DESCRIPTION OF THE PHYLUM XENOTURBELLIDA (BOURLAT ET AL. 2003)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>DEUTEROSTOMATA>XENOTURBELLIDA |
PHYLUM XENOTURBELLIDA LINKS
Hierarchical Classification
| Xenoturbellida (ZE-no-tur-BEL-i-duh) is derived from a Greek root that means stranger (ξένος – zenos) and a Latin root (turba) that means turbulence (turbellida is the Latin diminutive of turba). The reference is to an odd animal that resembles a free-living flatworm (called Turbellaria because the ciliated epithelium causes turbulence around the body of the animal). These are the strange or odd Turbellarians because, although they superficially resemble free-living flatworms, the Xenoturbellarians have been demonstrated to be quite different. The whole phylum has only two recognized species. The phylum name was coined by Bourlat et al. (2003). |
| INTRODUCTION TO THE XENOTURBELLIDA Telford (2008) describes Xenoturbella as a “flatworm” with a ventral mouth and a blind gut (see Figure 1). In fact, its digestive system almost has only a ventral mouth and an internal gastric cavity that has no pharynx. The nervous system is almost nothing more than a nerve net with four longitudinal nerves and no major nerve cords or ganglia. The anterior end of the animal, which is about 3 cm long and 0.5 cm wide, is denoted by a field of pigmented dots. The middle of the body has an obvious groove (Figure 2). The extreme simplicity of the animal prompted Westblad (1949), who discovered the organism off of the coast of Sweden, to consider Xenoturbella a basal free-living platyhelminth. As of now, the group contains only two recognized species in the same genus. Xenoturbellida has taken a winding road through the animal kingdom. It has been treated as a basal metazoan (Ehlers and Sopott-Ehlers 1997), an acoel flatworm (Franzen and Afzelius 1987; Lundin 1998, 2001; Raikova et al. 2000; and Rohde et al. 1988), a paedomorphic larva of an unknown deuterostome (Reisinger 1960), a bivalve mollusk (Israelsson 1997, 1999; and Israelsson and Budd 2005), and a basal deuterostome (Bourlat et al. 2003, 2006; Perseke et al. 2007; and Telford 2008)! Israelsson (1999) provided a detailed study of the animals together with a description of a new species in the same genus. He was convinced that Xenoturbella was a bivalve in which the organization of the trochophore larva was lost in the simplified adult. Bourlat et al. (2003) showed conclusively that the connection with the bivalve mollusks was an artifact of the Xenoturbella diet because they fed on common bivalve mollusks. Bourlat et al. (2006), in an analysis using 170 nuclear proteins and 13 mitochondrial proteins showed that Xenoturbellida emerged within the Deuterostomata and were basal to the Echinoderms and Hemichordates, a group called Ambulacria (see Figure 3). Perseke et al. (2007) in analyzing mitochondrial comparisons conclude that Xenoturbellida is basal to the deuterostomes (Figure 4). Telford (2008) in attempting to reconcile the obvious similarities with the Acoels and the dawning understanding of the relationship of Xenoturbellida with the deuterostomes, suggested that Xenoturbellida and Acoelomata might be sister groups, which together are basal in the Ambulacria clade (Figure 5). The implications of these scenarios are not trivial. Consider the occurrence of gill slits, a character shared by members of both major clades of the deuterostomes. Because it is very unlikely that such structures could have evolved independently, the presumed deuterostome ancestor must have possessed that character. If so, Xenoturbellida is a secondarily simplified deuterostome. Figures 3 and 5 can be explained by invoking secondary simplification. The simplification also could explain why xenoturbellids have fewer HOX genes than the presumed deuterostome ancestor (Fritzsch et al. 2007). Furthermore, Figure 5 solves the problem of the similarities between the acoelomorphs and the xenoturbellids by the simplification of their common ancestor. Figure 4 also could explain the similarities between the acoelomorphs and the xenoturbellids. However, in this scenario, the xenoturbellids and acoelomorphs are primitive. The last common ancestor of the Ambulacria and Chordata appeared after the xenoturbellids. |
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| FIGURE 1. A longitudinal section of Xenoturbella showing the statocyst (st), mouth (m), gastric cavity (gc), and the ring furrow (rf). Image from: Telford (2008) | FIGURE 2. A drawing of Xenoturbella showing the ring furrow and the pigmented cells near the anterior end. The bar is 2 mm. Image from: Israelsson (2005) |
FIGURE 3. The current working hypothesis concerning the relationship between Xenoturbellida and the rest of the deuterostomes (clade D). In this view (e.g. Bourlat et al. 2006; Dunn et al 2008), Xenoturbellida is basal to the clade of Ambulacria (Hemichordata + Echinodermata).
