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Patrick D Reynolds - One of the best experts on this subject based on the ideXlab platform.
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The Scaphopoda.
Advances in marine biology, 2002Co-Authors: Patrick D ReynoldsAbstract:The Scaphopoda are marine infaunal carnivores that feed on foraminiferans and other microorganisms selected and manipulated by their unique feeding tentacles or captacula. Their tusk-like shell is open at both ends; the burrowing foot and captacula protrude anteriorly, while respiratory currents pass through the posterior opening. Although the scaphopods comprise one of the smallest molluskan classes in terms of species diversity, they have a worldwide distribution ranging from intertidal to depths in excess of 6000 m. Despite detailed monographic work from the late nineteenth and early twentieth centuries, however, the biology of scaphopods is today among the least understood of mollusks. To some extent, this is related to a significant departure from more typical molluscan organization inferred from studies of the larger classes. For example, the mantle cavity is elongate and restricted in space, and several associated organs are lost, including the osphradium and ctenidia; the latter is associated with the loss of auricles from the scaphopod heart. There is also little record of ontogeny in the adult shell, as the older portion is periodically decollated to maintain the posterior aperture for passage of inhalant and exhalant currents. This and other constraints on scaphopod diversification are reflected in the range limits of shell shape within the class. In contrast, the "Dentalium" egg has been a model system for experimental embryology, particularly in studies of cell lineage and early morphogenesis. Later larval development and most organogenesis is nonetheless poorly known, although recent studies have considerably enhanced our knowledge in this area. Global biogeographic patterns of scaphopod diversity have been studied only preliminarily. A general decrease in diversity with depth is broken by small diversity peaks at bathyal depths. There is also evidence for a marked latitudinal diversity gradient in the world's oceans, near equatorial in the Pacific but at approximately 20 degrees N in the Atlantic. Scaphopods have a wide diversity of ecto- and endo-symbiotic associations with other organisms, including commensal bacteria and ciliates, mutualistic anemones and corals, and parasitic algae and platyhelminths. Other documented associations include predation by naticid gastropods and ratfish, and the inhabitation of empty scaphopod shells by a variety of sipunculans and hermit crabs. Phylogeny within the class is still unresolved, although significant progress has been made recently in documenting morphological variation among families, genera, and species for application in a cladistic context. The evolutionary relationships of Scaphopoda within Mollusca are farther from resolution, with every conchiferan class a proposed sister group among modern analyses. Molecular avenues should provide significant progress in scaphopod phylogenetics, as indicated by recent comparative sequence analysis and gene expression patterns. Scaphopods comprise the most recent class of mollusks to appear in the fossil record, dating at least from the Mississippian Carboniferous (362.5 My). They appear from first-order occurrence data to have a substantially documented fossil record, with several sharp extinction and radiation events evident during their comparatively short geologic history. The unresolved position of the Scaphopoda within molluscan phylogeny, and their relatively limited range of morphological disparity but substantial derivation from other conchiferans, renders the scaphopods an enigmatic group. They represent a significant evolutionary avenue within the Mollusca, but one that is poorly examined. Nonetheless, comprehensive phylogenetic analysis and documentation of diversity within this small, monophyletic, class-level taxon, well-preserved in the fossil record, is imminently tractable, and presents a singular opportunity in the study of molluscan evolution.
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Phylogenetic relationships among families of the Scaphopoda (Mollusca)
Zoological Journal of the Linnean Society, 1999Co-Authors: Patrick D Reynolds, Akiko OkusuAbstract:Phylogenetic relationships among families in the molluscan class Scaphopoda were analysed using morphological characters and cladistic parsimony methods. A maximum parsimony analysis of 34 discrete characters, treated as unordered and equally weighted, from nine ingroup terminal taxa produced a single most parsimonious tree; supplementary analyses of tree length frequency distribution and Bremer support indices indicate a strong phylogenetic signal from the data and moderate to minimally supported clades. The traditional major division of the class, the orders Dentaliida and Gadilida, is supported as both taxa are confirmed as monophyletic clades. Within the Dentaliida, two clades are recognized, the first comprised of the families Dentaliidae and Fustiariidae, the second of the Rhabdidae and Calliodentaliidae; together, these groups comprise a third clade, which has the Gadilinidae as sister. Within the Gadilida, a nested series of relationships is found among [Entalinidae, [Pulsellidae, [Wemersoniellidae, Gadilidae]]]. These results lend cladistic support to earlier hypotheses of shared common ancestry for some families, but are at variance with other previous hypotheses of evolution in the Scaphopoda. Furthermore, analysis of constituent Gadilinidae representatives provide evidence for paraphyly of this family. The relationships supported here provide a working hypothesis that the development of new characters and greater breadth of taxonomic sampling can test, with a suggested primary goal of establishing monophyly at the family level.
