Hammerhead sharks include about 10 species in the family Sphyrnida. These species include the largest hammerhead - the great hammerhead, and the winghead shark, scalloped bonnethead, whitefin hammerhead, scalloped hammerhead (mallethead), scoophead, bonnethead, smalleye hammerhead, smooth hammerhead and recently discovered Carolina hammerhead. Hammerhead sharks are thought to have evolved about 20 million years ago.
How Is the "Hammer" Used?:
This family of sharks is easily recognizable from other sharks by to their hammer-shaped heads. The head is called a cephalofoil. There are many theories as to the function of the cephalofoil. It may be used for better detection of prey by spreading out ampullae of Lorenzini, sensory organs that detect the electrical fields of other marine creatures. It may also provide "lift" while the shark is swimming, allow for better vision through the shark's wide-set eyes, and may be used as a tool to help the sharks pin their prey to the seafloor. You can read more about hammerhead shark head shape here and here.
Hammerhead sharks range in size from about 3 feet (scalloped bonnethead/mallethead) to 20 feet (great hammerhead). Their heads differ in shape depending upon the species - winghead sharks have a hammer with very narrow blades, while the head of a bonnethead shark is shaped more like a shovel. The eyes of a hammerhead are located at each side of their "hammer."
Hammerheads have relatively large first dorsal fins and small second dorsal fins. They have gray, brown or greenish backs and light undersides. They have 5 gill slits.
- Kingdom: Animalia
- Phylum: Chordata
- Subphylum: Gnathostomata
- Superclass: Pisces
- Class: Elasmobranchii
- Subclass: Neoselachii
- Infraclass: Selachii
- Superorder: Galeomorphi
- Order: Carcharhiniformes
- Family: Sphyrnidae
Habitat and Distribution:
Hammerhead sharks are found around the world in temperate and tropical waters, on or near continental shelves and seamounts. They may be found in large schools.
Hammerhead sharks eat fish, smaller sharks, rays, cephalopods and invertebrates. Hammerheads have a better visual range than other sharks due to their wide-set eyes. Their unique head allows for a wide-spread electro-reception system as well, which may give them the ability to detect their prey by electrical fields and to triangulate their prey by scent.
Hammerhead sharks are viviparous (give birth to live young), and the young are nourished via a yolk sac placenta. Female hammerhead sharks apparently can also give birth without mating with a male, as demonstrated in a study published in 2007 regarding a female shark in an aquarium, who gave birth without ever having contact with a male shark. So, if females do not have access to males they can mate with, they may switch to this form of asexual reproduction. While interesting, this strategy may lead to a lower genetic diversity in hammerhead populations overall, which is a concern for these dwindling populations.
Hammerhead sharks are generally not dangerous to humans, but large hammerheads, especially, should be avoided because of their size.
The International Shark Attack File lists hammerhead sharks as #8 on its list of species responsible for shark attacks from the years 1580 to 2011. During this time, hammerheads were responsible for 17 non-fatal, unprovoked attacks and 20 fatal, provoked attacks.
References and Further Information:
- Bester, Cathleen.Great Hammerhead Shark. Florida Museum of Natural History. Accessed June 30, 2012.
- Compagno, L., Dando, M. and S. Fowler. 2005. Sharks of the World. Princeton University Press.
- Elasmodiver. Shark Senses. Accessed June 30, 2012.
- FishBase. 9 Species in Family Sphyrnidae. Accessed June 30, 2012.
- Florida Museum of Natural History. 2012. ISAF Statistics on Attacking Species of Shark. Accessed June 30, 2012.
- Krupa, D. 2002. Why the Hammerhead Shark's Head is In the Shape It's In. American Physiological Society. Accessed June 30, 2012.
- National Geographic. Hammerhead Shark. Accessed June 30, 2012.
- ScienceDaily. 2010. Hammerhead Shark Study Shows Cascade of Evolution Affected Size, Head Shape. Accessed June 30, 2012.
- ScienceDaily. 2007. No Sex Please, We're Female Sharks. Accessed June 30, 2012.
- WoRMS. 2012. Sphyrnidae. In: Nicolas Bailly (2012). FishBase. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=105694 on June 30, 2012.