B = Bilateria
D = Deuterostomata.
FIGURE 4. Perseke et al. (2007) find that the Xenoturbellida is a sister group to the rest of the deuterostomes.
FIGURE 5. Telford (2008) summarizes arguments that suggest that Xenoturbellida, which has been demonstrated to be a deuterostome, is a sister group to the Acoelomorpha.
| LITERATURE CITED Bourlat, S. J., T. Juliusdottir, C. J. Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S. Lander, M. Thorndyke, H. Nakano, A. B. Kohn, A. Heyland, L. L. Moroz, R. R. Copley, and M. J. Telford. 2003. Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 444: 85-88. Bourlat, S. J., C. Nielsen, A. E. Lockyer, D. T. Littlewood, and M. J. Telford. 2003. Xenoturbella is a deuterostome that eats molluscs. Nature. 424: 925-928. Bourlat S. J., T. Juliusdottir, C. J. Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S. Lander, M. Thorndyke, H. Nakano, A. B. Kohn, A. Heyland, L. L. Moroz L. L., R. R. Copley, M. J. Telford. 2006a. Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 444: 85–88. Bourlat, S. J., H. Nakano, M. Akerman, M. J. Telford, M. Thorndyke, and M. Obst. 2006b. Feeding ecology of Xenoturbella bocki (phylum Xenoturbellida) revealed by genetic barcoding. Molecular Ecology Notes. 8: 18-22. Dunn, C. W., A. Hejnol, D. Q. Matus, K. Pang, W. E. Browne, S. A. Smith, E. Seaver, G. W. Rouse, M. Obst, G. Edgecomb, M. V. Sorensen, S. H. D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R. Mobjerg-Kristensen, W. C. Wheeler, M. Q. Martindale, and G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 452: 745-749. Franzen, A. and B. A. Afzelius. 1987. The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations. Zoologica Scripta. 16: 9-17. Fritzsch, G., M. U. Bohme, M. Thorndyke, H. Nakano, O. Israelsson, T. Stach, M. Schlegl, T. Hankeln, and P. F. Stadler. 2007. A PCR survey of Xenoturbella bocki HOX genes. Journal of Experimental Zoology B Molecular and Developmental Evolution. 310B: 278-284. Israelsson, O. 1997. …and molluscan embryogenesis. Nature. 390: 32. Israelsson, O. 1999. New light on the enigmatic Xenoturbella (phylum uncertain): Ontogeny and phylogeny. Proceedings of the Royal Society of London. B. 266: 835-841. Israelsson, O. and G. E. Budd. 2005. Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey. Development Genes and Evolution. 215: 358-363. Lundin, K. 1998. The epidermal ciliary rootlets of Xenoturbella bocki (Xenoturbellida) revisited: new support for a possible kinship with the Acoelomorpha (Platyhelminthes). Zoologica Scripta. 27(3): 263-270. Lundin, K. 2001. Degenerating epidermal cells in Xenoturbella bocki (phylum uncertain). Nemertodermatida and Acoela (Platyhelminthes). Belgian Journal of Zoology. 131: 153-157. Perseke, M., T. Hankeln, B. Weich, G. Fritzsch, P. F. Stadler, O. Israelsson, D. Bernhard, and M. Schlegel. 2007. The mitochondrial DNA of Xenoturbella bocki: Genomic architecture and phylogenetic analysis. Theory in Biosciences. 126: 35-42. Raikova, O. I., M. Reuter, U. Jondelius, and M. K. S. Gustafsson. 2000. An immuno-cytochemical and ultrastructural studyof the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.) Zoomorphology. 120: 107-118. Rohde, K., N. Watson, L. R. G. Cannon. 1988. Ultrastructure of epidermal cilia of Pseudactinoposthia sp. (Platyhelminthes Acoela) – implications for the phylogenetic status of the Xenoturbellida and Acoelomorpha. Journal of Submicroscopic Cytology and Pathology. 20: 759-767. Telford, M. J. 2008. Xenoturbellida: the fourth deuterostome phylum and the diet of worms. Genesis. 46: 580-586. Westblad, E. 1949. Xenoturbella bocki n.g., n.sp., a peculiar, primitive turbellarian type. Arkiv Zoologi. 1: 3-29. |
| By Jack R. Holt. Last revised: 04/07/2013 |
