This week’s featured species is a very unique and beautiful species, and one of my favorites (have you lost track how many favorites I have yet?!). The Sixgill Sawshark (Pliotrema warreni) is a slender shark species endemic to the southern tip of Africa. Like other sawsharks, they are named for their long, saw-like rostra; but unlike other sawsharks, the sixgill sawshark is the only species to have more than five pairs of gills (Ebert, Fowler, & Dando, 2015). Nearly all species of elasmobranchs have five pairs of gills, with a few exceptions like the sixgilled frilled shark (Chlamydoselachus anguineus), the sixgilled cowsharks in genus Hexanchus, the sevengill sharks of genus Heptranchias, and the sixgill stingray (Hexatrygon bickelli) (Klimey, 2013).
The sixgill sawshark is a member of the family Pristiohoridae, which currently has eight described species (Skomal, 2016).
- Sixgill Sawshark Plioterma warreni
- Japonese Sawshark Pristiophorus japonicus
- Shortnose Sawshark Pristiophorus nudipinnis
- Bahamas Sawshark Pristiophorus schroederi
- Tropical Sawshark Pristiophorus delicatus
- Lana’s Sawshark Pristiophorus lanae
- African dwarf sawshark Pristophorus nancyae
- Longnose (or Common) Sawshark Pristiophorus peroniensis
The sixgill sawshark is the only member of genus Pliotrema within this family; the other sawsharks, like the Longnose Sawshark (Pristiophorus cirratus), belong to the genus Pristiophorus (Parker, 2008). The sawsharks first appear in the fossil record around the middle of the Paleogene some 33.4 million years ago (Klimley, 2013). Their distribution was worldwide by the end of the Paleogene, ranging from Japan to Morocco, from Oregon to Belgium, and from Holland to California (Klimley, 2013).
Sawsharks are often confused with their cousins, the sawfishes of family Pristidae. That is mostly to do with the sawfish’s biology: their strong dorsal fins, broad pectoral fins, and a vertical causal fin (Tricas, et al., 1997). However, once you learn the differences between these two elasmobranchs, it’s very easy to distinguish them.
- Both sawfish and sawsharks have pairs of gills. However, the gills of sawsharks are located on the sides of their heads, while the gills of swfishes are located underneath their bodies just like other skates and rays (Parker, 2008; Tricas, et al., 1997).
- Their rostra are also structured differently. Sawsharks have teeth of varying sizes along both sides of the rostra and fleshy tendrils that trail down from about half way up their rostrum (Parker, 2008). Sawfishes have long, strong rostra with teeth that are uniform along both sides and no tendrils (Klimley, 2013).
- But one of the most telling characteristics, is size. Most species of sawsharks grow no more than 4.5 feet (1.4 m), while the some species of sawfishes, like the green sawfish (Pristis zijsron), can grow more than 20 feet (6 m) (Simpfendorfer, 2013).
Sawsharks and sawfish are the perfect example of convergent evolution: two unrelated species that evolve similar structures along their own evolutionary path ways. Essentially two species are out doing their own thing for millions of years, evolve to fit into their ecological niches, and show up to the party wearing very similar outfits! In the photo below, you can see the differences between the sawshark on the left and the sawfish on the right.
The sixgill sawshark spends most of its life near the bottom of the continental shelf and upper slopes off South Africa (Fowler, 2004). They primarily feed on small benthic fishes, crustaceans, and squids (Fowler, 2004). While a sawshark has not bee observed using its rostrum to hunt like the sawfishes, morphological evidence suggests that use of their rostra for predation may be similar (Nevatte, Williamson, Vella, Raoult, & Wueringer, 2017). The rostrum is covered in electrosensory cells that may aid the shark in finding food hidden in bottom substrates. The rostrum is lined with rostral teeth, also referred to as rostral spines. These teeth are modified from placoid scales, and developed independently of the oral teeth (Welton, Smith, Underwood, & Johanson, 2015). The rostral teeth have a crown that is slightly bent to the rear. This forms a cutting edge on both the front and the back edge of the tooth. The rear edge of tooth is serrated by barbed hooks (Herman, Hovestadt-Euler, & Hovestadt, 1992). The teeth in the upper and lower jaws have similar morphology. They have a wide base with a relatively short and narrow cusp (Herman, Hovestadt-Euler, & Hovestadt, 1992).
