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The excretory organs in Sphaerodorum flavum (Phyllodocida, Sphaerodoridae): A rare case of co-occurrence of protonephridia, coelom and blood vascular system in Annelida
Abstract
The excretory organs of Sphaerodorum flavum (Sphaerodoridae) were investigated by TEM and reconstructed from serial ultrathin sections. These organs are segmentally
arranged paired protonephridia, which are in close association with a well-developed blood vascular system. Each protonephridium
consists of a terminal part made up of two monociliary terminal cells (solenocytes), and a nephridioduct, formed by two cells.
The two solenocytes lie close together. Each cilium is surrounded by 12 microvillar rods projecting from the perikaryon of
each solenocyte. These rods form a weir-like structure in the coelomic space. The distal part of the weir is embedded in the
proximal nephridioduct. The largest part of the cell bodies of the solenocytes, containing the nucleus, is lateral or basal
to the weir-like structures. The lumen of the nephridioduct is formed by two multiciliated cells, which enclose the extracellular
nephridial canal one behind the other. The canal opens through the nephropore beneath the cuticle without penetrating the
cuticle. Both nephridioduct cells are surrounded by a blood vessel, which is partially folded into several layers. The significance
of a simultaneous occurrence of protonephridial excretory organs and a well-developed blood vascular system as well as coelomic
cavities is discussed. The results of this investigation indicate a close relationship of Sphaerodoridae to Phyllodocidae
instead of to Syllidae within the Phyllodocida.
... 7.13.14.9A-D and 7.13.14.10). This cuticle is formed by an electron-dense epicuticle and a basal cuticle lacking collagen fibers on the trunk (Kuper and Purschke 2001), unlike most polychaetes where the cuticle comprises layers of parallel collagen fibers crossing at about 90° (e.g., Hausen 2005). Irregularly arranged collagen fibers have been observed only in the protuberances (Purschke unpublished observation). ...
... F) and TEM ( Fig. 7.13.14.9A-D). The epidermis lying beneath the cuticle is a thin monolayer, separated from the underlying muscles by a thin extracellular matrix (Kuper and Purschke 2001;Fig. 7.13.14.9D). ...
... The musculature apparatus in sphaerodorids is very complex, especially at the anterior end, compared with the apparently simple external morphology. Sphaerodoropsis sp. and Sphaerodoridium balticum (Reimers, 1933) are provided with longitudinal muscles forming four pairs of discrete bands that run along the whole body length, the ventralmost pair being the most prominent, the dorsalmost, and two pairs of lateral and laterodorsal bands being considerably smaller in dimension (Reimers 1933, Kuper and Purschke 2001, Filippova et al. 2010, Helm and Capa 2015Fig. 7.13.14.11A, ...
... They are characterized, among other morphological features, by the presence of epithelial tubercles arranged in more or less longitudinal and transversal rows Pleijel and Dahlgren, 1998;Filippova et al., 2010;Fig. 1) and an epithelium protected by a thick cuticle without collagen, and provided with small disc-like projections covering its surface (Ruderman, 1911;Kuper and Purschke, 2001;Hausen, 2005;Capa et al., 2014). ...
... They are characterized, among other morphological features, by the presence of epithelial tubercles arranged in more or less longitudinal and transversal rows Pleijel and Dahlgren, 1998;Filippova et al., 2010;Fig. 1) and an epithelium protected by a thick cuticle without collagen, and provided with small disc-like projections covering its surface (Ruderman, 1911;Kuper and Purschke, 2001;Hausen, 2005;Capa et al., 2014). ...
... Close relationships of Sphaerodoridae and Phyllodocidae have also previously been suggested based on the presence of a muscular axial pharynx with ter-minal papillae (Fauvel, 1911), excretory organs as protonephridia with solenocytes, and a well-developed blood vascular system (Kuper and Purschke, 2001). A further indication of this sister group relationship was recovered after analyses of morphological and molecular data (Pleijel and Dahlgren, 1998;Aguado et al., 2007, respectively) but was not well supported in either of these studies. ...
Sphaerodoridae is a small, morphologically well-defined group of annelids, with remarkable spherical tubercles covering their body surface. They have generally been considered as part of Phyllodocida, but there is no consensus about its sister group relationships. Monophyly and internal phylogenetic relationships have never been tested. We present the first phylogenetic analysis of the family, including 33 individuals of 19 species in five genera of Sphaerodoridae, and a wide representation of other Phyllodocida, based on molecular data from nuclear 18S rDNA (2240 bp), mitochondrial 16S rDNA (614 bp), and cytochrome c oxidase subunit I (657 bp). Mitochondrial and nuclear loci were analysed separately and in combination using maximum parsimony, maximum likelihood and Bayesian inference. Results show maximum support for the monophyly of Sphaerodoridae, but sister group relationships remain unclear. The genera Sphaerodoropsis and Sphaerodoridium are found to be paraphyletic. Monophyly of Clavodorum, Ephesiella and Sphaerodorum could not be tested because only one species was included for each of these genera. At least six of the morphotypes included in the study do not fit within current species descriptions, suggesting undescribed species. Finally, the transfer of Sphaerodoropsis minuta back to Sphaerodoridium is proposed.
