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The digestive system of xenacoelomorphs

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Abstract

Interest in the study of Xenacoelomorpha has recently been revived due to realization of its key phylogenetic position as the putative sister group of the remaining Bilateria. Phylogenomic studies have attracted the attention of researchers interested in the evolution of animals and the origin of novelties. However, it is clear that a proper understanding of novelties can only be gained in the context of thorough descriptions of the anatomy of the different members of this phylum. A considerable literature, based mainly on conventional histological techniques, describes different aspects of xenacoelomorphs’ tissue architecture. However, the focus has been somewhat uneven; some tissues, such as the neuro-muscular system, are relatively well described in most groups, whereas others, including the digestive system, are only poorly understood. Our lack of knowledge of the xenacoelomorph digestive system is exacerbated by the assumption that, at least in Acoela, which possess a syncytial gut, the digestive system is a derived and specialized tissue with little bearing on what is observed in other bilaterian animals. Here, we try to remedy this lack of attention by revisiting the different studies of the xenacoelomorph digestive system, and we discuss the diversity present in the light of new evolutionary knowledge.

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Acknowledgments

We would like to thank Dr. Olga Raikova (Saint Petersburg, Russia) for helping us clarifying some ultrastructural details of the Acoela that were needed for this review. Prof. Julian P.S. Smith III (Winthrop University, USA) kindly sent us a copy of his, comprehensive, PhD thesis on acoel digestive systems. Dr. Kennet Lundin (Gothenburg Natural History Museum, Sweden) was also instrumental in helping us to understand some aspects of the nemertodermatids’ digestive system.

The Meara stichopi stained section was provided by Dr. John Buckland-Nicks (Antigonish, Nova Scotia, Canada). Sections of Xenoturbella bocki were obtained from Michaela Egertova and Dr. Maurice Elphick (London, UK) and processed in house.

We would like to thank the anonymous referees for helping us to improve the manuscript.

Funding

V: H. would like to acknowledge the financial support provided by the NIH (NIH Grant 1RO1 R01 NS054814 to V.H.). PM and SGS would like to thank the financial support provided by the “KVA fund for internationalization and scientific renewal at the Sven Lovén Centre” that allowed them to collect Xenoturbella specimens in Sweden. In addition, SGS would like to thank the support provided by the COST SNF Grant IZCOZ0_182957.

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Authors and Affiliations

  1. Departament de Genètica, Microbiologia i Estadística, Universitat de Barcelona, Av. Diagonal 643, 08028, Barcelona, Spain

    B. Gavilán & P. Martinez

  2. Department of Biology, University of Fribourg, 10, ch. Du Musée, 1700, Fribourg, Switzerland

    S. G. Sprecher

  3. Department of Biology, University of California, Los Angeles, CA, USA

    V. Hartenstein

  4. Institut Català de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

    P. Martinez

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  1. B. Gavilán
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Gavilán, B., Sprecher, S.G., Hartenstein, V. et al. The digestive system of xenacoelomorphs. Cell Tissue Res 377, 369–382 (2019). https://doi.org/10.1007/s00441-019-03038-2

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