Sixgill sharks are almost like living fossils, that belong to the order Hexanchiformes. The sharks in this order are the most primitive type of shark, and there are only seven (most sources will say six since this is true until a few days ago) different species alive. Despite this very low diversity these were once abundant in the ocean, since they are kind of living fossils that slowly died out during the mass extinctions that took place millions of years ago.
The unique feature of the Hexanchiformes order is that they have six or seven gills, while modern sharks only have five. They also only have a single dorsal fin, and lack something called the nictitating membrane – a third eyelid that is present in lots of animals. All in all, they have features that look more like the fossils we find than the other sharks, which is not really strange when you consider that they live very deep in the ocean where evolution tend to take much longer.
The head of a frilled shark (Chlamydoselachus anguineus), a member of the Hexanchiformes order. image by OpenCage, posted with the Creative Commons Attribution-Share Alike 2.5 Generic license.
The sixgill sharks
The real sixgill sharks are found in the genus Hexanchus, which consists of 3 (previously 2) living species, and a lot of extinct species. All these sharks have a broad, pointed head, yellow lower teeth, a long tail, and as you might have guessed by their name – six gills instead of five. They live deep down in the ocean, and most sixgill sharks so far have been found on the sea floor between 90 and 600 meters deep. These sharks do not really swim around on the vertical axis like other sharks, but rather stick close to the bottom of the sea.
They are keen scavengers that are quick to arrive at the carrion due to their good ability to smell dead fish from far away.
Hexanchus griseus, the most common species of sixgill shark. Image is Public Domain.
Two or three species of sixgill sharks?
The debate that has been going on for a long time is whether or not the sharks found in the northwest Atlantic Ocean is “just” a bigeye sixgill shark (Hexanchus nakamurai), or if it is a species of its own. The bigeye sixgill shark has typically been found in the Pacific Indian Oceans, which is far away from the northwestern Atlantic Ocean where the other alleged sightings of the bigeye sixgill sharks have been.
You would never be able to tell the two apart from each other by looking at them, and they are deemed next to impossible to identify in situ, but you obviously can since they are found in either the Pacific and Indian Oceans (Japan, La Reunion, and Madagascar), or in the northwestern Atlantic Ocean (Belize, Gulf of Mexico, and Bahamas).
Hexanchus nakamurai. Image by Jean-Lou Justine, posted with the Creative Commons Attribution-Share Alike 3.0 Unported license.
However, the debate that has been going is if they should be classified as two species or not, and most people have argued to keep them as one species, since they look so much alike. A new study “proved” this wrong, and sampled the mitochondrial genes of both sharks from the two different locations.
The results were pretty clear; there was a 7.037% sequence divergence. For comparison for with us humans, the expected sequence divergence from us to chimpanzees or orangutans is only 2-3 %. The genetic distance between the other sixgill sharks (Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark) is also just around 8 %, so it makes total sense to use this as enough evidence to confirm that this is in fact a new species.
Hexanchus nakamurai. Image by Jean-Lou Justine, posted with the Creative Commons Attribution-Share Alike 3.0 Unported license.
The three sixgill shark species
Now that we know that these two are separate species, the total amount of sixgill sharks alive are three. This includes the newly confirmed Atlantic sixgill shark (H. vitulus), the one expected to be the same species that keeps the name bigeye sixgill shark (H. nakamurai), and a final third species called the bluntnose sixgill shark (H. griseus). The last one if the most common one, and can grow to be huge (over 8 meters, compared to the H. vitulus and H. nakamurai's only 1.2 meters).
Taxonomy can be pretty complicated, so don’t worry if you are a bit confused by now, but the important stuff to take away from it is how DNA analyses were used to determine that we have three different sixgill sharks instead of only two. This will allow biologists to further study them with more precision, and perhaps even allow conservationists to be more aware of the status of each separate species.
This is not the first time genetic tests have been used to determine one species to in fact be two or more species, and it really opens up possibilities that we didn't have before we invented the technology to analyze genetic material. This certainly won't be the last time we see a new species being confirmed due to genetic tests!
Could there be more sixgill sharks?
The sixgill sharks are always found at the deep sea, which is an ecosystem that has not really been explored all that well yet, and some scientists believe that there are sixgill sharks all over the world, but we just haven’t encountered them yet. It’s difficult to argue against, so I guess we will need to wait and see if we find more living species of the sixgill sharks in the coming decades.
Thanks for reading
About @valth
Hey, I'm @valth - the author of this post. I love to write about nature, biology, animals, nature conservation, ecology and other related branches of science, and I occasionally write about my life, about Steem or about random stuff as well. I'm trying my best to write at least one post every day, so make sure to give me a follow if you enjoy my content and want to see more of it.
