Jellies and Sponges
ScienceDaily reports on an interesting phylogenetic study in their imaginatively named article Tree Of Animal Life Has Branches Rearranged, By Evolutionary Biologists (who else?).
The big shocker: Comb jellyfish — common and extremely fragile jellies with well-developed tissues — appear to have diverged from other animals even before the lowly sponge, which has no tissue to speak of. This finding calls into question the very root of the animal tree of life, which traditionally placed sponges at the base.
To clarify, comb jellies (Ctenophora) are not grouped with the other jellies, corals, anemones, hydras etc. (Cnidaria), and at least on the course I very recently took on systematic zoology, we were taught that Cnidaria is most likely the sister group of Bilateria (the rest of the animals), Ctenophora is the next group down, and then Porifera (sponges) sits at the very base of the animal tree. The question was whether either Ctenophora or Cnidaria were more closely related to Bilateria, or if they were a monophyletic group that itself was the sister group of Bilateria - but Porifera’s place at the base of the tree was never in question.
Although I have the original Letter to Nature I haven’t had the energy to actually read it yet, although I did look at the pretty pictures. The figures suggest that Cnidaria and Porifera compose a taxon that is the sister group of the rest of the animals, and Ctenophora is now the basal group.
I must say I instinctively feel very skeptical of this, but since I haven’t reviewed the evidence I’ll obviously reserve judgement. Sponges have no real tissues and can only barely be considered animals. Also they share a very interesting characteristic with some colonial organisms that might very well be the sister group of animals, the Choanoflagellates. Pharyngula recently blogged about this.
I’m hoping more accomplished scientist bloggers will pick up on this story as I’m interested what they have to say - if anyone sees a post about it, please don’t hesitate to point me in the right direction!
On a sidenote, ScienceDaily has illustrated their article with a picture of Cnidarians, apparently thinking that “comb jelly” means “any ol’ jellyfish”. [They’ve now changed the illustration - I suspect I wasn’t the only one who mailed them about it.] Comb jellies don’t swim by pumping clumsily like jellyfish - they’re the ones with the combs of beating cilia that refract light like rainbows.
biology, evolution, systematics, tree of life, animals, jellyfish, sponges