... 9A-D, 10). This cuticle is formed by an electron-dense epicuticle and a basal cuticle lacking collagen fibers on the trunk (Kuper & Purschke 2001), unlike most polychaetes where the cuticle comprises layers of parallel collagen fibers crossing at about 90° (e. ...
... These electron-dense projections very likely are responsible for the granulated appearance of (Fig. 3E, F) and TEM ( Fig. 9A-D). The epidermis lying beneath the cuticle is a thin monolayer, separated from the underlying muscles by a thin extracellular matrix (Kuper & Purschke 2001; Fig. 9D). The glands in the epidermis, in connection with tubercles and papillae have been regarded to have a sensory role (Ruderman 1911). ...
... According to Filippova et al. (2010), who recently studied in detail the musculature apparatus in Sphaerodoropsis sp., the musculature in sphaerodorids is very complex, especially at the anterior end, compared with the apparently simple external morphology. Sphaerodoropsis sp. and Sphaerodoropsis baltica (Reimers 1933) are provided with longitudinal muscles forming four pairs of discrete bands that run along the whole body length, the ventral-most pair being the most prominent, the dorsal most, and two pairs of lateral and latero-dorsal bands being considerably smaller in dimension (Reimers 1933, Kuper & Purschke 2001, Filippova et al. 2010Fig. 11A, B). ...
The family Sphaerodoridae is a well-defined group of annelids characterized by the presence of spherical tubercles over their surface (Ruderman 1911, Fauchald & Rouse 1997, Pleijel & Dahlgren 1998). Species described in the family are of relatively small size, generally ellipsoid in shape and with certain exceptions measuring between 0.5 and 5 mm in length. Sphaerodorids have a characteristic external morphology, with surface tubercles, an epithelium protected by a thick cuticle, segments poorly delineated, small and short prostomial appendages and simple or compound chaetae. Monophyly of the family has yet not being assessed in a phylogenetic analysis but is generally assumed due to their characteristic external morphology, i.e. presence of inflated capsules and tubercles present in two or more rows on the dorsum (Ruderman 1911, Fauchald & Rouse 1997, Pleijel & Dahlgren 1998). They are commonly considered as belonging to Phyllodocida although sistergroup relationships are still far from being understood (Rouse & Fauchald 1997, Pleijel & Dahlgren 1998, Worsaae et al. 2005, Aguado & Rouse 2006, Zrzavý et al. 2009).
... Goodrich 1945;Kuper & Purschke 2001) and deserve further detailed ultrastructural studies(Kuper & Purschke 2001). For instance, in Sphaerodorum gracilis (Rathke, 1843) (=flavum;Kuper & Purschke 2001) the nephridiopore opens beneath the cuticle with no external nephridial papilla, while in others there are conspicuous papillae with nephridiopore at the distal end. ...
... Goodrich 1945;Kuper & Purschke 2001) and deserve further detailed ultrastructural studies(Kuper & Purschke 2001). For instance, in Sphaerodorum gracilis (Rathke, 1843) (=flavum;Kuper & Purschke 2001) the nephridiopore opens beneath the cuticle with no external nephridial papilla, while in others there are conspicuous papillae with nephridiopore at the distal end. ...
... Goodrich 1945;Kuper & Purschke 2001) and deserve further detailed ultrastructural studies(Kuper & Purschke 2001). For instance, in Sphaerodorum gracilis (Rathke, 1843) (=flavum;Kuper & Purschke 2001) the nephridiopore opens beneath the cuticle with no external nephridial papilla, while in others there are conspicuous papillae with nephridiopore at the distal end. Furthermore, in males of Sphaerodoropsis arctowskyensisHartmann- Schröder and Rosenfeldt, 1988 there is only one pair of nephridia which open in the inflated ventral parapodia cirri of chaetiger 6 ...
