Cellular and intracellular pathways mediating the metamorphic stimulus in hydrozoan planulae

Odd family reunion: DNA barcoding reveals unexpected relationship between three hydrozoan species

Knowledge of life histories is crucial for understanding ecological and evolutionary processes, but for many hydrozoan species only incomplete life cycles have been described due to challenges in linking hydromedusae with their polyp stages. Using a combination of DNA barcoding, morphology, and ecological information, we describe for the first time the polyp stage of Halopsis ocellata Agassiz, 1865 and re-describe that of Mitrocomella polydiademata (Romanes, 1876). Campanulinid hydroids referable to Lafoeina tenuis Sars, 1874 and collected in the same biogeographic region as the type locality of this species are shown to be the polyp stage of these two mitrocomid hydromedusae. The nominal species L. tenuis thus is a species complex that includes the polyp stage of medusae belonging to at least two genera currently placed in a different family. Consistent morphological and ecological differences were found between the polyps linked to each of these two hydromedusae, but molecular results suggest that yet other species may have morphologically similar hydroids. Polyps morphologically identified to L. tenuis are therefore better referred to as Lafoeina tenuis-type until further associations are resolved, particularly when occurring outside of the area of distribution of H. ocellata and M. polydiademata. Molecular identification integrated with traditional taxonomy is confirmed as an effective approach to link inconspicuous stages of marine invertebrates with hitherto unknown life cycles, especially in often-overlooked taxa. Disentangling the relationships between L. tenuis, H. ocellata, and M. polydiademata lays the ground for future research aimed at resolving the taxonomy and systematics of the enigmatic families Mitrocomidae and Campanulinidae.

Two-Round Ca2+ transient in papillae by mechanical stimulation induces metamorphosis in the ascidian Ciona intestinalis type A

Marine invertebrate larvae are known to begin metamorphosis in response to environmentally derived cues. However, little is known about the relationships between the perception of such cues and internal signalling for metamorphosis. To elucidate the mechanism underlying the initiation of metamorphosis in the ascidian, Ciona intestinalis type A (Ciona robusta), we artificially induced ascidian metamorphosis and investigated Ca²⁺ dynamics from pre- to post-metamorphosis. Ca²⁺ transients were observed and consisted of two temporally distinct phases with different durations before tail regression which is the early event of metamorphosis. In the first phase, Phase I, the Ca²⁺ transient in the papillae (adhesive organ of the anterior trunk) was coupled with the Ca²⁺ transient in dorsally localized cells and endoderm cells just after mechanical stimulation. The Ca²⁺ transients in Phase I were also observed when applying only short stimulation. In the second phase, Phase II, the Ca²⁺ transient in papillae was observed again and lasted for approximately 5-11 min just after the Ca²⁺ transient in Phase I continued for a few minutes. The impaired papillae by Foxg-knockdown failed to induce the second Ca²⁺ transient in Phase II and tail regression. In Phase II, a wave-like Ca²⁺ propagation was also observed across the entire epidermis. Our results indicate that the papillae sense a mechanical cue and two-round Ca²⁺ transients in papillae transmits the internal metamorphic signals to different tissues, which subsequently induces tail regression. Our study will help elucidate the internal mechanism of metamorphosis in marine invertebrate larvae in response to environmental cues.

Warm temperature alters the chemical cue preference of Acropora tenuis and Heliopora coerulea larvae

Larvae released into the water column rely on chemical cues from the benthos for successful settlement. However, larval preference for substrates may be affected by rising seawater temperature brought about by global climate change. In this study, we examined the effect of elevated temperature on chemical cue preference by larvae of the scleractinian coral, Acropora tenuis, and the octocoral, Heliopora coerulea, collected from northwestern Philippines. At ambient temperature (28 °C), both H. coerulea and A. tenuis larvae showed preference for substrates containing either crustose coralline algae or crude ethanolic extracts from conspecific or congeneric corals. In contrast, at higher temperature (30 °C), greater preference was shown for substrates containing the crude extract from conspecific or congeneric corals. These results demonstrate that elevated temperature can change larval substrate preference, which will have downstream impacts on crucial biological processes, such as larval settlement and recruitment.

