February Elasmobranch of the Month: Bonnethead Shark
Author: Linda Weiss
The Bonnethead shark reaches a maximum length of 1.2 m (4 feet), making it the smallest species in the hammerhead family. Like all hammerheads, the bonnethead’s head expands on each side(cephalofoil), wider than its body with one eye located on each end. The Bonnethead’s cephalofoil is flat and crescent-shaped. The shape more closely resembles a rounded shovel than a hammer. The Bonnethead is the only known shark species with a variation in head feature occurring according to sex. Mature male Bonnetheads have a bulging cephalofoil, while females do not. The first dorsal fin is large and originates just behind the pectoral fins. The second dorsal fin is rounded and much smaller than the first. Pectoral fins originate immediately behind the gills. These fins are relatively short and broad. The forked caudal fin is comprised of two lobes, with the upper lobe being nearly three times larger than the lower lobe. The color of this shark’s top and sides varies from gray to grayish-brown and occasionally dark spots visible on its sides. The underside is light gray to white. The Bonnethead’s teeth are not serrated. The front teeth are small and slightly triangularly shaped, for grasping and cutting, while teeth in the back of the jaw are larger and flattened for grinding.
Habitat and Distribution
Bonnetheads are found in estuaries, bays and over coral reefs, as well as shallow, sandy flats. They are inhabitants of water depths between 10-80 m (33-262 ft.). Their range includes tropical and subtropical waters on both North American coasts. On the East coast they are found from North Carolina to Brazil and on the West coast they range from Southern California to Ecuador. Within these ranges, includes the Gulf of Mexico and the Caribbean. Schools upwards of fifteen sharks are commonly seen. During migrations schools as large as thousands may occur.
Bonnethead sharks are daytime feeders whose main prey items are crustaceans such as crabs and shrimp. Their diet also includes octopi, fish, clams, and mollusks. They use their shovel-shaped cephalofoil to dig in the sea floor to uncover prey.
Reproduction of the Bonnethead is viviparous (live-bearing). Eggs are produced and nourished from a yolk-sac inside the female, for a gestation period of four to five months. This is the shortest gestation known in sharks. Yolk sac are attached to the uterine wall, forming the placenta, which provide nourishment to the embryo. The live-born pups are approximately 21.5- 29.7 cm (0.7-1 ft.) long and are born in litter sizes averaging four to fourteen.
The IUCN Red List of Threatened Species currently lists the Bonnethead as Endangered, with a decreasing population trend.
This shark’s natural predators include other sharks such as Tigers and Lemons. The commercial fishing industry targets the Bonnethead for its meat and fins and they are also killed as bycatch. Habitat destruction from coastal development is also a likely danger to this shark species.
Florida Fish and Wildlife Conservation Commission
The IUCN Red List of Threatened Species
Pollom, R., Carlson, J., Charvet, P., Avalos, C., Bizzarro, J., Blanco-Parra, MP, Briones Bell-lloch, A., Burgos-Vázquez, M.I., Cardenosa, D., Cevallos, A., Derrick, D., Espinoza, E., Espinoza, M., Mejía-Falla, P.A., Navia, A.F., Pacoureau, N., Pérez Jiménez, J.C. & Sosa-Nishizaki, O. 2020. Sphyrna tiburo. The IUCN Red List of Threatened Species 2020: e.T39387A124409680. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T39387A124409680.en. Downloaded on 07 February 2021.