A new species of Sphaerodoridium Lützen, 1961 (Polychaeta: Sphaerodoridae) collected off Iceland during the BIOICE programme is described. Sphaerodoridium guerritai sp. nov. is mostly present in waters around the northern half of Iceland, and seems to be common in soft bottoms at depths of 49–1253 m. It is mainly characterized by having macrotubercles which are provided with a long stalk which bears 1–3 small papillae; this seems an unique feature in Sphaerodoridium and the closely related genus Clavodorum Hartman and Fauchald, 1971. Furthermore, the new species is also characterized by having one transversal row of 11–12 dorsal macrotubercles per chaetiger in midbody; 10–16 spherical papillae in front of each row of macrotubercles, somewhat arranged in a dorsal zig-zag; 10–18 ventral papillae per chaetiger arranged
following a non-random pattern: two transversal rows on parapodial areas and one on interparapodial area, of usually 4, 6 and 5 papillae respectively. Short paired tubular structures with distal opening were found on the ventrum of most chaetigers and are interpreted as nephridial papillae and nephridiopores, respectively. Females show special ventral structures between chaetigers 9–10 which may represent genital openings as suggested for other sphaerodorids; males do not show any apparent copulatory structures. A comprehensive table comparing some diagnostic features regarding macrotubercles and body papillae of all known species of Sphaerodoridium and Clavodorum is provided.
Key words: Sphaerodoridium guerritai sp. nov., Clavodorum, BIOICE, morphology, distribution
... This is mainly caused by missing homology statements of some of the key morphological characters comparable between sphaerodorids and other Phyllodocida Pleijel and Dahlgren, 1998) and the absence or scarce representation of members of this group in molecular phylogenies (Worsaae et al., 2005;Aguado et al., 2007;Böggemann, 2009;Aguado and Bleidorn, 2010;Weigert et al., 2014). Nevertheless, previous indications concerning potential sistergroups of Sphaerodoridae were represented by the families Syllidae (Ruderman, 1911;Glasby, 1993;Pleijel, 2001;Aguado and Rouse, 2006;Zrzavý et al., 2009), Phyllodocidae (Pleijel and Dahlgren, 1998;Kuper and Purschke, 2001) and Glyceriformia (Mileikovskii, 1967;Worsaae et al., 2005;Böggemann, 2009;Capa et al., submitted; see also Figure 1). However, detailed analyses supporting a reliable sistergroup relationship between Sphaerodoridae and other Phyllodocida are missing so far. ...
... Since very early on, sphaerodorids were classified into two distinct groups according to their general body shape (e.g., Levinsen, (Fauvel, 1911;Kuper and Purschke, 2001), whereas both molecular and morphological analyses support the glyceriformia as sister taxon (Worsaae et al., 2005;Böggemann, 2009;Capa et al., submitted). A possible close relationship between Sphaerodoridae and Syllidae is based on both morphological and molecular data (Claparède, 1863;Ruderman, 1911;Pleijel, 2001;Aguado and Rouse, 2006) and molecular analyses (Zrzavý et al., 2009). ...
... Sphaerodorids, unlike most other Phyllodocida, do not show conspicuous external segmentation, and the epithelium is covered by a thick cuticle (Webster and Benedict, 1887;Ruderman, 1911;Capa et al., submitted), and lacking collagen fibers (Kuper and Purschke, 2001;Hausen, 2005), unlike most polychaetes. ...
Sphaerodoridae is a group of benthic marine worms (Annelida) characterized by the presence of spherical tubercles covering their whole surface. They are commonly considered as belonging to Phyllodocida although sistergroup relationships are still far from being understood. Primary homology assessments of their morphological features are lacking, hindering the appraisal of evolutionary relationships between taxa. Therefore, our detailed morphological investigation focuses on different Sphaerodoridae as well as on other members of Phyllodocida using an integrative approach combining scanning electron microscopy (SEM) as well as immunohistochemistry with standard neuronal (anti-5-HT) and muscular (phalloidin-rhodamine) markers and subsequent CLSM analysis of whole mounts and sections. Furthermore, we provide histological (HES) and light microscopical data to shed light on the structures and hypothetical function of sphaerodorid key morphological features. We provide fundamental details into the sphaerodorid morphology supporting a Phyllodocida ancestry of these enigmatic worms. However, the muscular arrangement and the presence of an axial muscular pharynx is similar to conditions observed in other members of the Errantia too. Furthermore, nervous system and muscle staining as well as SEM and histological observations of different types of tubercles indicate a homology of the so called microtubercles, present in the long-bodied sphaerodorids, to the dorsal cirri of other Errantia. The macrotubercles seem to represent a sphaerodorid autapomorphy based on our investigations. Therefore, our results allow comparisons concerning morphological patterns between Sphaerodoridae and other Phyllodocida and constitute a starting point for further comparative investigations to reveal the evolution of the remarkable Sphaerodoridae.
