Featured Species: Smooth Hammerhead (Sphyrna zygaena)

This week’s featured species is one of the larger species of hammerhead sharks. The Smooth Hammerhead (Sphyrna zygaena) is a large bodied hammerhead that grows up to 13 feet (4 m) (Ebert, Fowler, & Dando, 2015). They have a tall first dorsal fin with a low second dorsal and pectoral fins. The smooth hammerhead can vary in coloration from olive-gray to dark gray dorsally with a white underbelly and dusky pectoral tips. They are named for their smooth cephalofoil, which is broad and narrow bladed with little-to-no median indentation (Ebert, Fowler, & Dando, 2015).

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Brunetti, J. (Photographer). (2018). Big mama smooth hammerhead ( 11 ft) [Digital Image]. Retrieved from https://www.facebook.com/cabosharkdive/

There are 10 species of hammerhead sharks known to science today:

Common Name

Scientific Name

IUCN Status

Winghead Shark Eusphyra blochii Endangered
Great Hammerhead Sphyrna mokarran Endangered
Whitefin Hammerhead Sphyrna couardi Endangered
Scalloped Hammerhead Sphyrna lewini Endangered
Smalleye Hammerhead Sphyrna tudes Vulnerable
Smooth Hammerhead Sphyrna zygaena Vulnerable
Scalloped Bonnethead Shark Sphyrna corona Near Threatened
Bonnethead Shark Sphyrna tiburo Least Concern
Scoophead Shark Sphyrna media Data Deficient
Carolina Hammerhead Sphyrna gilberti Yet To Be Assessed

 

Each species – with the exception of the cryptic species the Carolina Hammerhead (Sphyrna gilberti) – is identifiable by their unique head shape (Quattro, Driggers III, Grady, Ulrich, & Roberts, 2013). The smooth hammerhead is known for its relatively smooth cephalofoil compared to other species of hammerhead which typically have a central indentation. The smooth hammerhead has little-to-no median indentation.

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Drawings 1 to 9: Hammerhead sharks (fam. Sphyrnidae) are easily recognizable by the typical highly depressed, wide head with eyes on the outer side of the two lobes. The individual species, whose common denominator is this unusual head, can be distinguished by differences in horizontal profile, which can be almost straight or markedly curved. (1) Eusphyra blochii (2) Sphyrna tudes (3) Sphyrna couardi (4) Sphyrna corona (5) Sphyrna tiburo (6) Sphyrna mokarran (7) Sphyrna lewini* (8) Sphyrna zygaena (9) Sphyrna media (Mojetta, 1997). *Since publication Sphyrna gilberti has also been discovered as a cryptic species, and is only distinguishable from S. lewini by number of precaudual vertebrae (Quattro, Driggers III, Grady, Ulrich, & Roberts, 2013).

 

The smooth hammerhead is found worldwide in warm temperate and tropical waters. They are also known to visit freshwater in the Indian River in Florida, as well as the Rio de la Plata estuary in Uruguay (Doño Melleras, 2008). However, the extent of their distribution in tropical waters may be incomplete due to confusion with two other large hammerhead species which share the same habitat: the scalloped hammerhead and the great hammerhead (Tricas et al., 1997; Casper et al., 2009). These three hammerheads are relatively close in size and have many overlapping home ranges. The scalloped hammerhead is a more abundant species than the smooth hammerhead, which may cause confusion in identifications between species (Casper et al., 2009).

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Left: Scalloped Hammerhead Geographic Range Center: Smooth Hammerhead Geographic Range Right: Great Hammerhead Geographic Range [Digital Images]. Retrieved from https://www.iucnredlist.org/

The smooth hammerhead is a specialized predator, preferring a diet composed of stingrays and skates, small sharks, and schooling bony fishes (Tricase et al., 1997; Casper et al., 2009). Within their jaw, they have 13 to 15 triangular teeth on each side of the upper jaw. These teeth are smooth-edged with a slight lateral hook. Their lower jaw holds 12 to 14 teeth on each side of the jaw. These teeth are lightly serrated and have a more narrow cusp than the teeth in the upper jaw (Casper et al., 2009).

