JOURNAL OF MORPHOLOGY 269:698–712 (2008) Ultrastructure and Function of the Mastax in Dicranophorus forcipatus (Rotifera: Monogononta) Ole Riemann* and Wilko H. Ahlrichs Zoosystematik and Morphologie, Institut fu¨ r Biologie und Umweltwissenschaften, Carl von Ossietzky Universita¨t Oldenburg, 26111 Oldenburg, Germany ABSTRACT Rotifers are characterized by a complex The morphology of the jaw apparatus in rotifers set of cuticularized jaw elements in the pharynx. The has always played an important role in rotifer fine structure of the jaw elements has been the subject research. Together with features of the rotatory of SEM studies for some time, but only very limited apparatus, the mastax has been the most impor- information exists on the ultrastructure of the jaw tant character in traditional rotifer systematics elements and their function beyond taxonomic consider- ations. Drawing on SEM and TEM techniques, the pres- (Remane, 1933; De Beauchamp, 1965). Different ent study presents a detailed analysis of the mastax in mastax types and feeding modes have been identi- Dicranophorus forcipatus, a carnivorous monogonont fied. Correct species identification very often is rotifer species from freshwater habitats characterized by impossible without careful analysis of the fine an extrusible, grasping jaw apparatus. Based on ultra- structure of the trophi elements. In contemporary thin serial sections, the jaw elements are reconstructed rotifer taxonomy, scanning electron microscopy and, in total, nine paired and two unpaired muscles (SEM) of isolated trophi elements has become a identified. Possibly homologous muscles in other rotifer standard procedure (Kleinow et al., 1990; De species are discussed and functional considerations of Smet, 1998). Based primarily on such prepara- the forcipate mastax are suggested. J. Morphol. 269: tions, cladistic analyses of the taxon Rotifera have 698–712, 2008. Ó 2008 Wiley-Liss, Inc. been carried out (Sørensen, 2002). KEY WORDS: forcipate mastax; ultrastructure; feeding However, while our understanding of the fine mode structure of the hard elements based on SEM stud- ies is fairly good, we know only very little about how these jaw elements function in living speci- Rotifers are a remarkably diverse group of mens and how they interact with the surrounding aquatic micrometazoans with about 2,000 species musculature. Hardly anything is known about the described to date (Wallace et al., 2006; Segers, structural and functional basis of the different mas- 2007). They are most abundant, both in terms of tax types and different modes of feeding they make species diversity and biomass, in freshwater habi- possible. So far, only very few investigations into tats. A number of species also occur in marine envi- the histology and ultrastructure of the rotifer mas- ronments and in moist terrestrial habitats such as tax have been carried out. Most of them date back wet moss cushions and damp soil. One of the most to the early part of the 20th century and are re- characteristic features of rotifers is a complex set stricted to traditional light microscopic histological of pharyngeal, cuticularized jaw elements used in techniques (De Beauchamp, 1909; Martini, 1912; various ways for food uptake and processing. The Seehaus, 1930; Stoßberg, 1932). pharyngeal bulb together with the jaw elements is Recent advances in confocal laser scanning mi- called the mastax; the jaw elements as such are croscopy investigations of the muscular system of referred to as trophi. The ultrastructure of the tro- rotifers have produced superb results for body phi is remarkably similar to jaw elements in Gna- musculature, but, due to limited resolution, failed thostomulida and Limnognathia maerski. Based on to unravel the complex arrangement of pharyngeal this common ultrastructural feature and other shared characters, rotifers together with gnathos- Contract grant sponsors: Evangelisches Studienwerk Villigst; tomulids, Limnognathia maerski, Seison and the Deutsche Forschungsgemeinschaft (DFG). parasitic acanthocephalans have been suggested to comprise a monophyletic taxon Gnathifera (Ahl- *Correspondence to: Ole Riemann, Zoosystematik and Morpholo- richs, 1995; Rieger and Tyler, 1995). Studies rely- gie, Institut fu¨ r Biologie und Umweltwissenschaften, Carl von ing on molecular data do not produce unambiguous Ossietzky Universita¨tOldenburg,26111Oldenburg,Germany. results regarding the monophyly of Gnathifera: E-mail: [email protected] While some support its monophyly including also Published online 26 February 2008 in Cycliophora (Giribet et al., 2000), it is questioned Wiley InterScience (www.interscience.wiley.com) by others (Giribet et al., 2004). DOI: 10.1002/jmor.10616 Ó 2008 WILEY-LISS, INC. MASTAX OF DICRANOPHORUS