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The phylogeny and classification of Scaphopoda (Mollusca): an assessment of current resolution and cladistic reanalysis
Zoologica Scripta, 1997Co-Authors: Patrick D ReynoldsAbstract:The first cladistic analysis of phylogeny in the class Scaphopoda (Steiner 1992a,1996) examined relationships among family and selected sub-family taxa using morphological data. A preferred/ consensus tree of relationships illustrated monophyly of the orders Dentaliida and Gadilida, partial resolution among dentaliid families, and complete resolution among gadilid taxa. However, several alternative replications of the analysis, including use of a revised data matrix, did not produce the reported tree number or level of resolution; in all cases, monophyly of the Dentaliida was not supported by strict consensus of resultant parsimonious trees. Reanalysis, using unordered characters and outgroup rooting, only clearly resolves monophyly of the Gadilida and the sister relationship of the Entalinidae with the remaining gadilid families. These analyses emphasize the need for more comparative data and thorough parsimony analysis in scaphopod cladistic phylogenetics, as relationships in this class are still some way from resolution.
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Distribution and ultrastructure of ciliated sensory receptors in the posterior mantle epithelium of Dentalium rectius (Mollusca, Scaphopoda)
Acta Zoologica, 1992Co-Authors: Patrick D ReynoldsAbstract:The scaphopod mantle cavity opens posteriorly via the pavilion, a siphon-like extension of the posterior mantle through which the respiratory currents pass. The pavilion was examined for ciliated sensory cells in Dentalium rectius Carpenter, 1865, using scanning and transmission electron micropscopy. Three types of sensory receptor were distinguished on the basis of number, length and ultrastructure of the associated cilia. Receptors with 2–5 cilia of ˜ 1.7 μm length lined the pavilion edge. A second type, possessing 1–2 cilia, ˜ 8.2 μm in length, was found throughout the internal and on part of the external surface of the pavilion. The third receptor type consisted of a rigid bundle of 16–40 cilia with a length of ˜ 14.4 μm, and was present close to the periphery and at the base of the pavilion near the entrance to the mantle cavity. The structure and distribution of these cells are similar to peripheral chemo- and mechanoreceptors which sample respiratory currents and the surrounding environment in other molluscs, but they may assume a greater functional significance in scaphopods due to the absence of an osphradium in this class.
Rudolf Kilias - One of the best experts on this subject based on the ideXlab platform.
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Scaphopoda‐Typen und ‐Typoide (Mollusca) im Zoologischen Museum in Berlin
Mitteilungen aus dem Museum für Naturkunde in Berlin. Zoologisches Museum und Institut für Spezielle Zoologie (Berlin), 2008Co-Authors: Rudolf KiliasAbstract:Catalogue of the Types of Scaphopoda (Mollusca) in the Collections of the Zoological Museum in Berlin This paper provides a list of the type specimens of Scaphopoda (Mollusca) in the collections of the Zoological Museum in Berlin. It includes thirty-two types of species, described by Wilhelm Dunker, Frederick Wollaston Hutton, Siegfried H. F. Jaeckel, Eduard von Martens, Ludwig Plate, George Brettingham Sowerby and Henry Suter. Collection and bibliographical data are provided for the specimens.
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Scaphopoda typen und typoide mollusca im zoologischen museum in berlin
Mitteilungen aus dem Museum für Naturkunde in Berlin. Zoologisches Museum und Institut für Spezielle Zoologie (Berlin), 1995Co-Authors: Rudolf KiliasAbstract:Catalogue of the Types of Scaphopoda (Mollusca) in the Collections of the Zoological Museum in Berlin This paper provides a list of the type specimens of Scaphopoda (Mollusca) in the collections of the Zoological Museum in Berlin. It includes thirty-two types of species, described by Wilhelm Dunker, Frederick Wollaston Hutton, Siegfried H. F. Jaeckel, Eduard von Martens, Ludwig Plate, George Brettingham Sowerby and Henry Suter. Collection and bibliographical data are provided for the specimens.
