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Organization of the Epistome in Phoronopsis Harmeri (Phoronida) and Consideration of the Coelomic Organization in Phoronida

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Organization of the Epistome in Phoronopsis Harmeri (Phoronida) and Consideration of the Coelomic Organization in Phoronida Zoomorphology (2011) 130:121–134 DOI 10.1007/s00435-011-0126-z ORIGINAL PAPER Organization of the epistome in Phoronopsis harmeri (Phoronida) and consideration of the coelomic organization in Phoronida Elena N. Temereva • Vladimir V. Malakhov Received: 16 September 2010 / Revised: 28 April 2011 / Accepted: 30 April 2011 / Published online: 17 May 2011 Ó Springer-Verlag 2011 Abstract Results provided by modern TEM methods Keywords Lophotrochozoa Á Phylogeny Á Phoronida Á indicate the existence of the lophophoral and trunk coelo- Coelom Á Ultrastructure mes but not of the preoral coelom in Phoronida. In the present work, the epistome in Phoronopsis harmeri was studied by histological and ultrastructural methods. Two Introduction kinds of cells were found in the frontal epidermis: sup- porting and glandular. The coelomic compartment is shown The Phoronida has since long been in focus of debates on to be inside the epistome. This compartment has a complex Metazoan evolution. The investigations of Masterman shape, consists of a central part and two lateral branches, (1898), who advanced the archicoelomate concept, stimu- and contacts the lophophoral coelom, forming two complete lated further study on the anatomy of the phoronid coelo- dissepiments on the lateral sides and a partition with many mic system. Several important works were dedicated to the holes in the center. TEM reveals that some portions of the organization and fate of the preoral coelom in the acti- incomplete partition are organized like a mesentery, with notroch larva (Menon 1902; Cowles 1904; Zimmer 1978, the two layers of cells separated by ECM. The myoepithe- 1980; Siewing 1974; Herrmann 1980, 1986) and in adult lial cells of the coelomic lining form the circular and radial phoronids (Pross 1974, 1978; Siewing 1974, 1980; Emig musculature of the epistome. Numerous amoebocytes occur and Siewing 1975). These classical studies provided evi- in the coelom lumen. The tip of the epistome and its dorso- dence phoronids possess three coelomic compartments: the lateral parts lack a coelomic cavity and are occupied by preoral coelom inside the epistome, the lophophoral coe- ECM and muscle cells. The fine structure of the T-shaped lom inside the lophophore and tentacles, and the trunk vessel is described, and its localization inside lophophoral coelom. Thus, the organization of coelomic system in coelom is demonstrated. We assert that the cavity inside the phoronids is similar to deuterostomian coelomic system, epistome is the preoral coelom corresponding to the true which also consists of three sets of coelomic compartments preoral coelom of the larva of this species. Proving this arranged along the fronto-caudal axis. Recent ultrastruc- assertion will require additional study of metamorphosis in tural studies, however, have shown that the actinotroch this species. To clarify the patterns of coelom organization larva and adult phoronids lack a preoral coelom (Bartolo- in phoronids, we discuss the bipartite coelomic system in maeus 2001 for Phoronis muelleri Selys-Longchamps, Phoronis and the tripartite coelomic system in Phoronopsis. 1903; Gruhl et al. 2005 for Phoronis ovalis Wright, 1856). These recent studies challenge the idea of a closer relation between phoronids and deuterostomes and does not longer contradict the Lophotrochozoa hypothesis, according to Communicated by T. Bartolomaeus. which the phoronids belong to the Protostomia (Halanych et al. 1995; Peterson and Eernisse 2001). This hypothesis & E. N. Temereva ( ) Á V. V. Malakhov has been supported by recent molecular studies (Dunn et al. Department of Invertebrate Zoology, Biological Faculty, Moscow State University, Moscow 119992, Russia 2008; Hejnol et al. 2009; Paps et al. 2010). However, the e-mail: [email protected] new morphological evidence that larvae of Phoronopsis 123 122 Zoomorphology (2011) 130:121–134 species have a distinct preoral coelom has been obtained by Results Temereva and Malakhov (2006). The epistome in adults of Phoronopsis species has never been studied by TEM Shape methods, so that the occurrence and characteristics of the preoral coelom in adult Phoronopsis species are unclear. The epistome is situated between the mouth or lophophoral To fill this gap, the current study provides new histo- grooves and inner rows of tentacles (Figs. 1a, 2a–c). This logical and ultrastructural information on the structure of organ is very long and has a complex shape with central, the epistome in Phoronopsis harmeri. The coelomic system lateral, and dorso-lateral parts (Figs. 2c, 3a, c). In dorso- organization among phoronids is also discussed. lateral and lateral parts of the lophophore, the epistome looks like a small epidermal fold covering the lophophoral groove (Fig. 4b, c). The height of epistome is 76 ± 1 lm. Methods In the center of the lophophore, the height of the epistome considerably increases to 190 ± 2 lm (Fig. 4d–f). At the Adults of Phoronopsis harmeri Pixell, 1912 (Phoronida) lophophore median line, the epistome looks like a large lobe were collected from August to October 2005 from sandy covering the mouth opening (Fig. 3b). The central part of sediments at 3–4 m depth in the Vostok Bay, Sea of Japan, the epistome can be moved up and down whereas the lateral and from May to June 2010 in the intertidal zone in Coos and dorso-lateral parts always remain motionless (Fig. 1b). Bay (USA, Oregon). Some live animals were photographed The frontal and abfrontal sides of the epistome differ in using a Panasonic DMC-TZ10 digital camera mounted on a position relative to the mouth and also differ in epidermis binocular light microscope. Specimens extracted from organization (Figs. 3b, 4). The frontal side faces the mouth tubes were dissected at the anterior end to obtain a loph- or lophophoral groove and is covered by thick columnar ophore with tentacles and epistome. Pieces of animals were epidermis. The abfrontal side faces the inner row of ten- fixed for routine histology, scanning (SEM) and transmis- tacles and is covered by squamose epithelium. The shallow sion (TEM) electron microscope. For routine histology and SEM, a 4% paraformaldehyde solution in filtered sea water was used as a fixative. Specimens were rinsed in distilled water, dehydrated in ethanol, and embedded in Paraplast Regular (Sigma). Sagittal and cross- sections (7 lm thick) made with a Leica rotary microtome (Leica RM 2125) were stained with Caracci hematoxylin. Sections were examined with a Zeiss Axioplan2 microscope and photographed with an AxioCam HRm camera. For SEM, fixed specimens dehydrated in ethanol fol- lowed by an acetone series were critical point dried and then sputter coated with platinum-palladium alloy. Speci- mens were examined with a Camscan S2 Scan scanning electron microscope. For TEM, specimens were fixed at 4°C in 2.5% glutar- aldehyde in 0.05 M cacodylate buffer containing 21 mg/ml NaCl and then postfixed in 2% osmium tetroxide in the same buffer containing 23 mg/ml NaCl. Postfixation was followed by en bloc staining for 2 h in 1% uranyl acetate in distilled water and then specimens were dehydrated in ethanol followed by an acetone series, and embedded in Spurr resin. Semi-thin and thin sections were cut with a Reichert Ultracut E ultramicrotome. Semi-thin sections were stained with methylene blue and observed with a Reichert Polyvar light microscope. Thin sections were stained with lead citrate and then examined with a JEOL JEM 100B electron microscope. Fig. 1 Phoronopsis harmeri For all measurements, we used three different specimens The lophophore of (photographs of living animals). a The epistome (ep) is in a placid condition. b The and calculated the mean by standard formula. In the text, central part of the epistome is raised. Abbreviations: ert external row only the means are given. of tentacles, irt internal row of tentacles, m mouth 123 Zoomorphology (2011) 130:121–134 123 b Fig. 2 Schemes of the Phoronopsis harmeri anterior body part (a) and 3D reconstruction of coelom organization (b). The lophophore structure is simplified; tentacles are partly removed in a. White, gray, dark gray, and black indicate the preoral coelom, the lophophoral coelom, trunk coelom, and the coelom contour and blastocoel, respectively. In order, parts of blastocoel correspond to descending (di) and ascending (ai) branches of intestine. Hatching indicates the border between the central of the epistome cavity and the lophophoral coelom. Parts of the base and lateral sides of the epistomal cavity are hidden by the lophophoral coelom. ap anal papilla, cf collar fold, cpe epistome central part, ef epistome latero-frontal epidermal fold, ert external row of tentacles, irt inner row of tentacles, lg lophophoral groove, m mouth, np nephridiopore mouth, cells are up to 35 ± 1 lm high (Fig. 4e, f), whereas they measure merely 17 ± 1 lm in height in the dorso- lateral and lateral parts (Fig. 4b). Two kinds of cells were found in the frontal epidermis: supporting cells and glan- dular cells (Fig. 5a). The apical cell membrane extends into numerous thin, 2-lm-long microvilli bearing a glycocalix; one cilium emanates from the apical cell membrane of the supporting cells (Fig. 5a). The axoneme has the usual 9 9 2 ? 2 pattern of microtubule arrangement. The basal structure of the cilium includes vertical and horizontal striated rootlets and an accessory centriole that points toward the mouth opening (Fig. 5b). The Golgi complex is situated next to the vertical rootlet. Because of a high concentration of free ribosomes, the cytoplasm appears electron-dense. Several large granules occur in the apical cytoplasm (Fig. 5a, c). These have an irregular shape and a complex structure with electron-transparent inclusions in the electron-dense material (Fig. 5c). The elongate nucleus occupies the middle part of the cell and contains a large nucleolus (Fig. 5a, c). Many specialized retort-shaped gland cells reside between the other epidermal cells (Fig. 5a, c, d). The expanded basal part of the glandular cell contains numerous granules, vesicles, and a small spherical nucleus (Fig.
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