Ammonia induces metamorphosis of the oral half of buds into polyp heads in the scyphozoan Cassiopea

Summary The scyphozoan polyp Cassiopea forms vegetative free swimming buds that metamorphose into sessile polyps. In sterile sea water metamorphosis does not take place. Buds keep swimming for weeks. Application of millimolar quantities of NH ₄ ⁺ causes the buds to metamorphose within one day. The resulting animals bear hypostome and tentacles, however, only occasionally peduncle and foot. Almost all transform either completely into solitary polyp head or only the oral half of the bud developes into a head while the aboral half remains bud tissue which becomes constricted off. Under suited conditions this small bud is able to transform into a normal shaped polyp.     

Metamorphosis and acquisition of symbiotic algae in planula larvae and primary polyps of Acropora spp.

Coral planulae settle, then metamorphose and form polyps. This study examined the morphological process of metamorphosis from planulae into primary polyps in the scleractinian corals Acropora nobilis and Acropora microphthalma, using the cnidarian neuropeptide Hym-248. These two species release eggs that do not contain Symbiodinium. The mode of acquisition of freshly isolated Symbiodinium (zooxanthellae) (FIZ) by the non-symbiotic polyp was also examined. Non-Hym-248 treated swimming Acropora planulae did not develop blastopore, mesenteries or coelenteron until the induction of metamorphosis 16 days after fertilization. The oral pore was formed by invagination of the epidermal layer after formation of the coelenteron in metamorphosing polyps. At 3 days after settlement and metamorphosis, primary polyps exposed to FIZ established symbioses with the Symbiodinium. Two–four days after exposure to FIZ, the distribution of Symbiodinium was limited to the gastrodermis of the pharynx and basal part of the polyps. Eight–ten days after exposure to FIZ, Symbiodinium were present in gastrodermal cells throughout the polyps.     

Groups travel further: pelagic metamorphosis and polyp clustering allow higher dispersal potential in sun coral propagules

We report that planulae produced by Tubastraea coccinea can metamorphose and aggregate in groups of up to eight polyps in the water column, without previous settlement on benthic substrate. We also evaluated the survival of propagules to test whether different levels of aggregation allowed for longer planktonic life and, therefore, higher dispersal potential. Our results show that pelagic polyps live longer than planulae, probably because they can feed and meet the presumably high-energy demands of swimming. Clusters of two or more individuals lived longer than solitary polyps. However, mortality did not differ between small (2–3 polyps) and large (4–8 polyps) clusters, suggesting the existence of an upper limit to cluster size. Most swimming clusters (80 %) remained alive after 6 months, suggesting that pelagic metamorphosis and cluster formation can be a key life-history feature increasing dispersal potential, population connectivity, and the colonization of new habitats in this invasive species.     

Using KCl to determine size at competence for larvae of the marine gastropod Crepidula fornicata (L.)

Competent larvae of the marine gastropod Crepidula fornicata (L.) were induced to metamorphose (i.e., lose the velum) by elevating sea-water [KCl] by 5–50 mM. The response was optimal at 15–20-mM elevations, at which 50% metamorphosis was obtained in <4 h. Larvae that did not metamorphose during brief exposures (1–5 h) to elevated [KCl] generally maintained the larval form following transfer to control sea water, suggesting that competent larvae must be continuously immersed in the test solutions for metamorphosis to occur. The smallest larvae to respond to elevated [KCl] had shell lengths of ≈700–800 μm, the range of shell lengths within which larvae of this species become responsive to natural inducers. All larvae >≈1125 μm shell length metamorphosed in response to increased [KCl]. Rearing temperature may affect the size at which larvae of this species become responsive to K⁺. CaCl₂ (20-mM concentration elevations), GABA (4×10⁻⁷, 4×10⁻⁶ M), and NaCl (10–20-mM concentration elevations) generally failed to trigger metamorphosis. Twenty-mM elevations of [RbCl] and [CsCl] induced 100% metamorphosis but the juveniles were immobile and died after several days. Elevating [KCl] appears to be a reliable way to assess competence and trigger metamorphosis in larvae of C. fornicata.     