Gerhard Steiner - One of the best experts on this subject based on the ideXlab platform.
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catalog of species group names of recent and fossil Scaphopoda mollusca
Zoosystema, 2004Co-Authors: Gerhard Steiner, Alan R. KabatAbstract:Ce catalogue contient les noms des taxons recents et fossiles du groupe-espece des mollusques de la classe des Scaphopoda. Sur un total de 1965 entrees, 517 concernent des taxons valides recents, 816 des taxons valides fossiles, 543 sont des noms invalides et 89 ont ete subsequemment exclus des Scaphopoda. L'autorite et les references bibliographiques completes sont indiquees pour chaque nom. L'allocation generique originale et actuelle, la localite type et le lieu de depot du materiel type sont indiques autant que possible. Pour les taxons recents, les synonymes, distributions geographique et bathymetrique sont indiques. Les references croisees avec les synonymes plus recents sont basees sur les opinions publiees. Huit especes sont nouvellement synonymisees dans ce travail : Dentalium tessellatum synonyme plus recent de Entalinopsis habutae ; Dentalium caudani synonyme plus recent de Fissidentalium candidum ; F. ergasticum, F. milneedwardsi et F. scamnatum synonymes plus recents de F. capillosum ; F. exuberans synonyme plus recent de F. paucicostatum ; et Cadulus halius synonyme plus recent de C. podagrinus. Trois taxons subspecifiques sont mis en synonymie avec les especes nominales correspondantes : Antalis cerata tenax, Polyschides rushii arne et Gadila agassizii hatterasensis. De plus, huit nouvelles combinaisons generiques sont proposees : Paradentalium americanum n. comb., Coccodentalium cancellatum n. comb., Fissidentalium peruvianum n. comb., Pulsellum teres n. comb., Polyschides poculum n. comb., Polyschides foweyensis n. comb., Polyschides portoricensis n. comb. et Polyschides nitidus n. comb. Treize homonymes plus recents sont renommes et listes en annexe 1.
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molecular phylogeny of Scaphopoda mollusca inferred from 18s rdna sequences support for a Scaphopoda cephalopoda clade
Zoologica Scripta, 2003Co-Authors: Gerhard Steiner, Hermann DreyerAbstract:The phylogenetic relationships of the Scaphopoda, one of the ‘lesser’ molluscan classes, with the other conchiferan taxa are far from clear. They appear either as the sister-group to the Bivalvia (Diasoma concept) or to a Gastropoda–Cephalopoda clade or to the Cephalopoda alone (helcionellid concept). We compiled a 18S rDNA sequence dataset of 48 molluscan species containing 17 scaphopods to test these hypotheses and to address questions regarding high-level relationships with the Scaphopoda. Both parsimony and maximum likelihood trees show low branch support at the base of the Conchifera, except for the robust clade uniting Scaphopoda and Cephalopoda. This result is corroborated by spectral analysis and likelihood mapping. We also tested alternative topologies which scored significantly worse both in tree length and in likelihood. The 18S rDNA data thus reject the Diasoma in favour of a Scaphopoda–Cephalopoda clade as proposed in the helcionellid concept. When plotted on the molecular tree, the pivotal morphological characters associated with the burrowing life style of the Bivalvia and Scaphopoda, i.e. mantle/shell enclosure of the body and the burrowing foot with true pedal ganglia, appear convergent in these groups. In contrast, the prominent and tilted dorsoventral body axes, multiple cephalic tentacles and a ring-shaped muscle attachment on the shell are potential synapomorphies of Scaphopoda and Cephalopoda. Most of the higher taxa within the Scaphopoda are supported by the molecular data. However, there is no support for the families Dentaliidae and Gadilidae. The basal position of the Fustiariidae within the Dentaliida is confirmed.