... Sphaerodoropsis sp. and Sphaerodoropsis baltica (Reimers 1933) are provided with longitudinal muscles forming four pairs of discrete bands that run along the whole body length, the ventral-most pair being the most prominent, the dorsal most, and two pairs of lateral and latero-dorsal bands being considerably smaller in dimension (Reimers 1933, Kuper & Purschke 2001, Filippova et al. 2010; Fig. 11A, B). By contrast in Sphaerodorum flavum, Ruderman (1911) and Kuper & Purschke (2001) described only three pairs of longitudinal bundles and the dorsal and dorsolateral bundles being either fused or closely apposed (e.g. Fig. 1A in Kuper & Purschke 2001). ...
... These bands are formed by large flattened cells lying in single rows and with the nuclei located on the distal part facing the coelomic cavity. These fibers are not covered by a coelomic epithelium (Tzetlin 1987, Kuper & Purschke 2001, Tzetlin & Filippova 2005). Small bands of transverse muscle fibers encircle the body between the epidermis and the longitudinal muscles except for a narrow gap in the mid-ventral line (Ruderman 1911, Reimers 1933, Kuper & Purschke 2001, Filippova et al. 2010; Fig. 11C). ...
... As a result the body cavity forms a more or less continuous cavity in which the brain, gut, developing gametes etc. are freely moveable. The coelom is filled with fluid and a few coelomocytes, granulocytes and developing and mature gametes (Ruderman 1911, Reimers 1933, Kuper & Purschke 2001; Figs. 1D, 10). ...
http://www.degruyter.com/view/Zoology/bp_029147-6_63
Sabellariidae Johnston, 1865 is a well-defined, and highly specialized group of marine annelids commonly known as honeycomb or sandcastle worms. They live in characteristic tubes of cemented sand grains, other mineral or biogenic particles such as foraminifera (Kirtley 1994), sometimes attached to one another forming large reefs that can extend over several kilometers. They cannot survive out of their tubes and are unable to build new ones if removed from them. Sabellariids have a well-developed operculum with rows of golden paleae that can seal the entrance of the tube when the animal withdraws into it (Figs. 1A, B, Fig. 2A, C). Both structures, tube and operculum, provide protection from desiccation, silt deposition, and predators......
... Such pattern is, in fact, not unique; similar "inversed" body musculature was reported for some annelids and Acoela (Filippova et al., 2010;Kerbl et al., 2015;Tekle et al., 2005). Moreover, in spherodorids, there are differences in the patterns of body wall muscle organization between shortbodied and elongated species (Helm & Capa, 2015;Kuper & Purschke, 2001). Short-bodied animals, like Spaerodoropsis species, possess transversal muscles located between the longitudinal ones, while elongated species have a "conventional" muscular organization with outer transversal and inner longitudinal layers. ...
Orthonectida is an enigmatic group of parasitic invertebrates with an unclear taxonomic position. Recent molecular studies demonstrated that Orthonectida belongs to Annelida; however, the lack of morphological data does not allow to follow the evolutionary pathway from free-living annelids to parasitic orthonectids. Here, we studied the nervous and the muscular systems in the male and female orthonectid Rhopalura litoralis using confocal laser scanning microscopy and immunohistochemistry. The muscular system is formed by four outer longitudinal muscular bundles and several inner transversal muscles. The nervous system of females is represented by a well-developed cerebral ganglion and a nerve plexus in the body. In males, the cerebral ganglion is significantly smaller, and the body plexus is absent. Instead, a pair of nerves with three pairs of serially organized nerve cells runs posteriorly from the ganglion along the lateral sides of the body. Analyses of the structure of all the orthonectids studied so far suggest that reduction and simplification of the free-living males and females are the dominant mode of evolution in orthonectids.
... They are more frequent in mud and sand, but can be found also on hardbottoms (Fauchald 1974) and algal mats (Sardá-Borroy 1987). There are few studies on their behavior and biology (Ruderman 1911, Christie 1984, Kuper & Purschke 2001. Most species are free-living depositfeeders (Fauchald & Jumars 1979); however, some may be associated with ophiuroids, starfish and gorgonians, or live on the tentacles of terebellid polychaetes (Rouse and Pleijel 2001). ...
Sphaerodorids belonging to the genus Ephesiopsis were collected on the outer continental shelf and slope off the coast of São Paulo State, during the REVIZEE/Score Sul/Benthos Program. Ephesiopsis guayanae Hartman and Fauchald 1971 from off Guyana is the only species previously known in this genus. Ephesiopsis shivae sp. nov., differs from E. guayanae in having a longer body, macrotubercles with smaller terminal papillae, and simple chaetae not as pointed distally; eyes are absent in the new species. Four macrotubercles with terminal papillae were observed on the pygidium of E. shivae sp. nov. Such macrotubercles were not reported in E. guayanae, although the specimens are complete; the presence of these structures in the new species necessitating emendation of the genus diagnosis.
