Larval development of Brachiopod <Emphasis Type="Italic">Coptothyris grayi</Emphasis> (Davidson, 1852) (Brachiopoda,Rhynchonelliformea)

The larval development of the Brachiopod Coptothyris grayi (Davidson, 1852) from the Sea of Japan is described for the first time. Ciliated blastula proved to represent the first free-swimming stage. The blastopore is initially formed as a rounded hole stretching later along the anteroposterior axis. The larva is first divided into two lobes (the apical lobe and the trunk); the mantle lobe is formed later as two lateral folds. Two pairs of seta bundles appear in the late stage larvae. The apical larval lobe in brachiopods is supposed to match the pre-oral lobe and anterior part of the trunk with tentacles in phoronids.     

Vestigial prototroch in a basal nemertean, Carinoma tremaphoros (Nemertea; Palaeonemertea)

Nemerteans have been alleged to belong to a protostome clade called the Trochozoa that includes mollusks, annelids, sipunculids, echiurids, and kamptozoans and is characterized by, among other things, the trochophore larva. The trochophore possesses a prototroch, a preoral belt of specialized ciliary cells, derived from the trochoblast cells. Nemertea is the only trochozoan phylum for which presence of the trochophore larva possessing a prototroch had never been shown. However, so little is known about nemertean larval development that comparing it with development of other trochozoans is difficult. Development in the nemertean clade Pilidiophora is via a highly specialized planktonic larva, the pilidium, and most of the larval body is lost during a drastic metamorphosis. Other nemerteans (hoplonemerteans and palaeonemerteans) lack a pilidium, and their development is direct, forming either an encapsulated or planktonic "planuliform" larva, producing a juvenile without a dramatic change in body plan. We show that early in the development of a member of a basal nemertean assemblage, the palaeonemertean Carinoma tremaphoros, large squamous cells cover the entire larval surface except for the apical and posterior regions. Although apical and posterior cells continue to divide, the large surface cells cleavage arrest and form a contorted preoral belt. Based on its position, cell lineage, and fate, we suggest that this belt corresponds to the prototroch of other trochozoans. Lack of differential ciliation obscures the presence of the prototroch in Carinoma, but differentiation of the trochoblasts is clearly manifested in their permanent cleavage arrest and ultimate degenerative fate. Our results allow a meaningful comparison between the development of nemerteans and other trochozoans. We review previous hypotheses of the evolution of nemertean development and suggest that a trochophore-like larva is plesiomorphic for nemerteans while a pilidium type of development with drastic metamorphosis is derived.     

Apical sensory neurones mediate developmental retardation induced by conspecific environmental stimuli in freshwater pulmonate snails

Freshwater pond snails Helisoma trivolvis and Lymnaea stagnalis undergo larval development and metamorphosis inside egg capsules. We report that their development is permanently under slight tonic inhibitory influence of the anterior sensory monoaminergic neurones, which are the remnants of the apical sensory organ. Conspecific juvenile snails, when reared under conditions of starvation and crowding, release chemical signals that are detected by these neurones in encapsulated larvae and reversibly suppress larval development, thus providing a link between environmental signals and developmental regulation. Induced retardation starts from the trochophore stage and results in up to twofold prolongation of the larval lifespan. Upon stimulation with the signal, the neurones increase synthesis and release of monoamines [serotonin (5-HT) in Helisoma and dopamine in Lymnaea] that inhibit larval development acting via ergometrine-sensitive internal receptors. Thus, the novel regulatory mechanism in larval development of molluscs is suggested and compared with the phenomenon of dauer larvae formation in the nematode Caenorhabditis elegans.     