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Molecular phylogeny of Scaphopoda (Mollusca) inferred from 18S rDNA sequences: support for a Scaphopoda–Cephalopoda clade
Zoologica Scripta, 2003Co-Authors: Gerhard Steiner, Hermann DreyerAbstract:The phylogenetic relationships of the Scaphopoda, one of the ‘lesser’ molluscan classes, with the other conchiferan taxa are far from clear. They appear either as the sister-group to the Bivalvia (Diasoma concept) or to a Gastropoda–Cephalopoda clade or to the Cephalopoda alone (helcionellid concept). We compiled a 18S rDNA sequence dataset of 48 molluscan species containing 17 scaphopods to test these hypotheses and to address questions regarding high-level relationships with the Scaphopoda. Both parsimony and maximum likelihood trees show low branch support at the base of the Conchifera, except for the robust clade uniting Scaphopoda and Cephalopoda. This result is corroborated by spectral analysis and likelihood mapping. We also tested alternative topologies which scored significantly worse both in tree length and in likelihood. The 18S rDNA data thus reject the Diasoma in favour of a Scaphopoda–Cephalopoda clade as proposed in the helcionellid concept. When plotted on the molecular tree, the pivotal morphological characters associated with the burrowing life style of the Bivalvia and Scaphopoda, i.e. mantle/shell enclosure of the body and the burrowing foot with true pedal ganglia, appear convergent in these groups. In contrast, the prominent and tilted dorsoventral body axes, multiple cephalic tentacles and a ring-shaped muscle attachment on the shell are potential synapomorphies of Scaphopoda and Cephalopoda. Most of the higher taxa within the Scaphopoda are supported by the molecular data. However, there is no support for the families Dentaliidae and Gadilidae. The basal position of the Fustiariidae within the Dentaliida is confirmed.
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Catalogue of supraspecific taxa of Scaphopoda (Mollusca)
Zoosystema, 2001Co-Authors: Gerhard Steiner, Alan R. KabatAbstract:This annotated catalogue of Recent and fossil supraspecific names of the molluscan class Scaphopoda contains 15 class- and ordinal-level names, 33 family-level names, 114 genus-group names, and 12 non-scaphopod names erroneously assigned to this class. For the suprageneric names, this catalogue provides their original rank, their current systematic status and, if applicable, their type genus. For the genus-group names, this catalogue provides the type species, mode of designation, systematic position, and the listing of the Recent species. Complete bibliographic references are provided for all supraspecific names. The current validity of each nominal taxon is assessed based upon the literature, with junior synonyms cross-referenced.
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Spawning behaviour of Pulsellum lofotensis (M. Sars) and Cadulus subfusiformis (M. Sars) (Scaphopoda, Mollusca)
Sarsia, 1993Co-Authors: Gerhard SteinerAbstract:Abstract The spawning behaviour of two scaphopods of the order Gadilida in Norwegian waters, Pulsellum lofotensis and Cadulus subfusiformis, differs from that reported for other scaphopods. Only sperm is expelled through the posterior mantle aperture in P. lofotensis, oo-cytes leave via the anterior opening. In C. subfusiformis both types of gametes exit through the anterior aperture. Eggs are generally laid individually and are not connected to each other by mucus. Cleansing behaviour in regard to egg-laying is described. A correlation between habitats and different ways of spawning in Scaphopoda is discussed.
Gerhard Haszprunar - One of the best experts on this subject based on the ideXlab platform.
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The development of the serotonergic and FMRF-amidergic nervous system in Antalis entalis (Mollusca, Scaphopoda)
Zoomorphology, 2003Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:The morphogenesis of serotonin- and FMRF-amide-bearing neuronal elements in the scaphopod Antalis entalis was investigated by means of antibody staining and confocal laser scanning microscopy. Nervous system development starts with the establishment of two initial, flask-like, serotonergic central cells of the larval apical organ. Slightly later, the apical organ contains four serotonergic central cells which are interconnected with two lateral serotonergic cells via lateral nerve projections. At the same time the anlage of the adult FMRF-amide-positive cerebral nervous system starts at the base of the apical organ. Subsequently, the entire neuronal complex migrates behind the prototroch and the six larval serotonergic cells lose transmitter expression prior to metamorphic competence. There are no strictly larval FMRF-amide-positive neuronal structures. The development of major adult FMRF-amide-containing components such as the cerebral system, the visceral loop, and the buccal nerve cords, however, starts before the onset of metamorphosis. The anlage of the putative cerebral system is the only site of adult serotonin expression in Antalis larvae. Establishment of the adult FMRF-amidergic and serotonergic neuronal bauplan proceeds rapidly after metamorphosis. Neurogenesis reflects the general observation that the larval phase and the expression of distinct larval morphological features are less pronounced in Scaphopoda than in Gastropoda or Bivalvia. The degeneration of the entire larval apical organ before metamorphic competence argues against an involvement of this sensory system in scaphopod metamorphosis. The lack of data on the neurogenesis in the aplacophoran taxa prevent a final conclusion regarding the plesiomorphic condition in the Mollusca. Nevertheless, the results presented herein shed doubts on general theories regarding possible functions of larval "apical organs" of Lophotrochozoa or even Metazoa.