... Revision of this specimen did not provide evidence about this because sperm is no longer distinguishable. The structures that seem to have been considered embryos could be the segmental ganglia (called perikarya according to Filippova et al., 2010), that are well developed in sphaerodorids (Reimers, 1933;Kuper & Purschke, 2001). Sphaerodorum Ørsted, 1843: 42. ...
Background
Long-bodied sphaerodorids (Annelida, Sphaerodoridae) is the common name for members of the three closely and morphologically homogenous currently accepted genera of benthic marine bristle worms: Ephesiella , Ephesiopsis and Sphaerodorum . Members of this group share the presence of two dorsal and longitudinal rows of macrotubercles with terminal papillae, and two longitudinal rows of microtubercles, features that are unique among sphaerodorids. Genera are distinguished by the chaetae morphology. Members of Ephesiella are characterised by having compound chaetae (except, sometimes, simple chaetae in the first chaetigers), Sphaerodorum bear only simple chaetae, and Ephesiopsis have both compound and simple chaetae in all parapodia.
Methods
Mitochondrial (partial COI and 16S rDNA) and nuclear (partial 18S rDNA and 28S rDNA) sequence data of long-bodied sphaerodorids with compound and simple chaetae, and an outgroup of additional seven sphaerodorid species were analysed separately and in combination using Bayesian inference (BA), and Maximum Likelihood (ML) methods. Long-bodied sphaerodorids from around the world (including type specimens) were examined under a range of optical equipment in order to evaluate putative generic and specific diagnostic features, in addition to intraspecific variability.
Results
Phylogenetic analyses of mitochondrial and nuclear DNA sequences of specimens identified as Ephesiella and Sphaerodorum, based on chaeta morphology, were performed. Sphaerodorum and Ephesiella were recovered as paraphyletic and nested within each other. Revision of current nominal species diagnostic features are performed and discussed.
Discussion
Results contradict current generic definitions. Recovery of paraphyletic compound and simple chaetae clades urge the synonymization of these two genera of long-bodied sphaerodorids. Morphological data also suggest the synonymization of Ephesiopsis .
... They are more frequent in mud and sand, but can be found also on hardbottoms (Fauchald 1974) and algal mats (Sardá-Borroy 1987). There are few studies on their behavior and biology (Ruderman 1911, Christie 1984, Kuper & Purschke 2001 ). Most species are free-living depositfeeders (Fauchald & Jumars 1979); however, some may be associated with ophiuroids, starfish and gorgonians, or live on the tentacles of terebellid polychaetes (Rouse and Pleijel 2001). ...
Sphaerodorids belonging to the genus Ephesiopsis were collected on the outer continental shelf and slope off the coast of São Paulo State, during the REVIZEE/Score Sul/Benthos Program. Ephesiopsis guayanae Hartman and Fauchald 1971 from off Guyana is the only species previously known in this genus. Ephesiopsis shivae sp. nov., differs from E. guayanae in having a longer body, macrotubercles with smaller terminal papillae, and simple chaetae not as pointed distally; eyes are absent in the new species. Four macrotubercles with terminal papillae were observed on the pygidium of E. shivae sp. nov. Such macrotubercles were not reported in E. guayanae, although the specimens are complete; the presence of these structures in the new species necessitating emendation of the genus diagnosis.
... The presence of numerous coated vesicles in the canal cell suggests transport processes (Rupert and Smith, 1988). Protonephridia occur only rarely in coelomate animals (e.g., Kuper and Purschke, 2001). ...
Molecular data for nephridial development in polychaetes are not available yet. The scope of our work was to establish a reference system for future investigations using two markers for nephridial development: beta-tubulin as marker for cilia and alkaline phosphatase (AP) activity for secretory epithelia. The markers identified, unexpectedly, three consecutively forming generations of nephridia: (1) a transitory unciliated, but AP-positive head kidney, (2) a transitory larval nephridium, which undergoes a morphological transition from a protonephridium to a funnelled nephridium concomitant with the development of the coelomic cavity and finally, (3) the serially arranged metanephridia. The spatial arrangement of larval and definitive nephridia, revealed an up to now unknown developmental boundary between the synchronously forming larval and the serially proliferating definitive segments. Development of three consecutive sets of nephridia with different morphology and biochemical properties was unexpected and reveals an interesting multistep process in the development of excretory structures in Platynereis.
