Signal transduction in barnacle settlement: Calcium re-visited

Influence of calcium concentration on larval adhesion in a highly invasive fouling ascidian: From morphological changes to molecular mechanisms

Calcium ion (Ca2+) is involved in the protein-mediated larval adhesion of fouling ascidians, yet the effects of environmental Ca2+ on larval adhesion remain largely unexplored. Here, the larvae of fouling ascidian C. robusta were exposed to different concentrations of Ca2+. Exposures to low-concentration (0 mM and 5 mM) and high-concentration (20 mM and 40 mM) Ca2+ significantly decreased the adhesion rate of larvae, which was primarily attributed to the decreases in adhesive structure length and curvature. Changes in the expressions of genes encoding adhesion-, microvilli-, muscle contraction-, and collagen-related proteins provided a molecular-level explanation for adhesion rate reduction. Additionally, larvae likely prioritized their energy towards immunomodulation in response to Ca2+ stresses, ultimately leading to adhesion reduction. These findings advance our understanding of the influencing mechanisms of environmental Ca2+ on larval adhesion, which are expected to provide references for the development of precise antifouling strategies against ascidians and other fouling species.

TRPM7-Mediated Ca2+ Regulates Mussel Settlement through the CaMKKβ-AMPK-SGF1 Pathway

Many marine invertebrates have planktonic larval and benthic juvenile/adult stages. When the planktonic larvae are fully developed, they must find a favorable site to settle and metamorphose into benthic juveniles. This transition from a planktonic to a benthic mode of life is a complex behavioral process involving substrate searching and exploration. Although the mechanosensitive receptor in the tactile sensor has been implicated in sensing and responding to surfaces of the substrates, few have been unambiguously identified. Recently, we identified that the mechanosensitive transient receptor potential melastatin-subfamily member 7 (TRPM7) channel, highly expressed in the larval foot of the mussel Mytilospsis sallei, was involved in substrate exploration for settlement. Here, we show that the TRPM7-mediated Ca2+ signal was involved in triggering the larval settlement of M. sallei through the calmodulin-dependent protein kinase kinase β/AMP-activated protein kinase/silk gland factor 1 (CaMKKβ-AMPK-SGF1) pathway. It was found that M. sallei larvae preferred the stiff surfaces for settlement, on which TRPM7, CaMKKβ, AMPK, and SGF1 were highly expressed. These findings will help us to better understand the molecular mechanisms of larval settlement in marine invertebrates, and will provide insights into the potential targets for developing environmentally friendly antifouling coatings for fouling organisms.

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.

Proteome of larval metamorphosis induced by epinephrine in the Fujian oyster Crassostrea angulata

Background

The Fujian oyster Crassostrea angulata is an economically important species that has typical settlement and metamorphosis stages. The development of the oyster involves complex morphological and physiological changes, the molecular mechanisms of which are as yet unclear.

Results

In this study, changes in proteins were investigated during larval settlement and metamorphosis of Crassostrea angulata using epinephrine induction. Protein abundance and identity were characterized using label-free quantitative proteomics, tandem mass spectrometry (MS/ MS), and Mascot methods. The results showed that more than 50% (764 out of 1471) of the quantified proteins were characterized as differentially expressed. Notably, more than two-thirds of the differentially expressed proteins were down-regulated in epinephrine-induced larvae. The results showed that “metabolic process” was closely related to the development of settlement and metamorphosis; 5 × 10^− 4 M epinephrine induced direct metamorphosis of larvae and was non-toxic. Calmodulin and MAPK pathways were involved in the regulation of settlement of the oyster. Expression levels of immune-related proteins increased during metamorphosis. Hepatic lectin-like proteins, cadherins, calmodulin, calreticulin, and cytoskeletal proteins were involved in metamorphosis. The nervous system may be remodeled in larval metamorphosis induced by epinephrine. Expression levels of proteins that were enriched in the epinephrine signaling pathway may reflect the developmental stage of the larvae, that may reflect whether or not larvae were directly involved in metamorphosis when the larvae were treated with epinephrine.

Conclusion

The study provides insight into proteins that function in energy metabolism, immune responses, settlement and metamorphosis, and shell formation in C. angulata. The results contribute valuable information for further research on larval settlement and metamorphosis.

Graphical abstract

Transcriptome analyses suggest a molecular mechanism for the SIPC response of Amphibalanus amphitrite

Barnacles are notorious marine fouling organisms. Their successful attachment to a substrate requires that they search for an appropriate habitat during their cyprid stage. A chemical cue called SIPC (Settlement-Inducing Protein Complex) has been shown to play a key role in the induction of cyprid gregarious settlement; however, the underlying biochemical mechanism remains unclear. Here, RNA-seq was used to examine the gene expression profiles of Amphibalanus amphitrite cyprids in response to SIPC and to identify SIPC-activated intracellular signaling pathways. A total of 389 unigenes were differentially expressed in response to SIPC, and cement protein genes were not among them. KEGG enrichment analysis suggested that SNARE interactions in the vesicular transport pathway were significantly influenced by SIPC treatment, indicating a possible role for SIPC in triggering protein transportation and secretion. Several genes with specific functions in metamorphosis were found among the differentially expressed genes (DEGs). GO (Gene Ontology) enrichment analysis revealed that the DEGs were significantly enriched in enamel mineralization pathways, suggesting that SIPC may also be involved in the activation of mineralization.