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Muscle development in Antalis entalis (Mollusca, Scaphopoda) and its significance for scaphopod relationships.
Journal of morphology, 2002Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:We applied fluorescence staining of F-actin, confocal laser scanning microscopy, as well as bright-field light microscopy, SEM, and TEM to examine myogenesis in larval and early juvenile stages of the tusk-shell, Antalis entalis. Myogenesis follows a strict bilaterally symmetrical pattern without special larval muscle systems. The paired cephalic and foot retractors appear synchronously in the early trochophore-like larva. In late larvae, both retractors form additional fibers that project into the anterior region, thus enabling retraction of the larval prototroch. These fibers, together with the prototroch, disappear during metamorphosis. The anlagen of the putative foot musculature, mantle retractors, and buccal musculature are formed in late larval stages. The cephalic captacula and their musculature are of postmetamorphic origin. Development of the foot musculature is dramatically pronounced after metamorphosis and results in a dense muscular grid consisting of outer ring, intermediate diagonal, and inner longitudinal fibers. This is in accordance with the proposed function of the foot as a burrowing organ based on muscle-antagonistic activity. The existence of a distinct pair of cephalic retractors, which is also found in basal gastropods and cephalopods, as well as new data on scaphopod shell morphogenesis and recent cladistic analyses, indicate that the Scaphopoda may be more closely related to the Gastropoda and Cephalopoda than to the Bivalvia. J. Morphol. 254:53–64, 2002. © 2002 Wiley-Liss, Inc.
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The expression of an engrailed protein during embryonic shell formation of the tusk-shell, Antalis entalis (Mollusca, Scaphopoda).
Evolution & development, 2001Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:SUMMARY This study presents the first detailed account of the larval and early post-metamorphic development of a scaphopod species, Antalis entalis, since 1883. Special reference is given to the expression pattern of an engrailed protein during the formation of the embryonic (protoconch) and adult shell (teleoconch). We found that in the trochophore-like larva the engrailed protein is expressed in shell-secreting cells at the margin of the protoconch close to the mantle edge. During metamorphosis the growth of the protoconch and expression of the engrailed protein along its margin stop and the teleoconch starts to form. These data suggest a different genetic background regarding protoconch and teleoconch formation in the Scaphopoda and possibly all Conchifera, thus inferring a different evolutionary origin of both organs. The single anlage of the scaphopod protoconch contradicts earlier hypotheses of a monophyletic taxon Diasoma (Scaphopoda + Bivalvia), which has been mainly based on the assumption of a primarily bilobed shell in both taxa. Comparative data on engrailed expression patterns suggest nervous system patterning as the basic function of engrailed in the Bilateria. However, there are several independent gain-of-function events, namely segment compartmentation in the Annelida and Arthropoda, protoconch formation in the Mollusca, skeletogenesis in the Echinodermata, and limb formation in vertebrates. These findings provide further evidence that homologous genes may act in very different pathways of bilaterian body plan formation in various animal phyla.
Andreas Wanninger - One of the best experts on this subject based on the ideXlab platform.