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Left: Robertson, R.D. (Photographer). (2016). Sphyrna zygaena upper teeth [Digital Image]. Retrieved from https://commons.wikimedia.org/ Right: Robertson, R.D. (Photographer). (2016). Sphyrna zygaena lower teeth [Digital Image]. Retrieved from https://commons.wikimedia.org/

Like other hammerheads, the smooth hammerhead’s cephalofoil is extremely sensitive. Smooth hammerheads utilize the electrosensitive cells in their cephalofoil to to actively seek out prey and navigate the oceans. The ampullae of Lorenzini were first described in 1678 by Italian physician and ichthyologist Stefano Lorenzini. These small gel-filled chambers are complete with sensory cells and a tube-like canal from the surface into the skin. These sophisticated sensory organs enable sharks to detect low frequency electrical fields created by living organisms. Sharks are able to detect electrical fields as low as one billionth of a volt! To put that into perspective, that is the equivalent of sensing a few small batteries spread thousands of miles apart (Skomal, 2016)! Despite this incredible ability, sharks use this electrosense at close distances, typically within just a few feet to locate prey (Skomal, 2016; Parker, 2008). Actively hunting species, such as the bull (Carcharhinus leucas) or hammerhead sharks, can have 1,500 or more ampullae located in their snouts and across their heads. While sluggish bottom dwelling species that do not actively hunt may only have a few hundred ampullae (Parker, 2008).

 

Florida Shark Diving (Videographer). (2014). Rare Smooth Hammerhead in Florida [Video Clip]. Retrieved from https://www.youtube.com/

 

Smooth hammerheads are known to migrate to cooler latitudes in the summer months and return to warmer tropical waters in the winter months (Tricas et al., 1997). By using their electrosenses, sharks are able to detect the Earth’s weak magnetic field to orientate themselves and navigate migrations (Skomal, 2016). These magnetic fields bend and dip across the ocean floor. The bends and dips are influenced by the type of rock beneath the Earth’s surface (Parker, 2008). The magnetic field generates minuscule electrical currents, which the smooth hammerhead can then detect through their electrosense. They can then deduce the size and direction of the magnetic field and use this to navigate.

World_Magnetic_Field_2015.pdf
NOAA/NGDC/CIRES (Creators). (2014). Map of intensity of magnetic Earth field in 2015 [Digital Image]. Retrieved from https://wikipedia.org/

The smooth hammerhead is caught in a number of fisheries including longlines, handlines, gillnets, seines, and pelagic and bottom trawls (Bonfil, 1994; Maguire, 2006). Unfortunately, their fins are highly prized on the Far East fin trade. When they are caught, they are often finned and their bodies thrown back to the sea where they are unable to pass oxygen over their gills and eventually drown (Casper et al., 2009). In Hong Kong, the smooth hammerhead and scalloped hammerhead represent 4 – 5% of the fins at auction (Clarke, Magnussen, Abercrombie, McAllister, & Shivji, 2006). Luckily, many states now have shark finning bans including the United States, Canada, Australia, Brazil, the EU, and other regional fisheries organizations (Casper, et al., 2009). While enforcing these types of fisheries is difficult, the move to ban these practices will hopefully decrease or even prevent the capture of oceanic species specifically for their fins. All hammerheads in family Sphyrnidae are listed as an Annex I Highly Migratory Species of the UN Convention on the Law of the Sea. This convention urges states to cooperate over the management of these species; however, to date there is no such management plan in place. To conserve the smooth hammerhead, successful management plans and fishing regulations are needed throughout its range, as well as constant monitoring of population size and structure (Casper et al., 2009).

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U.S. Fish and Wildlife Service Headquarters (Photographer). (2019). Hammerhead Fins [Digital Image]. Retrieved from https://www.flickr.com/

I’m delighted to announce a partnership with the wonderfully talented Julius Csotonyi! Julius creates stunning shark coloring sheets that are fun and educational for all ages. From time to time you’ll see Julius’ work featured right here and on my other social media platforms! You can find more coloring sheets in the store! You are welcomed to download these beautiful sheets and enjoy them with family and friends.

