This week’s Featured Species takes a closer look at the first known shark species to be omnivorous: the Bonnethead Shark (Sphyrna tiburo) (Leigh, Papastamatiou, & German, 2018). The bonnethead shark is the smallest member of family Sphyrnidae, which includes the hammerhead sharks:
|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|
The bonnethead shark is a social shark, typically traveling in groups ranging from 3 to 15 individuals (Skomal, 2016). Like other social animals, the bonnethead shark has a host of complex social interactions. Males are known to display a threat display called a “hunch.” This posture is a warning to other sharks to back off. They have also been observed bumping, head shaking, jaw snapping, patrolling, and even biting other members in their group to establish and maintain dominance (Tricas, et al., 1997).
The bonnethead shark is one of the most common species found in the Western Atlantic and Eastern Pacific. The distribution of this species in tropical and temperate waters on either side of the Americas suggests that once these two populations were one stock (Stevens, 1997). Prior to about 3 million years ago, the Atlantic and Pacific Oceans were one body of water. However a geological upheaval caused a land bridge to form between the Americas, splitting these sharks into different populations. It is likely that given enough time, these populations will genetically diverge further, leading to speciation – or the formation of a new species (Stevens, 1997). In fact, recent studies have shown some genetic variation between populations in the Gulf of Mexico and the Western Atlantic Ocean (Escatel-Luna, et al., 2015; SEDAR, 2013).
Like other members of Spyrnidae, bonnethead sharks have a shovel-like head known as a cephalofoil. Each species of hammerhead has a cephalofoil unique to that species. The exception is the Carolina Hammerhead and the Scalloped Hammerhead. The Carolina Hammerhead is considered a cryptic species, meaning without genetic testing and internal examination of vertebrae, these two species appear identical (Quattro, Driggers III, Grady, Ulrich, & Roberts, 2013).
The shape of a hammerhead’s cephalofoil allows of delicate electroreceptors, the ampullae of lorenzini, to be spread out over a wider area than other sharks. These sharks use their head similar to a metal detector, swinging their heads back and forth over the sand to find hidden prey items (Klimley, 2013). Watch these sharks detect minute electrical signals buried in the sand in the video below!
BBC Studios (Videographer). (2008 September 8) Amazing! Bonnethead Shark hypnosis – Perfect Shark – BBC [Video Clip]. Retrieved from https://www.youtube.com/
Bonnethead sharks inhabit shallow, coastal waters, bays, estuaries, and harbors where they hunt small bony fishes and invertebrates like squid (Tricas, et al., 1997; Skomal, 2016). Earlier this month a new study revealed that these sharks consume and utilize the nutrients from seagrass, making them the first known omnivorous shark species (Leigh, et al., 2018). This new find actually raises many questions about the roles bonnethead sharks play in the seagrass ecosystem. Omnivores are able to adjust their diets, switching from prey items to plant material based on their environment. Because of this biological adaptation, omnivores are typically able to adjust within their environments easier than herbivores and carnivores. Bonnethead sharks are very common despite pressures from targeted and bycatch fisheries (SEDAR, 2013). It is possible that, in addition to a relatively quick reproduction cycle, these sharks use this omnivorous adaptation to compensate when their typical prey items come under threat from fisheries. At this time, more research is needed to truly understand how these sharks use this adaptation to their advantage and the additional roles they play in their ecosystem as omnivores.