Molecular characterization of larval development from fertilization to metamorphosis in a reef-building coral

BACKGROUND

Molecular mechanisms underlying coral larval competence, the ability of larvae to respond to settlement cues, determine their dispersal potential and are potential targets of natural selection. Here, we profiled competence, fluorescence and genome-wide gene expression in embryos and larvae of the reef-building coral Acropora millepora daily throughout 12 days post-fertilization.

RESULTS

Gene expression associated with competence was positively correlated with transcriptomic response to the natural settlement cue, confirming that mature coral larvae are "primed" for settlement. Rise of competence through development was accompanied by up-regulation of sensory and signal transduction genes such as ion channels, genes involved in neuropeptide signaling, and G-protein coupled receptor (GPCRs). A drug screen targeting components of GPCR signaling pathways confirmed a role in larval settlement behavior and metamorphosis.

CONCLUSIONS

These results gives insight into the molecular complexity underlying these transitions and reveals receptors and pathways that, if altered by changing environments, could affect dispersal capabilities of reef-building corals. In addition, this dataset provides a toolkit for asking broad questions about sensory capacity in multicellular animals and the evolution of development.

Sensory Flask Cells in Sponge Larvae Regulate Metamorphosis via Calcium Signaling

The Porifera (sponges) is one of the earliest phyletic lineages to branch off the metazoan tree. Although the body-plan of sponges is among the simplest in the animal kingdom and sponges lack nervous systems that communicate environmental signals to other cells, their larvae have sensory systems that generate coordinated responses to environmental cues. In eumetazoans (Cnidaria and Bilateria), the nervous systems of larvae often regulate metamorphosis through Ca(2+)-dependent signal transduction. In sponges, neither the identity of the receptor system that detects an inductive environmental cue (hereafter "metamorphic cues") nor the signaling system that mediates settlement and metamorphosis are known. Using a combination of behavioral assays and surgical manipulations, we show here that specialized epithelial cells-referred to as flask cells-enriched in the anterior third of the Amphimedon queenslandica larva are most likely to be the sensory cells that detect the metamorphic cues. Surgical removal of the region enriched in flask cells in a larva inhibits the initiation of metamorphosis. The flask cell has an apical sensory apparatus with a cilium surrounded by an apical F-actin-rich protrusion, and numerous vesicles, hallmarks of eumetazoan sensory-neurosecretory cells. We demonstrate that these flask cells respond to metamorphic cues by elevating intracellular Ca(2+) levels, and that this elevation is necessary for the initiation of metamorphosis. Taken together, these analyses suggest that sponge larvae have sensory-secretory epithelial cells capable of converting exogenous cues into internal signals via Ca(2+)-mediated signaling, which is necessary for the initiation of metamorphosis. Similarities in the morphology, physiology, and function of the sensory flask cells in sponge larvae with the sensory/neurosecretory cells in eumetazoan larvae suggest this sensory system predates the divergence of Porifera and Eumetazoa.

Characterization of receptor of activated C kinase 1 (RACK1) and functional analysis during larval metamorphosis of the oyster Crassostrea angulata

During a large-scale screen of the larval transcriptome library of the Portuguese oyster, Crassostrea angulata, the oyster gene RACK, which encodes a receptor of activated protein kinase C protein was isolated and characterized. The cDNA is 1,148 bp long and has a predicted open reading frame encoding 317 aa. The predicted protein shows high sequence identity to many RACK proteins of different organisms including molluscs, fish, amphibians and mammals, suggesting that it is conserved during evolution. The structural analysis of the Ca-RACK1 genomic sequence implies that the Ca-RACK1 gene has seven exons and six introns, extending approximately 6.5 kb in length. It is expressed ubiquitously in many oyster tissues as detected by RT-PCR analysis. The Ca-RACK1 mRNA expression pattern was markedly increased at larval metamorphosis; and was further increased along with Ca-RACK1 protein synthesis during epinephrine-induced metamorphosis. These results indicate that the Ca-RACK1 plays an important role in tissue differentiation and/or in cell growth during larval metamorphosis in the oyster, C. angulata.