Morphogenesis and phenotypic divergence in two developmental morphs of Streblospio benedicti (Annelida,Spionidae)

Abstract. The morphology of marine invertebrate larvae is strongly correlated with egg size and larval feeding mode. Planktotrophic larvae typically have suites of morphological traits that support a planktonic, feeding life style, while lecithotrophic larvae often have larger, yolkier bodies, and in some cases, a reduced expression of larval traits. Poecilogonous species provide interesting cases for the analysis of early morphogenesis, as two morphs of larvae are produced by a single species. We compared morphogenesis in planktotrophic and lecithotrophic morphs of the poecilogonous annelid Streblospio benedicti from the trochophore stage through metamorphosis, using observations of individuals that were observed alive, with scanning electron microscopy, or in serial sections. Offspring of alternate developmental morphs of this species are well known to have divergent morphologies in terms of size, yolk content, and the presence of larval bristles. We found that some phenotypic differences between morphs occur as traits that are present in only one morph (e.g., larval bristles, bacillary cells on the prostomium and pygidium), but that much of the phenotypic divergence is based on heterochronic changes in the differentiation of shared traits (e.g., gut and coelom). Tissue and organ development are compared in both morphs in terms of their structure and ontogenetic change throughout early development and metamorphosis.     

Larval Development and Metamorphosis in Sipuncula

In a brief review of development of the phylum Sipuncula, fourpatterns of development are recognized: (1) direct with no pelagicstage; (2) one larval stage, a lecithotrophic trochophore; (3)two larval stages, a lecithotrophic trochophore and a lecithotrophicpelagosphera; (4) two larval stages, a lecithotrophic trochophoreand a planktotrophic pelagosphera. Larval types and their metamorphosesare described, with special attention to the development andmorphology of the larval cuticle. In the majority of speciesstudied, the egg envelope is transformed into the larval cuticleat metamorphosis of the trochophore. The cuticle of many planktotrophicpelagosphera larvae is characterized by surface papillae ofdiverse form and pattern. The underlying cuticle in some speciesis composed of layers of fibers at right angles to one another.     

Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel <Emphasis Type="Italic">Dreissena polymorpha</Emphasis>

Bivalvia is a taxon of aquatic mollusks that includes clams, oysters, mussels, and scallops. Within heterodont bivalves, Dreissena polymorpha is a small, mytiliform, freshwater mussel that develops indirectly via a planktotrophic veliger larva. Currently, only a few studies on bivalve neurogenesis are available, impeding the reconstruction of a ground pattern in Bivalvia. In order to inject novel data into this discussion, we describe herein the development of the serotonin-like and α-tubulin-like immunoreactive (lir) neuronal components of D. polymorpha from the early trochophore to the late veliger stage. Neurogenesis starts in the early trochophore stage at the apical pole with the appearance of one flask-shaped serotonin-lir cell. When larvae reach the veliger stage, four flask-shaped serotonin-lir cells are present in the apical organ. At the same time, the anlagen of the cerebral ganglia start to form at the base of the apical organ. From the apical organ, one pair of cerebro-visceral connectives projects posteriorly and connects to a posterior larval sensory organ that contains serotonin- and α-tubulin-like flask-shaped cells. Additional, paired serotonin-lir neurites originate from the apical organ and project into the velum. One unpaired stomatogastric serotonin-lir cell develops ventrally to the stomach at the veliger stage. The low number of serotonin-lir cells in the apical organ of bivalve veligers is shared with larvae of basally branching gastropods and scaphopods and is thus considered a feature of the last common ancestor of Conchifera, while the overall simplicity of the larval neural architecture appears to be a specific trait of Bivalvia.     

Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa

‘Trochophore’ is a term used in a strict sense for larvae having an opposed-band method of feeding, involving a prototroch and metatroch. Other ciliary bands such as a telotroch and neurotroch may be present. The trochophore has been proposed to represent the ancestral larval form for a group of metazoan phyla (including all members of the Spiralia). The name trochophore is also often applied to larvae that do not conform to the above definition. A cladistic analysis of spiralian taxa (with special reference to polychaete annelids), based on a suite of adult and larval characters, is used to assess several hypotheses: (1) that the trochophore (in a strict sense) is a plesiomorphic form for the Spiralia; (2) that die stricdy defined trochophore is plesiomorphic for members of the Spiralia such as the Polychaeta. The homology of each of the various separate ciliary bands of spiralian larvae, and features such as the apical tuft and protonephridia is also assessed. The results favour the conclusion that the trochophore, if defined as a feeding larval form using opposed bands, should not be regarded as an ancestral (= plesiomorphic) type for the Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The evidence suggests that the various ciliary bands have differing evolutionary histories, and only the Echiura (possibly an annelid group) has members with the classical trochophore. The trochophore is re-defined as a larval form with a prototroch. This broad definition covers a wide variety of larvae, and matches the current usage more accurately than the restricted term. Features such as the neurotroch, telotroch and opposed-band feeding show convergence and reversals. The nature of the metatroch requires further investigation. The presence of a prototroch (and hence trochophore larvae) is used to identify an apomorphy-based taxon, Trochozoa, that includes the first ancestor to have evolved a prototroch and all its descendants. This minimally includes the Annelida [sensu lato), Echiura, Entoprocta, Mollusca and Sipuncula and is a less inclusive taxon than the Spiralia.     

