Field of Science

Loss of anus?

ResearchBlogging.orgIt's nice when we are somewhat certain that evolutionary trees are accurate, so it is unsettling when researchers suggest that they aren't. This time it's Acoels, which were are thought to be older than both Protostomes (mouth develops before the anus) and Deuterostomes (anus first). But now a team of evolutionary biologists is suggesting that Acoles belong within the group of Deuterostomes. This even though Acoles don't have a separate mouth and anus at all, but an opening that serves both functions.
Zoologists have generally placed acoels on the earliest branch of the bilaterians — before the split between protostomes and deuterostomes — because the worms lack so many key features such as a separate mouth and anus, a central nervous system and organs to filter waste.
(From Nature News).

Click for larger version.

After analysing sequences from nuclear DNA, the group made a separate evolutionary tree based on genes in mitochondria. They also studied microRNAs, which regulate gene expression but do not code for proteins. According to co-author Kevin Peterson, a palaeontologist at Dartmouth College in Hanover, New Hampshire, microRNAs are particularly useful for studying deep evolutionary relationships. The team found that acoels have a type of microRNA known to be specific to deuterostomes, suggesting that they are related. [Emphasis added.]
Really? This microRNA evidence weighs more than the fact that Acoels have no anus, etc.? The researchers did not only use microRNA:
Here we assemble three independent data sets—mitochondrial genes, a phylogenomic data set of 38,330 amino-acid positions and new microRNA (miRNA) complements—and show that the position of Acoelomorpha is strongly affected by a long-branch attraction (LBA) artefact. When we minimize LBA we find consistent support for a position of both acoelomorphs and Xenoturbella within the deuterostomes.
Okay, but since I don't give much credence to microRNA compared to presence/absence of separate mouth/anus and central nervous system, what would happen to the inference if the microRNA data were taken out?

Unlike some other researchers who like very much that we have a living form ancestral to all bilaterians (organisms with bilateral symmetry), my problem here is that we are asked to believe that an ancestor of Acoels had separate mouth and anus, etc., but then lost those later on. The hell?! One biology lab manual I used once had the text "the critical invention of the anus", and I think this is apt. What happens to a lineage that makes it lose it's anus, and instead start barfing out excrements?

Very strange.

Philippe, H., Brinkmann, H., Copley, R., Moroz, L., Nakano, H., Poustka, A., Wallberg, A., Peterson, K., & Telford, M. (2011). Acoelomorph flatworms are deuterostomes related to Xenoturbella Nature, 470 (7333), 255-258 DOI: 10.1038/nature09676


  1. Ophiuroids (brittle stars) lack an anus, despite their ancestors undoubtedly having one, and use the mouth for excretion as well as feeding. Some prostigmatan mites also lack an anus, though I don't know whether they actually excrete at any time during their very short lives so they may not be a good example. Most researchers also now accept a derived position for Platyhelminthes among the lophotrochozoans, in which case they would have also had to lose their anus at some point.

  2. Didn't Cartman suggest doing something like that on an episode of South Park? Or was his idea to ingest food through the anus and excrete through the mouth? But then again, Cartman could be a rather large, roundish Acoel himself.

  3. Christopher, thanks for sharing that. I didn't know that about Ophiuroids and Platyhelminthes. Do you know if anyone has proposed why they would have lost their anus?

  4. Lack of an anus really isn't that compelling in my view. Convergent evolution that sheds features previously acquired isn't that uncommon. For example, skinks, dolphins and whales lost limbs. Dolphins and whales also lost almost all hair. Mammals with good color vision lost their sense of smell.

    Many vertebrates including cats and birds barf as a compliment to their excretory system. Doing so serves a similar function to a drain catcher in the shower -- it prevents stuff that would clog up the plumbing from getting stuck in the digestive system. Perhaps having a dual purpose mouth-anus prevents digestive system clogging since anything small enough to get in will always be small enough to get out.

    An anus provides one more big hole that allows outside material to enter the body, so perhaps losing one reduces incidence of parasitic infection. By analogy, a large percentage of women with both vaginas and anuses experience (i.e. one more big hole in their bodies) yeast infections and other health problems related to infectious agents entering the body there that men do not. This isn't a bid deal if your body can cope with the infection, but if the anal equivalent of a yeast infection in Acoelomorpha were deadly to it, not having an anus might be a very good thing.