Comparative fine structural studies of planulae and primary polyps of identical age of the sea pen,Ptilosarcus gurneyl

Electron microscopic study of an 18-day-old planulae and primary polyps of the sea pen, Ptilosarcus gurneyi, reveals 14 cell types: sustentacular cell A, sustentacular cell B, nerve cell, sensory cell, cnidoblast, interstitial cell, five types of gland cell (A, B, C, D and E), amoebocyte, style cell and endodermal cell. Of these, 9 are found in the planula, 12 in polyps and 7 are common to both stages. The fine structure of all cell types is described. Since the planulae and polyps in this study were identical in age of development, the gaining and losing of certain types of cells in the polyp are attributed to changes associated with settlement and metamorphosis. Modifications of the seven common cell types during metamorphosis can also be attributed to the change of life style from pelagic to benthic.     

Serotonin involvement in the metamorphosis of the hydroid Eudendrium racemosum

Hydroid planulae metamorphose in response to an inducing external stimulus, usually a bacterial cue. There is evidence that neurotransmitters participate in the signal transduction pathway of hydroid metamorphosis. Eudendrium racemosum is a colonial hydroid common in the Mediterranean Sea. It lacks the medusa stage and the planulae develop on female colonies during the fertile season. In this work, serotonin (5-HT) was localized in some planula ectodermal cells. Co-localization of serotonin and beta-tubulin suggested that 5-HT was present in sensory nervous cells and in different ectodermal cells. To investigate the role of neurotransmitters in metamorphosis, E. racemosum planulae were treated with serotonin and dopamine and with agonists and antagonists of the corresponding receptors. Serotonin and a serotonin receptor agonist induced metamorphosis, while a 5-HT receptor antagonist inhibited it. Dopamine and all dopaminergic drugs used did not show any significant effect on the onset of metamorphosis. Results from this work showed that 5-HT could stimulate metamorphosis in E. racemosum planulae in the presence of a natural inducer. A mechanism by which this neurotransmitter could act in this phase is proposed.     

Morphological, cytochemical and neuropharmacological evidence for the presence of catecholamines in hydrozoan planulae

Planula larvae of Halocordyle disticha were examined for the presence of catecholamines using a multipronged approach. Transmission electron micrographs of planular sensory cells and ganglionic cells demonstrated dense-cored vesicles and electron-dense droplets in both cell types. These vesicles and droplets were similar in morphology to catecholamine-containing granules of vertebrates. Planulae processed with the SPG histofluorescence technique, specific only for catecholamines, exhibited blue-green fluorophores which were most prominent in the anterior ectoderm. Such fluorescence was associated with sensory cells, ganglionic cells and the neural plexus. Pretreatment of planulae with neuropharmacological agents which prevent reuptake (reserpine) or cause release (nicotine, ephedrine) of catecholamines caused a diminution of the fluorophores. Pretreatment of animals with 6-hydroxydopamine, which causes destruction of catecholamine-containing cells, prevented any fluorescent response. Ultrastructural examination of reserpine-treated planulae revealed a dramatic reduction in the populations of dense-cored vesicles and electron-dense droplets. Furthermore, many of the vesicles and droplets remaining in reserpinized animals appeared washed out, i.e. stained faintly. Exposure of planulae to exogenous norepinephrine caused premature, rapid metamorphosis and produced polyps with slightly stunted tentacles and pitted, irregular hypostomes. Exposure of planulae to nicotine caused similar effects. Rearing planulae in sea water containing alpha blockers, phentolamine and tolazoline, had no discernible effect on behaviour (motility, phototactic response) or gross morphology. However, planulae raised in sea water containing propranolol, a beta blocker, ceased all movement, became tack-shaped and died within 72 h. These results meet multiple criteria for the identification of catecholamines in hydrozoan planulae and suggest that such catecholamines may function as neurotransmitters, neurohormones or neuromodulators during larval development.     

