A home for Xenoturbella

Abstract

Lacking a centralized nervous system, coelom, anus and reproductive organs, the deep-sea flatworm Xenoturbella presents problems when it comes to its classification and teasing out its evolutionary history. Despite its simplicity, some of Xenoturbella's features appear to align it among the deuterostomes, the group of animals that includes ourselves. If true, this implies either a radical simplification of the body plan or the acquisition of many key deuterostome features independently by the various deuterostome groups. Two papers in this issue tackle different aspects of Xenoturbella, but together, move the field on a notch. Greg Rouse et al. add four new deep-sea species of Xenoturbella from the eastern Pacific Ocean to the two already known from the Atlantic. Phylogenomic analysis aligns them at the base of the Protostomia or even as basal bilaterians — much as would be expected from their simple morphology and not invoking radical simplification. Andreas Hejnol and colleagues come to a broadly similar conclusion based on robust phylogenetic analysis using eleven transcriptomes of Xeonturbella and acoel worms.