Ultrastructure of the calcium-sequestering gastrodermal cell in the hydroidHydractinia symbiolongicarpus (Cnidaria, Hydrozoa)

Large, free-floating crystals of calcium carbonate occur in vacuoles of gastrodermal cells of the hydroid Hydractinia symbiolongicarpus. Here, morphological details about the process by which these cells accumulate and sequester calcium are provided by a cytochemical method designed to demonstrate calcium at the ultrastructural level. Electron-dense material presumably indicative of the presence of calcium was EGTA-sensitive and was shown by parallel electron energy loss spectroscopy (EELS) and energy spectroscopic imaging (ESI) to contain calcium. Calcium occurred in only one cell type, the endodermally derived gastrodermal cell. In these cells, the electron-dense material appeared first as a fine precipitate in the cytosol and nucleus and later as larger deposits and aggregates in the vacuole. During the life cycle, gastrodermal cells of the uninduced planula and the planula during metamorphic induction sequestered calcium. In primary polyps and polyps from established colonies, gastrodermal cells sequestered calcium, but the endodermal secretory cells did not. Our observations support the hypothesis that gastrodermal cells function as a physiological sink for calcium that enters the organism in conjunction with calcium-requiring processes such as motility, secretion, and metamorphosis.

What's new in cnidarian biology?

This introduction to the following collection of eight review articles on aspects of cnidarian biology looks at reasons why people study these animals, their economic importance, and their conceptual interest as highlighted in the reviews.

Metamorphosis in the Cnidaria

The free-living stages of sedentary organisms are an adaptation that enables immobile species to exploit scattered or transient ecological niches. In the Cnidaria the task of prospecting for and identifying a congenial habitat is consigned to tiny planula larvae or larva-like buds, stages that actually transform into the sessile polyp. However, the sensory equipment of these larvae does not qualify them to locate an appropriate habitat from a distance. They therefore depend on a hierarchy of key stimuli indicative of an environment that is congenial to them; this is exemplified by genera of the Anthozoa (Nematostella, Acropora), Scyphozoa (Cassiopea), and Hydrozoa (Coryne, Proboscidactyla, Hydractinia). In many instances the final stimulus that triggers settlement and metamorphosis derives from substrate-borne bacteria or other biogenic cues which can be explored by mechanochemical sensory cells. Upon stimulation, the sensory cells release, or cause the release of, internal signals such as neuropeptides that can spread throughout the body, triggering decomposition of the larval tissue and acquisition of an adult cellular inventory. Progenitor cells may be preprogrammed to adopt their new tasks quickly. Gregarious settlement favours the exchange of alleles, but also can be a cause of civil war. A rare and spatially restricted substrate must be defended. Cnidarians are able to discriminate between isogeneic and allogeneic members of a community, and may use particular nematocysts to eliminate allogeneic competitors. Paradigms for most of the issues addressed are provided by the hydroid genus Hydractinia.

Why and how marine-invertebrate larvae metamorphose so fast

It is argued that larviparous development has evolved at least eight times among extant animals. A 'need for speed hypothesis' is proposed to explain profound convergence on a pattern of small larvae and rapid metamorphosis across six marine invertebrate clades. Shared selection pressures include limits to larval size, the plankton-to-benthos transition, extreme hazards on the benthos, and the profound helplessness of metamorphosing animals. The adaptive mechanisms include: (1) development of juvenile structures in larvae before they are metamorphically competent; (2) external cues trigger metamorphosis; and (3) rapid cell-to-cell conductance of the metamorphic signal to bring about rapid loss of larval structures and release of juvenile structures. Both pattern and mechanisms contrast in every regard with those of the other two major larviparous clades, Insecta and Amphibia.

Serotonin plays an early role in the metamorphosis of the hydrozoan Phialidium gregarium

Hydrozoan larvae normally metamorphose in response to an obligate external environmental cue. Application of certain artificial chemical stimuli will also induce metamorphosis. These chemicals and their inhibitors have been used to define and order some of the signal transduction events involved in this process. Results from this study show that exogenous application of serotonin (5-HT) will induce metamorphosis and that 5-HT immunoreactive cells are present in larvae when they are competent to metamorphose. The 5-HT inhibitors ketanserin, clozapine, and 5,7-DHT prevent metamorphosis from occurring as a response to a natural inducing stimulus. Additionally, 5-HT signaling occurs prior to both an influx of external Ca2+ from seawater and activation of protein kinase C, two other steps in the metamorphic signal transduction pathway. The neuropeptide LWamide, previously shown to induce metamorphosis in a related hydrozoan, Hydractinia echinata, also induced metamorphosis in Phialidium. When larvae were cotreated with LWamide and the 5-HT antagonist ketanserin, settlement occurred but was not followed by polyp morphogenesis. These results are used to present a model for the action of 5-HT during metamorphosis in Phialidium gregarium.

