Diversity of form and behaviour among nemertean pilidium larvae

A selection of pilidium larvae are described from plankton collected from Bamfield Inlet in British Columbia, Canada, the Florida Gulf Coast and the Great Barrier Reef of Australia. Among the Bamfield larvae are examples attributable to Cerebratulus and Lineus, the former being among the largest pilidia known, up to 1.45 mm in height, and two previously undescribed types. Pilidium brachiatum from Florida is re‐described to highlight differences from Hubrechtella larvae, which are superficially similar, and an assortment of tropical forms is included to illustrate the predominance of the compressed gyrans‐type larva in this location. Pilidium larvae have typically been categorized by their shape as seen in lateral view. This account emphasizes the importance of three‐dimensional shape, pigmentation, overall architecture and behaviour in distinguishing between the larvae of different species.     

The peculiar nemertean larva pilidium recurvatum belongs to Riserius sp., a basal heteronemertean that eats Carcinonemertes errans,a hoplonemertean parasite of Dungeness crab

A typical nemertean pilidium larva resembles a hat with ear flaps. But one type, called pilidium recurvatum, looks more like a sock, swimming heel first. This distinctive larva was discovered in 1883 off the coast of Rhode Island and subsequently found in plankton samples from other parts of the world. Despite the long time since discovery, and its significance in discussions of larval evolution, this larva remained unidentified even to the family level. We collected pilidium recurvatum larvae from plankton samples in Coos Bay, OR, and identified them as belonging to the heteronemertean genus Riserius based on juvenile morphology and DNA sequence data. Phylogenetic analysis suggests that two distinct types of pilidium recurvatum from Oregon represent two new species within this currently monotypic genus. We describe the morphology of pilidium recurvatum using confocal microscopy and compare it to that of the typical pilidium, discussing possible implications for larval feeding. We also report our surprising discovery that juveniles of Riserius sp. from Oregon prey on another nemertean, Carcinonemertes errans, an egg predator of Cancer magister (Dungeness crab), a commercially important species. We speculate that the species‐level diversity and geographic distribution of Riserius may be much greater than currently appreciated.     

The invention of the pilidium larva in an otherwise perfectly good spiralian phylum Nemertea

One of the most remarkable larval types among spiralians, and invertebrates in general, is the planktotrophic pilidium. The pilidium is found in a single clade of nemerteans, called the Pilidiophora, and appears to be an innovation of this group. All other nemerteans have either planktotrophic or lecithotrophic juvenile-like planuliform larvae or have direct development. The invention of the pilidium larva is associated with the formation of an extensive blastocoel that supports the delicate larval frame and elaborate ciliary band. Perhaps the most striking characteristic of the pilidium is the way the juvenile worm develops inside the larva from a series of isolated rudiments, called the imaginal discs. The paired cephalic discs, cerebral organ discs, and trunk discs originate as invaginations of larval epidermis and subsequently grow and fuse around the larval gut to form the juvenile. The fully formed juvenile ruptures the larval body and, more often than not, devours the larva during catastrophic metamorphosis. This review is an attempt to examine the pilidium in the context of recent data on development of non-pilidiophoran nemerteans, and speculate about the evolution of pilidial larval development. The author emphasizes the difference between the planuliform larvae of Palaeonemerteans and Hoplonemerteans, and suggest a new name for the hoplonemertean larvae--the decidula.     

Ultrastructure of multiciliated collar cells in the pilidium larva of Lineus bilineatus (Nemertini)