Development of the muscle system and contractile activity in the mussel Mytilus trossulus (Mollusca, Bivalvia)

The development of contractile apparatus was subjected to comparative analysis during ontogenesis of the mussel Mytilus trossulus. Indirect immunofluorescence with the polyclonal antibody against mussel twitchin, a protein of thick filaments, and florescent phalloidin as a marker of filamentous cell actin were used to monitor changes in the developing muscle system at different larval stages. The first definitive muscle structures were found at the late trochophore stage (36 h after fertilization) and starting from the midveliger stage (96 h), striated muscles, which are never present in adult mussels, were distinctly seen. The striated muscle periodicity was 1.25 microm in both mussle larvae and adult scallop. The contractile activities of veliger and adult muscles were measured using an electronic signal-processing videosystem. This work is the first complex study of morphological, biochemical, and physiological characteristics of the muscle system in the larvae and adult mollusks.     

扁玉螺早期发育的实验观察

在实验室条件下人工孵化扁玉螺(Neverita didyma)的卵块,观察了其胚胎发育和幼虫发育过程.扁玉螺的早期发育属间接发生型,其胚胎发育包括卵裂期、囊胚期、原肠胚、膜内担轮幼虫、膜内面盘幼虫;幼虫发育包括面盘幼虫、后期面盘幼虫和匍匐幼虫;匍匐幼虫经变态后发育为稚螺.在水温25～26℃条件下,受精卵发育至膜内面盘幼虫约需38h,5～6d后面盘幼虫冲破卵膜而孵化.扁玉螺面盘幼虫的显著特点是具有1对眼点和1对平衡囊,面盘呈双叶状;后期面盘幼虫的面盘为4叶,呈蝴蝶状,足发达,幼虫既能浮游,又能爬行.后期面盘幼虫进一步生长发育,逐渐转入匍匐生活.     

The development of the serotonergic and FMRF-amidergic nervous system in<Emphasis Type="Italic"> Antalis entalis</Emphasis> (Mollusca,Scaphopoda)

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.     

Development of the muscle system and contractile activity in the mussel <Emphasis Type="Italic">Mytilus trossulus</Emphasis> (Mollusca,Bivalvia)

The development of contractile apparatus was subjected to comparative analysis during ontogenesis of the mussel Mytilus trossulus. Indirect immunofluorescence with the polyclonal antibody against mussel twitchin, a protein of thick filaments, and fluorescent phalloidin as a marker of filamentous cell actin were used to monitor changes in the developing muscle system at different larval stages. The first definitive muscle structures were found at the late trochophore stage (36 h after fertilization) and starting from the midveliger stage (96h), striated muscles, which are never present in adult mussels, were distinctly seen. The striated muscle periodicity was 1.25 μm in both mussel larvae and adult scallop. The contractile activities of veliger and adult muscles were measured using an electronic signal-processing video workstation. This work is the first complex study of morphological, biochemical, and physiological characteristics of the muscle system in the larvae and adult molluscs.     