Metamorphosis of <Emphasis Type="Italic">Hydractinia echinata</Emphasis>—natural versus artificial induction and developmental plasticity

Many marine invertebrates reproduce through a larval stage. The settlement and metamorphosis of most of the species are synchronised and induced by environmental organisms, mainly bacteria. The hydrozoan Hydractinia echinata has become a model organism for metamorphosis of marine invertebrates. In this species, bacteria, e.g. Pseudoalteromonas espejiana, are the natural inducers of metamorphosis. Like in other species of marine invertebrates, metamorphosis can be induced artificially by monovalent cations, e.g. Cs⁺. In this study, we present systematic data that metamorphosis—with both inducing compounds, the natural one from bacteria and the artificial one Cs⁺—are indeed similar with respect to (a) the morphological progression, (b) the localisation of the primary induction signal in the larva, (c) the pattern of apoptotic cells occurring during the initial 10 h of metamorphosis and (d) the disappearance of RFamide-dependent immunocytochemical signals in sensory neurons during this process. However, a difference occurs during the development of the anterior end, insofar as apoptotic cells and settlement appear earlier in planulae induced with bacteria. Thus, basically, Cs⁺ may be used as an artificial inducer, mimicking the natural process. However, differences in the appearance of apoptotic cells and in settlement raise the question of how enormous developmental plasticity in hydrozoans actually can be, and how this is related to the absence of malignant devolution in hydrozoans.     

Analysis of pattern formation during embryonic development of Hydractinia echinata

Summary Patterning processes during embryonic development of Hydractinia echinata were analysed for alterations in morphology and physiology as well as for changes at the cellular level by means of treatment with proportioning altering factor (PAF). PAF is an endogenous factor known to change body proportions and to stimulate nerve cell differentiation in hydroids (Plickert 1987, 1989). Applied during early embryogenesis, this factor interferes with the proper establishment of polarity in the embryo. Instead of normal shaped planulae with one single anterior and one single posterior end, larvae with multiple termini develop. Preferentially, supernumerary posterior ends, which give rise to polyp head structures during metamorphosis, form while anterior ends are reduced. The formation of such polycaudal larvae coincide with an increase in the number of interstitial cells and their derivatives at the expense of epithelial cells. Treatment of further advanced embryonic stages causes an increase in length, presumably due to the general stimulation of cell proliferation observed in such embryos. Also, the spatial arrangement of cells (i.e. cells in proliferation and RFamide (Arg-Phe-amide immunopositive nerve cells) is altered by PAF. Larvae that develop from treated embryos display altered physiological properties and are remarkably different from normal planulae with respect to their morphogenetic potential: (1) Larvae lose their capacity to regenerate missing anterior parts; isolated posterior larva fragments form regenerates of a bicaudal phenotype. (2) In accordance with the frequently observed reduction of anterior structures, the capacity to respond to metamorphosis-inducing stimuli decreases. (3) The morphogenetic potential to form basal polyp parts is found to be reduced. In contrast, the potential to form head structures during metamorphosis increases, since primary polyps with supernumerary hypostomes and tentacles metamorphose from treated animals.     

An endogenous peptide is involved in internal control of metamorphosis in the marine invertebrate<Emphasis Type="Italic"> Cassiopea xamachana</Emphasis> (Cnidaria: Scyphozoa)

In the metagenetic life-cycle of the scyphozoan Cassiopea xamachana metamorphosis of planula-larvae or larva-like buds to polyps is triggered by specific external cues which are transmitted inside the larva or bud where internal signals finally coordinate the initiation of metamorphosis. This study deals with an endogenous metamorphosis inducer present in planulae and buds of Cassiopea. The inductive cue is localized in the basal part of the buds and can be characterized as a peptide with an apparent molecular weight of about 7,000 Da. Further purification was performed via reversed phase HPLC on a C18 column. Additional inhibitor assays revealed that protein kinase C and PI3 kinase, two known elements of the metamorphosis-inducing signal transduction cascade in Cassiopea, may act downstream of the endogenous inducing peptide.     