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Electronic supplementary material including a detailed Material and method section, Figures and Tables from Staggered Hox expression is more widespread among molluscs than previously appreciated
2018Co-Authors: Tim Wollesen, Sonia Victoria Rodríguez Monje, André Luiz De Oliveira, Andreas WanningerAbstract:This file contains a detailed Material and method section, in addition to micrographs on Hox, ParaHox, and Elav gene expression during scaphopod development (Figures S1-22). A phylogenetic analysis on the Elav gene is provided in Figues S23-24. Table 1 summarizes Hox expression in mollusks
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The development of the serotonergic and FMRF-amidergic nervous system in Antalis entalis (Mollusca, Scaphopoda)
Zoomorphology, 2003Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:The morphogenesis of serotonin- and FMRF-amide-bearing neuronal elements in the scaphopod Antalis entalis was investigated by means of antibody staining and confocal laser scanning microscopy. Nervous system development starts with the establishment of two initial, flask-like, serotonergic central cells of the larval apical organ. Slightly later, the apical organ contains four serotonergic central cells which are interconnected with two lateral serotonergic cells via lateral nerve projections. At the same time the anlage of the adult FMRF-amide-positive cerebral nervous system starts at the base of the apical organ. Subsequently, the entire neuronal complex migrates behind the prototroch and the six larval serotonergic cells lose transmitter expression prior to metamorphic competence. There are no strictly larval FMRF-amide-positive neuronal structures. The development of major adult FMRF-amide-containing components such as the cerebral system, the visceral loop, and the buccal nerve cords, however, starts before the onset of metamorphosis. The anlage of the putative cerebral system is the only site of adult serotonin expression in Antalis larvae. Establishment of the adult FMRF-amidergic and serotonergic neuronal bauplan proceeds rapidly after metamorphosis. Neurogenesis reflects the general observation that the larval phase and the expression of distinct larval morphological features are less pronounced in Scaphopoda than in Gastropoda or Bivalvia. The degeneration of the entire larval apical organ before metamorphic competence argues against an involvement of this sensory system in scaphopod metamorphosis. The lack of data on the neurogenesis in the aplacophoran taxa prevent a final conclusion regarding the plesiomorphic condition in the Mollusca. Nevertheless, the results presented herein shed doubts on general theories regarding possible functions of larval "apical organs" of Lophotrochozoa or even Metazoa.
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Muscle development in Antalis entalis (Mollusca, Scaphopoda) and its significance for scaphopod relationships.
Journal of morphology, 2002Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:We applied fluorescence staining of F-actin, confocal laser scanning microscopy, as well as bright-field light microscopy, SEM, and TEM to examine myogenesis in larval and early juvenile stages of the tusk-shell, Antalis entalis. Myogenesis follows a strict bilaterally symmetrical pattern without special larval muscle systems. The paired cephalic and foot retractors appear synchronously in the early trochophore-like larva. In late larvae, both retractors form additional fibers that project into the anterior region, thus enabling retraction of the larval prototroch. These fibers, together with the prototroch, disappear during metamorphosis. The anlagen of the putative foot musculature, mantle retractors, and buccal musculature are formed in late larval stages. The cephalic captacula and their musculature are of postmetamorphic origin. Development of the foot musculature is dramatically pronounced after metamorphosis and results in a dense muscular grid consisting of outer ring, intermediate diagonal, and inner longitudinal fibers. This is in accordance with the proposed function of the foot as a burrowing organ based on muscle-antagonistic activity. The existence of a distinct pair of cephalic retractors, which is also found in basal gastropods and cephalopods, as well as new data on scaphopod shell morphogenesis and recent cladistic analyses, indicate that the Scaphopoda may be more closely related to the Gastropoda and Cephalopoda than to the Bivalvia. J. Morphol. 254:53–64, 2002. © 2002 Wiley-Liss, Inc.
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MICROMORPHOLOGY AND GENE EXPRESSION IN MUSCLE AND SHELL DEVELOPMENT OF THE MOLLUSCA
2001Co-Authors: Andreas WanningerAbstract:This work comprises detailed studies by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fluorescence staining combined with confocal laser scanning microscopy (CLSM), as well as serial sectioning analyses and reconstruction techniques to elucidate the development of the larval and adult musculature of several basal representatives of the molluscan classes Polyplacophora, Bivalvia, Scaphopoda, and Gastropoda. Special reference is given to the shell musculature. In addition, aspects of the myo-anatomy of adult Solenogastres are reconsidered. A further part of this study deals with scaphopod shell morphogenesis and expression of the homeobox gene engrailed (en), in order to gain insights regarding the scaphopod-bivalve relationship. The results enable far reaching conclusions regarding the evolution and the phylogeny of the Mollusca. Solenogastres TEM analysis of adult Solenogastres revealed a mesenchymate body wall musculature which consists of outer ring, intermediate diagonal, and inner longitudinal muscles and resembles the condition of