Differential mRNA accumulation and translation during Spisula development

The patterns of proteins synthesized in developing Spisula embryos and larvae were compared with in vitro translation products by one-dimensional gel electrophoresis. Major changes in the in vivo pattern occur at fertilization; these are regulated at the translational level (Rosenthal, Hunt, and Ruderman, 1980, Cell 20, 487-494). The pattern is further altered by midcleavage, and subsequent development is accompanied by frequent changes in the kinds of proteins made. By midcleavage many of the in vivo changes are paralleled by alterations in mRNA levels. Three cDNA clones containing developmentally regulated, nonmitochondrial sequences were isolated from a library constructed from veliger larval RNA. Clone 3v4 encodes alpha-tubulin. Clone 12v4 encodes a 35,000-D protein of unknown function. The protein product of clone 10v8 has not been identified. The concentration of alpha-tubulin RNA is relatively low through midcleavage, increases by the swimming gastrula stage, and is maintained at a moderately high level throughout larval development. 10v8 and 12v4 RNAs first appear in trochophore larvae; their concentrations peak 10-12 hr later, and then decline. The proportions of alpha-tubulin and 10v8 RNA that are translated vary with developmental stage. During early cleavage very little alpha-tubulin RNA is on polysomes; in swimming gastrulae 64% of this mRNA is polysomal. Seventy percent of 10v8 RNA is translated in the trochophore larva, while only approximately 40% is polysomal in the 21-hr veliger. These results show that translational regulation may be superimposed on changes in cytoplasmic mRNA concentrations to determine the level of gene expression during embryogenesis.     

Potential role of cathepsin B in the embryonic and larval development of clam Meretrix meretrix

This study was designed to investigate the possible role of Meretrix meretrix cathepsin B (MmeCB) in embryonic and larval development. MmeCB mRNA expression profile was revealed by semi-quantitative RT-PCR. The level of MmeCB mRNA expression was low in trochophore stage but high in pedveliger stage. MmeCB protein expression was detected in the digestive gland, velum, and epidermis along the edges of the shell in D-larvae and pedveligers by immunocytochemistry. In post larvae, MmeCB protein expression was noticed abundant in the digestive gland, whereas a modest expression was identified in the gill filament. The average shell length of larvae hatched from embryos treated with 0.01, 1, and 10?μmol/L Ca074Me (a cathepsin B inhibitor) was significantly shorter than that of control groups. The metamorphosis rates of larvae treated with 0.01 and 1?μmol/L Ca074Me were significantly lower than that of control groups in 4-day larvae, but not in 5-day larvae. Taken together, these results indicated that MmeCB may have stimulatory effects on embryonic development, metamorphosis, and larval growth during M. meretrix larval development.     

The role of MAPK signaling in patterning and establishing axial symmetry in the gastropod Haliotis asinina

Gastropods are members of the Spiralia, a diverse group of invertebrates that share a common early developmental program, which includes spiral cleavage and a larval trochophore stage. The spiral cleavage program results in the division of the embryo into four quadrants. Specification of the dorsal (D) quadrant is intimately linked with body plan organization and in equally cleaving gastropods occurs when one of the vegetal macromeres makes contact with overlying micromeres and receives an inductive signal that activates a MAPK signaling cascade. Following the induction of the 3D macromere, the embryo begins to gastrulate and assumes a bilateral cleavage pattern. Here we inhibit MAPK activation in 3D with U0126 and examine its effect on the formation and patterning of the trochophore, using a suite of territory-specific markers. The head (pretrochal) region appears to maintain quadri-radial symmetry in U0126-treated embryos, supporting a role for MAPK signaling in 3D in establishing dorsoventral polarity in this region. Posterior (posttrochal) structures - larval musculature, shell and foot - fail to develop in MAPK inhibited trochophores. Inhibition of 3D specification by an alternative method - monensin treatment - yields similar abnormal trochophores. However, genes that are normally expressed in the ectodermal structures (shell and foot) are detected in U0126- and monensin-perturbed larvae in patterns that suggest that this region has latent dorsoventral polarity that is manifested even in the absence of D quadrant specification.     

