Love them or hate them, sharks are critically important to the health of our oceans. That’s just an undeniable fact. As apex predators, they have far reaching effects that help regulate the ecosystem down to the level of organisms on which they do not even directly prey. These incredible, ancient fish have been around for many millions of years longer than any dinosaur and have remained relatively unchanged since. Currently, however, more than 60 percent of all shark species on the planet are somewhere on the spectrum of threatened to critically endangered. This is extremely wrong. Animals that have survived for so long and through so much should not be pushed to the edge of extinction by the ridiculous blunders of such a self-absorbed species. Thankfully, there has recently been a worldwide effort to protect sharks and a decline of practices like the slaughtering of sharks for their fins or livers and recreational shark fishing.
Here in Puget Sound we are very fortunate to have bluntnose sixgill sharks (Hexanchus griseus). These Sound sharks are now protected after a closure on recreational sixgill fishing was put into action by the Washington State Department of Fish and Wildlife (WDFW). This ban was in response to public outrage over the capture of several local sixgills from Elliot Bay fishing piers. The WDFW also initiated a research program with the Seattle Aquarium, the National Oceanographic and Atmospheric Association (NOAA) Fisheries Service, and other scientific partners such as the University of Washington, Point Defiance Zoo and Aquarium, and Vancouver Aquarium in an effort find more about these little known, deep water sharks.
Thanks to efforts like these, we now have more information about sixgills, especially in Puget Sound. Sixgills, as with other deep water animals, have consistent daily patterns. They migrate down to great depths during the day and rise to shallower water at night. This is called diel vertical migration and is largest mass movement of organisms on the planet at one time. However, Puget Sound sixgills are often found in much shallower water than is typical elsewhere – sometimes as shallow as about ten feet during the day. Fortunately, this makes them easier to research. Through capture and tagging studies, it has been determined that most of the sharks in Puget Sound are sub-adults. It is suggested that Puget Sound may serve as a nursery for these animals until they have reached sexual maturity and leave to lead a more pelagic lifestyle. Not only are these sixgills young, but there is a high level of relatedness among juveniles that inhabit the same area. DNA studies found that sharks that were punch biopsied within the same set were significantly more likely to be related to each other than not. These sets consisted mostly of siblings and half siblings. From all the sharks sampled in Puget Sound during this study, analysis resulted in the identification of 33 cohorts. The stranding of a large adult female carrying 71 full-term pups in southern Puget Sound gave researchers an opportunity to look at relatedness within a litter, and confirmed the suspicion that females are polyandrous, that is, mating with more than one male during a breeding season. Six male sharks contributed to the genetics of this litter, but the contribution was unequal because only a few of them contributed the majority of the genotypes found.
Litters can range from 22 to 108 pups and gestation is hypothesized to be no less than 12 months and quite likely closer to 24 months or more. During breeding, male sharks appear to nip at the female’s gill area to get her attention and to entice her to mate as evidenced by white marks observed by biologist divers only during this time of the year. Similar behavior is seen in other shark species. Female sixgills mature at around 14 feet and males at closer to 10. As with many sharks, sixgills seem to grow slowly, but not a lot is known about age at maturity or rate of growth. One shark was found to double in size over its first year of life and then was recaptured later appearing to have grown around a third of an inch per month since. This is just one individual, however, and a much larger sample across different populations would be needed to fully understand sixgill growth rate.
At full size, the biggest sixgills can grow to a little over 15 feet, making this shark the largest fish in Puget Sound and one of the largest living sharks in the world. As their name suggests, they have six gill slits behind the head rather than the usual five found in most sharks. Their snouts are large and rounded, hence bluntnose, and protrude in front of jaws containing very unique teeth. The upper jaw has rows of thin, hook-like teeth that are common in many shark species. However, the lower jaw contains teeth that differ highly from those of other sharks. These teeth from rows of six on either side of the jaw and are deeply serrated like saw blades. Teeth like this are very similar to those seen in Jurassic sharks, suggesting that this species is quite ancient and primitive. Since these animals normally spend their time in very deep water (up to over 8,000 feet down – to put this in perspective, a mile is 5,280 feet) where food is scarce, they take every opportunity to scavenge when they can. Having saw like teeth on the lower jaw and puncturing teeth on the top jaw help them hold and saw through large chunks of flesh such as whale blubber. This allows them to remove more manageable pieces from huge carcasses, which they then swallow whole.
Not only are sixgills adaptable, deep sea scavengers, they are also skilled predators. These fish are capable of surprisingly great bursts of speed that contrasts with their sluggish appearance and behavior. Prey can include anything from crabs, mollusks, and teleosts (bony fish) to other cartilaginous fish or even marine mammals. Despite this, there has never been a serious injury or fatality recorded as a result of interaction with a sixgill shark. On the contrary, people often go on dives in Puget Sound specifically to see them. Sixgills do not appear to fear humans and show inquisitive behavior when they encounter one. If a person gets too close for the animal’s comfort, it will calmly swim away. Touching a sixgill may cause it to whip around and nip at the diver in warning, but no injuries usually occur and those that do are minor. Still, sixgill sharks are very big and powerful animals that should always be treated with caution. Even biologists who study sixgill behavior and are very knowledgeable will conduct their research from the safety of a shark cage or a boat and give the sharks their space when diving with them.
For the research that has been done on bluntnose sixgill sharks, we have barely touched the surface when it comes to understanding their lives and the ecological roles they play. What we do know gives us even more incentive to protect and study these amazing creatures. If Puget Sound is in fact a nursery for pups and young adult sharks, then it is a valuable resource for maintaining the genetic diversity of this species. Many shark species suffer from low genetic diversity caused by human actions and the fact that they reproduce slowly. Puget Sound sixgills, even with the relatively high number of related individuals, still shows moderate genetic diversity. Preserving safe and productive areas like Puget Sound is crucial to the survival of shark species worldwide. Sharks are excellent indicators of environmental health and where they do well, other species will undoubtedly thrive as well. Helping sharks like Puget Sound sixgills helps improve our planet’s oceans little by little, and in turn, our lives.
1. Martin, R. Aidan. “Swimming with Jurassic Sharks.” ReefQuest Centre for Shark Research (2003). http://www.elasmo-research.org/education/topics/d_jurassic_shark.htm
2. Larson, Shawn, et al. “Relatedness and polyandry of sixgill sharks, Hexanchus griseus, in an urban estuary.” Conservation Genetics 12.3 (2011): 679-690.
3. Ebert, David A. “Biological aspects of the sixgill shark, Hexanchus griseus.” Copeia (1986): 131-135.
4. Andrews, Kelly S., et al. “Diel activity patterns of sixgill sharks, Hexanchus griseus: the ups and downs of an apex predator.” Animal Behaviour 78.2 (2009): 525-536.
5. Andrews, K. S., et al. “Acoustic monitoring of sixgill shark movements in Puget Sound: evidence for localized movement.” Canadian Journal of Zoology 85.11 (2007): 1136-1143.
6. Rupp, J. “A natural history of the sixgill shark, Hexanchus griseus.” Proc Puget Sound Res (2001).
7. Bauml, J. “Hexanchus griseus.” Animal Diversity Web (2004). Web. 11 Feb. 2016 http://animaldiversity.org/accounts/Hexanchus_griseus/