And the deep sea surprises us yet again (photos of the type specimen of Dendrogramma enigmatica from Just et al. ).
I totally ignored the original hype about these beasties. I saw them pop up on I Fucking Love Science the other day, read the headline, decided it was probably another annoyingly sensationalised news story about a moderately strange new species and went on with my life. (The fact that they kinda look like weird flatworms didn’t help) Well, now that I’ve seen the paper, I… nah, I don’t regret the decision to ignore the news story, because hyperbole like that headline about rewriting the tree of life drives me up the wall, but I am glad that I finally checked what the hype was all about.
It’s really cool, after all these years of humanity cataloguing the living world, to find something so weird that basically all we can say about it is that it’s an animal. At this point it’s not clear to me how much of that is genuine weirdness and how much is simply down to the lack of data. The organisms were found in bulk seafloor samples brought up from depths of 400 and 1000 m somewhere off Tasmania nearly thirty years ago, and they are apparently quite poorly preserved. There’s no DNA, though commenters on the PLoS article seem to think it might be possible to get some out of the specimens. (That would be nice!)
According to the authors’ description, the general organisation of Dendrogramma species can be discerned and is much like a cnidarian or a ctenophore – two basic germ layers with thick jelly in between, and a blind gut – but they appear to lack anything that would clearly identify them as a member of either group, such as comb rows or stinging cells. Because they appear to have only two germ layers, the authors conclude they are probably not bilaterians, but because they don’t have diagnostic features of any other kind of animal, and because there’s so much more we don’t know about them, they don’t feel brave enough to place them beyond that.
The beasties are made of a stalk and a flat disc; the mouth opens at the tip of the stalk and the gut extends into the disc, where it bifurcates repeatedly to form dozens of branches. Two comments on the PLoS website point out that this arrangement is a bit like a flatworm – many of which have a long pharynx that they can poke out to feed, and a highly branched intestine occupying most of the body (a lovely diagram and photo can be found in the bottom half of this page).
Superficially at least, it sounds possible that Dendrogramma‘s “stalk” is an extended pharynx. However, flatworms are bilaterians, and between their skin and their gut wall they are full of the tissues of the mesoderm, the third germ layer – muscles, simple kidneys, reproductive organs and quite a lot of cell-rich connective tissue. Just et al.‘s description of Dendrogramma states that the equivalent space in these creatures is filled with mesogloea, i.e. jelly with few or no cells. If Dendrogramma really lacks mesodermal tissues, then it wouldn’t make a very good flatworm! (The paper itself doesn’t discuss the flatworm option at all, presumably for similar reasons.)
Of course, the thing that piqued my interest in Dendrogramma is its supposed resemblance to certain Ediacaran fossils, specifically these ones. It would be awesome if we could demonstrate that the living and the fossil weirdos are related, since then determining what Dendrogramma is would also classify the extinct forms, but I’m not holding my breath on this count. The branching… whatevers in the fossils in question may look vaguely like the branching gut of Dendrogramma, but, as discussed above, so do flatworm guts. The similarity to the fossils may well have nothing to do with actual phylogenetic relatedness, which the authors sound well aware of.
Nature, helpful as always. >_>
It seems all we can do for the moment is wait for more material to come along, hopefully in a good enough state to make detailed investigations including genetic studies. My inner developmental biologist is also praying for embryos, but the gods aren’t generally kind enough to grant me these sorts of wishes 😛
I do quite like the name, though. Mmmmm, Dendrogramma. 🙂
Just J et al. (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 9:e102976