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