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Xenoturbella is a deuterostome that eats molluscs

Abstract

Xenoturbella bocki, first described in 1949 (ref. 1), is a delicate, ciliated, marine worm with a simple body plan: it lacks a through gut, organized gonads, excretory structures and coelomic cavities. Its nervous system is a diffuse nerve net with no brain. Xenoturbella's affinities have long been obscure and it was initially linked to turbellarian flatworms1. Subsequent authors considered it variously as related to hemichordates and echinoderms owing to similarities of nerve net and epidermal ultrastructure2,3, to acoelomorph flatworms based on body plan and ciliary ultrastructure4,5,6 (also shared by hemichordates7), or as among the most primitive of Bilateria8. In 1997 two papers seemed to solve this uncertainty: molecular phylogenetic analyses9 placed Xenoturbella within the bivalve molluscs, and eggs and larvae resembling those of bivalves were found within specimens of Xenoturbella10,11. This molluscan origin implies that all bivalve characters are lost during a radical metamorphosis into the adult Xenoturbella. Here, using data from three genes, we show that the samples in these studies were contaminated by bivalve embryos eaten by Xenoturbella and that Xenoturbella is in fact a deuterostome related to hemichordates and echinoderms.

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References

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Acknowledgements

We thank M. Akam and R. Jenner for comments on the manuscript, I. Ruiz Trillo for sharing unpublished results, and the scientists of Kristineberg Marine Station for help in sample collection. We are grateful for support from the Wellcome Trust to M.J.T. and to D.T.J.L.

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Competing interests

The authors declare that they have no competing financial interests.

Correspondence to Maximilian J. Telford.

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Supplementary Information: Figures and Tables (PDF 1264 kb)

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Further reading

Figure 1: Best ML tree found using SSU sequences.
Figure 2: Position of Xenoturbella within the deuterostomes as suggested by our analyses of SSU and mitochondrial data.

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