Overview

Brief Summary

Hammerhead sharks (Sphyrna)

Hammerhead sharks (Sphyrna) belong to the family Sphyrnidae.

Hammerheads occur worldwide at depths of 0-4,050 m in warmer waters along coastlines and continental shelves. They usually swim in schools by day and become solitary hunters at night.

Hammerheads range from 0.9-6 m (3-20 ft) long and weigh from 3-580 kg (6.6-1,300 lb). While several hammerhead species are quite large, these are more slender and streamlined compared to other large sharks, increasing their speed and manoeuverability in the water.[1] They are usually light gray with a greenish tint. Their bellies are white which lets them be close to the bottom of the ocean and blend in to sneak up on prey.[2] Their heads have lateral projections which giving them a hammer-like shape or "cephalofoil". This shape may have evolved to enhance vision and other sensory reception, manoeuvering and prey manipulation. [3] The eyes are mounted on the sides of the hammer head and provide good 360 degree vision in the vertical plane, so the sharks can see above and below at all times.[4] The shape of the head was thought to help the shark find food, aiding in close-quarters manoeuverability and allowing sharp turning movement without losing stability. It was found that the unusual structure of its vertebrae was instrumental in making the turns correctly, more often than the shape of its head, though it would also shift and provide lift. Hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively.[5]

Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They form schools by day, sometimes in groups of over 100. In the evening, they become solitary hunters.

Reproduction occurs once a year. The male usually bites the female violently until she agrees to mate with him.[7] The shark is viviparous; females give birth to live young. Fertilization is internal with the male transferring sperm to the female through one of two intromittent organs (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 similar to a mammalian placenta and called a "yolk sac placenta" or "pseudoplacenta", through which the mother delivers sustenance until birth. Once the baby sharks are born, the parents do not take care of them. There is usually a litter of 12-15 pups, but the great hammerhead has litters of 20-40 pups. The baby sharks huddle together and swim toward warmer water and stay together until they are older and big enough to be on their own.[7]In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, where a female's ovum fuses with a polar body to form a zygote without needing a male. This was the first shark known to do this.[8]

Hammerheads have a varied diet including sharks and other fish, squid, octopus and crustaceans. Stingrays are a favourite. The sharks often swim along the bottom of the ocean, stalking their prey. The head is used as a weapon when hunting down prey. The shark uses its head to pin down stingrays and eats the ray when the ray is weak and in shock.[7] The great hammerhead tends to be more aggressive and can eat other hammerhead sharks and eat their own young.[9]

As do not have mineralized bones and rarely fossilize, only their teeth are commonly found as fossils. The hammerheads seem closely related to the carcharhinid sharks, which have similar teeth. Using mitochondrial DNA, Andrew Martin constructed a phylogenetic tree of hammerhead sharks, which suggests that the first ancestral hammerhead sharks had large hammers.[6]

Three species of hammerhead can be dangerous to humans: the scalloped, great and smooth hammerheads. The great and scalloped hammerhead are listed on the World Conservation Union's (IUCN) 2008 Red List as endangered, while the smalleye hammerhead is listed as vulnerable. This status is due to over-fishing and demand for their fins, an expensive delicacy. Fishermen typically cut off the fins and often toss the rest of the fish back into the sea.[10]

In Native Hawaiian culture, sharks are considered to be gods of the sea, also caled aumakua, protectors of humans, and cleaners of excessive ocean life. Some sharks are believed to be family members who died and have been reincarnated into shark form. Some sharks are considered man-eaters (niuhi). The hammerhead shark or mano kihikihi is not considered a man-eater or niuhi; but is considered to be one of the most respected sharks of the ocean, an aumakua. Many Hawaiian families believe they have an aumakua watching over them and protecting them from the niuhi. The hammerhead is thought to be the birth animal of some children. Hawaiian children who are born with the hammerhead shark as an animal sign are believed to be warriors and are meant to sail the oceans. It is very rare for hammerheads to pass through the waters of Maui, but many Maui natives believe that when the sharks pass by, it is a sign that the gods are watching over the families, and the oceans are clean and balanced.[11]

