This week’s featured species is another one of my favorite species (have you been counting how many “favorites” I have?!): the Scalloped Hammerhead (Sphyrna lewini). The first time I ever saw a scalloped hammerhead was at the Monterey Bay Aquarium in their breath taking Open Sea exhibit. The way she glided by so effortlessly with just a smooth swish of her caudal fin, and could turn on a dime around that bizarre head, she captivated me. Hammerhead sharks really are just out of this world!
There are 10 species of hammerhead sharks known to science:
|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 one exception I will cover in a moment- is identifiable by their unique head shape. The scalloped hammerhead is known for their broad leading edge that arches backwards. Along the edge is a prominent central indentation with two smaller ones on to either side. This gives the scalloped hammerhead its classic “scalloped” look (Tricas et al, 1997). However, there is another species of hammerhead shark that is almost impossible to tell apart of the the scalloped hammerhead. The Carolina Hammerhead (Sphyrna gilberti) was recently described in 2013 as a separate species from the scalloped hammerhead, even though they look identical externally (Quattro et al, 2013). However, after examining 54 specimens in South Carolina, the research team was able to determine that the Carolina Hammerhead was in fact a separate species based on DNA analysis and by counting the number of precaudal vertebrae. This inability to differentiate species by sight is what is known as a cryptic species.
Unlike other solitary species of hammerhead, like the Great Hammerhead, the scalloped hammerhead is known to form large schools, sometimes containing hundreds of individuals (Skomal, 2016). These schools are segregated by size and sex. Females venture off into deeper waters offshore, while the males stay closer inshore (Klimley, 2013). This segregation of habitats among the sexes means that females rely on a wider range of mesopelagic and epipelagic prey items in their diets, compared to males that mostly feed on benthic- or bottom dwelling- organisms (Klimley, 2013). Females may have evolved to migrate offshore in search of these prey items in order to support their larger bodies and to nourish their embryos. While the females form large, socially complex schools during the day, they break away from the school to hunt alone at night.
Galapagos Conservation Trust (Videographer). (2017 April 6). Hammerhead schooling behaviour in Galapagos [Video Clip]. Retrieved from https://www.youtube.com/
Pregnant females can gestate their pups for up to 12 months. When they are ready to pup, the females migrate from deep offshore waters into shallow, coastal waters (Stevens & Lyle, 1989). Here they give birth to as many as 40 pups (Skomal, 2016). The pups will stay in relative safety of the shallow nursery for up to 2 years before moving out into deeper waters. But during this time, they face many challenges, including high predation rates from other carcharchinids and even adult scalloped hammerheads (Clarke, 1971). This is the most significant source of natural mortality of this species, and it may help explain the high fecundity rate when compared to other sharks (Holland et al, 1993).
National Geographic (Videographer). (2018 February 16). See a Hidden Hammerhead Shark Nursery in the Galapagos | National Geographic [Video Clip]. Retrieved from https://www.youtube.com/
Like many elasmobranchs, scalloped hammerheads as highly valued for their fins. Their schooling behavior makes them a prime target for the fin trade (Maguire, Sissenwine, Csirke, & Garcia, 2006). This makes them highly susceptible to localized stock depletion (Baum et al, 2009). It is estimated that between 1.3 to 2.7 million scalloped hammerhead and smooth hammerhead fins are represented in the shark fin trade every year (Clarke et al, 2006). The trade is appalling. If it sickens or enrages you, you are not alone. But you can use that emotion to help make a difference. There is a petition to strength the bill before the House of Commons in Canada to not only eliminate the import/export of shark fins from the country, but also to make the sale of shark fins within the country illegal. There is another petition going before the US Congress to eliminate the shark fin trade within the country. It is illegal to fin a shark in US waters, but the sale of shark fins is not illegal. Help make a change and add your name to the petitions! For the Sharks.
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.
Authority: Griffith & Smith, 1834
Family: Sphyrnidae; 10 species
Length: Maximum of 12 feet (3.6 m)
Weight: 180 lbs (81 kg)
Habitat: Continental and island shelves; shallow coastal and offshore waters; juveniles utilize bays and estuaries
Depth: Surface to 1,000 feet (300 m)
Reproduction: Placental viviparous
Gestation: 9 – 12 months
Litter Range: 12 – 38 pups
Home Range: Worldwide tropical waters and seasonally warm temperate waters
Diet: Bony fishes, invertebrates, other sharks and rays
IUCN Status: Endangered
(Funicelli, 1975; Snelson Jr, Williams-Hooper, & Schmid, 1988;
Johnson, & Snelson Jr, 1996; Piercy, Snelson, & Grubbs ,2016)
Thanks so much for checking out this week’s awesome shark: the Scalloped Hammerhead! If you missed last week’s flat shark, be sure to pop back and check out the Atlantic Stingray. Find out what this species is telling us about the health of the Gulf of Mexico eight years after the Deepwater Horizon oil spill. If there is a species of shark 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.
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. Remember proceeds benefit shark research and conservation with a donation to Project AWARE!
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 finatics!
Featured Image Source
Klapfer, A. (Photographer). (2015). Scalloped Hammerhead [Digital Image]. Retrieved from http://qcostarica.com/
Baum, J., Clarke, S., Domingo, A., Ducrocq, M., Lamónaca, A.F., Gaibor, N., … & Vooren, C.M. (2009). Sphyrna lewini. The IUCN Red List of Threatened Species 2009. Retrieved from https://www.iucnredlist.org/species/39385/10190088
Clarke, S. C., McAllister, M. K., Milner‐Gulland, E. J., Kirkwood, G. P., Michielsens, C. G., Agnew, D. J., … & Shivji, M. S. (2006). Global estimates of shark catches using trade records from commercial markets. Ecology letters, 9(10), 1115-1126.
Clarke, T. A. (1971). The Ecology of the Scalloped Hammerhead Shark, Sphyrna lewini, in Hawaiil.
Holland, K. N., Wetherbee, B. M., Peterson, J. D., & Lowe, C. G. (1993). Movements and distribution of hammerhead shark pups on their natal grounds. Copeia, 495-502.
Klimley, A. P. (2013). The biology of sharks and rays. University of Chicago Press.
Maguire, J. J., Sissenwine, M., Csirke, J., & Garcia, S. (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.
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. Zootaxa, 3702(2), 159-178.
Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.
Stevens, J. D., & Lyle, J. M. (1989). Biology of three hammerhead sharks (Eusphyra blochii, Sphyrna mokarran and S. lewini) from northern Australia. Marine and Freshwater Research, 40(2), 129-146.
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.