Elsevier

Tissue and Cell

Volume 39, Issue 3, June 2007, Pages 171-177
Tissue and Cell

Ultrastructural aspects of the ‘statocyst’ of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

https://doi.org/10.1016/j.tice.2007.03.002Get rights and content

Abstract

The “statocyst” in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body (“statolith”), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.

Introduction

Xenoturbella is an enigmatic marine animal that is merely a ciliated bag with epithelial epidermis and gastrodermis and a ventral opening (“mouth”), but without any distinct organs except for a “statocyst” containing flagellated cells (Westblad, 1949, Ehlers, 1991, Israelsson, 1999, Stach et al., 2005). The two known species, Xenoturbella bocki Westblad (1949) and X. westbladi Israelsson (1999), are rapidly moving, free-living, and marine. Its simplicity in organization has led to diverse phylogenetic interpretations. Recent nucleotide sequence data support a deuterostome relationship (Bourlat et al., 2003, Bourlat et al., 2006, Israelsson and Budd, 2005).

The statocyst has been described at light microscopical level by Westblad (1949) and briefly at ultrastructural by Ehlers (1991). The aim of this study is to describe its ultrastructure in detail.

Section snippets

Material and methods

Several hundred specimens of X. bocki and X. westbladi were collected off the west coast of Sweden during 1997–2004 as described in Israelsson (2006). Specimens for light microscopy were fixed in 2.5% glutaraldehyde and 3.5% paraformaldehyde in 0.05 M phosphate buffer (pH 7.4) with 0.6 M NaCl at 4 °C, dehydrated via ethanol series, cleared with Histoclear or xylene series, paraffin sectioned, and stained by various standard procedures (Romeis, 1989), such as Harris haematoxylin, Heidenhain iron

Light microscopy

A statocyst was found in every examined specimen of X. bocki and X. westbladi. The statocyst is identical in X. bocki and X. westbladi, irrespectively of sex, size, or season. Therefore, the following description will refer to both species. The traditional name of this structure (‘statocyst’) will be used, although it should be remember that its function is unknown.

Statocyst

The statocyst (Fig. 1, Fig. 2) is an unpaired oval vesicle (50–80 μm in diameter) in the anterior end of the body beneath the

Statocyst

The results of this study confirm the earlier descriptions by Westblad (1949) and Ehlers (1991) in general. However, these papers did not describe the vesicular system of the motile cells.

Westblad (1949) inferred that the capsule of the statocyst was formed as a continuation of the subepidermal membrane complex (SMC). To the contrary, Ehlers (1991) stated that the statocyst is located within the epidermis surrounded by the intraepidermal nerve plexus. The data reported in this paper support the

Acknowledgments

I thank D. Lindberg, K. McDonald, R. Zalpuri and W.P. Chan (University of California Berkeley), G.E. Budd (Department of Earth Sciences, Paleobiology, Uppsala University), A. Warén (Swedish Museum of Natural History, Stockholm, Sweden), the staff at Kristineberg Marine Laboratory, the crew of Oscar von Sydow, and an anonymous referee. The work was supported by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT).

References (13)

  • O. Israelsson

    Observations on some unusual cell types in the enigmatic worm Xenoturbella (phylum uncertain)

    Tissue Cell

    (2006)
  • S.J. Bourlat et al.

    Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida

    Nature

    (2006)
  • S.J. Bourlat et al.

    Xenoturbella is a deuterostome that eats molluscs

    Nature

    (2003)
  • U. Ehlers

    Comparative morphology of statocysts in the Plathelminthes and the Xenoturbellida

    Hydrobiologia

    (1991)
  • U. Ehlers et al.

    Ultrastructure of the subepidermal musculature of Xenoturbella bocki, the adelphotaxon of the Bilateria

    Zoomorphology

    (1997)
  • D.W. Fawcett

    A Textbook to Histology

    (1986)
There are more references available in the full text version of this article.

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