Abstract
BackgroundXenoturbella is a group of marine benthic animals lacking an anus and a centralized nervous system. Molecular phylogenetic analyses group the animal together with the Acoelomorpha, forming the Xenacoelomorpha. This group has been suggested to be either a sister group to the Nephrozoa or a deuterostome, and therefore it may provide important insights into origins of bilaterian traits such as an anus, the nephron, feeding larvae and centralized nervous systems. However, only five Xenoturbella species have been reported and the evolutionary history of xenoturbellids and Xenacoelomorpha remains obscure.ResultsHere we describe a new Xenoturbella species from the western Pacific Ocean, and report a new xenoturbellid structure - the frontal pore. Non-destructive microCT was used to investigate the internal morphology of this soft-bodied animal. This revealed the presence of a frontal pore that is continuous with the ventral glandular network and which exhibits similarities with the frontal organ in acoelomorphs.ConclusionsOur results suggest that large size, oval mouth, frontal pore and ventral glandular network may be ancestral features for Xenoturbella. Further studies will clarify the evolutionary relationship of the frontal pore and ventral glandular network of xenoturbellids and the acoelomorph frontal organ. One of the habitats of the newly identified species is easily accessible from a marine station and so this species promises to be valuable for research on bilaterian and deuterostome evolution.
Highlights
Xenoturbella is a group of marine benthic animals lacking an anus and a centralized nervous system
Recent molecular phylogenetic analyses support a close affinity with the Acoelomorpha [12,13,14], a group of marine worms originally suggested to belong to the Platyhelminthes, but later suggested to be the sister group to the Nephrozoa [15,16,17,18,19,20,21]
Similarities in their overall morphologies and position tempts us to speculate that the frontal pore and ventral glandular network of xenoturbellids and the frontal organ of acoelomorphs are homologous and that the structure is a synapomorphy for Xenacoelomorpha
Summary
Xenoturbella is a group of marine benthic animals lacking an anus and a centralized nervous system. Molecular phylogenetic analyses group the animal together with the Acoelomorpha, forming the Xenacoelomorpha This group has been suggested to be either a sister group to the Nephrozoa or a deuterostome, and it may provide important insights into origins of bilaterian traits such as an anus, the nephron, feeding larvae and centralized nervous systems. Xenoturbella is a group of marine benthic worms, first described in 1949 as a ‘strange’ platyhelminth [1] It has a mouth but lacks an anus, the digestive organ is a sack rather than a tube. Recent molecular phylogenetic analyses support a close affinity with the Acoelomorpha [12,13,14], a group of marine worms originally suggested to belong to the Platyhelminthes, but later suggested to be the sister group to the Nephrozoa (all remaining Bilateria) [15,16,17,18,19,20,21].
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