A fine structural study of metamorphosis of the hydrozoan Mitrocomella polydiademata

This report is a comprehensive fine structural analysis of the morphological changes occurring during metamorphosis of the marine hydrozoan Mitrocomella polydiademata. Five stages are recognized during metamorphosis: planulae just prior to settlement, ball and filiform stages, immature polyps, and primary feeding polyps. Settlement and metamorphosis of cnidarian planulae involve such changes as ciliary arrest, discharge of nematocytes, secretion of glandular cells, differentiation of cells, and changes in cell and body shape.     

Is the remodeling of the polyp prepattern in a hydrozoan planula a function of larval age or size and is it of “adaptive” significance?

Eggs of medusae develop into lecithotrophic planulae that undergo metamorphosis at different ages to form polyps. As planulae age they decrease in size as their yolk stores are utilized. The planulae of most Phialidium medusae develop into polyps where there is a decrease in the size of the holdfast region and a relative increase in the size of the hydranth region as they age. These changes occur independently of the decrease in planula size. In planulae with a decrease in the size of the holdfast region and an increase in the size of the hydranth-forming region there was a 50% decline in polyps that successfully stayed attached to the substrate after metamorphosis. These aged planulae produced an initial hydranth with the same number of tentacles as polyps from full-sized young planulae while young half-sized planulae produced hydranths where the tentacle number was smaller. The first phase of polyp colony growth with a small initial hydranth was slower than growth of a colony with a larger initial hydranth. Predation during this period led to more death in colonies with a small initial hydranth. The decline in successful attachment in aged planulae was not offset by the higher rate of growth and a smaller window of time where predation leads to death, suggesting that this age-related developmental change in planulae was not adaptive.     

Early Life History of the ‘Irukandji’ Jellyfish Carukia barnesi

Adult medusae of Carukia barnesi were collected near Double Island, North Queensland Australia. From 73 specimens, 8 males and 15 females spawned under laboratory conditions. These gametes were artificially mixed which resulted in fertilized eggs. Post fertilization, most eggs developed to an encapsulated planula stage and then paused for between six days and six months prior to hatching as ciliated planulae. The paused stage planulae were negatively buoyant and adhered to substrate. The first planula was produced six days post fertilization, lacked larval ocelli, remained stationary, or moved very slowly for two days prior to metamorphosis into primary polyps. Mature polyps reproduced through asexual reproduction via lateral budding producing ciliated swimming polyps, which in turn settled and developed into secondary polyps. Medusae production for this species was in the form of monodisc strobilation, which left behind polyps able to continue asexual reproduction.     

The Embryonic Origin of Neurosensory Cells and the Role of Nerve Cells in Metamorphosis in Phialidium gregarium (Cnidaria,Hydrozoa)

Summary

The embryonic origin of the nervous system in Phialidium gregarium was investigated. Entoderm-free planulae, surgically produced by bisection at mid-gastrulation, and normal planulae were examined by light and electron microscopy to determine their cellular composition. The cell types that occur in the epidermis of the normal planula were described. The entoderm-free planulae were found to be devoid of interstitial cells and their derivatives, the nematocytes and ganglion cells. Neurosensory cells were present, however, indicating that they are derivatives of the ectodermal epithelium.

The role of nerve elements in the initiation of metamorphosis was also examined. Normal and entoderm-free planulae treated for four hours with 0.4% colchicine at two, three, or four days of development fail to undergo cesium-induced metamorphosis. Since such treatment in other hydrozoans eliminates interstitial cells and their derivatives [1-3], it might be argued that ganglion cells are necessary to initiate metamorphosis. The observation that entoderm-free planulae, devoid of interstitial cell derivatives, are capable of responding to induction by bacteria or cesium, however, indicates that in Phialidium the colchicine effect is on other cell types. The results are compared with findings for other Cnidaria.     