Summary The ultrastructure of the apical plate of the free-swimming pilidium larva of Lineus bilineatus (Renier 1804) is described with particular reference to the multiciliated collar cells. In the multiciliary collar cells there are several, up to 12, cilia surrounded by a collar of about 20 microvilli extending from the cells' apical surface. The cilia have the typical 9+2 axoneme arrangement and are equipped with striated caudal rootlets extending from the basal bodies. No accessary centriole or rostral rootlet were observed. Microvilli surrounding the cilia are joined in a cylindrical manner by a mucus-like substance to form a collar. In comparison with many sensory receptor cells built on a collar cell plan the multiciliary collar cells of the pilidium larva apical plate are rather simple and unspecialized. In other pilidium larvae monociliated collar cells are found in the apical plate. The possible function and phylogenetic implications of multiciliated collar cells in Nemertini are briefly discussed.List of Abbreviations a axoneme - b basal body - c cilia or flagella - d desmosome - G Golgi apparatus - m mitochondria - mf microfilaments - mu mucus - mv microvilli - n nucleus - nt neurotubules - pm plasma membrane - r rootlet - ri ribosomes - v secretory vesicles     

Development of the tropical fish Xenomelaniris brasiliensis (Pisces: Atherinidae) from embryo to larva and time to metamorphosis

Embryonic-larval development, and metamorphosis larval time, were studied in the tropical fish Xenomelaniris brasiliensis. Twenty nine sexually mature specimens were used, 16 females (10.86+/-1.01 cm and 7.63+/-2.62 g) and 13 males (10.43+/-0.57 cm and 6.54+/-1.44 g) which produced gametes through abdominal massage. Fertilized eggs were spherical (1.18 +/-0.44 mm diameter), greenish, transparent, benthonic and vitelus-rich; rugose striated chorion with numerous external filaments randomly distributed and abundant oil globules (0.11 + 0.07 mm diameter). The embryonic development was finished at 26.36+/-2.03 degrees C, 39.67+/-0.58 PSU and pH 8.30+/-0.10. Larvae (4.56+/-0.97 mm total length) hatched at 143 hours and 19 minutes. with vitteline sac vestiges and a single oil globule. The larvae were fed on Brachionus plicatilis and Isochrysis galbana. After the second week .4rtemia nauplii were added and I. galbana maintained. Flexion started 13 days after larvae hatched (6.10+/-1.54 mm total length) and was completed 32 days later (11.25+/-1.87 mm total length) with the hipural complex completely developed. In conclusion, X brasiliensis showed direct larval development and started larval metamorphosis (13.08+/-2.07 mm total length) to juvenile 40 days after hatching.     

Morphogenetic effect of cAMP during the metamorphosis of the frog larva

In an in vitro system, the tail absorption of the frog larva was significantly stimulated by cAMP more intensely than by T3. Theophylline, an inhibitor of cPDE, had no effect and failed to enhance, even decreased the effect of T3. It may be supposed that T3 does not act through a cAMP mechanism, but the tail absorption can be influenced by cAMP in a direct way.     

Catecholamine-containing,serotonin-like and neuropeptide FMRFamide-like immunoreactive cells and processes in the nervous system of the pilidium larva (Nemertini)

