Myogenesis in Aplysia californica (Cooper, 1863) (Mollusca,Gastropoda, Opisthobranchia) with special focus on muscular remodeling during metamorphosis

To date only few comparative approaches tried to reconstruct the ontogeny of the musculature in invertebrates. This may be due to the difficulties involved in reconstructing three dimensionally arranged muscle systems by means of classical histological techniques combined with light or transmission electron microscopy. Within the scope of the present study we investigated the myogenesis of premetamorphic, metamorphic, and juvenile developmental stages of the anaspidean opisthobranch Aplysia californica using fluorescence F‐actin‐labeling in conjunction with modern confocal laser scanning microscopy. We categorized muscles with respect to their differentiation and degeneration and found three true larval muscles that differentiate during the embryonic and veliger phase and degenerate during or slightly after metamorphosis. These are the larval retractor, the accessory larval retractor, and the metapodial retractor muscle. While the pedal retractor muscle, some transversal mantle fibers and major portions of the cephalopedal musculature are continued and elaborated during juvenile and adult life, the buccal musculature and the anterior retractor muscle constitute juvenile/adult muscles which differentiate during or after metamorphosis. The metapodial retractor muscle has never been reported for any other gastropod taxon. Our findings indicate that the late veliger larva of A. californica shares some common traits with veligers of other gastropods, such as a larval retractor muscle. However, the postmetamorphic stages exhibit only few congruencies with other gastropod taxa investigated to date, which is probably due to common larval but different adult life styles within gastropods. Accordingly, this study provides further evidence for morphological plasticity in gastropod myogenesis and stresses the importance of ontogenetic approaches to understand adult conditions and life history patterns. J. Morphol., 2008. © 2007 Wiley‐Liss, Inc.     

Development of the musculature in the limpet Patella (Mollusca, Patellogastropoda)

 Whole-mount technique using fluorescent-labelled phalloidin for actin staining and confocal laser scanning microscopy as well as semi-thin serial sectioning, scanning and transmission electron microscopy were applied to investigate the ontogeny of the various muscular systems during larval development in the limpets Patella vulgata L. and P. caerulea L. In contrast to earlier studies, which described a single or two larval shell muscles, the pretorsional trochophore-like larva shows no less than four different muscle systems, namely the asymmetrical main head/foot larval retractor muscle, an accessory larval retractor with distinct insertion area, a circular prototroch/velar system, and a plexus-like pedal muscle system. In both Patella species only posttorsional larvae are able to retract into the shell and to close the aperture by means of the operculum. Shortly after torsion the two adult shell muscles originate independently in lateral positions, starting with two fine muscle fibres which insert at the operculum and laterally at the shell. During late larval development the main larval retractor and the accessory larval retractor become reduced and the velar muscle system is shed. In contrast, the paired adult shell muscles and the pedal muscle plexus increase in volume, and a new mantle musculature, the tentacular muscle system, and the buccal musculature arise. Because the adult shell muscles are entirely independent from the various larval muscular systems, several current hypotheses on the ontogeny and phylogeny of the early gastropod muscle system have to be reconsidered. Received: 23 June 1998 / Accepted: 25 November 1998     

Developmental dynamics of myogenesis in the shipworm <Emphasis Type="Italic">Lyrodus pedicellatus</Emphasis> (Mollusca: Bivalvia)

Background

The shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Although its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Here, we describe myogenesis of L. pedicellatus revealed by filamentous actin labelling and discuss the data in a comparative framework in order to test for homologous structures that might be part of the bivalve (larval) muscular ground pattern.

Results

Five major muscle systems were identified: a velum retractor, foot retractor, larval retractor, a distinct mantle musculature and an adductor system. For a short period of larval life, an additional ventral larval retractor is present. Early in development, a velum muscle ring and an oral velum musculature emerge. In late stages the lateral and dorsal mantle musculature, paired finger-shaped muscles, an accessory adductor and a pedal plexus are formed. Similar to other bivalve larvae, L. pedicellatus exhibits three velum retractor muscles, but in contrast to other species, one of them disappears in early stages of L. pedicellatus. The remaining two velum retractors are considerably remodelled during late larval development and are most likely incorporated into the elaborate mantle musculature of the adult.

