Strange worm, Xenoturbella


This odd marine worm, Xenoturbella bocki, is in the news right now, and I had to look it up in Pechenik’s Biology of the Invertebrates(amzn/b&n/abe/pwll) to remind myself of what it was. Here’s the complete entry:

Xenoturbella bocki

This marine worm, first described in 1949 as an acoel flatworm and later claimed as either an early metazoan offshoot or a primitive deuterostome, has recently been affiliated with primitive bivalve molluscs, based upon a study of gamete development (oogenesis) and an analysis of sequence data from both 18S rRNA and mitochondrial genes. Little is known about its reproductive mode, and developmental studies that might help to resolve the phylogenetic issues are just starting to be reported. A second species was described in 1999.

The animals are up to 4 cm long, vermiform (worm-shaped), and covered by locomotory cilia. They have no digestive tract, and indeed no organs at all. Their only conspicuous morphological feature, other than their cilia, is a statocyst for determining orientation. To date, they have been collected only off the coasts of Sweden and Scotland, in sediments at depths of 20 m to 100 m.

That’s it. Part of that is now known to be wrong: the data showing an affinity to the molluscs is an artifact, caused by the fact that it somehow eats bivalves, and partly digested clam material contaminated the samples. Otherwise, not much is known; I’ve found papers describing the presence of oocytes inside the animal, but no one as far as I know has actually observed its development. It’s a strange, mysterious blob of a worm.

We now know a little more about it, though. A rather thorough molecular study of the organism has accumulated sufficient sequence information to place it in a phylogenetic tree, and it falls into the deuterostome clade, along with us chordates, echinoderms, and hemichordates. At the same time, it’s different enough that the investigators believe it warrants its very own phylum, the Xenoturbellida, bringing the number of extant deuterostome phyla up to four. If only there were a parliament of metazoans, our party would now have an additional vote, although I suspect the Xenoturbellida would always be siding with the flatworms, anyway.

Here’s the summary tree from their data:

Bayesian analysis of nuclear data after the addition of asteroid, hemichordate and xenoturbellid data. The new sequences join the branch to the echinoderm, and the cephalochordates now join the chordate branch. This indicates that the previous result is due to systematic error. Xenoturbella is the sister group of the Ambulacraria (echinoderms plus hemichordates).

You may notice that there are two major branches of that tree. The authors also propose two new clades to represent those groups: the Olfactores, consisting of the vertebrates and urochordates, and the Ambulacria (hemichordates and echinoderms).

Another purpose of the paper was to resolve a recent issue, the possibility suggested in some previous work that the cephalochordates were more closely related to the echinoderms than to the other chordates. That was a strange result, and would imply that echinoderms had to have lost a great many characters common to cephalochordates and vertebrates, such as a notochord. This seemed most improbable, and this new paper suggests it was an artifact of the analysis. They threw in more data (more data is always good) and carried out a few manipulations to remove details known to skew results, and got more detailed trees that fit the expectations from morphology much better, and, of course, are stronger and more consistent in themselves.

Bayesian analysis of mitochondrial data with the amino acids M, I, N and K recoded as missing data places the cephalochordates with vertebrates; Xenoturbella is the sister group of Ambulacraria. Support for critical nodes is shown in the order BPP (%), TF (%) and MLBP (%). Long branches leading to the urochordates have been shortened as indicated by a break (/) in the branch.

I do have two reservations about the work on this animal, though.

One is that it is all well and good to have more solid molecular analyses, but I have this little bias…it bothers me to see an animal called a deuterostome when no one has looked at a gastrula. I’d say at this point that the deuterostomy of Xenoturbellida is a hypothesis, one likely to be found correct, but I’d sure like to see some developmental biology. I’d like to see more biology of this organism, period. We’ve got its place in the tree of history marked, but we still don’t know anything about its life history, its physiology, its development—and given its place in the family tree, the embryo and larva could be very informative.

Another problem I have is more with the press reports than in the work itself. For example, look at this press release, Enigmatic worm identified as mankind’s long lost relative:

Scientists have discovered one of mankind’s closest invertebrate relatives in the shape of a rare 3cm worm that resides in mud at the bottom of a Swedish fjord.

The research, published in today’s edition of Nature (21 August), is the first conclusive proof that humans and the ‘Xenoturbella’ worm, whose Latin name means strange flatworm, derive from a common ancestor, thereby placing Xenoturbella in the same division of the animal kingdom as man.

Ick, ick, ick. Everything is about this animal’s relationship to human beings, and it implies so much that is wrong. Look at the trees. This creature is closer to the lines of starfish and acorn worms than to the vertebrates. The appeal of Xenoturbella is that it represents a group that branched off within the deuterostome clade long, long ago. It’s the distance of this relative that’s interesting, but it’s not unique. Fruit flies are also invertebrate relatives of humans, and are even more distant than this strange worm; shouldn’t the media be just as excited about the relationship of its readers to sea urchins, flies, clams, bacteria, and trees?

Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR, Telford MJ. (2006) Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 444(7115):85-8.

Bourlat SJ, Nielsen C, Lockyer AE, Timothy D, Littlewood J, Telford MJ (2003). Xenoturbella is a deuterostome that eats molluscs. Nature 424: 925-928.