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Shark Stats

Authority: Linnaeus, 1758

Family: Sphyrna, 10 species

Length: Up to 13 feet (4.0 m)

Weight: Up to 180 lbs (81.6 kg)

Habitat: Continental and insular shelves, close inshore to well offshore

Depth: Surface to 656 feet (200 m)

Reproduction: Ovoviviparity

Gestation: 10 -11 months

Litter Range:  29 – 50 pups

Home Range: Worldwide in warm temperate and tropical seas

Diet: Schooling bony fishes, stingrays, small sharks, skates, large crustaceans

IUCN Status: Vulnerable

(Stevens, 1984; Casper et al., 2009; Ebert, Fowler, & Dando, 2015)

The new Ocean For Sharks Shop is open! There’s handmade ocean inspired plush animals, canvas paintings, and of course my children’s book, Winifred the Wondrous Whale Shark, available in print and PDF. Be sure to stop by. Proceeds benefit shark research and conservation with a donation to Project AWARE!

Thanks so much for checking out the wild Smooth Hammerhead! If you missed last week’s featured species, please be sure to check out the elusive Longfin Mako! If there is a species of elasmobranch you’d love to know more about, leave me a comment or send me a message! I would love to do a feature on your favorite species. Also connect with me on Instagram and Facebook for even more elasmo fun!

Conservation legislation needs public support in order to become law and help protect the environment and wildlife. Tell your representatives that you care about environmental and wildlife conservation. It only takes a moment to make a change that will last a lifetime. Until next time finactics!

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Featured Image Source

Bove, B. (Photographer). (2019)  Smooth hammerhead from a video shot while free diving with the sharks off Jupiter, Fl. [Digital Image]. Retrieved from https://www.facebook.com/

Literature Cited

Bonfil, R. (1994). Overview of world elasmobranch fisheries(No. 341). Food & Agriculture Org..

Casper, B.M., Domingo, A., Gaibor, N., Heupel, M.R., Kotas, E., Lamónaca, A.F., Pérez-Jimenez, J.C., Simpfendorfer, C., Smith, W.D., Stevens, J.D., Soldo, A. & Vooren, C.M. (2009). Sphyrna zygaenaThe IUCN Red List of Threatened Species. Retrieved from https://www.iucnredlist.org/species/39388/10193797

Clarke, S. C., Magnussen, J. E., Abercrombie, D. L., McAllister, M. K., & Shivji, M. S. (2006). Identification of shark species composition and proportion in the Hong Kong shark fin market based on molecular genetics and trade records. Conservation Biology20(1), 201-211.

Doño Melleras, F. (2008). Identification and characterization of breeding areas of the hammerhead shark (Sphyrna spp) in the coasts of Uruguay (No. 597.3 (899) DOÑ)..

Ebert, D. A., Fowler, S. L., & Dando, M. (2015). Sharks of the world: a fully illustrated guide. Wild Nature Press.

Maguire, J. J. (2006). The state of world highly migratory, straddling and other high seas fishery resources and associated species (No. 495). Food & Agriculture Org..

Mojetta, A. (1997). Sharks: History and biology of the lords of the sea. (E. McNulty, Ed.). San Diego: Thunder Bay Press.

Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.

Quattro, J. M., Driggers III, W. B., Grady, J. M., Ulrich, G. F., & MA, R. (2013). Sphyrna gilbert sp. Nov., a new hammerdead shark (Carcharhiniformes, Sphyrnidae) from the western Atlantic Ocean. Zootaxa3702(2), 159-178.

Stevens, J. D. (1984). Biological observations on sharks caught by sport fisherman of New South Wales. Marine and Freshwater Research35(5), 573-590.

Tricas, T. C., Deacon, K., Last, P., McCosker, J. E., Walker, T. I., & Taylor, L. (1997). The Nature Company Guides: Sharks and Rays. (L. Taylor, Ed.). Hong Kong: The Nature Company, Time Life Books.

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