Substratum preferences and planulae settling of two red sea alcyonaceans: Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl)

The settling behaviour and substratum preferences of the planulae of the Red Sea soft corals Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl) were examined in the laboratory. The planulae of the two species have a short pelagic phase and they tend to settle immediately upon leaving the parent colonies. Mucous secretion is used by the larvae for crawling and adhering to the substratum. They exhibit an aggregated pattern of settlement. The developing polyps are found in depressions or pits of the substratum. The planulae preferentially settle on rough substrata and avoid smooth surfaces. They search for substrata covered with an organic coating, composed of turf or crustose coralline algae. Such substrata create better conditions for larval settlement and metamorphosis. The planulae of P. f. fulvum exhibit a striking preference for upside-down attachment on undersides of the substrata, while Xenia macrospiculata utilizes both substratum faces for settlement. Light intensity seems insignificant in determining attachment sites. The findings of the experiments correspond well with the distributional patterns of juveniles of the two species as found in the natural environment. The specific requirements for settling of both species increase their chances of successful development and thus enhance their survival.     

Bud formation in the scyphozoan Cassiopea andromeda: epithelial dynamics and fate map

Cassiopea andromeda scyphistomae reproduce asexually by forming spindle shaped buds which, after detachment, metamorphose into polyps. Parent polyps appear to contribute to the buds' ecto- and endodermal epithelial cells, septal muscle cells, nematocytes and some zooxanthellae. Herein we describe bud morphogenesis, define 5 bud stages, and investigate the recruitment of bud ectoderm. India ink vital marking experiments reveal permanent apicobasal displacement of ectoderm. Labelled polyp cells are displaced towards and incorporated into the emerging bud. Ectoderm is recruited from all angular positions and cells labelled at increasing distances from the bud center are traced at increasingly more proximal positions on the buds. Unlike in Hydra attenuata, the recruitment area appears to be asymmetric since the zone contributing ectoderm from below is smaller than the zones above and lateral to the buds.     

Cellular and intracellular pathways mediating the metamorphic stimulus in hydrozoan planulae

Summary Both the natural metamorphic stimulus (an unidentified bacterial product) and an artificial trigger of metamorphosis (Cs⁺) cause large calcium transients in planula cells of the hydrozoanMitrocomella polydiademata. When these transients are inhibited with calcium channel blockers, metamorphosis is also inhibited. All cells of theMitrocomella planula contain a calcium-specific photoprotein. The cells where the calcium transients occur during natural- and Cs⁺-induced metamorphosis have been visualized in normal and entoderm free planulae that lack ganglion cells, using a compound microscope coupled to an image intensifier and video camera. During bacteria- and Cs⁺-induced metamorphosis, groups of contiguous cells, occupying from about 10% to the entire visible surface of the planula, simultaneously exhibit calcium transients. When the cells that initiate a transient comprise only part of the planula surface, the calcium transient frequently propagates and can eventually involve every cell on the visible planula surface. There is no special site on the planula surface where calcium transients are more apt to be initiated. There is no indication that propagation of a flash in one direction is more likely than in another. The velocity of propagation is virtually the same in all directions. The only feature of the spatial distribution of bacteria- and Cs⁺-induced calcium transients that appears to be necessary for the induction of metamorphosis is that at least one transient must involve all of the surface cells of the planula. The spatial behavior of calcium transients is the same in entoderm free planulae (lacking ganglion cells) as in normal planulae. The propagation of these calcium transients most probably occurs via epithelial conduction. This metamorphic step involving calcium transients is probably the intercellular communication system that informs the cells of the planula that metamorphosis will commence.Metamorphosis inMitrocomella planulae can also be induced with phorbol esters. Calcium transients do not occur during phorbol ester-induced metamorphosis, indicating that they act at a different point in the metamorphic pathway. Calcium channel blockers do not inhibit phorbol ester-induced metamorphosis. Inhibitors of protein kinase-C, inhibit both phorbol ester-induced metamorphosis and Cs⁺- and bacteria-induced metamorphosis, but have no effect on the calcium transients induced by Cs⁺. This indicates that the calcium transient mediated step in the metamorphic pathway occurs prior to protein kinase-C activation. Calcium transients probably play a major role in activating protein kinase-C.     