Signal transduction in barnacle settlement: Calcium re-visited

The search for marine natural product antifoulants is being hampered by problems associated with conventional settlement assays. Yet it has been recognised that the study of how chemical cues are perceived by fouling organisms may offer clues to settlement inhibitors and may identify novel biochemical assays for antifoulants based on signal transduction pathways. Here the role of calcium in barnacle settlement is re-examined. A requirement for calcium in settlement of the cypris larva of Balanus amphitrite has been confirmed; settlement was inibited in low calcium, and calcium-free, seawater. Although 10 mM (19.27 mM) excess calcium had no effect and higher concentrations were inhibitory, a 5 mM excess stimulated settlement. Stimulation is proposed to be effected by an increase in intracellular calcium. The release of calcium from intracellular pools with thapsigargin (but not cyclopiazonic acid) induced settlement and an antagonist of intracellular calcium, TMB-8, generally inhibited settlement. Nevertheless, the calcium ionophore A23187 did not induce settlement at the concentrations tested. Consequently, the relative importance of external calcium and intracellular pools to increased intracellular calcium has yet to be determined. Pharmacological manipulations of calcium channels with organic and inorganic channel blockers strongly indicate calcium channel involvement in barnacle settlement. The data are summarised in an hypothetical scheme for signal transduction at settlement and are compared to those obtained for other marine invertebrate larvae.

Protein kinase C in hydrozoans: involvement in metamorphosis of Hydractinia and in pattern formation of Hydra

A wealth of information has suggested the involvement of protein kinase C (PKC) in metamorphosis of Hydractinia echinata and in pattern formation of Hydra magnipapillata. We have identified a Ca²⁺- and phospholipid-dependent kinase activity in extracts of both species. The enzyme was characterized as being similar to mammalian PKC by ion exchange chromatography. Gel filtration experiments revealed a molecular weight of about 70 kD. In phosphorylation assays of endogenous Hydractinia proteins, a protein with a molecular weight of 22.5 kD was found to be phoshorylated upon addition of phosphatidylserine. Bacterial induction of metamorphosis of Hydractinia echinata caused an increase in endogenous diacylglycerol, the physiological activator of PKC, suggesting that the bacterial inducer acts by activating receptor-regulated phospholipid metabolism. Exogenous diacylglycerol leads to membrane translocation of PKC, indicative of an activation. On the basis of our results and those of Freeman and Ridgway (1990) a model for the biochemical events during metamorphosis is presented.

Calcium control of metamorphosis in polychaete larvae

The importance of Ca2+ in the control of metamorphosis of a marine invertebrate larva was investigated. An excess of [Ca2+] in the external medium induced metamorphosis of Phragmatopoma californica (polychaete) larvae in a concentration-dependent manner. This effect is specific for calcium, and not simply the result of osmotic changes, as an excess of Mg2+ did not induce metamorphosis. Consistent with this finding, the calcium ionophore, A23187, also induced metamorphosis in a concentration-dependent manner. Paradoxically, however, the aromatic compounds diltiazem, verapamil, D600, and nifedipine, known to block Ca2+ channels in other systems, also induced metamorphosis. When exposed to diltiazem for only 20 h and subsequently washed free of this compound, 95% of the larvae metamorphosed and developed normally. Previous studies have demonstrated that the induction of metamorphosis in Phragmatopoma californica is controlled by chemosensory recognition of an exogenous morphogen and mediated by an excitatory pathway that involves adenyl cyclase and cyclic AMP. Because cellular excitation and cyclic AMP-dependent signal transduction generally involve the participation of calcium ion, the most parsimonious explanation for the results reported here include (1) direct control of the morphogenetic pathway by calcium ion, and (2) complexities of the calcium regulation of this process, or a functional similarity between the structurally related aromatic effectors tested and the natural inducer of metamorphosis.