Conclusions

To our knowledge, this is the first account of any larval retractor system that might contribute to the adult bodyplan of a (conchiferan) mollusk. A comparative analysis shows that a pedal plexus, adductors, a larval velum ring, velum retractors and a ventral larval retractor are commonly found among bivalve larvae, and thus most likely belong to the ground pattern of the bivalve larval musculature.

     

Musculature in sipunculan worms: ontogeny and ancestral states

SUMMARY Molecular phylogenetics suggests that the Sipuncula fall into the Annelida, although they are morphologically very distinct and lack segmentation. To understand the evolutionary transformations from the annelid to the sipunculan body plan, it is important to reconstruct the ancestral states within the respective clades at all life history stages. Here we reconstruct the ancestral states for the head/introvert retractor muscles and the body wall musculature in the Sipuncula using Bayesian statistics. In addition, we describe the ontogenetic transformations of the two muscle systems in four sipunculan species with different developmental modes, using F-actin staining with fluorescent-labeled phalloidin in conjunction with confocal laser scanning microscopy. All four species, which have smooth body wall musculature and less than the full set of four introvert retractor muscles as adults, go through developmental stages with four retractor muscles that are eventually reduced to a lower number in the adult. The circular and sometimes the longitudinal body wall musculature are split into bands that later transform into a smooth sheath. Our ancestral state reconstructions suggest with nearly 100% probability that the ancestral sipunculan had four introvert retractor muscles, longitudinal body wall musculature in bands and circular body wall musculature arranged as a smooth sheath. Species with crawling larvae have more strongly developed body wall musculature than those with swimming larvae. To interpret our findings in the context of annelid evolution, a more solid phylogenetic framework is needed for the entire group and more data on ontogenetic transformations of annelid musculature are desirable.     

Muscle development in Antalis entalis (Mollusca,Scaphopoda) and its significance for scaphopod relationships

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.     

Anatomy and innervation of the abdominal segmental muscles in larval and adult Tenebrio molitor (Coleoptera)

The external morphology, musculature, and the innervation of the abdominal segments were examined in larvae and adult Tenebrio molitor. In the larva, there are 26 pairs of muscles arranged at four different levels in the ventral, lateral, and dorsal region of each segment. In the adult, the number of muscles has been dramatically reduced and is limited to six pairs of muscles located at the dorsal and lateral region of the segment. These muscles, in either larval or adult stages, are innervated by two main nerves, n1 and n2, which originate from the segmental ganglia. The cell bodies of the motoneurons innervating the muscles of the 3rd abdominal segment are located in the 3rd and 2nd abdominal ganglia. Some cell bodies are retained throughout metamorphosis, but others disappear during the larva-pupa transition.     

Major muscle systems in the larval caenogastropod,Ilyanassa obsoleta,display different patterns of development

This study describes the anatomical and developmental aspects of muscular development from the early embryo to competent larval stage in the gastropod Ilyanassa obsoleta. Staining of F‐actin revealed differential spatial and temporal patterns of several muscles. In particular, two major muscles, the larval retractor and pedal retractor muscles originate independently and display distinct developmental patterns similar to observations in other gastropod species. Additionally, together with the larval retractor muscle, the accessory larval muscle developed in the embryo at the trochophore stage. Therefore, both these muscles develop prior to ontogenetic torsion. The pedal retractor muscle marked the most abundant growth in the mid veliger stage. Also during the middle stage, the metapodial retractor muscle and opercular retractor muscle grew concurrently with development of the foot. We show evidence that juvenile muscles, such as the buccal mass muscle and siphon muscle develop initially during the late veliger stage. Collectively, these findings substantiate that larval myogenesis involves a complex sequence of events that appear evolutionary conserved within the gastropods, and set the stage for future studies using this model species to address issues concerning the evolution and eventual fates of larval musculature in molluscs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Mechanisms of rapid morphogenetic movements in the metamorphosis of the bryozoan Bugula neritina (cheilostomata,cellularioidea). I. Attachment to the substratum