Summary Pilidium larvae at different developmental stages have been investigated for the occurrence of glyoxylic acid induced fluorescence in catecholamines (CA), and serotonin-like (5-HT) and neuropeptide FMRFamide-like (FMRFamide) immunoreactivity (ir). The distribution of CA, 5-HT-ir and FMRFamide-ir cells and processes was compared with the location of nerve processes as found by transmission electron microscopy (TEM). In the pilidium larvae the marginal and oral nerves contain CA and 5-HT-ir processes and 5-HT-ir unipolar cells. The posterior suboral nerve contain CA and 5-HT-ir processes, whereas in the anterior suboral nerve neither CA nor 5-HT-ir and FMRFamide-ir were observed. The lateral helmet nerve contains FMRFamide-ir processes and unipolar cells. In the epidermis CA and 5-HT-ir multipolar cells were found. The juvenile worm that develops inside the pilidium larva was found to contain only 5-HT-ir. A pair of lateral cords extent the whole length of the juvenile and anteriorly they form the anterior ventral cerebral commissure. Also, from the anterior part of the lateral cords projects a pair of circumrhynchodeal processes which dorsally form the dorsal cerebral commissure. A pair of proboscis processes originate from the circumrhynchodeal processes and extend the whole length of the probosics. From the dorsal cerebral commissure cephalic processes project rostrally and ventrally. Only unipolar 5-HT-ir cells were observed, and they were located along the lateral cords into which their processes extend.Abbreviations AEC 3-amino-9-ethylcarbazole - ap apical plate - arp anterior accessory ridge processes - ason anterior suboral nerve - CA catecholamines - cd cephalic discs - cp cephalic processes - crp circumrhynchodeal processes - DAB 3,3'-diaminobenzidine - dc dorsal cerebral commissure - epi epidermis - es oesophagus - fl fore lobe - FMRFamide phe—met—arg—phe—NH₂ - Go goat - GS goat serum - hl hind lobe - int intestine of the juvenile - lhn lateral helmet nerve - lhp lateral helmet processes - ll lateral lobe - lp lateral processes of the juvenile - mcb marginal ciliary band - me mesoderm - mn marginal nerve - moc monociliary cell - mp marginal processes - mu muscle - muc multiciliary cell - n 1, n 2, n 3 division of marginal nerve - on oral nerve - op oral processes - pb proboscis - pp proboscis processes - pson posterior suboral nerve - psop posterior suboral processes - Ra rabbit - sd stomodeum - st stomach - td trunk discs - tr trunk - TRITC tetramethylrhodamine isothiocyanate - vc ventral cerebral commissure - z 1, z 2 ciliary zones of marginal ciliary band - 5-HT serotonin     

The morphology of the apical organ and adjacent epithelium of pilidium prorecurvatum, a pelagic larva of an unknown heteronemertean (Nemertea)

The morphology of pilidia ex gr. recurvatum from Peter the Great Bay (Sea of Japan) was studied by confocal laser scanning and transmission-electron microscopy. The studied pilidium larvae differ from pilidium recurvatum in lacking a posterior ciliary ring and by the presence of a caudal tuft. On this basis, pilidium prorecurvatum is proposed as a new name for the lavae. The apical organ of pilidium prorecurvatum is represented by a thickened epithelium, which consists of uniform columnar monociliary collar cells and is innervated by a pair of serotonergic intraepithelial neurons. The bodies of the serotonergic neurons are located outside of the apical organ, but occasional axons were found at the organ base. The rest of the pilidial epithelium is represented by flattened polygonal multiciliated cells with sparse microvilli; the bodies of two neurons lie in the helmet epithelium immediately adjacent to the apical organ. Morphologically, the apical organ of the pilidium corresponds well to that of other lophotrochozoan larvae, but their homology remains unclear.     

Effect of weak electromagnetic radiation on larva development and metamorphosis of grain beetle Tenebrio molitor

The effect of weak electromagnetic radiation (36 GHz, 100 mu W/cm2) on the development of the grain beetle Tenebrio molitor was studied. Insects were irradiated in different larval instars and at the pupal stage. It was found that weak electromagnetic radiation stimulated the molting and pupation of larvae and the metamorphosis of pupae. The stimulating effect of radiation was weak when animals were exposed in the initial period of the instar and the pupal stage and was more pronounced if the irradiation was carried out in the second half of the current instar and the pupil stage. The effect of weak electromagnetic radiation on the development of beetle can be related to the function of the hormones of metamorphosis.     

Neural system reorganization during metamorphosis in the planula larva of Clava multicornis (Hydrozoa, Cnidaria)