Two different kinds of filtration nephridia, protonephridia and metanephridia, are described in Polychaeta. During ontogenesis
protonephridia generally precede metanephridia. While the latter are segmentally arranged, protonephridia are characteristic
for the larva and are the first nephridial structure formed during ontogenesis. There is strong evidence that both organs
depend on the same information and that their specific structure depends on the way in which the coelom is formed and which
final expansion it gains. While metanephridia are regarded to be homologous throughout the polychaetes, protonephridia seem
to have evolved in several lineages. Some of the protonephridia closely resemble less differentiated stages of metanephridial
development, so that protonephridial evolution can be explained by truncation of the metanephridial development. Nevertheless,
structural details are large enough to allow us to expect information on the polychaete evolution if the database on polychaete
nephridia increases. A comparison of the polychaete metanephridia with those of the Clitellata and Sipuncula reveals some
surprising details. In Clitellata the structure of the funnel is quite uniform in microdrilid oligochaetous Clitellata and
resembles that of the aeolosomatids. Like the nephridia in the polychaete taxa Sabellida and Terebellida, those of the Sipunucla
possess podocytes covering the coelomic side of the duct.
Die Segmentierung, auch als Metamerie bezeichnet, wird dadurch definiert, dass in der Längsachse sich wiederholenden Grundeinheiten mit gleichem anatomischem Aufbau auftreten. Anneliden, Arthropoden und Chordaten sind wohl die artenreichsten Tierstämme, die in der Anatomie das Merkmal der Segmentierung aufweisen. Eine aktuelle Fragestellung im Zusammenhang der Segmentierung ist die des potenziell gemeinsamen phylogenetischen Ursprunges aller metameren Tiere. Im Gegensatz zu Arthropoden und Chordaten gibt es nur wenige gut untersuchte ursprüngliche Anneliden Modelle. Platynereis dumerilii ist ein ursprünglicher Annelid. Diese Arbeit zeigt morphologische und molekulare Befunde zur Segmententstehung in der Entwicklung und der Regeneration von Platynereis dumerilii.
Die Ergebnisse dieser Arbeit zeigen, dass die Segmentierung in Platynereis einem wohl geordneten komplexen Wachstumsprozess zugrundeliegt. Segmente wachsen nicht einfach in einer bestimmten Zeitfolge aus der PZ aus, sondern werden nach bislang unbekannten Faktoren individuell mit sehr variablen Zeiten aus neuem Zellmaterial gebildet. Da neu gebildete Segmentanlagen in einem bestimmten Differenzierungs¬zustand verharren, solange die vorhergebildete Segmentanlage nicht vollständig ausdifferenziert ist, müssen Signale aus anteriorer Richtung die PZ in der Proliferation und die Segmentanlage im Wachstum inhibieren. In der Regeneration fehlt diese Inhibition, und es findet eine ungebremste Proliferationsphase statt, solange bis wieder Segmente beginnen auszudifferenzieren. Die innere Organisation ist durch paarig angelegte Coelomräume, dem ventralen paarigen Nervenstrang und dem Exkretionssystem segmental gegliedert. Das Exkretionssystem, bestehend aus paarigen Metanephridien, die bereits embryonal im mittleren Tritomer angelegt werden, unterscheiden sich jedoch von den Deutomer Nephridien. Deutomer Nephrostome befinden sich ventral und zeigen eine deutlich bewimperten Trichter, während Tritomernephrostome median und eine im KLSMPräparat eher kapselförmigen Aufbau zeigen.Die Regeneration beginnt mit dem Wundverschluss, dem Kontakt zwischen Darm und Epidermis. Die Kontaktstelle wird durch eingewanderte Eleocyten ausgefüllt und ein Zellgerüst aufgebaut, welches nach dem Wundverschluss und der Bildung eines Regenerationsblastems abgeworfen wird. Ursprung des neuen Blastems sind dedifferenzierte epidermale und mesodermale Zellen. Segmente werden rasch nach der Bildung des Pygidums und der Analcirren gebildet. Zusätzlich gibt es unterschiedliche Wachstumsgeschwindigkeiten. Während Nervensystem und Ringmuskulatur von Beginn der Regeneration an schnell wachsen und ausdifferenzieren, folgen Cuticula, Metanephridien und Parapodialstrukturen erst nach 5 Tagen und etwa 5 gebildeten Segmentanlagen. Dabei werden linke und rechte Seite des Segmentes unabhängig voneinander ausdifferenziert, was auf eine direkte Steuerung der Segmententwicklung durch das paarig aufgebaute Nervensystem zurückzuführen ist. Diese morphologischen Befunde werden durch Proliferationsstudien erweitert. Segmente werden sowohl in Normalentwicklung als auch in der Regeneration aus Zellen der epidermalen Proliferationszone gebildet. Diese proliferieren in anteriorer Richtung. Die Tochterzellen proliferieren weiter in anteriorer Richtung. Nach ca. 1 Tag ändert sich die Proliferationsrichtung nach ventral, sodass auch mesodermale Strukturen ihren Ursprung in der Epidermis haben, was auf ektomesodermale Stammzellen schließen lässt. Pduwnt-1 spielt für den Aufbau von larvalen- postlarvalen- und Regenerations- Segmentgrenzen eine zentrale Rolle. Daneben erfüllt dieses Gen auch andere Funktionen, wie die Expression am Anus und dem Stomadeum zeigen. Die hohe Übereinstimmung von Proliferations-und Expressionsschwerpunkten der PZ und den posterioren Segmentanlagenrändern (in der Regeneration) sprechen auch für eine mitogene Funktion von pduwnt-1. Diese Ergebnisse zeigen damit dass die Segmentierungsmechanismen in Platynereis viele Ähnlichkeiten mit Arthropoden, aber auch große Unterschiede zu Clitellaten und auch vielen Polychaten besitzen, was der phylogenetischen Diskussion um den Ursprung der Segmentierung aber auch der Einteilung der Anneliden einen neuen interessanten Baustein hinzufügt.