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@valth, this was something new to me, thanks. the picture of this "fossil" shark is stunning, it looks more like a character from SF movies! Even with our expensive machinery we still find a new species, this is inspiring! 2 thoughts;) 1st, today i somehow read your article with more effort; but it could easily be my sleepy brain who needs a coffee:) 2nd: the comments on your blog are so much more interesting than mine; i usually get some bulsh** like "good my friend", haha, which is exhausting ;) so keep the great work :D
Yeah, it looks really strange to me as well! I find it so cool that we still find new species in the ocean to this day, and there are probably thousands of still undiscovered species down there that we have never observed. My opinion is that we really need to spend more money on marine discovery in the future :)
Hehe, that's often a problem with taxonomy; it is pretty easy to get confused by three or four (scientific) names that are almost identical. I had to spend some time to even grasp which is which of these myself when I wrote this.
Yeah, I love that so many knowledgeable people share their experiences in my comment section. It really adds an extreme amount of value to the post as a whole in my opinion. So thank you so much for taking the time to write good comments, @k-a-s-i-a; it really means a lot to me :D
Yeah, DNA analysis is really shaking up taxonomy. My own area of expertise is microbiology, where the concept of a 'species' is already kinda of shaky and where differences based on appearance and metabolism are very rough guidelines. The advent of high throughput sequencing has convinced a lot of us to move a bunch of bugs into different (or new!) genera.
A lot of time, we don't even bother with species, and instead use something called an OTU (operational taxonomic unit), which is essentially us just giving up and saying they're different because of some % difference in their DNA (usually 3% or more in part of the DNA sequence which codes for their 16S rRNA).
In terms of macrobiology, DNA is doing the same thing. As you've pointed out, they've settled a longstanding debate in six-gilled sharks, but that's not the only case. My favorite example is when they discovered multiple species of lemurs based on DNA evidence - the lemurs appear otherwise identical even to trained researchers in the lab.
I can only imagine how difficult it my be to keep up with taxonomy within microbiology! I guess there is a lot of work to be done there, and seeing as the traditional taxonomy was based on morphology, there must be lots and lots of mistakes for microbes.
Oh, that's very interesting! It makes a lot of sense though; the traditional taxonomy does not fit very well to the diversity of the microbes.
This is really cool! I wonder how many more examples of this we will find in the next decades; surely there will be a lot of species that will be divided into several new species.
Thanks again for leaving a great comment, @effofex!
@valth I presume this is an adaptive variation to get optimum amounts of extracting oxygen at depths for the sharks. I guess the interesting fact is that we have discovered more about space than we have about the deep sea. There may be many many more previous unknown species lurking in the dark of the deep sea.
Perhaps time for us to push the boundaries of exploration on that from too!
Yeah, that makes a lot of sense! I didn't think of this myself when I wrote this post, but I definitely think this could be right.
I believe that we will see lots of new species in the deep sea in the next few decades. There is only a very small part of it that has been explored, so there could still be huge surprises for us. My favorite point to back up this point is how the 10m+ giant squid (Architeuthis sp.) was not seen alive until 2004.
I would love nothing more than to see more deep sea exploration, but unfortunately it is very expensive and pretty complicated to explore.
Yes we certainly need the elon musk version of Jacque Cousteau to invest lots of money into deep sea exploration, and i agree completely with especially near the ocean floor I suspect there are many a species down there which haven’t been discovered and may never be. I guess you could say its a proverbial needle in a haystack.
If Prof Google serves me well its only less than 5% we have actually explored, so many mnay more decades of exploration to go. Maybe Norway and their seafaring history could set up the first base for aquanauts.
That would be great! Unfortunately I think that there is very little to potentially gain from exploring it (in terms of money), so I guess businessmen are less likely to invest in this compared to space travel (which can potentially be worth billions).
Yeah, most of it is unexplored. I think Norway has plans to actually set up a permanent research ship that will travel around the globe and collect data from the ocean, so we are already working on something like what you suggest :)
A very peculiar tail indeed...Since they live so deep, is it possible that they are blind???
What's the maximum depth for humans and camera can reach without imploding???
Advancements in material and mechanical engineering will continue to increase our reach into deep dark oceanic abysses. From 20 years from now who knows....
My friend I need to ask you for something...can we have some private chat???
Fish that live deep in the ocean are often blind, but these sharks are not :)
A remote controlled submarine can go as deep as we want to, so we could theoretically explore all the ocean if we wanted to, but it's really expensive to use such a submarine.