The planula larva of the hydroid Clava multicornis (Forskål, 1775) has a complex nervous system, characterized by the presence of distinct, anteriorly concentrated peptidergic populations of amidated neurons, presumably involved in the detection of environmental stimuli and metamorphic signals. Differently from other hydrozoan larvae in C. multicornis planulae GLW-positive cells with putative sensory role have a peculiar dome-shaped forefront organization, followed by a belt of RF-positive nerve cells. By immunohistochemistry, we investigated the transformation of the peptidergic (GLW-amide and RF-amide) larval neuroanatomy at different stages of metamorphosis and the subsequent development of the primary polyp nervous system. By terminal transferase-mediated dUTP nick end-labeling assay, apoptotic nuclei were first identified in the anterior pole of the settled larva, in the same region occupied by GLW-amide positive putative sensory cells. In primary polyps, GLW-amide positive signals first encircled the hypostome area, later extending downwards along the polyp column or upwards over the hypostome dome, whereas RF-amide positive sensory cells initially appeared at the tentacles base to later extend in the tentacles and the polyp column. In spite of the possession of distinct neuroanatomies, different cnidarian planulae may share common developmental mechanisms underlying metamorphosis, including apoptosis and de novo differentiation. Our data confirm the hypothesis that the developmental dynamics of tissue rearrangements may be not uniform across different taxa.     

<Emphasis Type="Italic">In vivo</Emphasis> studies of development of the main functional systems in the heteronemertean pilidium larva

There is performed in vivo morphological study of the White Sea heteronemertines belonging to the type of Pilidium pyramidale (conussoidale). Based on the layer-by-layer microshooting with subsequent computer processing, development of the pilidium digestive, nervous, and muscle systems is described from the stage following at once the gastrula till the premetamorphosis larva. Peculiarities of structural organization of the main functional systems are revealed depending on the larva size and the stage of formation of imaginal discs. It is first shown that even in the not completely formed pilidium, neurons are located not only in envelopes and walls of the digestive tract, but also in the depth of cupola along the central muscle retractor. Their processes are distributed between the main body parts and organs by seeming to perform connections of the apical organ and central muscle retractor with the digestive tract, blades, and the nerve plexus of the cupola wall. In the digestive tract between pharynx and stomach in the formed pilidium, the sphincter is first revealed. It has been shown that in the course of larva development, the non-orderly arranged and poorly developed muscle fibers gradually form in the blade the fan-like, whereas in the cupola wall, the net-like structure.     

昆虫变态发育类型与调控机制

昆虫变态发育使得昆虫成为地球上种类最多、分布最广的动物种群。它特指末龄幼虫化蛹,或者蛹向成虫的转变过程。根据变态剧烈程度,可将昆虫变态发育简单分为增节变态、表变态、原变态、不完全变态及全变态5种类型。此外,昆虫变态发育是一个极其复杂的生物学过程,受到激素、基因、营养等多种因素的精密调控。本文简要介绍了昆虫变态发育的类型和分子调控机理方面的研究进展。     

Development of the renal corpuscle during metamorphosis in the lamprey.

The renal corpuscle of the adult lamprey, Petromyzon marinus L., is formed during the programmed period of metamorphosis. Development is initiated early in this metamorphic period and is marked by the synchronous formation and growth of rudimentary nephron units (RNU) from longitudinal cord of nephrogenictissue extending from the posterior tip of the degenerating larval kidney to the cloaca and connected to the peritoneal epithelium. Detachment of the RNU from the peritoneum involves autolysis and cell death and is accompanied by their branching into five or six hexagonally-arranged nephrons which radiate from the original point of attachment. Differentiation of the epithelial cells at the proximal ends of the nephrons is preceded by the widening of lateral intercellular spaces, the formation of tubular lumina (primitive urinary spaces), the loss of apical cell junctions, and the development of a capillary network with its associated mesangium. With the extension of the capillaries and mesangium between the proximal ends of adjacent undifferentiated nephrons, visceral epithelial cells (podocytes), with long cell processes (trabeculae) and slit membranes, make their appearance. The urinary spaces resulting from this form of development are lined by the epithelium of the dilated ends of the nephrons (nephric capsules). The cells of these capsules differentiate mainly into podocytes, but a few parietal cells connect to the draining tubule. This method of development explains the unique form of the renal corpuscle in the adult lamprey. Despite the type of morphogenesis, this renal corpuscle possesses the fine-structural features seen in the renal corpuscles of other vertebrates.     

