Time for a correction. On September 7, 2014, I put up a post about a weird new creature, Dendrogramma, two species of which were dredged up from the deep seas of Australia. Here’s one of them:
Thse species, which had stalks and inflexible disks, weren’t considered members of existing phyla like ctenophores (comb jellies) because, as the original paper (Just et al., reference and link below) noted, they lack features present in other similar phyla (my emphasis in this original quote):
Dendrogramma shares a number of similarities in general body organisation with the two phyla, Ctenophora and Cnidaria, but cannot be placed inside any of these as they are recognised currently. We can state with considerable certainty that the organisms do not possess cnidocytes, tentacles, marginal pore openings for the radiating canals, ring canal, sense organs in the form of e.g., statocysts or the rhopalia of Scyphozoa and Cubozoa, or colloblasts, ctenes, or an apical organ as seen in Ctenophora. No cilia have been located. We have not found evidence that the specimens may represent torn-off parts of colonial Siphonophora (e.g., gastrozooids). Neither have we observed any traces of gonads, which may indicate immaturity or seasonal changes. No biological information on Dendrogramma is available.
DNA data, which would have been very useful, weren’t available for these specimens as they were collected in 1986 and fixed in formalin, which destroys DNA. While the authors didn’t name a new phylum, they suggested that these two species were indeed representatives of a new phylum, and that caused a lot of excitement. (New phyla aren’t often described.)
However, a 2015 expedition, whose results are described in a new paper in Current Biology (O’Hara et al., reference and free link below), produced RNA that could be sequenced. And that RNA shows that Dendrogramma isn’t a new phylum at all, but a siphonophore. Siphonophores are well known, an order that falls in the class Hydrozoa, itself in the phylum Cnidaria. Siphonophores are a bizarre group consisting of specialized individual animals that band together as a group to form a “superorganism”; the most familiar member is the Portuguese man of war, and here’s another, the pelagic (free swimming) siphonophore Marrus orthocanna:
As the new paper notes:
Siphonophores are bizarre pelagic colonial cnidarians in the class Hydrozoa. They are complex elongate or spherical organisms with specialised locomotive and feeding zooids, and a net of tentacles that can be extended to catch prey or attach to the seafloor. There are 175 described species, living in a range of habitats from the sea surface (e.g., Physalia physalis, the Portuguese Man O’War) to the deep-sea. Larger, more delicate species have been found mainly in the non-turbulent mesopelagic (300–1000 m) or bathypelagic zones (1000–3000 m).
The RNA analysis places Dendrogramma (probably just one species, not two), firmly in the siphonophores: it’s the red species in the phylogny below.
So, move along folks, nothing more to see here. It’s just the usual advance of science, when we can better identify a bizarre form using DNA—or in this case, RNA. The earlier speculations that Dendrogramma may be a living remnant of the bizarre Ediacaran fauna that went extinct about 540 million years ago is no longer tenable.
h/t: Matthew Cobb, Casey Dunn
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REFERENCES:
Just, J., R. M. Kristensen, and J. Olesen. 2014. Dendrogramma, New Genus, with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia, Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara. PLOS One DOI: 10.1371/journal.pone.0102976
O’Hara, T. D. et al. 2016. Dendrogramma is a siphonophore. Current Biol. 26: R457-458.
The branching channels in Dendrogramma looked a lot like those in some of the Ediacaran fossils. Could they be siphonphores too?
I think that suggestion has been made before. However, remember that the Ediacarans cover a number of quite distinct morphologies, and are quite likely to represent several “phyla.”
Yes, the suggestion that these might be Ediacaran animals was made when Dendrogramma was first described. Showing that they are in fact siphonophores does not rule that out.
How does one tell if one is a committed cladist, anyway? Wouldn’t one need DNA to do that?
Ummmm, ability to recite 3 or more volumes of Wili Hennig’s work on demand?
I don’t get your point.
Since cladism is all about relationships of descent, not morphology, isn’t the “acid test” of descent the DNA relationships (the nested hierarchy)?
How would you resolve a series of microbes which indulged in rampant horizontal gene transfer? As, for example is certainly involved in some inter-species transmissions of antibiotic resistance, and is strongly suspect to be involved in the early history of all of the prokaryotes.
Cladistics is good for tree-like models of descent. But if your phylogeny is not tree-like …
DNA analysis is good at generating a LOT of data, and therefore for getting high statistical significance for your inferred phylogeny. But if there is HGT, then the violation of the terms of the model makes the whole interpretation much more fraught. To bring the question closer to home – just how are we going to sort out the “Cro Magnon” vs “Neanderthal” vs “Denisovan” phylogeny. There is nothing to have prevented a SE Chinese with 20% Denisovan ancestry interbreeding with a European “Cro Magnon” and leaving little genetic bootprints throughout the Middle East.