FORCIPATUS 699 muscles and jaw elements (Sørensen et al., 2003; coverslip. The position of the jaws was manipulated by carefully Santo et al., 2005; Sørensen, 2005). Transmission touching the edges of the coverslip with a fine needle. Digital electron microscopic (TEM) investigations based on images were taken with an Olympus color view I digital camera. ultrathin serial sections and careful reconstruc- tions seem to be indispensable for a detailed Terminology understanding of the complex mastax. Unfortu- nately, however, there are very few such studies To avoid terminological confusion, the terms defined in the following are used consistently throughout this article (see also (Koehler and Hayes, 1969a,b; Cle´ment and Amsel- Fig. 2A). lem, 1989; Ahlrichs, 1995). Both the unpaired incus (paired rami, unpaired fulcrum) and The present study is intended to present a more the paired malleus (manubrium, uncus) consist of distinct jaw comprehensive understanding of the forcipate elements that are connected at one point. This connection (grasping) mastax type. It is based on SEM prepa- serves as a point of reference for the terms proximal–distal denoting relative position. For a description of the position of rations and TEM studies of complete ultrathin the jaw apparatus relative to the specimen, the terms frontal– serial sections of the forcipate mastax in Dicrano- caudal, dorsal–ventral and median–lateral are used. phorus forcipatus, a cosmopolitan, predatory roti- Besides the fulcrum, rami, manubria and unci constituting fer species commonly occurring in the periphyton the basic jaw elements that can be found in almost every rotifer species, there are a number of accessory jaw elements charac- of freshwater habitats. For the first time our teristic only of certain subtaxa of Rotifera. In Dicranophorus approach reveals the complex interplay of mastax forcipatus, a pair of rod-shaped accessory jaw elements ventral hard elements, epithelial cells, and individual to the rami can be found. Traditionally, these are called epi- muscles responsible for the coordinated movement pharynges. However, given their position ventral to the rami of the jaw apparatus in a representative of Dicra- and considering the fact that in certain species accessory jaw elements dorsal to the rami exist as well, the term hypophar- nophoridae. Based on these reconstructions and ynges (sg. hypopharynx) is suggested and is introduced here for supported by observations of living specimens, we the first time. suggest functions of individual muscles in the overall grasping movement of the jaw apparatus. Identifying individual muscles, we expect that RESULTS future studies may reveal potentially homologous The Cuticularized Jaw Elements muscles in other species and that in such a way, General organization. The mastax with its the musculature of the rotifer mastax may become characteristic, grasping jaw apparatus is very con- another tool in refining our knowledge of rotifer spicuous in living specimens observed under the phylogeny. light microscope (Fig. 1A). When retracted, it is positioned caudal to the mouth opening, through which it can be extruded to grasp various items of MATERIALS AND METHODS food. In vivo, the longitudinal axis of the jaw appa- ratus is slightly tilted against the body axis. Specimens of Dicranophorus forcipatus (O. F. Mu¨ller, 1786) were sampled in shallow ditches covered with Lemna sp. near The mastax in Dicranophorus forcipatus, com- Oldenburg, North-West Germany. For SEM preparations, indi- prising the trophi, individual muscles, epithelial vidual specimens were dissolved following the protocol given by cells, paired salivary glands and sensory cilia, is Kleinow et al. (1990) leaving only the cuticularized jaw appara- organized in a strictly bilaterally symmetrical way. tus. These jaws were carefully rinsed with distilled water, air- dried, and coated with gold. SEM was carried out on a Zeiss The cuticularized jaw apparatus consists of the DSM 940 scanning electron microscope. In total, four specimens unpaired fulcrum and the paired rami (together were prepared for SEM. For TEM studies, specimens were referred to as the incus), paired manubria with anaesthetized for 5 min in an aqueous solution of 0.25% bupiva- proximally attached unci (together referred to as 1 caine (Bucain ) and subsequently fixed with 1% OsO4 in 0.1 m the malleus) and paired hypopharyngeal rods posi- NaCaCodylate buffer at 08C. After fixation, specimens were dehydrated in an increasing acetone series, subsequently embed- tioned ventral to the rami (Figs. 1B,C and 2A). ded in Araldite hardened at 608C for 72 h and ultrasectioned The cuticularized jaw elements are extracellular (70 nm) on a Reichert ultracut followed by automatic staining and surrounded