Development of the nervous system of the pluteus larva of Strongylocentrotus droebachiensis

Summary Development of the nervous system of the pluteus larva of Strongylocentrotus droebachiensis was investigated using indirect immunofluorescence with antibodies against dopamine, GABA, and serotonin, and glyoxylic acid-induced fluorescence of catecholamines. Serotonergic cells first appear in full gastrulae; dopaminergic and GABAergic cells are present in early four-arm plutei. The number of neurons and the complexity of the nervous system increases through development of the pluteus. In the pluteus the dopaminergic component of the nervous system includes a ganglion in the lower lip of the mouth and a pair of ganglia at the base of the post-oral arms which extend axons along the base of the circumoral ciliary band. The distribution of cells visualized by glyoxylic acid-induced fluorescence is similar to that of dopaminergic cells. GABAergic neurons occur in the upper lip and in the wall of the esophagus. Serotonergic neurons are present in the lower lip; the pre-oral hood contains an apical ganglion which extends axons along the base of the epidermis overlying the blastocoel. The dopaminergic and GABAergic components of the nervous system are associated with effectors involved in feeding and swimming. The serotonergic component is not associated with any apparent effectors but may have a role in metamorphosis.     

Development and metamorphosis of the sea-star, Astropecten scoparius Valenciennes.

1. Astropecten scoparius develops to a bipinnaria with simple ciliary bands and short bipinnari arms through a wrinkled blastula by holoblastic, radial cleavage. 2. About seven days after insemination, the posterior portion of the bipinnaria becomes swollen and fine spicules appear on it, while the anterior portion, the stalk, remains unchanged. 3. Metamorphosis takes place gradually at the posterior portion, while the metamorphosing bipinnaria is pelagic. Two weeks after insemination, the stalk rapidly shrinks and the larva sinks to the bottom. 4. About 18 days after insemination, the juvenile completes metamorphois with the opening of the mouth. The newly metamorphosed juvenile is 600 mu in diameter and each arm bears two pairs of the tube-feet, each having a sucker at the tip, and one terminal tentacle with red eye-spot. 5. The aboral skeletal system of the juvenile immediately after metamorphosis is composed of one central, one madreporic, ten radial and interradial plates, in addition to five terminal plates on the arms. 6. The juveniles smaller than about one cm in R do not bear some of the diagnostic features in this species. 7. A characteristic feature of the development of Astropecten, i.e., the lack of a brachiolaria stage, is stressed. The term "nonbrachiolarian type" is tentatively proposed to distinguish the development of Astropecten and Luidia, which do not pass through a brachiolaria stage, from the usual indirect type of development.     

Comparative biology of oogenesis in nemertean worms

Stricker, S. A., Smythe, T. L., Miller, L. and Norenburg, J. L. 2001. Comparative biology of oogenesis in nemertean worms. — Acta Zoologica (Stockholm) 82 : 213–230
In order to supplement previous analyses of oogenesis in nemertean worms, this study uses light and electron microscopy to compare the ovaries and oocytes in 16 species of nemerteans that represent various taxa within the phylum. Nemertean ovaries comprise serially repeated sacs with an ovarian wall that characteristically includes myofilament-containing cells interspersed among the germinal epithelium. Each oocyte can attach to the germinal epithelium by a vegetally situated stalk and resides in the ovarian lumen without being surrounded by follicle cells. In the ovary, oocytes arrest at prophase I of meiosis and contain a hypertrophied nucleus ('germinal vesicle') that often possesses multiple nucleoli. Intraovarian growth apparently involves an autosynthetic mode of yolk formation in most nemerteans and generates oocytes that measure ~60 µm to 1 mm. When fully developed, oocytes can be discharged through a short gonoduct and are either spawned freely or deposited within egg cases. In most species, oocytes released from the ovary possess extracellular coats and resume maturation by undergoing germinal vesicle breakdown (GVBD). Such post-GVBD specimens also form a punctate endoplasmic reticulum that may facilitate fertilization and development.