    Since polygenomic data confirm the microRNA and mitochondrial genes, it is safe to assume that the genetic classification is correct, but had the genetic classification come only from mitochondrial genes, one interesting alternative possibility might have been that there was a mitochondria transfer at some point in the beast's ancesteral history while the rest of it remained the same. This would arguably be a bacterial analogy to the kind of lateral gene transfer that retroviruses can cause.

    The general assumption is that mitochondria are bacteria that were captured by an early unicellular animal first in symeosis and eventualy as a degraded mere organ. A mitochondria transfer incident doesn't seem that different in terms of the realm of possibility. Perhaps for a while there could have been two mitochondria in the beast, but the more evolutionarily advanced version led the original mitochondria to take on a secondary role and then disappear entirely.

  5. Yes, some characters are lost in evolution, I am well aware of that. However, here we are talking about more than one character (anus, CNS, and filter organs). Also, I am not saying it is impossible, but that I would really like to hear suggestions for how it could happen. We quite well understand the selection pressures that made marine mammals lose or transform their limbs and hair, and ditto for sense of smell. And getting rid of a hole to prevent infectious agents entering sounds a little far-fetched, given that so many other organisms manage with an anus.

  6. A number of years back, I suggested that the reduced anatomy of Xenoturbella might be related to a size-bottleneck in the past, as loss of organ systems is quite common in microscopic animals, but at the time there was no evidence of such micro-xenoturbellids. Acoels may support this idea to some extent, as they are really tiny. However, I don't know of any other cases where loss of the anus has happened due to size reduction; it may only explain features such as the diffuse nervous system. In the case of Platyhelminthes, it is interesting that the Catenulida, one of the two basalmost divisions in Platyhelminthes (the other, the Rhabditophora, includes all other Platyhelminthes) and the ones with the most reduced digestive system, get the majority of their nutrients via endosymbiotic unicellular algae and largely don't actually eat (I suspect that they must eat a little, though, if only to take up the algae in the first place). This may be related to anus loss in Platyhelminthes (maybe the rhabditophoran digestive system has been re-evolved to replace one lost by catenulid-type ancestors?), or it may be a unique feature of catenulids. It may be interesting to see whether nutritive symbioses exist in acoelomorphs.

    I should note that I remain skeptical of acoelomorphs as deuterostomes, though I'm not inherently opposed to the idea. A derived nature for Platyhelminthes seems pretty solid at present, though.

    Re mitochondria replacement: the idea seems credible to me, though I don't know of any actual examples. There are known cases of chloroplast replacement in dinoflagellates that could work as an analogy.

  7. So I was thinking about this post, and it occurs to me that one of the very few (mostly) irreversible arrows of evolution appears to be "more orifices" -- or at least, "if this orifice is serving multiple functions, divide it up". Granted, we're only up to three now, but as far as I know, the process virtually never reverses itself. It's easy to cite examples of animals that have lost ancestral limbs, but not so easy to cite examples of animals that went back to having a single mouth/anus, or that went back to having a cloaca.

    Which makes me wonder, in a few more hundred million years, will we see species that have two penises, one for mating and one for urinating? heh...

  8. You're forgetting ears and nose. That's sort of three more.

  9. animals that went back to... having a cloaca

    Birds, quite likely. Hedgehogs, definitely.

  10. Birds, quite likely.

    Really?? Huh, my ignorance is display I guess. Well forget my whole point then!

    What common ancestor are you thinking of?

  11. Same here. Which bird ancestor lost the anus? Same for hedgehogs.

  12. Not lost the anus, reverted to having a cloaca. Hedgehogs and neoavian birds both have a single combined reproductive and excretory opening, despite probably having ancestors where they were separate (in the case of birds, palaeognaths and Galloanserae have large phalli while neoavians don't; it's possible that the large phalli devloped independently in the former two groups, but most researchers seem to incline to thinking that the neoavian condition is more likely a reversion).

  13. Hello! Can you send me the pdf of paper "Acoelomorph flatworms are deuterostomes related to Xenoturbella". I'm interesting in this group of bilaterian invertebrates. Thank you!

  14. Sure, Lisandro, if you give me your email...


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