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.

Quantitative proteomics study of larval settlement in the Barnacle Balanus amphitrite

Barnacles are major sessile components of the intertidal areas worldwide, and also one of the most dominant fouling organisms in fouling communities. Larval settlement has a crucial ecological effect not only on the distribution of the barnacle population but also intertidal community structures. However, the molecular mechanisms involved in the transition process from the larval to the juvenile stage remain largely unclear. In this study, we carried out comparative proteomic profiles of stage II nauplii, stage VI nauplii, cyprids, and juveniles of the barnacle Balanus amphitrite using label-free quantitative proteomics, followed by the measurement of the gene expression levels of candidate proteins. More than 700 proteins were identified at each stage; 80 were significantly up-regulated in cyprids and 95 in juveniles vs other stages. Specifically, proteins involved in energy and metabolism, the nervous system and signal transduction were significantly up-regulated in cyprids, whereas proteins involved in cytoskeletal remodeling, transcription and translation, cell proliferation and differentiation, and biomineralization were up-regulated in juveniles, consistent with changes associated with larval metamorphosis and tissue remodeling in juveniles. These findings provided molecular evidence for the morphological, physiological and biological changes that occur during the transition process from the larval to the juvenile stages in B. amphitrite.

MKK3 was involved in larval settlement of the barnacle Amphibalanus amphitrite through activating the kinase activity of p38MAPK

The p38 mitogen-activated protein kinase (p38MAPK) plays a key role in larval settlement of the barnacle Amphibalanus amphitrite. To study the signaling pathway associated with p38MAPK during larval settlement, we sought to identify the upstream kinase of p38MAPK. Three MKKs (MKK3, MKK4 and MKK7) and three MAPKs (p38MAPK, ERK and JNK) in A. amphitrite were cloned and recombinantly expressed in E. coli. Through kinase assays, we found that MKK3, but not MKK4 or MKK7, phosphorylated p38MAPK. Furthermore, MKK3 activity was specific to p38MAPK, as it did not phosphorylate ERK or JNK. To further investigate the functional relationship between MKK3 and p38MAPK in vivo, we studied the localization of phospho-MKK3 (pMKK3) and MKK3 by immunostaining. Consistent with the patterns of p38MAPK and phospho-p38MAPK (pp38MAPK), pMKK3 and MKK3 mainly localized to the antennules of the cyprids. Western blot analysis revealed that pMKK3 levels, like pp38MAPK levels, were elevated at cyprid stage, compared to nauplii and juvenile stages. Moreover, pMKK3 levels increased after treatment with adult barnacle crude extracts, suggesting that MKK3 might mediate the stimulatory effects of adult barnacle extracts on the p38MAPK pathway.

Barnacles and biofouling

Biofouling, the attachment and growth of organisms on submerged, man-made surfaces, has plagued ship operators for at least 2500 years. Accumulation of biofouling, including barnacles and other sessile marine invertebrates, increases the frictional resistance of ships' hulls, resulting in an increase in power and in fuel consumption required to make speed. Scientists and engineers recognized over 100 years ago that in order to solve the biofouling problem, a deeper understanding of the biology of the organisms involved, particularly with regard to larval settlement and metamorphosis and adhesives and adhesion, would be required. Barnacles have served as an important tool in pursuing this research. Over the past 20 years, the pace of these studies has accelerated, likely driven by the introduction of environmental regulations banning the most effective biofouling control products from the market. Research has largely focused on larval settlement and metamorphosis, the development of new biocides, and materials/surface science. Increased research has so far, however, failed to result in commercial applications. Two recent successes (medetomidine/Selektope(®), surface-bound noradrenaline) build on our improving understanding of the role of the larval nervous system in mediating settlement and metamorphosis. New findings with regard to the curing of barnacle adhesives may pave the way to additional successes. Although the development of most current biofouling control technologies remains largely uninfluenced by basic research on, for example, the ability of settling larvae to perceive surface cues, or the nature of the interaction between organismal adhesives and the substrate, newly-developed materials can serve as useful probes to further our understanding of these processes.

Expression of calmodulin and myosin light chain kinase during larval settlement of the Barnacle Balanus amphitrite