other worm-shaped taxa. The ventrally inter-crossing dorso-ventral musculature, which is diagnostic for the Mollusca, is arranged in multiple serial units along the anterior-posterior body axis. Polyplacophora During development, the chiton larva undergoes an intermediate stage in which the dorso-ventral musculature is serially arranged as in adult Solenogastres. The concentration into seven (and later eight) functional shell plate muscle units is a secondary condition which takes place after metamorphosis. Thus, assumptions of a primarily "segmented" (i.e. annelidlike) character of the polyplacophoran shell plate musculature are rejected. In addition, the anterior (i.e. pre-trochal) body region of chiton larvae shows a muscular grid which is lost at metamorphosis and resembles the body wall musculature of adult aplacophoran (Solenogastres + Caudofoveata) molluscs. Both, the multiple seriality of the dorso-ventral muscles and the apical muscle grid are regarded as ontogenetic recapitulation of the basal molluscan condition which is fully expressed in the adult body plan of Solenogastres. This infers a non-segmented, worm-shaped ancestor at the base of molluscan evolution. The existence of a larval ring-shaped muscle that underlies the prototroch cells (prototroch muscle ring) is a shared feature of polyplacophoran, gastropod, and bivalve larvae (see below) and suprataxic homology of this organ is proposed. Bivalvia Besides a rather complicated set of larval retractor muscles, the veligers of autobranchs (i.e. all Bivalvia except the Protobranchia, the latter with a test-cell larva) exhibit a distinct prototroch muscle ring similar to chitons and gastropods. Both systems are entirely larval and are resorbed during metamorphosis. Scaphopoda The general ontogeny and especially myogenesis in the dentaliid scaphopod Antalis entalis proceeds much more direct than in polyplacophorans or gastropods. Accordingly, distinct larval muscle systems are lacking. However, the paired cephalic and pedal retractors both form additional fibers which project into the region of the prototroch and are lost at metamorphosis. The existence of a distinct, paired cephalic retractor system, which is also found in the basal gastropod and cephalopod bauplan but not in the Bivalvia, suggests a clade comprising the Scaphopoda and Gastropoda + Cephalopoda. This is strengthened by expression data of the homeobox gene engrailed, which plays a significant role in molluscan shell formation. While two dorso-lateral centers of engrailed expression, which correspond to the two centers of initial shell calcification, are found in early bivalve veligers, engrailed is exclusively found in mantle margin cells surrounding the single anlage of the embryonic scaphopod shell. In contrast to bivalves, the scaphopod shell is thus formed from a single center of calcification, and a scaphopod-bivalve sistergroup relationship is therefore rejected. Gastropoda Primitive gastropods, such as the patellogastropods Patella vulgata and Patella caerulea, show one pair of asymmetrically positioned larval retractor muscles which have distinct insertion sites at the embryonic shell. Another strict larval muscle system is the prototrochal muscle ring. All these muscle are lost before, during, or shortly after metamorphosis. Parts of the adult mantle musculature as well as the muscles of the cephalic tentacles are formed prior to metamorphosis, while the buccal musculature is of entire postmetamorphic origin. The process of gastropod ontogenetic torsion is mainly caused by muscular activity of the larval retractors, while the adult shell musculature arises after the completion of torsion. Thus, ontogenetic torsion is regarded as an entirely larval process inferring that the arrangement of the adult shell musculature - which can often be reconstructed by muscle scars on fossilized shells - is not indicative for the question whether paleozoic univalved molluscs were torted or not.
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The expression of an engrailed protein during embryonic shell formation of the tusk-shell, Antalis entalis (Mollusca, Scaphopoda).
Evolution & development, 2001Co-Authors: Andreas Wanninger, Gerhard HaszprunarAbstract:SUMMARY This study presents the first detailed account of the larval and early post-metamorphic development of a scaphopod species, Antalis entalis, since 1883. Special reference is given to the expression pattern of an engrailed protein during the formation of the embryonic (protoconch) and adult shell (teleoconch). We found that in the trochophore-like larva the engrailed protein is expressed in shell-secreting cells at the margin of the protoconch close to the mantle edge. During metamorphosis the growth of the protoconch and expression of the engrailed protein along its margin stop and the teleoconch starts to form. These data suggest a different genetic background regarding protoconch and teleoconch formation in the Scaphopoda and possibly all Conchifera, thus inferring a different evolutionary origin of both organs. The single anlage of the scaphopod protoconch contradicts earlier hypotheses of a monophyletic taxon Diasoma (Scaphopoda + Bivalvia), which has been mainly based on the assumption of a primarily bilobed shell in both taxa. Comparative data on engrailed expression patterns suggest nervous system patterning as the basic function of engrailed in the Bilateria. However, there are several independent gain-of-function events, namely segment compartmentation in the Annelida and Arthropoda, protoconch formation in the Mollusca, skeletogenesis in the Echinodermata, and limb formation in vertebrates. These findings provide further evidence that homologous genes may act in very different pathways of bilaterian body plan formation in various animal phyla.