Modulation of Mytilus trossulus (Bivalvia: Mollusca) larval survival and growth in culture. Short communication

Commercial importance and ability to live in a wide range of salinities have made the common mussel, Mytilus trossulus, a relevant model to study modulation of larval growth and development. We investigated the effects of various salinities combined with neomycin and ampicillin application on Mytilus larvae survival and growth. Both neomycin and ampicillin enhanced trochophore and veliger survival under condition of low salinity. The average veliger size was increasing in accordance with the increase of salinity. In case of neomycin treatment 3.6% of the larvae reached the pediveliger stage. No abnormalities of larval morphology of the FMRFamide and 5-HT systems occurred after 7 days of culturing with both antibiotics.     

Larval development and post‐larval growth of Branchiomma bairdi (Annelida: Sabellidae) from a Mediterranean population

Branchiomma bairdi is a Caribbean fan worm introduced in several localities worldwide, including the Mediterranean Sea, where the species’ range has rapidly expanded. Reproduction in B. bairdi was previously investigated in both extra‐Mediterranean and Mediterranean areas, but no information is available on larval development and post‐larval growth. In the present article, we examined these features for a population from the Mar Grande of Taranto (Ionian Sea). The species is hermaphrodite, and fertilization occurs in situ. Mucus seems to play an important role in fertilization, and also in preserving eggs before fertilization. The trochophore stage develops within the mucus and after hatching, larvae swim for about 3 d before settlement. The trochophore showed a distinct prototroch and two red dorsolateral larval eyes. The pelagic stage takes only 96 h even though prototroch is maintained after settlement, disappearing at 5 d, when larvae showed three chaetigers and branchial crown consisted of four radioles. Some interesting observations concerning changes in the morphology of chaetae and in the number of uncini during growth are also reported, together with discussion of the development of stylodes, an important diagnostic feature in Branchiomma species identification.     

The larval stages of the sea urchin, Strongylocentrotus purpuratus

The adult body plan of Strongylocentrotus purpuratus is established within the imaginal rudiment during the larval stages. To facilitate the study of these stages, we have defined a larval staging scheme, which consists of seven stages: Stage I, four-arm stage; Stage II, eight-arm stage; Stage III, vestibular invagination stage; Stage IV, rudiment initiation stage; Stage V, pentagonal disc stage; Stage VI, advanced rudiment stage; and Stage VI, tube-foot protrusion stage. Each stage is characterized by significant morphological features observed for the first time at that stage. This scheme is intended as a guide for determining the degree of larval development, and for identifying larval and adult structures. Larval anatomy was visualized using light and confocal microscopy as required on living material, whole mount fixed specimens, and serial sections. Antibody staining to localize specific gene products was also used. Detailed analysis of these data has furthered our understanding of the morphogenesis of the rudiment, and has suggested provocative questions regarding the molecular basis for these events. We intend this work to be of use to investigators studying gene expression and morphogenesis in postembryonic larvae.     

Development and neural organization of the tornaria larva of the Hawaiian hemichordate, Ptychodera flava

We report scanning and transmission electron microscopic studies of the early development of the Hawaiian acorn worm, Ptychodera flava. In addition, we provide an immunohistochemical identification of the larval nervous system. Development occurs and is constrained within the stout chorion and fertilization envelope that forms upon the release of the cortical granules in the cytoplasm of the egg. The blastula consists of tall columnar blastomeres encircling a small blastocoel. Typical gastrulation occurs and a definitive tornaria is formed compressed within the fertilization envelope. The young tornaria hatches at 44 hr and begins to expand. The major circumoral ciliary band that crosses the dorsal surface and passes preorally and postorally is well developed. In addition, we find a nascent telotroch, as well as a midventral ciliary band that is already clearly developed. The epithelium of tornaria is a mosaic of monociliated and multiciliated cells. Immunohistochemistry with a novel neural marker, monoclonal antibody 1E11, first detects nerve cells at the gastrula stage. In tornaria, 1E11 staining nerve cells occur throughout the length of the ciliary bands, in the apical organ, in a circle around the mouth, in the esophageal epithelium and in circumpylorus regions. Axon(s) and apical processes extend from the nerve cell bodies and run in tracks along the ciliary bands. Axons extending from the preoral and postoral bands extend into the oral field and form a network. The tornaria nervous system with ciliary bands and an apical organ is rather similar to the echinoderm bipinnaria larvae.