Sure! Sorry I didn't see this earlier. Just send an encrypted memo, @xabi :)
It's possible that sixgill sharks exists in many places because knowledage of deep sea ecosystem is not as developed as it should be. In near future we have a better understanding /knowledge of deep sea ecosystem, who know we find shark of sevengill or eightgill. A lot of marine life is unknow. Lets hope to learn more secrets of marine life in future. keep sharing @valth
Yeah, that's right. We already have found one with seven gills, so it's possible to find these strange sharks. Who knows what interesting things are hiding down in the dark ocean? :)
Its strange that they cannot swim vertically and remain at the bottom of the sea.and there is huge divergence of 7% as compared to humans divergence.
By the way its very informative article.☺☺
I'm glad to hear that you found it informative :) But they are able to swim vertically, they just prefer to stay close to the bottom :)
Best regards from @valth
The real depths of the ocean are another world entirely. Truly the last unexplored frontiers of our amazing planet.
Some real beauty in their... ugliness (lol) as well. I love the way that species like this have evolved to adapt to living at these depths with no light.
Hope you're well mate, have a good one!
Yeah, we have no idea what animals might be lurking deep down in the ocean. I hope we get more deep-sea discoveries in the next decades, because I think it is very important to learn more about it. Thanks for stopping by and commenting; I hope you're well too :)
Excellent work @valth is a new species for me it is fantastic that fossil looks like a monster of the movie Harry Potter super information very good images a full post as always congratulations learn something new is very positive in life thanks for sharing greetings
Haha, that's true!
I'm glad you liked the post, @andreina89 :)
I feel how i used to feel back in school, it's truly so fascinating, wonder why i never click on your link until now:). I am 100% resteeming:). Thank you @valth, hope you will always post about sea life. Btw...are mermaids (images that can be found online)real?
Thank you very much!
No, mermaids are unfortunately not real :(
you welcome:), it's my pleasure to do so. Oh..would have been great if they were:).
Hehe, yeah, that would be really cool! :)
Well I had seen a documentary on Mariana trench, it says the scientist has barely discovered 1℅ of the aquatic life. Literally it's a huge and sometimes it's beyond our imaginations.
Yeah, there is still so much more to discover in the oceans!
The aquatic world holds thousands of living species amongst which some are unidentified.
I think most are still unidentified; we have only discovered a very small percentage of the ocean so far!
oh is this the bluntnose you are talking about, i know its sexual behavior its oviparous i.e the eggs hatches in side the female also the distinct size of the female is larger than male and also the time period estimated for giving birth can go upto 1 to 2 years
Thanks for adding more info about the shark, @chaitanya.mhatre :)
Oh im glad i could help @valth sir you have been sharing some amazing information :)
A very Good research ,you are working hard ,I am learning from your work. the head of frilled shark is looking scary thank God they are in deep sea
I'm glad you liked it! Yeah, I guess it can look scary to some :P
This is a effective composition about Sixgill sharks. I knew reading your post about different types of Sixgill sharks.Description of Species of world really impressed me. Thank you very much for your information.
You're welcome :)
The frilled shark is quite different i have never seen this type of shark. It seems you have lot of information about species.
Yeah, it's very unique compared to most living sharks, such as it having six gills and only one dorsal fin.
wawoow thats scary and really an amazing discovery.
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You don't have to be afraid of them unless you like to dive 100+ meters deep :P
nice and new information about shark
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Doesn't look so scary
No, I agree that it's not scary. Most sharks are harmless, so there's not often a need to fear them at all.
I have learned a lot about this post.
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That's good to hear :)
it's looking so scary i don't saw it before this @valth
No need to be scared of it ;) It won't attack humans.
wow its really amazing really thanks for such incredible things to know..
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Yeah, it really is :)
A team of scientists led by Florida Institute of Technology’s Toby Daly-Engel has confirmed after decades of uncertainty that sixgill sharks residing in the Atlantic Ocean are a different species than their counterparts in the Indian and Pacific oceans.
Good job with copy-paste from https://www.sciencedaily.com/releases/2018/02/180220122928.htm
Amazing how sharks differ from eachother, underwater worlds are still a big black box
Yeah, absolutely. I bet have have only discovered a few % of the species that are found down there.
It's possible that sixgill sharks exists in many places because knowledage of deep sea ecosystem is not as developed as it should be. In near future we have a better understanding /knowledge of deep sea ecosystem, who know we find shark of sevengill or eightgill. A lot of marine life is unknow. Lets hope to learn more secrets of marine life in future. keep sharing @valth