The larval morphology, settlement behavior, and the rapid morphogenetic movements that occur during the first 60 sec of metamorphosis of the cellularioid cheilostome bryozoan Bugula neritina have been examined and analyzed by light and electron microscopy. The larva attaches to the substratum at the onset of metamorphosis by the eversion of the internal sac. At the same time, the coronal cilia reverse their direction of beat, spreading an adhesive secreted by the neck region of the everting sac over the metamorphosing larva. During attachment, the larva goes through several configurations that coincide with the sequential contraction and relaxation of certain larval muscles. Histological and ultrastructural evidence indicates that the neck and wall regions of the internal sac are everted by the contraction of the muscles in the equatorial plane of the larva at the same time that the roof region in pulled toward the larval equator by the contraction of the axial muscles. The subsequent relaxation of the axial muscles allows the roof region to be everted by the antagonistic force generated by the sustained contraction of the equatorial musculature. After the roof region attaches to the substratum, the apical disc is temporarily retracted by a second contraction of the axial muscles. The apical disc subsequently reextends as the axial muscles relax just before coronal involution. A comparison of the ontogenetic sequence of rapid morphogenetic movements in the metamorphoses of cheilostome and ctenostome bryozoans indicates that cellularioid cheilostomes have undergone peramorphosis in the aspect of development.     

Out-of-the tropics or trans-tropical dispersal? The origins of the disjunct distribution of the gooseneck barnacle Pollicipes elegans

Myogenesis is currently investigated in a number of invertebrate taxa using combined techniques, including fluorescence labeling, confocal microscopy, and 3D imaging, in order to understand anatomical and functional issues and to contribute to evolutionary questions. Although developmental studies on the gross morphology of bivalves have been extensively pursued, organogenesis including muscle development has been scarcely investigated so far. The present study describes in detail myogenesis in the scallop Nodipecten nodosus (Linnaeus, 1758) during larval and postmetamorphic stages by means of light, electron, and confocal microscopy. The veliger muscle system consists of an anterior adductor muscle, as well as four branched pairs of striated velum retractors and two pairs of striated ventral larval retractors. The pediveliger stage exhibits a considerably elaborated musculature comprising the velum retractors, the future adult foot retractor, mantle (pallial) muscles, and the anterior and posterior adductors, both composed of smooth and striated portions. During metamorphosis, all larval retractors together with the anterior adductor degenerate, resulting in the adult monomyarian condition, whereby the posterior adductor retains both myofiber types. Three muscle groups, i.e., the posterior adductor, foot retractor, and pallial muscles, have their origin prior to metamorphosis and are subsequently remodeled. Our data suggest a dimyarian condition (i.e., the presence of an anterior and a posterior adductor in the adult) as the basal condition for pectinids. Comparative analysis of myogenesis across Bivalvia strongly argues for ontogenetic and evolutionary independence of larval retractors from the adult musculature, as well as a complex set of larval retractor muscles in the last common bivalve ancestor.     

The Development of Teeth and Associated Feeding Structures During the Metamorphosis of the Lamprey,Geotria australis

The pattern of development of the teeth, laminae, piston, muscles, cartilages and fimbriae associated with the suctorial disc of lampreys has been investigated histologically during the seven stages of metamorphosis in the Southern Hemisphere species, Geotria australis. The cirrhi-bearing hood of ammocoetes and the earliest stage of metamorphosis (S1) were indistinguishable. In stages S2, S3, S4, these cirrhi regressed and the supraoral lamina, piston and infraoral lamina primordial regions began to differentiate. The fifth stage (S5) was characterised by an elaboration of the annular cartilage and disc musculature, deposition of the tectal cartilage, initiation of tooth development, formation of oral fimbriae, and eruption of the keratin cone of the transverse lingual lamina. Subsequently (S6), the keratin cusps of the supraoral and infraoral laminae became exposed at the surface, and distinct retractor and protractor muscles formed around the lingual cartilage. In the latter part of the terminal stage in metamorphosis (S7), just prior to the time when the animal migrates downstream, the primary tooth cones and the keratin cusp of the longitudinal lingual laminae began breaking through the epithelial surface of the disc.     

Three‐dimensional reconstruction of the musculature of various life cycle stages of the cycliophoran Symbion americanus