Returning to the Ediacarans, we have morphology. And we have hypothesised relationships between modern biological (i.e. breeding) species which are linked to Palaeozoic morphoclades themselves suggested to be linked to morphoclades of Ediacarans. That’s long chain of inference, and a long way from settling question.
Isn’t there some pop-sci book recently out titled something like “It’s not as simple as that”?
This is an example of a science post that I like but have nothing to say about, so this post is merely to encourage Jerry to keep up the good work.
correction: “comment”, not “post”.
Here, here. I don’t have the expertise to comment, but I enjoy the science posts – a lot. Keep up the good work, and thanks.
Yup, me too.
Seriously, this is endlessly fascinating. I was totally unaware of this type of life structure. I had heard of the Portuguese Man O’War, but knew nothing beyond the name. This joining of individuals to create a super individual is very intriguing. The photo of the the pelagic siphonophore looks like some sort of CGI creature capable of escaping a planet’s atmosphere under it’s own fuel combustion process. The fact it’s a real organism is both curious and captivating.
Also, the overall story here, in which an older discovery that had suggested the finding was a new phylum but was superseded by recent discoveries ruling that possibility out, is a wonderful testament to the discipline of science and scientists. I suppose the original scientists who made the discovery could’ve pushed for the find being a new phylum, but instead logically left the door open for the concrete proof they did not truly have. So, when proof was found and determined, there was no controversy, only an increase in knowledge. This whole process is refreshing in the midst of most of what I read and hear on a daily basis which involves opinions and deep seated selfishness, regardless of topic. While my understanding of the technical details of this article on dendrogramma is minimal, the overarching tale is quite satisfying.
I concur. A lot of time can be killed, web crawling from site to site learning about these types of organisms. It brings up so many questions some which I can’t find answers for.
It also brought to my awareness these pictures of beautiful drawings by Ernst Haeckel of Siphonophorae, from his book Kunstformen der Natur:
https://www.pinterest.com/pin/223843043957156106/
Which serves to remind me that when it comes to drawing I can not even aspire to be a talent-less hack.
Sorry, I did not intend to include a picture, I just wanted the link.
I think it’s an excellent illustration and worth including.
(I don’t think Prof CC minds pictures, it’s just imbedded Youtube videos that guzzle his bandwidth)
cr
You could be a highly talented draughtsman and artist and still be leagues below Haeckel.
Some examples : https://www.google.co.uk/search?q=Ernst+Haeckel+of+Siphonophorae
Ditto! I don’t have the in-depth scientific
education to appreciate all the detail, but I
comprehend enough to make it eminently worthwhile for me to read. And, it’s worthwhile for the beauty alone. This animal looks like a transparent water lily leaf from the “top” view. Endlessly, amazing, fascinating life-forms!
Same here. I have to admit I don’t understand a lot of it, but I find these things stunningly beautiful.
These guys do to the ocean what we would like these things to do to our living rooms:
RoombaCat
Well, look at that…truth, found by applying science.
Ribonucleic acid rules! How many such classification errors were made before we had this tool, I wonder?
Makes you wonder many fossil-based classifications are totally bunk…
Depends on what you mean by “totally bunk”. Wrong genus is nothing new – but considering the repeated reorganisations of botanical nomenclature in the last couple of generations, I don’t see that as being anything to worry about over much.
Order level … that’s worried about. However if the fossils simply don’t have the necessary parts for a definitive classification, then people may make a tentative assignment, but are generally explicit that this is a proposal, and may not be correct.
Which specific “fossil-based classifications” are you concerned about?
Genetic analysis has of course corrected many mistakes about phylogeny, and resolved quite a few mysteries. But it too can produce erroneous results that are later resolved by more genetic analysis.
I am curious what sort of collection methodology turns up RNA rather than DNA. However I am pressed for time at present and may not remember to dig up the paper later.
I hear that it is an unusually large specimen, by about an order of magnitude, so it is still exciting.
But it put a serious bract on my expectations.
It is nice to see is follow up. Too often a story makes a big splash and turns out not to be so. A new phylum is a big splash. Still a neat organism, tho.
This is totally cool, and I am glad to see that the mystery is solved. Well, maybe a slight twinge of regret over the mundane answer, but that is how things go, sometimes.
It always feel dazzling to see such rare deep sea creatures. What shapes! They are a reminder that the more easily accessible animals we see around us are a tame and customary set of forms. The next thought has to be, what about life on planets in other solar systems across the galaxy and beyond? You’d need a dose of LSD to imagine them.
Wonderful stuff, sir. Thanks for your continuing efforts to help us understand the science. I love it.
agreed 🙂
Science corrects its mistake
++++++++++++++++++++++++++++
That is what shouts out to me in this post.
TBH I am not that interested in these animals, but I do applaud Jerry for promptly publishing a correction – Oh that some other groups would do that 😉
To me, this news is exciting. I like when things fall into their proper places.