Little of the sixgill sawshark’s biology and life history characteristics are known. They are ovoviviparous, meaning the give birth to live young which get their nutrients from a yolk sac. Though their gestation period is known, females give birth to an average of 5 to 7 pups. And these pups are born with their saw rostra ready to use! At least Nature has a sense of humor! The teeth of the rostra are actually folded back along the blade in the womb and through the birth canal in order to avoid injuring mom during birth (Parker, 2008). Once these pups are born, they leave mom and are ready to begin the hunt. Their populations are segregated by size, with the young sharks preferring shallower waters than the adults (Fowler, 2004). Pupping grounds are suspected off Kwazulu-Natal in South Africa, near the Eastern Cape. Overall, the sixgill sawshark is less fecund than other species of sawsharks (Fowler, 2004).
Due to the morphology of their rostra, sawsharks are vulnerable to entanglement in fishing nets and lines. The sixgill sawshark is not targeted specifically by any fishery, however, they are taken as bycatch in demersal trawlers (Fowler, 2004). When they are taken, they are often discarded and thus bycatch data is limited and often unmonitored. Throughout the sixgill sawshark’s home range, there is extensive offshore trawl fisheries. It is suspected that the current rate of bycatch is unsustainable for this naturally rare species (Fowler, 2004). In recent years, the South African fisheries have made a push towards more sustainable fishing practices, which aims to decrease the number of bycatch species in their hauls. At present, the sixgill sawshark is categorized as a Near Threatened species by the International Union for the Conservation of Nature (IUCN) due to concerns regarding their vulnerability to these unmonitored fishing practices and their limited home range (Fowler, 2004). No conservation legislation currently exists specifically for the sixgill sawshark. Continued study and monitoring of their population size, structure, and trends, as well as their life history characteristics is required to conserve this species.
Marine Stewardship Council – Sustainable Seafood (Videographer). (2015). South African Seep Sea Trawl Fishery and MSC [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: Regan, 1906
Family: Pristiophoridae, 8 species
Length: Females reach at least 4.4 feet (1.36 m); Males reach up to 3.6 feet (1.12 m)
Weight: Up to 20 lbs (9 kg)
Habitat: Offshore continental shelves and upper slopes
Depth: 121 – 1640 feet (37 – 500 m)
Litter Range: 5 – 7 pups
Home Range: Southeast Atlantic Ocean and Southwest Indian Ocean
Diet: Small bony fish, crustaceans
IUCN Status: Near Threatened
(Fowler, 2004; Ebert, Fowler, & Dando, 2015; Skomal, 2016)
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 Sixgill Sawshark! If you missed last week’s featured species, please be sure to check out the amazing Bigeye Thresher! 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
Regan, C. T. (Author). (1906). Pilotrema warreni [Digital Image]. Retrieved from https://commons.wikimedia.org/wiki/File:Pliotrema_warreni_REGAN,_1906.jpg
Ebert, D. A., Fowler, S. L., & Dando, M. (2015). Sharks of the world: a fully illustrated guide. Wild Nature Press.
Fowler, S.L. (2004). Pliotrema warreni. The IUCN Red List of Threatened Species. Retrieved from https://www.iucnredlist.org/species/44496/10901776
Herman, J., Hovestadt-Euler, M., & Hovestadt, D. C. (1992). Contributions to the study of the comparative morphology of teeth and other relevant ichthyodorulites in living supraspecific taxa of Chondrichthyan fishes. Part A: Selachii, (4), 193-254.
Klimley, A. P. (2013). The biology of sharks and rays. University of Chicago Press.
Nevatte, R. J., Williamson, J. E., Vella, N. G. F., Raoult, V., & Wueringer, B. E. (2017). Morphometry and microanatomy of the barbels of the common sawshark Pristiophorus cirratus (Pristiophoridae): implications for pristiophorid behaviour. Journal of Fish Biology, 90(5), 1906–1925.
Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.
Simpfendorfer, C. (2013). Pristis zijsron The IUCN Red List of Threatened Species 2013. Retrieved from http://dx.doi.org/10.2305/IUCN.UK.2013-1.RLTS.T39393A18620401.en
Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.
Welton, M., Smith, M. M., Underwood, C. J., & Johanson, Z. (2015). Teeth outside the mouth? Evolution and development of the sawfish and sawshark rostral dentitions (Elasmobranchii; Chondrichthyes). Royal Society Open Science, 2.