • Syn: ↑hammerhead • Hypernyms: ↑shark • Hyponyms: ↑smooth hammerhead, ↑Sphyrna zygaena, ↑ … Useful english dictionary
hammerhead shark — noun a) Any of various sharks of the genus Sphyrna having the eyes set on projections from the sides of the head, which gives it a hammer shape. b) A winghead shark … Wiktionary
hammerhead shark — Manō kihikihi … English-Hawaiian dictionary
round-headed hammerhead shark — paprastasis kūjaryklis statusas T sritis zoologija | vardynas taksono rangas rūšis atitikmenys: lot. Sphyrna zygaena angl. balance fish; common hammerhead; hammerhead shark; round headed hammerhead shark; smooth hammerhead rus. молот рыба;… … Žuvų pavadinimų žodynas
bronze hammerhead shark — bronzinis kūjaryklis statusas T sritis zoologija | vardynas taksono rangas rūšis atitikmenys: lot. Sphyrna lewini angl. bronze hammerhead shark; scalloped hammerhead rus. бронзовая молот рыба; южная молот рыба ryšiai: platesnis terminas –… … Žuvų pavadinimų žodynas
Hammerhead — may refer to:* The head of a hammer.In zoology: * Hammerhead sharks (genus Sphyrna ), of which there are nine known species. * Hammerkop ( Scopus umbretta ), a bird of order Ciconiiformes, may also be known as a hammerheadIn music: * Black… … Wikipedia
HammerHead — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Hammerhead peut se traduire par « tête de marteau » et désigne le requin marteau en anglais. Le terme peut aussi se référer à : Sommaire 1… … Wikipédia en Français
Hammerhead (Dino-riders) — Hammerhead is a Rulon character from the TV cartoon Dino riders. He looks like a hammerhead shark and leads a race called the shark people. He is the most competent of all Rulon commanders. Normally he follows his emperor Krulos, but he helps him … Wikipedia