This week’s featured species is the last, but certainly not least, of the weird and wild thresher sharks: the Bigeye Thresher Shark (Alopias superciliosus). The bigeye thresher distinguished from the thresher sharks by its distinct, huge eyes that extend along its flat topped head into deep groves (Ebert, Fowler, & Dando, 2015). The big eye thresher also has deep horizontal grooves that extend from its head over the gill arches to its very long pectoral fins.
The big eye thresher shark is a member of the Alopiidae family. This family contains three species in one genus, Alopias. The big eye thresher is found globally in tropical and subtropical waters. It shares its home range with the largest of the thresher sharks, the the Common Thresher Shark (Alopias vulpinus), which can reach lengths of 20 feet (6 m) from snout to tip of the tail (Skomal, 2016). While the bigeye thresher can reach lengths of 16 feet (4.8 m) in length. The third member of the Alopiidae family is the Pelagic Thresher Shark (Alopias pelagicus), which is the smallest of the thresher sharks, measuring around 12 feet (3.65 m). These sharks are found in tropical and subtropical waters of the Indo-Pacific; there have been no reports of this species in the Atlantic Ocean (Ebert, Fowler, & Dando, 2015).
The bigeye thresher, like many species of elasmobranch, take several years to reach sexual maturity. It can take 12 to 13 years before females are sexually mature, and 9 to 10 years for males (Liu, Chiang, & Chen, 1998). When females are ready to reproduce, they are not a highly fecund species. Females gestate 2 to 4 pups for up to 12 months before giving birth to live pups (Klimley, 2013). This slow method of reproduction leaves the bigeye thresher vulnerable to exploitation due to unsustainable fishing practices. Even under moderate levels of exploitation, it is suspected bigeye thresher populations would be under pressure and see declines in populations (Amorim et al., 2009).
The bigeye thresher is a formidable hunter of ocean going bony fishes like herring and billfishes, as well as squids (Tricas et al., 1997). The morphology of the teeth in the upper and lower jaws are very similar. They have slender, slightly curving cusps with smooth edges. These teeth belong to a predator that catches and swallows its prey in a single bite rather than tearing away chunks of flesh a piece at a time. The upper jaw has between 19 to 24 teeth, while the lower jaw has 20 to 24 teeth.
The bigeye thresher shark doesn’t just rely on its teeth to hunt and catch its prey. Thresher sharks have been observed working in pairs to herd their prey into tighter and tighter bait balls (Parker, 2008). Then each shark takes a turn striking out in the most unique fashion in the shark – and possibly the animal – world. The use their extreme heterocercal tail to slash forward with lightning speed and stun their prey (Parker, 2008). This technique is called tail-smacking. Until recently, it was only speculated that the thresher sharks used their tails to hunt in this way. But in 2010, a team of researchers captured this behavior for the first time by the pelagic thresher shark at Pescador Island in the Philippines (Oliver, Turner, Gann, Silvosa, & Jackson, 2013). The study was subsequently published in 2013.
PLOS Media (Videographer). (2013). Hunting Strategies of Thresher Sharks: Overhead Tail-Slap [Video Clip]. Retrieved from https://youtu.be/
Unlike other species of sharks which are cold blooded and rely on the surrounding water temperature to thermoregulate their bodies, the sharks in the family Alopiidae, which as well as family Alopiidae, as well as the sharks of family Lamnidae, which includes the Great White and the shortfin mako, are warm-blooded – sort of. These endothermic sharks have a modified circulatory system that allows them to elevate the temperatures of certain organs (such as the eyes, brain, heart, stomach, and trunk muscles) through a process called counter-current heat exchange. The arteries and veins run parallel over the trunk muscles. The incoming cold blood in the veins is warmed by the outgoing warm blood in the arteries (Klimley, 2013).
The bigeye thresher shark is a highly migratory species. Tagging studies have tracked a single individual from New York to the Eastern Gulf of Mexico, nearly 1720 miles (2767 km) (Kohler, & Turner, 2001). But these sharks also perform diel migrations, meaning they spend their days between 650 to 1650 feet (200 to 500 m) and migrate vertically in the water column to shallower waters between 260 to 425 feet (80 to 130 m) at night to feed (Nakano, Matsunaga, Okamoto, & Okazaki, 2003; Weng, & Block, 2004). These daily vertical migrations is one of the reasons for the bigeye thresher’s big eyes. Their eyes are exceedingly large to maximize the amount of light or irradiance that can reach their photo-receptive cells in the back of the eye in their low light environments (Klimley, 2013).