  • 1) Castro, José I., The Sharks of North America. Oxford University Press (2011), ISBN 978-0-19-539294-4
  • 2) Hammerhead Shark. Sharks-world.com
  • 3) Enhanced visual fields in hammerhead sharks. doi:10.1242/jeb.032615
  • 4) McComb, D. Michelle et al. (2009-11-27). "Hammerhead shark mystery solved". BBC News.
  • 5) Martin, R. Aidan (1993). "If I Had a Hammer". Rodale's Scuba Diving August 1993.
  • 6) Martin, R. Aidan. "Origin and Evolution of the 'Hammer'". www.elasmo-research.org.
  • 7) Hammerhead Shark. Aquatic Community
  • 8) 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): 425–7. doi:10.1098/rsbl.2007.0189. PMC 2390672. PMID 17519185.
  • 9) HAMMERHEAD SHARK – Enchanted Learning Software. Enchantedlearning.com
  • 10) Panamanian officials find half ton of shark fins. Associated Press via Washingtonpost.com (2011-02-25)
  • 11) Sharks Highly respected in Hawaiian Culture. Moolelo.com (2004-09-28).
  • Other references
  • Hessing, S. (2000). Sphyrna tiburo. Animal Diversity Web.
  • Quattro, et al., J. M.; Stoner, D. S.; Driggers, W. B.; Anderson, C. A.; Priede, K. A.; Hoppmann, E. C.; Campbell, N. H.; Duncan, K. M. et al. (December 2005). "Genetic evidence of cryptic speciation within hammerhead sharks (Genus Sphyrna)". Marine biology (Springer Berlin / Heidelberg) 148 (5): 1143–1155. doi:10.1007/s00227-005-0151-x. |displayauthors= suggested (help)
  • Record Hammerhead Pregnant With 55 Pups". Discovery News. Associated Press. July 1, 2006.
  • "Scientist Finds 'Genetically Distinct' Shark". PhysOrg.com.
  • Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Chondrichthyes entry)". Bulletins of American Paleontology 364: 560.
  • Sharks Highly respected in Hawaiian Culture. Moolelo.com (2004-09-28).
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Ecology

Habitat

Depth range based on 1483 specimens in 9 taxa.
Water temperature and chemistry ranges based on 848 samples.

Environmental ranges
  Depth range (m): 0 - 4050
  Temperature range (°C): 2.233 - 28.199
  Nitrate (umol/L): 0.164 - 31.717
  Salinity (PPS): 32.419 - 36.580
  Oxygen (ml/l): 2.346 - 6.470
  Phosphate (umol/l): 0.038 - 1.926
  Silicate (umol/l): 0.380 - 27.833

Graphical representation

Depth range (m): 0 - 4050

Temperature range (°C): 2.233 - 28.199

Nitrate (umol/L): 0.164 - 31.717

Salinity (PPS): 32.419 - 36.580

Oxygen (ml/l): 2.346 - 6.470

Phosphate (umol/l): 0.038 - 1.926

Silicate (umol/l): 0.380 - 27.833
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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Evolution and Systematics

Functional Adaptations

Functional adaptation

Skin reduces drag: shark
 

The skin of sharks reduces drag by having a scales with longitudinal grooves.

   
  While a shark’s coarse shape is famously hydrodynamic, shark skin is anything but smooth. The very small individual scales of shark skin, called dermal denticles ("little skin teeth"), are ribbed with longitudinal grooves which result in water moving more efficiently over their surface than it would were shark scales completely featureless. Over smooth surfaces, fast-moving water begins to break up into turbulent vortices, or eddies, in part because the water flowing at the surface of an object moves slower than water flowing further away from the object. This difference in water speed causes the faster water to get "tripped up" by the adjacent layer of slower water flowing around an object, just as upstream swirls form along riverbanks. The grooves in a shark’s scales simultaneously reduce eddy formation in a surprising number of ways: (1) the grooves reinforce the direction of flow by channeling it, (2) they speed up the slower water at the shark’s surface (as the same volume of water going through a narrower channel increases in speed), reducing the difference in speed of this surface flow and the water just beyond the shark’s surface, (3) conversely, they pull faster water towards the shark’s surface so that it mixes with the slower water, reducing this speed differential, and finally, (4) they divide up the sheet of water flowing over the shark’s surface so that any turbulence created results in smaller, rather than larger, vortices. (Courtesy of The Biomimicry Institute)
  Learn more about this functional adaptation.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
Specimen Records: 604
Specimens with Sequences: 503
Specimens with Barcodes: 393
Species: 12
Species With Barcodes: 11
Public Records: 268
Public Species: 5
Public BINs: 9
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Barcode data

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Wikipedia

Sphyrna

Sphyrna (from the greek word σφυρί "hammer") is a genus of hammerhead sharks with a cosmopolitan distribution in the world's oceans.

Species[edit]

There are currently nine recognized species in this genus:[1][2]

References[edit]

  1. ^ Froese, Rainer, and Daniel Pauly, eds. (2013). Species of Sphyrna in FishBase. June 2013 version.
  2. ^ a b Quattro, J.M., Driggers, W.B. III, Grady, J.M., Ulrich, G.F. & Roberts, M.A. (2013): Sphyrna gilberti sp. nov., a new hammerhead shark (Carcharhiniformes, Sphyrnidae) from the western Atlantic Ocean. Zootaxa, 3702 (2): 159–178.
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