Cycliophora is a very recently described phylum of acoelomate metazoans with a complex life cycle and a phylogenetic position that has been under debate ever since its discovery in 1995. Symbion americanus, which lives attached to the mouthparts of the American lobster, Homarus americanus, represents the second species described for the phylum. Aiming to increase the morphological knowledge about this cryptic clade, the present study describes the muscle arrangement of the feeding stage, the attached Prometheus larva with the dwarf male inside, the free living male, the Pandora larva, and the chordoid larva of S. americanus using actin staining and confocal laser scanning microscopy. 3D reconstructions of the muscular systems are presented. In the feeding stage, circular muscles compose the buccal funnel aperture. In addition, a pair of muscles runs longitudinally in the buccal funnel. A complex sphincter was found just proximally to the anus, and six longitudinal muscles run from the trunk constriction (“neck”) in basal direction. The musculature of the larval stages and the dwarf male is very complex and includes longitudinal muscles that run dorsally and ventrally. In addition, we found dorso‐ventral muscles. The male has a complex posterior muscle apparatus in the vicinity of the penis. In this stage, X‐ and V‐shaped structures were identified on the dorsal and the ventral side, respectively. Pandora and chordoid larvae possess additional circular muscles. We discuss our findings with respect to muscle elements of other metazoan groups and the chordoid larva of Symbion pandora. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Induction of metamorphosis in the sand dollar Peronella japonica by thyroid hormones

The larva of the sand dollar Peronella japonica lacks a mouth and gut, and undergoes metamorphosis into a juvenile sand dollar without feeding. In the present study, it was found that thyroid hormones accelerate the metamorphosis of P. japonica larvae. The contents of thyroid hormones in larvae increased gradually during development. Thiourea and potassium perchlorate, inhibitors of thyroid hormone synthesis, delayed larval metamorphosis and simultaneously repressed an increase in the content of thyroxine in the larval body. These results suggest that the P. japonica larva has a system for synthesis of thyroid hormones that act as factors for inducing metamorphosis.     

Models of metamorphic timing: an experimental evaluation with the pond-dwelling salamander Hemidactylium scutatum (Caudata: Plethodontidae)

Amphibian larvae vary tremendously in size at metamorphosis and length of larval period. We raised pond-dwelling four-toed salamander (Hemidactylium scutatum) larvae to test two models that predict a larva’s age and size at metamorphosis. The Wilbur-Collins model proposes that the developmental rate of a larva responds to changes in growth rate in an adaptive manner throughout the larval period, and that metamorphosis can be initiated after a minimum size has been reached. The Leips-Travis or fixed-rate model states that developmental rate is set early in the larval period, perhaps by early growth rate or food availability and their positive correlation with developmental rate, and that changes in growth rate during the larval period affect size at metamorphosis, but have no effect on the age of an individual at metamorphosis. A modified version of the Wilbur-Collins model suggests that a larva’s developmental rate becomes fixed about two-thirds of the way through the larval period, with changes in growth rate after that point only affecting size at metamorphosis. Larvae were raised on eight different feeding regimes which created two constant and six variable growth histories. Growth history did significantly affect size at metamorphosis. However, an a posteriori statistical test revealed a group of seven and an overlapping group of six treatments with indistinguishable lengths of larval period, indicating a general picture of a fixed developmental rate regardless of growth history. This result is unique among similar studies on invertebrates, fish, and frogs. There was no association between early growth or food level and development rates. Neither the Wilbur-Collins nor the Leips-Travis fixed-rate models were supported. The invariable developmental rate of Hemidactylium and recent osteological evidence from the literature suggest that larvae begin the process of metamorphosis as soon as they hatch, probably a trait selected for by strong predation pressure in the aquatic environment. A variety of different approaches (ecological, developmental, phylogenetic) are necessary to fully evaluate the adaptive nature of the timing of transitions between life cycle stages. Received: 3 June 1999 / Accepted: 18 March 2000     

Facultative planktotrophy in the tropical echinoid Clypeaster rosaceus (Linnaeus) and a comparison with obligate planktotrophy in Clypeaster subdepressus (Gray) (Clypeasteroida: Echinoidea)

Larval development to metamorphosis and early juvenile growth and survivorship were examined in Clypeaster subdepressus (Gray) and C. rosaceus (Linnaeus). C. subdepressus has an obligatorily planktotrophic larva that metamorphoses after 16 to 28 days at 27°C. The larva of C. rosaceus can, but need not feed prior to metamorphosis, which occurs after 5 to 7 days at 27°C. Feeding by larvae of C. rosaceus does not change the time to metamorphosis but does increase size at metamorphosis, early juvenile growth and may increase juvenile survivorship relative to unfed larvae. Size at metamorphosis increases in larvae of C. rosaceus that feed for several days after they are competent to metamorphose, but there may be a limit to this increase because the condition of the rudiment degenerates after a period of time. The development of C. rosaceus may represent a transition between planktotrophy and lecithotrophy. This intermediate state has advantages for the juvenile stage that are not included in the trade of fecundity against risk to offspring usually considered in life history discussions of developmental mode of marine invertebrates.     