SharkTagger Organization [Videographer]. (2017). Bigeye Tresher Shark Tagging Hawaii [Video Clip]. Retrieved from https://www.youtube.com/watch?v=Mu5xtdXunfI
The bigeye thresher shark is currently categorized as a species that is Vulnerable to exitinction by the International Union for the Conservation of Nature (IUCN) due to their slow life history characteristics and low capacity to recover from exploitation (Amoriom et al, 2009). As we previously mentioned, the bigeye thresher shark takes a low time to reach sexual maturity and is not particularly fecund, only producing a handful of offspring every two years. This leaves them vulnerable to the high levels of largely unmanaged targeted and bycatch fisheries that exploit them (Clarke et al, 2006). The bigeye thresher shark, like so many species of shark, is directly targeted by the meat and fins fisheries. In Hong Kong, the thresher shark species is found to represent 2-3% of the fins sold at market (Clarke, Magnussen, Abercrombie, McAllister, & Shivji, 2006). It is estimated that between 350,000 to 3.9 million thresher sharks are represented in the fin trade each year (Clarke et al, 2006b). At these current harvest rates, the fisheries are likely unsustainable. the Food and Agriculture Organization of the United Nations (FAO) analysis of the species states, “unless demonstrated otherwise, it is prudent to consider these species as being fully exploited or over exploited globally” (Maguire, 2006). At present, the bigeye thresher shark protected in the Northwest Atlantic under the Fishery Management Plan of the Atlantic Tunas, Swordfish, and Sharks since 2000 as a prohibited species. All thresher sharks in family Alopiidae are included under Annex I of the Highly Migratory Species of UN Convention on the Law of the Sea, which urges states to cooperate over management of included species. To date there is no such management.
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: Lowe, 1841
Family: Alopiidae, 3 species
Length: Up to 16 feet (4.8 m)
Weight: Up to 440 lbs (199.5 kg)
Habitat: Close inshore to open sea
Depth: Surface to 2375 feet (723 m)
Gestation: 12 months
Litter Range: 2 – 4 pups
Home Range: Tropical and subtropical waters worldwide
Diet: Pelagic mackerel, tuna, herring, billfishes, squids, and bottom bony fishes
IUCN Status: Vulnerable
( Tricas et al., 1997; Compagno, 2001; Ebert, Fowler, & Dando, 2015; Skomal, 2016; Amorim et al., 2009)
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 Bigeye Thresher Shark! If you missed last week’s featured species, please be sure to check out the amazing Caribbean Rough Shark! 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!
Featured Image Source
WorldLifeExpectancy (Author). (n.d.). Alopias superciliosus [Digital Image]. Retrieved from https://www.worldlifeexpectancy.com/
Amorim, A., Baum, J., Cailliet, G.M., Clò, S., Clarke, S.C., Fergusson, I., Gonzalez, M., Macias, D., Mancini, P., Mancusi, C., Myers, R., Reardon, M., Trejo, T., Vacchi, M. & Valenti, S.V. (2009). Alopias superciliosus. The IUCN Red List of Threatened Species. Retrieved from https://www.iucnredlist.org/species/161696/5482468
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, 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 Biology, 20(1), 201-211.
Compagno, L. J. (2001). Sharks of the world. An annotated and illustrated catalogue of shark species known to date, vol 2. Bullhead, mackerel and carpet sharks (Heterodontiformes, Lamniformes and Orectolobiformes). FAO species catalogue for fishery purposes, 1, viii+-1.
Ebert, D. A., Fowler, S. L., & Dando, M. (2015). Sharks of the world: a fully illustrated guide. Wild Nature Press.
Klimley, A. P. (2013). The biology of sharks and rays. University of Chicago Press.
Kohler, N. E., & Turner, P. A. (2001). Shark tagging: a review of conventional methods and studies. In The behavior and sensory biology of elasmobranch fishes: an anthology in memory of Donald Richard Nelson (pp. 191-224). Springer, Dordrecht.
Liu, K. M., Chiang, P. J., & Chen, C. T. (1998). Age and growth estimates of the bigeye thresher shark, Alopias superciliosus. Fishery Bulletin, 96, 482-491.
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..
Nakano, H., Matsunaga, H., Okamoto, H., & Okazaki, M. (2003). Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean. Marine Ecology Progress Series, 265, 255-261.
Oliver, S. P., Turner, J. R., Gann, K., Silvosa, M., & Jackson, T. D. U. (2013). Thresher sharks use tail-slaps as a hunting strategy. PLoS One, 8(7), e67380.
Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.
Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.
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.
Weng, K. C., & Block, B. A. (2004). Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia. Fishery Bulletin, 102(1), 221-229.