This week’s featured species comes to us from the dark, inky depths of temperate and tropical waters world wide. The Bluntnoes Sixgill Shark (Hexanchus griseus) is a large, heavy bodied shark, known for its six pairs of gill slits (Ebert, Fowler, & Dando, 2015). They have broad heads with wide mouths and blunt snouts. Their pectoral fins are moderately sized, while their small single dorsal fin is set far back on its back just above the anal fin (Ebert, Fowler, & Dando, 2015). These shark are found at great depths, often found below 6500 feet (2000 m) (Parker, 2008; Ebert, Fowler, & Dando, 2015).
The bluntnose sixgill shark belongs to a family known as Hexanchidae in the class Hexanchiformes. This class contains some of the more primitive sharks we know today, including the two species of frilled sharks (Chlamydoselachus spp), which also have six pairs of gills; two species of sevengilled sharks: Heptrachias perlo and Notorynchus cepedianus; and three species of sixgilled sharks: the bluntnose sixgill, the bigeye sixgill shark (Hexanchus nakamurai), and the recently discovered Atlantic Sixgill Shark (Hexanchus vitulus) (Tricas et al., 1997; Daly-Engel, Baremore, Grubbs, Gulak, Graham, & Enzenauer, 2018). Along with the sixgill sawshark (Pilotrema warreni), these are the only known shark species to have six or seven pairs of gills instead of five. We know of only one ray with six pairs of gills, the sixgill stingray (Hexatrygon bickelli). There are several extinct members of the Hexanidae family that first appeared in the fossil record in the early Jurassic Period, nearly 200 million years ago (Klimley, 2013). These early ancestors are the first modern shark morphology, or squalomorph, to appear in the fossil record (Klimley, 2013). Evidence suggests Heterodonotiformes, early ancestors of the horn sharks, Port Jackson, and bullhead sharks, emerged around the same time period (Klimley, 2013).
Much of the bluntnose sixgill shark’s behavior and biology has yet to be studied. This is largely due to the depth of its preferred habitat. Large adults are most often observed from submersibles at depths between 650 – 3600 feet (200 – 1100 m), but they have been observed at even greater depths below 6500 feet (2000 m) (Cook & Compagno, 2009). We are still limited in our means to study the ocean’s marine life at these great depths for extended periods of time; hopefully with continued technological innovations we will continue to unlock their mysteries. One quirk of their biology that has been observed in some regions is increased light sensitivity with increased size. As they age and increase in size, the bluntnose sharks in some locations have been observed becoming increasing agitated when exposed to even moderately intense light (Cook & Compagno, 2009). This may be due to the adults preferring deeper, darker waters than their juvenile counterparts; however, the nature of these reactions are not fully understood.
EVNautilus (Videographer). (2017 July 12). Sleek Sixgill Shark Spotted… Again! | Nautilus Live [Video Clip]. Retrieved from https://www.youtube.com/
The bluntnose sixgill shark is a formidable hunter. They have a wide ranging diet which can include marine mammals and pinnipeds, but they can also feed on other sharks, skates and rays, dolphinfish, swordfish, halibut, marlin, anglerfish, squids, crabs, and even carrion (Ebert, 1994; Ebert, Fowler, & Dando, 2015). To handle such a wide range of prey items, they have saw-like teeth in their lower jaws. Each tooth in the lower jaw has a single large cusp with several small cusplets angled to one side. Along the top jaws, there are six rows of similar, narrower teeth. The teeth in the top jaw have a a larger single cusp with smaller recurved cusplets (Tricas et al., 1997). There is a slight morphological difference in tooth structure between males and females. It is hypothesized the change in the male’s tooth structure may give them an advantage when grabbing on to the females during courtship. There is some evidence to support this hypothesis as females are sometimes observed seasonally with mating scars over their gills.
The bluntnose sixgill shark is ovoviviparous, meaning it gives birth to live young which have developed inside the mother while attached to their own yolk sacs. Females don’t reach sexual maturity until they are at 14 feet (4.2 m) in length. But once they reach sexual maturity, they are relatively fecund. Females can produce 20 to 100 live young measuring 28 inches (70 cm) long (Tricas et al., 1997; Cook & Compagno, 2009). It is not certain how long their gestation period is; it is thought it could be two years or more. In early February 2019, a pregnant bluntnose sixgill shark washed up onto Vancouver Island beach; unfortunately, the pups died as well. It appeared to have died from natural causes. There was no evidence of entanglement, boat strike, or other outward injury (The Canadian Press, 2019). While occurrences like this are sad and often upsetting, they give the scientific community an opportunity to learn more about the species. In this case, not only by studying the female through the necropsy, but also study the development and biology of the pups.