Different developmental stages (trochophores, nectochaetae, non-mature and mature adults) of Anaitides mucosa were investigated ultrastructurally. A. mucosa has protonephridia throughout its life; during maturity a ciliated funnel is attached to these organs. The protonephridial duct cells are multiciliated, while the terminal cells are monociliated. The single cilium is surrounded by 14 microvilli which extend into the duct lumen without coming into any contact with the duct cells. Corresponding ultrastructure and development indicate that larval and adult protonephridia are identical in A. mucosa. Differences between various developmental stages can be observed only in the number of cells per protonephridium. A comparison between the funnel cells, the cells of the coelothel and the duct cells reveals that the ciliated funnel is a derivative of the duct. Due to the identical nature of the larval and postlarval protonephridia, such a funnel cannot be a secondary structure. In comparison with the mesodermally derived metanephridial funnel in phoronids it seems likely that the metanephridia of annelids and phoronids evolved convergently.
Nephridial diversity is high in Phyllodocida (Annelida) and ranges from protonephridia to metanephridia. The nephridia of
Tomopteris helgolandica (Tomopteridae) can be characterized as metanephridia which bear a multiciliated solenocyte. This cell is medially apposed
to the proximal part of the nephridial duct and bears several cilia, each of which is surrounded by a ring of 13 microvilli.
An extracellular matrix connects the microvilli and thus leads to the impression of a tube surrounding the central cilium.
Each tube separately enters a subjacent duct cell and the cilia extend into a cup-shaped compartment within the duct cell.
This compartment is not connected to the duct. The funnel consists of eight multiciliated cells and is connected to the nephridial
duct, which initially runs intercellularly and later percellularly. The last duct cell bears a neck-like process which pierces
the subepidermal basal membrane and is connected to epidermal cells forming a small invagination, the nephropore. The nephridia
of T. helgolandica develop from a band of cells and all structural components are differentiated at an early developmental stage. Further development
is characterized by enlargment of the funnel, ciliogenesis in the solenocyte, merging of different sections of the duct and,
finally, the formation of the nephropore. An evaluation of the nephridia of T. helgolandica leads to the hypothesis that the nephridial diversity in Phyllodocida can be explained by the retainment of different stages
in the transition of protonephridia into metanephridia; this is caused by the formation of a ciliated funnel at different
ontogenetic stages. Although the protonephridia in Phyllodocida are regarded as primary nephridial organs, protonephridia
are also presumed to have evolved secondarily in progenetic interstitial species of the Annelida by an incomplete differentiation
of the nephridial anlage.
The reproductive biology of Sphaerodorum gracilis was studied between August 1978 and July 1979. Gametogenesis in both sexes involves the production of unusual gonads consisting of small clusters of cells floating in the coelomic fluid. Oogenesis begins in October with a prolonged period of oocyte proliferation lasting at least five months followed by a period of rapid vitellogenic growth leading to a restricted spawning season in the following October. Mature oocytes measure 200–250 μm in diameter and contain large quantities of lipid droplets and yolk granules. Spermatozoa were only observed in abundance in October; they possess a simple, small, round head measuring 2 μm in diameter, and a long filamentous flagellum. Fertilization is assumed to be external and the larvae have a direct, lecithotrophic development in keeping with other sphaerodorids.
