Author: Bee Smith
Great Hammerhead Shark
Sphyrna mokarran
It’s a bird! It’s a plane! No, It’s a giant shark with wings on its head!
Key Features & Appearance
Hammerhead sharks are named after their recognizable heads which are laterally expanded and so resemble the shape of a hammer. This head shape is called a cephalofoil, and though it may look strange, it provides many advantages: it means the eyes are further apart, which increases the binocular field of vision, and the increased distance between nostrils improves the ability to determine the direction of scent trails, it also thought to enhance electroreceptive abilities by providing more space for electrosensory pores, as well as helping with maneuverability.
Credit: Jillian Morris
There are 10 species of hammerhead sharks, and the great hammerhead is the largest species, reaching up to 6m but typically around 3-5m, with females being larger than males. As well as its larger size, it can also be distinguished from other hammerheads by its straight rather than rounded head shape, with a notch in the center. Another feature is its very tall first dorsal fin. Most sharks use their pectoral fins to generate life, and the dorsal fin helps with stabilization, but great hammerheads have been shown to spend up to 90% of their time swimming on their side using their large dorsal fin to generate lift.
Habitat & Distribution
It is a coastal-pelagic and semi-oceanic species, being found in tropical and temperate seas worldwide. It can be found in the western and eastern Atlantic Ocean, Indian Ocean and Red Sea, Indo-Pacific and eastern Pacific Ocean. Exact ranges and the connectivity between populations is still being determined, but we do know that it is a highly mobile species, capable of long distance migrations. It is also thought to predominantly spend time in shallow waters, though it can be found from the surface to depths of 300m.
Diet
It is said to have a varied diet of cephalopods, crustaceans, fish, rays, and other shark species. However, it has been found that it may actually primarily eat rays and sharks, and another benefit of its head shape is that it uses it to hunt rays by pinning them against the seafloor.
Reproduction
Great hammerheads are usually solitary but come together to mate. We have seen them mate and found that unlike most sharks who mate near the seafloor, they mate near the surface. Female great hammerheads are viviparous, meaning they give birth to live young - and baby great hammerheads have much rounder heads than the adults (otherwise, the mummy hammerhead would not be too happy giving birth).
Credit: Jillian Morris
Threats
Great hammerheads are caught in many global fisheries and target catch and bycatch, from industrial longlines and purse seines to coastal gillnet and artisanal fishing. They are targeted for their large fins as these are worth a lot of money in the shark fin trade, and this also makes them a valuable incidental catch. Furthermore, even if they are released when caught as bycatch, they are unlikely to survive as they suffer very high post-release mortality compared to other sharks. This fishing has led to serious population declines of 90% in the Atlantic and as high as 99% in the Mediterranean. Great hammerheads are also targeted by recreational fishing, and suffer the same problem of very low survival rates even if they are returned to the ocean.
Status
The great hammerhead is classified as “Critically Endangered” according to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and it is on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix 2 which controls the trade of its products.
Fun Fact
It used to be believed that the first hammerheads had smaller “hammers” and larger “hammers” evolved later. However, we now know that the ancestral hammerheads had larger hammers, and smaller ones evolved later!
Team member Candace diving with a great hammerhead Credit: Jillian Morris
Works Cited
(No date) Fast facts: Available at: https://iucn.org/sites/default/files/import/downloads/fast_facts_iucn_pelagix_report_species_snapshots.pdf (Accessed: 27 August 2024).
Abercrombie, D.L., Clarke, S.C. & Shivji, M.S. Global-scale genetic identification of hammerhead sharks: Application to assessment of the international fin trade and law enforcement. Conserv Genet 6, 775–788 (2005). https://doi.org/10.1007/s10592-005-9036-2
CITES COP12 (no date) CITES Sharks and Rays. Available at: https://citessharks.org/cites-listed-shark-species (Accessed: 27 August 2024).
D. M. McComb, T. C. Tricas, S. M. Kajiura; Enhanced visual fields in hammerhead sharks. J Exp Biol 15 December 2009; 212 (24): 4010–4018. doi: https://doi.org/10.1242/jeb.032615
Forselledo, Rodrigo & Domingo, Andrés & Mas, Federico & Miller, Philip. (2022). Great hammerhead.
Gaylord, M.K., Blades, E.L. and Parsons, G.R. (2020) ‘A hydrodynamics assessment of the hammerhead shark cephalofoil’, Scientific Reports, 10(1). doi:10.1038/s41598-020-71472-2.
Great hammerhead shark - Sphyrna Mokarran (no date) Shark Research Institute. Available at: https://www.sharks.org/great-hammerhead-shark-sphyrna-mokarran (Accessed: 27 August 2024).
Guttridge, T. L., Müller, L., Keller, B. A., Bond, M. E., Grubbs, R. D., Winram, W., Howey, L. A., Frazier, B. S., & Gruber, S. H. (2022). Vertical space use and thermal range of the great hammerhead (Sphyrna mokarran), (Rüppell, 1837) in the western North Atlantic. Journal of Fish Biology, 101(4), 797–810. https://doi.org/10.1111/jfb.15185
Guttridge, T.L. et al. (2017) ‘Philopatry and regional connectivity of the great hammerhead shark, Sphyrna Mokarran in the U.S. and Bahamas’, Frontiers in Marine Science, 4. doi:10.3389/fmars.2017.00003.
Nammack, Marta F. et al. “Status review report : great hammerhead shark (Sphyrna mokarran).” (2014).
Payne, N., Iosilevskii, G., Barnett, A. et al. Great hammerhead sharks swim on their side to reduce transport costs. Nat Commun 7, 12289 (2016). https://doi.org/10.1038/ncomms12289
Raoult V, Broadhurst MK, Peddemors VM, Williamson JE, Gaston TF. Resource use of great hammerhead sharks (Sphyrna mokarran) off eastern Australia. J Fish Biol. 2019; 95: 1430–1440. https://doi.org/10.1111/jfb.14160
Stephen M. Kajiura, Kim N. Holland; Electroreception in juvenile scalloped hammerhead and sandbar sharks. J Exp Biol 1 December 2002; 205 (23): 3609–3621. doi: https://doi.org/10.1242/jeb.205.23.3609
Strong, W.R., Snelson, F.F. and Gruber, S.H. (1990) ‘Hammerhead shark predation on stingrays: An observation of prey handling by Sphyrna Mokarran’, Copeia, 1990(3), p. 836. doi:10.2307/1446449.
Комментарии