The bluntnose sixgill shark is currently categorized as a Near Threatened species by the International Union for the Conservation of Nature (IUCN). This is because the bluntnose sixgill is vulnerable to accidental capture longline, gillnet, handline, trammel net, and pelagic and bottom trawl fisheries (Cook & Compagno, 2009). When this species is taken as bycatch, its meat is smoked in the Pacific Northwest of the United States, most notably Washington state. Italy also produces fine cured meats from the bluntnose sixgill shark to be exported to the European markets. Australia also utilizes the meat and the liver oil (Last & Stevens, 2009). Their fins may be traded on the market, however they are not reported. Despite several attempts to create a targeted commercial fishery for the sixgill shark, it is widely believed within the scientific community that this species is not capable of supporting a commercial or recreational fishery (Cook & Compagno, 2009). Attempts to establish a directed fishery in California rapidly collapsed as sixgill shark populations in the San Francisco and Humbolt Bays in California and the Puget Sound in Washington seriously declined in 1995 due to fishery activity (Cook & Compagno, 2009).
The video below is a few years old, and so the information in the beginning is a little outdated; however, it does provide a decent recap and a neat look at sixgills in their deepwater environment. Note:: Class Hexanchiformes now includes the two species of frilled sharks (Chlamydoselachus spp), two species of sevengilled sharks (Heptrachias spp and Notorynchus spp), and three species of sixgilled sharks with the discovery of the Atlantic Sixgill Shark (Hexanchus vitulus) in 2018.
Science Filmmaking Tips [Videographer]. (2009). Sixgill Sharks [Video Clip]. Retrieved from https://www.youtube.com/
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: Bonnaterre, 1788
Family: Hexanchidae; 5 species
Length: At least 13 feet (4 m)
Weight: In excess of 1,100 lbs (499 kg)
Habitat: Juveniles found closer to shore, while adults found in deeper waters; deep-benthic species, littoral, semipelagic, not known to be epipelagic
Depth: Deep water species found between 650 – 3600 feet (200 – 1100 m); observed at depths as great as 6500 feet (2000 m)
Gestation: Unclear; possibly more than 2 years
Litter Range: 20 – 100 pups
Home Range: Patchy world wide distribution, absent in the polar regions
Diet: Cetaceans, pinnipeds, large bony fishes
IUCN Status: Near Threatened
(Parker, 2008; Cook & Compagno, 2009; Ebert, Fowler, & Dando, 2015)
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 sleek, deep water Bluntnose Sixgill Shark! If you missed last week’s featured species, please be sure to check out the Pacific Angelshark! 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!
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Featured Image Source
Ocean Treasures (Author). (n.d.). Bluntnose Sixgill Shark [Digital Image]. Retrieved from https://otlibrary.com/bluntnose-sixgill-shark/
Cook, S.F. & Compagno, L. J.V. (2009). Hexanchus griseus. The IUCN Red List of Threatened Species. Retrieved from https://www.iucnredlist.org/species/10030/3155348
Daly-Engel, T. S., Baremore, I. E., Grubbs, R. D., Gulak, S. J., Graham, R. T., & Enzenauer, M. P. (2018). Resurrection of the sixgill shark Hexanchus vitulus Springer & Waller, 1969 (Hexanchiformes, Hexanchidae), with comments on its distribution in the northwest Atlantic Ocean. Marine Biodiversity, 1-10.
Ebert, D. A. (1994). Diet of the sixgill shark Hexanchus griseus off southern Africa. South African Journal of Marine Science, 14(1), 213-218.
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.
Last, P. R., & Stevens, J. D. (2009). Sharks and rays of Australia. 2nd Edit.
Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.
The Canadian Press. (2019). Scientists intrigued by remains of pregnant sixgill shark on B.C. beach | National Post. Retrieved April 11, 2019, from https://nationalpost.com/news/canada/scientists-intrigued-by-remains-of-pregnant-sixgill-shark-on-b-c-beach
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.