The new species is described in detail. It lacks eyes, has saddle-shaped nuchal organs, and reproduces throughout the year. Some new morphological details are given for M. aberrans (Webster&Benedict, 1885) and M. sczelkowii Mecznikov, 1865. All three species possess two types of neurosetae, composite and simple transitory. While the number and position of the testes is fairly constant in M. ephippiophorus and M. sczelkowii, great variation is found in M. aberrans. The excretory organs are bifurcated in M. aberrans, but unbranched in the other species. They appear to be modified metanephridia with protonephridial character-istics. The new species is considered to be closely related to M. arenarius Westheide, 1973, M. sczelkowii, and M. southerni Westheide, 1967. Some notes on ecology are given.
Untersucht wurden Glycera unicornis Savigny und Branchiostoma lanceolatum Pallas. Bei Glycera sitzen die Solenocyten auf der äuβeren Oberfläche des Nephridialsackes. Jede Zelle bildet eine kleine, einseitig geschlossene Röhre, deren Wandung aus einer zarten Membran besteht. Sie wird durch meist 17 parallel zu ihrer Längsachse verlaufende Stäbe versteift. Die offene Mündung der Röhre taucht in eine unter dem Solenocyten liegende Zelle ein, die zu einer Zellschicht unterhalb der Solenocyten gehört. Im Lumen der Röhre schwingt eine Geiβel, die nicht an der Mündung der Röhre endet, sondern sich in einem von der darunter liegenden Zelle gebildeten endoplasmatischen Kanal fortsetzt.
Die Solenocyten von Branchiostoma bestehen immer aus 10 dreikantigen, parallel verlaufenden Stäben. Sie sind wie bei Glycera angeordnet, jedoch fehlt hier eine verbindende Membran. Die Stäbe sind also durch Lücken voneinander getrennt und stellen daher ein offenes Stabwerk dar. In ihrem Innern schwingt eine Geiβel. Der ganze Apparat wird von Zellen gebildet, die ihrer Form und Lage nach den Epicyten oder Podocyten der Wirbeltiere vergleichbar sind. Die Röhren münden in den Maschen eines plasmatischen Netzwerkes des Nephridiums.
Blood vessels in Nereis japonica were studied by electron microscopy. It was found that blood vessels regardless of location were similar in the basic organization of the basal lamina and the usual presence of collagen fibrils on the vessel wall. Differences arise, depending on whether the outside of the basal lamina is covered by peritoneal cells, by gut epithelium, or by epidermis. These relate to the location of the vessels in mesenteries, gut or epidermis, but do not reflect basic structural differences in the vessels themselves. Furthermore, it was concluded that true endothelial cells do not exist in the circulatory system of Nereis japonica and that, in this respect, the system is essentially different from that of vertebrates, in which endothelial cells line the vessels of a closed circulatory system. These considerations lead to the further conclusion that the vascular lumen in Nereis is essentially interstitial space and that the system, which has been known as a typical “closed” circulatory system in annelids, is actually an open circulatory system. Peritoneal cells covering the walls of internal vessels show various degrees of muscular differentiation and those possessing myofilaments may be called “myomesothelial cells.”
The small hesionid polychaete Hesionides arenaria possesses paired segmental excretory organs that closely resemble solenocytic protonephridia. The nephridium consists of one terminal cell and four tubule cells which form the emission channel. From the terminal cell, up to six flagella arise each surrounded by a weir of ten regularly arranged cytoplasmic rods. The structure of the cytoplasm of three of the following cells suggests that they function in active transport and storage. Because all of the larger, more primitive species of this family are equipped with metanephridia, the possibility is discussed that these organs have been developed out of metanephridia. The Hesionides arenaria nephridium may be a morphological stage in the evolutionary pathway from metanephridia to solenocytes. This would mean that solenocytes can no longer be considered to be homologous in every case with other protonephridial organs in polychaetes and may well be derived several times independently out of metanephridia or true protonephridia.
The protonephridia ofProtodrilus rubropharyngeus are described. They consist of a terminal cell, one nephridiopore cell, and different types of duct cells (proximal, medial,
distal) with the duct running intracellularly. Reabsorption takes place in the duct by means of very unique lamellar foldings.
An interesting characteristic of the nephridial system inP. rubropharyngeus is the presumed double filtration of the primary urine that occurs in the walls of both the lateral blood lacunae and the
terminal cell. The structure of excretory organs in relation to the particular coelomatic conditions found in different groups
of polychaetes is discussed.
During the development of the intermediate and late gastrula of Pomatoceros triqueter, the terminal and channel cells grow towards each other and form the protonephridium. They interdigitate through lateral microvilli. The junctions between the cells form filtration clefts, which are bridged by fibrils.