Ultrastructure of the Protonephridial System in Neodasys chaetonotoideus (Gastrotricha: Chaetonotida) and in the Ground Pattern of Gastrotricha Alexander Kieneke, 1,2 * Pedro Martı ´nez Arbizu, 2 and Wilko H. Ahlrichs 1 1 Zoosystematik & Morphologie, Institut fu ¨ r Biologie und Umweltwissenschaften, Carl von Ossietzky Universita ¨t Oldenburg, 26111 Oldenburg, Germany 2 Forschungsinstitut Senckenberg, DZMB, 26382 Wilhelmshaven, Germany ABSTRACT The taxon Neodasys has a basal position within Gastrotricha. This makes it very interesting for phylogenetic considerations in this group. To complete the reconstruction of the nephridial system in the stem species of Gastrotricha started earlier, we have studied the whole protonephridial system of Neodasys chaetono- toideus by means of complete sets of ultrathin sections and TEM. In many characters, protonephridia of N. chae- tonotoideus resemble those of macrodasyidan gastrotrich species. For example, each of the six protonephridia, arranged in three pairs, consists of three distinct cells that constitute the continuous protonephridial lumen. Espe- cially, the terminal cell of the protonephridia of N. chaeto- notoideus shows a striking pattern: The perforation of the filter region is a meandering cleft that is continuous with the seam of the enfolded lumen of that cell. With the results presented here and that of former TEM studies, we give a comprehensive idea of the excretory organs in the ground pattern of Gastrotricha. Moreover, we can elaborate on the hypothesized protonephridial system in the stem species of Bilateria. We suggest that a meandering filtration cleft is a feature of the ground pattern of the Bilateria. J. Morphol. 268:602–613, 2007 Ó 2007 Wiley-Liss, Inc. KEY WORDS: Gastrotricha; phylogenetic implications; protonephridia; TEM; Neodasys; Bilateria The marine taxon Neodasys Remane, 1927 is one of the most basal groups within Gastrotricha. Although Neodasys has occupied different system- atic positions within Gastrotricha (for a compre- hensive overview see Hochberg, 2005), it is very likely that it has this basal position within Gastro- tricha and is the sister group of all other Chaeto- notida (d’Hondt, 1971; Ruppert, 1982; Travis, 1983; Hochberg and Litvaitis, 2000). A recent anal- ysis based on sequences of the 18S rRNA gene placed Neodasys within a part of the Macroda- syida, which appear as a paraphyletic group in that study. However, the basal position of Neo- dasys among Gastrotricha is confirmed (Todaro et al., 2006). Neodasys has protonephridia, as all other gastro- trichs investigated so far possess these organs for excretion and osmoregulation (Brandenburg, 1962; Teuchert, 1967, 1973; Rieger et al., 1974; Neuhaus, 1987; Kieneke et al., in press). Apart from our knowledge of their existence, only a few structural data of these pressure-filtration nephridia in Neo- dasys sp. have been obtained (Ruppert, 1991). Con- sidering the putative basal position of Neodasys within Gastrotricha, we need to know the mor- phology of protonephridia in this taxon in order to reconstruct the protonephridial system of the stem species of Gastrotricha (Kieneke et al., in press). A complete analysis of the protonephridial system in Neodasys is necessary to figure out every single character state of the protonephridia in the ground pattern of Gastrotricha. MATERIALS AND METHODS The investigated specimens of the marine gastrotrich Neo- dasys chaetonotoideus Remane, 1927 were sampled on April 12, 2005 at the artificial intertidal flat Geniusbank in the north of Wilhelmshaven, Germany (Wadden Sea) and on September 4, 2005 at the beach (eulitoral zone) of the island Spiekeroog, Ger- many (North Sea). The specimens were extracted from the uppermost, oxygenated layer of the fine sandy (Geniusbank) or medium grain size sediment (beach of Spiekeroog) with the sea- water–ice treatment (Uhlig, 1964). For transmission electron microscope (TEM) observation, the extracted animals were narcotized in a 7% MgCl 2 solution for a few minutes. After complete relaxation of the specimens, they were fixed in 2.5% glutaraldehyde (a little ruthenium-red added) in 0.1 M sodium cacodylate buffer (pH 7.2) for 1 h at 48C. All specimens were postfixed for 60 min at 48C in 1% This article is dedicated to Reinhard Rieger. His tragic death means a serious loss to the community of zoologists. He was one of the most important scientists of our day in Morphology and Phylog- eny and, not least, in Gastrotricha research. Contract grant sponsor: German Science Foundation (DFG); Contract grant number: AH 94/3-1, AH 94/3-2, MA 2557/4-1, MA 2557/4-2. *Correspondence to: Alexander Kieneke, Zoosystematik & Mor- phologie, Institut fu ¨ r Biologie und Umweltwissenschaften, Carl von Ossietzky Universita ¨t, 26111 Oldenburg, Germany. E-mail: akieneke@senckenberg.de Published online 17 April 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10536 JOURNAL OF MORPHOLOGY 268:602–613 (2007) Ó 2007 WILEY-LISS, INC.