Bonnethead sharks are known to use seagrass beds as nurseries along the Gulf Coast of Florida (Hueter & Manire, 1994; McCallister, & Gelsleichter, 2013; Bethea, et al., 2014; ). The bonnethead is placental viviparous, meaning females give live birth to 4 to 14 pups per litter after a year long reproduction cycle. The reproduction cycle of a female bonnethead shark can be divided into 9 distinct phases (Klimley, 2013). In November of each year, males and females mate. A female will store a male’s sperm in her oviductal gland for several months. This allows the female to fertilize her eggs when she is ready to ovulate without relying on a receptive male. Several months later the female moves into the second stage of reproduction: preovulation. During this time the female’s uterus is still small, however follicle growth has begun. In April, the female’s uterus has begun to expand and several ova have fully developed. Shortly after ovulation, the female moves into post-ovulation where the sperm within her oviductal gland begins to decrease. It is likely that fertilization occurs during or between these two stages, however the exact timing of fertilization is now known. In May of each year, the female is the fifth stage of reproduction: early pregnancy. During this stage, the embryos first appear in her uterus. A few weeks later, during implantation, the placenta starts developed in each embryo. In late pregnancy, the placenta is fully developed and the pups are nearly ready for birth. Females typically give birth between July and August of each year. Following parturition, the female’s uterus becomes flaccid before beginning to grow follicles and the female is once again receptive (Klimley, 2013).
Authority: Linnaeus, 1758
Family: Sphyrnidae, 10 species
Length: 5 feet (1.5 m)
Weight: 13 lbs (5.9 kg)
Habitat: Continental and island shelves; shallow coastal waters, bays, harbors, esutaries
Depth: Surface to 260 ft (80 m)
Reproduction: Placental Viviparity
Gestation: 5 months
Litter Range: 4 – 14 pups
Home Range: Tropical to temperate waters in Western Atlantic Ocean, Gulf of Mexico, Caribbean, and Eastern Pacific Ocean
Diet: Small bony fishes, invertebrates, seagrass
IUCN Status: Least Concern
(Cortés, et al., 2016; Skomal, 2016; Leigh, et al., 2018)
Thanks for checking out the bonnethead shark! Be sure to take a look back at last week’s Featured Species: the Crocodile Shark. Have a species you’d love to see featured? Leave me a comment!
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Featured Image Source
Hisgett, T. (Photographer). (2011 September 20). Bonnethead shark (Sphyrna tiburo) [Digital Image]. Retrieved from https://commons.wikimedia.org/
Bethea, D., Ajemian, M.J., Carlson, J.K., Hoffmayer, E.R., Imhoff, J.L., Dean, R., Peterson, C.T. and Burgess, G.H. 2014. Distribution and community structure of coastal sharks in the northeastern Gulf of Mexico. Environmental Biology of Fishes 98(5): 1233-1254.
Cortés, E., Lowry, D., Bethea, D. & Lowe, C.G. 2016. Sphyrna tiburo. The IUCN Red List of Threatened Species 2016: e.T39387A2921446.
Escatel-Luna, E., Adams, D.H., Uribe-Alcocer, M., Islas-Villanueva, V. and Díaz-Jaimes, P. 2015. Population Genetic Structure of the Bonnethead Shark, Sphyrna tiburo, from the Western North Atlantic Ocean Based on mtDNA Sequences. Journal of Heredity 106(4): 355-365.
Hueter, R.E. and Manire, C.A. 1994. Bycatch and catch-release mortality of small sharks and associated fishes in the estuarine nursery grounds of Tampa Bay and Charlotte Harbor. Project Report. NOAA NMFS/ MARFIN Program NA17FF0378.
Leigh, S. C., Papastamatiou, Y. P., & German, D. P. (2018). Seagrass digestion by a notorious ‘carnivore.’ Proceedings of the Royal Society B: Biological Sciences, 285(1886), 2018158
McCallister, M., Ford, R. and Gelsleichter, J. 2013. Abundance and distribution of sharks in northeast Florida waters and identification of potential nursery habitat. Marine and Coastal Fisheries 5(1): 200-210.
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, W. B. I., 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.
Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.
Southeast Data, Assessment, and Review (SEDAR). 2013. SEDAR 34 Stock Assessment Report: HMS Bonnethead Shark. NOAA Fisheries, North Charleston, South Carolina, USA.
Stevens, J. D. (Ed.). (1997). Sharks (6th ed.). New York: Facts on File.
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.