Behavior,metamorphosis, and muscular organization of the predatory rotifer Acyclus inquietus (Rotifera,Monogononta)

Abstract. The atrochid rotifer, Acyclus inquietus, is a sedentary predator that lives within the colonies of its prey, the rotifer Sinantherina socialis. After larvae infiltrate and become associated with the colony, they secrete a permanent gelatinous tube and undergo metamorphosis to the adult stage. We followed settlement and metamorphosis using bright-field microscopy to document specific larval behaviors after eclosion, and used epifluorescence and confocal microscopy of phalloidin-labeled specimens to visualize some of the morphological changes that occur during metamorphosis. Upon eclosion, larvae possess paired eyespots and a ciliated corona that functions strictly in locomotion. After leaving the parent's gelatinous tube, larvae eventually settle on unoccupied colonies of S. socialis or on other substrates if colonies are unavailable. Settlement involves a period of gliding among colony members before attachment with the foot and the secretion of a gelatinous tube. After settlement, there is a drastic reconfiguration of the corona that involves loss of the eyespots, loss of the coronal cilia, and the formation of the cup-shaped infundibulum, a deep depression in the anterior of the head that leads to the mouth. The development of the infundibulum involves the expansion of tissues around the mouth and is accompanied by a reorientation of the underlying musculature that supplies the infundibulum and allows its use in prey capture. The arrangement of the muscles in the trunk and foot regions, which contain outer circular (complete and incomplete) and inner longitudinal bands, remains unchanged between ontogenetic stages, and reflects the condition characteristic of other rotifers.     

Neuromuscular development in Patellogastropoda (Mollusca: Gastropoda) and its importance for reconstructing ancestral gastropod bodyplan features

Within Gastropoda, limpets (Patellogastropoda) are considered the most basal branching taxon and its representatives are thus crucial for research into evolutionary questions. Here, we describe the development of the neuromuscular system in Lottia cf. kogamogai. In trochophore larvae, first serotonin‐like immunoreactivity (lir) appears in the apical organ and in the prototroch nerve ring. The arrangement and number of serotonin‐lir cells in the apical organ (three flask‐shaped, two round cells) are strikingly similar to those in putatively derived gastropods. First, FMRFamide‐lir appears in veliger larvae in the Anlagen of the future adult nervous system including the cerebral and pedal ganglia. As in other gastropods, the larvae of this limpet show one main and one accessory retractor as well as a pedal retractor and a prototroch muscle ring. Of these, only the pedal retractor persists until after metamorphosis and is part of the adult shell musculature. We found a hitherto undescribed, paired muscle that inserts at the base of the foot and runs towards the base of the tentacles. An apical organ with flask‐shaped cells, one main and one accessory retractor muscle is commonly found among gastropod larvae and thus might have been part of the last common ancestor.     

Structure and metamorphic remodeling of the larval nervous system and musculature of Phoronis pallida (Phoronida)

The structure of the larval nervous system and the musculature of Phoronis pallida were studied, as well as the remodeling of these systems at metamorphosis. The serotonergic portion of the apical ganglion is a U-shaped field of cell bodies that send projections into a central neuropil. The majority of the serotonergic cells are (at least) bipolar sensory cells, and a few are nonsensory cells. Catecholaminergic cell bodies border the apical ganglion. The second (hood) sense organ develops at competence and is composed of bipolar sensory cells that send projections into a secondary neuropil. Musculature of the competent larva includes circular and longitudinal muscle fibers of the body wall, as well as elevators and depressors of the tentacles and hood. The juvenile nervous system and musculature are developed prior to metamorphosis and are integrated with those of the larva. Components of the juvenile nervous system include a diffuse neural net of serotonergic cell bodies and fibers and longitudinal catecholaminergic fibers. The juvenile body wall musculature consists of longitudinal fibers that overlie circular muscle fibers, except in the cincture regions, where this pattern is reversed. Metamorphosis is initiated by the larval neuromuscular system but is completed by the juvenile neuromuscular system. During metamorphosis, the larval nervous system and the musculature undergo cell death, and the larval tentacles and gut are remodeled into the juvenile arrangement. Although the phoronid nervous system has often been described as deuterostome-like, these data show that several cytological aspects of the larval and juvenile neuromuscular systems also have protostome (lophotrochozoan) characteristics.