Planulation,larval biology,and early growth of the deep‐sea soft corals Gersemia fruticosa and Duva florida (Octocorallia: Alcyonacea)

Abstract. Although the reproductive biology and early life‐history stages of deep‐sea corals are poorly understood, such data are crucial for their conservation and management. Here, we describe the timing of larval release, planula behavior, metamorphosis, settlement, and early juvenile growth of two species of deep‐sea soft corals from the northwest Atlantic. Live colonies of Gersemia fruticosa maintained under flow‐through laboratory conditions released 79 planulae (1.5–2.5 mm long) between April and early June 2007. Peak planulation in G. fruticosa coincided with peaks in the chlorophyll concentration and deposition rates of planktic matter. Metamorphosis and settlement occurred 3–70 d post‐release. The eight primary mesenteries typically appeared within 24 h, and primary polyps grew to a height of ～6–10 mm and a stalk diameter of ～1 mm within 2–3 months. Planulae of Duva florida (1.5–2.5 mm long) were extracted surgically from several colonies and were successfully reared in culture. Primary polyps reached a height of ～3–4 mm within 2–3 months. No budding of primary polyps was observed in either species over 11–13 months of monitoring, suggesting a very slow growth rate.     

Signal transmission and covert prepattern in the metamorphosis of Hydractinia echinata (Hydrozoa)

Summary Planulae are simply structured larvae lacking an overt longitudinal organization. In the course of a rapid metamorphosis, however, they transform into polyps, which display striking structural patterns. Metamorphosis takes place only in response to external stimuli. Surgical removal and transplantation of larval parts reveal that external stimuli, including artificial inducers such as cesium ions, tumor promoters and diacylglycerol, act on the anterior quarter of the larva where sensory cells containing Arg-Phe-amide-like peptides are located. The external stimuli initiate the release of an internal signal, which is transmitted to the posterior end causing the successive transformation of larval into adult tissue. The transformation front moves from the anterior to the posterior quarter in 60 min. The internal signal can be released or bypassed by a transitory lowering of the Mg²⁺ content of the seawater. By using this procedure, or by administering an extract containing the putative internal signal substance, each isolated part of the larva can be induced to metamorphose separately. Provided there is no time for regeneration after cutting before metamorphosis is initiated, the most anterior fragment forms only stolons, the most posterior fragment forms only a head. The overt pattern of the polyp is, therefore, generated under the influence of a covert anterior-posterior prepattern of the larva.     

Control of head morphogenesis in an invertebrate asexually produced larva-like bud (<Emphasis Type="Italic">Cassiopea andromeda</Emphasis>; Cnidaria: Scyphozoa)

Scyphopolyps of Cassiopea andromeda propagate asexually by forming larva-like buds which separate from the parent in a developmentally quiescent state. These buds metamorphose into sessile polyps when exposed to specific biogenic, chemical inducers. Morphogenesis of transversely dissected buds indicates the presence of pattern-determining signals; whereas the basal bud fragments may still form a complete scyphistoma the apical bud fragments develop spontaneously in the absence of an inducer into a polyp head without stalk and foot. Based on these findings Neumann (dissertation, Cologne University, 1980) postulated a head-inhibiting signal which is released at the basal pole and inhibits head formation at the apical end. Contrary to this hypothesis dissection itself might induce the development of head structures. The present study deals with the control of polyp head formation in C. andromeda. It concentrates on two points, namely the postulated head inhibitor and the involvement of compounds known to act during metamorphosis (the enzyme protein kinase C and the specific metamorphosis inducer Z-GPGGPA). We found that compared to intact buds and apical bud fragments transversely incised buds reached an intermediate stage of head development. This confirms Neumann's hypothesis. Consequently we focused on the mode of action and the chemical nature of the head-inhibiting signal in C. andromeda. Our results indicate that the head inhibitor may be included in one of six pooled fractions isolated from bud homogenate via gel filtration on a Sephadex G-50 column. The inhibitor is supposed to be water-soluble and to have a molecular weight of 850-1,500 Da. Furthermore we prove that head formation is not promoted by the metamorphosis-inducer Z-GPGGPA but is prevented by the inhibitors psychosine, chelerythrine and RO-32-0432 showing the involvement of protein kinase C in this process.