Barnacles are one of the most common organisms in intertidal areas. Their life cycle includes seven free-swimming larval stages and sessile juvenile and adult stages. The transition from the swimming to the sessile stages, referred to as larval settlement, is crucial for their survivor success and subsequent population distribution. In this study, we focused on the involvement of calmodulin (CaM) and its binding proteins in the larval settlement of the barnacle, Balanus ( = Amphibalanus) amphitrite. The full length of CaM gene was cloned from stage II nauplii of B. amphitrite (referred to as Ba-CaM), encoding 149 amino acid residues that share a high similarity with published CaMs in other organisms. Quantitative real-time PCR showed that Ba-CaM was highly expressed in cyprids, the stage at which swimming larvae are competent to attach and undergo metamorphosis. In situ hybridization revealed that the expressed Ba-CaM gene was localized in compound eyes, posterior ganglion and cement glands, all of which may have essential functions during larval settlement. Larval settlement assays showed that both the CaM inhibitor compound 48/80 and the CaM-dependent myosin light chain kinase (MLCK) inhibitor ML-7 effectively blocked barnacle larval settlement, whereas Ca(2+)/CaM-dependent kinase II (CaMKII) inhibitors did not show any clear effects. The subsequent real-time PCR assay showed a higher expression level of Ba-MLCK gene in larval stages than in adults, suggesting an important role of Ba-MLCK gene in larval development and competency. Overall, the results suggest that CaM and CaM-dependent MLCK function during larval settlement of B. amphitrite.

Antifouling bastadin congeners target mussel phenoloxidase and complex copper(II) ions

Synthetically prepared congeners of sponge-derived bastadin derivatives such as 5,5'-dibromohemibastadin-1 (DBHB) that suppress the settling of barnacle larvae were identified in this study as strong inhibitors of blue mussel phenoloxidase that is involved in the firm attachment of mussels to a given substrate. The IC₅₀ value of DBHB as the most active enzyme inhibitor encountered in this study amounts to 0.84 μM. Inhibition of phenoloxidase by DBHB is likely due to complexation of copper(II) ions from the catalytic centre of the enzyme by the α-oxo-oxime moiety of the compound as shown here for the first time by structure activity studies and by X-ray structure determination of a copper(II) complex of DBHB.

Reversible anti-settlement activity against Amphibalanus (=Balanus) amphitrite, Bugula neritina, and Hydroides elegans by a nontoxic pharmaceutical compound, mizolastine

Mizolastine, an antihistamine pharmaceutical, was found to significantly inhibit larval settlement of the barnacle Amphibalanus (=Balanus) amphitrite, the bryozoan Bugula neritina, and the polychaete Hydroides elegans with EC(50) values of 4.2, 11.2, and 4.1 microg ml(-1), respectively. No toxicity against the larvae of these three species was observed at the concentration range tested during incubations with mizolastine. To determine whether the anti-settlement activity of mizolastine is reversible, recovery bioassays using these three species were conducted. More than 70% of the larvae that had been exposed for 4 h to mizolastine at concentrations four-fold greater than their respective EC(50) values completed normal metamorphosis. The results of the recovery bioassay provide evidence that the anti-settlement effect of mizolastine is reversible in addition to being nontoxic. The anti-settlement activities of several intermediates of the synthesis process of mizolastine were also examined. One of the intermediates, 2-chloro-1-(4-fluorobenzyl)-1H-benzo[d]imidazole, inhibited larval settlement and metamorphosis with low toxicity. These results may improve the understanding of the key functional group responsible for the anti-settlement activity of mizolastine.

2D gel-based proteome and phosphoproteome analysis during larval metamorphosis in two major marine biofouling invertebrates

Larvae of some benthic invertebrates respond (metamorphose or not) to chemical cues within minutes or hours and often without excessive transcription or translation. Although protein phosphorylation is one of the most important molecular switching mechanisms that govern variety of rapid cellular responses in higher organisms, this is the first study to analyze the global protein expression and protein phosphorylation status during larval metamorphosis in two major marine biofouling invertebrates (a bryozoan Bugula neritina and a barnacle Balanus amphitrite). Results indicate that larval proteomic response to metamorphosis (inhibiton or induction) involves substantial change in the phosphorylation status of proteins rather than de novo protein synthesis. An abundantly expressed and an unnamed phosphoprotein that appears to play key regulatory role in larval metamorphosis was identified. When larvae of bryozoan and barnacle were challenged with a metamorphosis (and kinase) inhibitor, the genistein, the number of phosphoproteins in bryozoan were substantially reduced but drastically increased in barnacle. Taken together, this is the first time that the usefulness of employing 2DE-based proteomic and phosphoproteomic approaches was demonstrated for us to understand the molecular mechanisms of larval metamorphosis and to study the mode-of-action of chemical cues in marine organisms.

Antifouling activity of bromotyrosine-derived sponge metabolites and synthetic analogues

Eighteen brominated sponge-derived metabolites and synthetic analogues were analyzed for antilarval settlement of Balanus improvisus. Only compounds exhibiting oxime substituents including bastadin-3 (4), -4 (1), -9 (2), and -16 (3), hemibastadin-1 (6), aplysamine-2 (5), and psammaplin A (10) turned out to inhibit larval settling at 1 to 10 microM. Analogues of hemibastadin-1 (6) were synthesized and tested for structure activity studies. Debromohemibastadin-1 (8) inhibited settling of B. improvisus, albeit at lower concentrations than hemibastadin-1 (6). Both 6 and 8 also induced cyprid mortality. 5,5'-dibromohemibastadin-1 (7) proved to be nontoxic, but settlement inhibition was observed at 10 microM. Tyrosinyltyramine (9), lacking the oxime function, was not antifouling active and was non-toxic at 100 microM. Hemibastadin-1 (6) and the synthetic products showed no general toxicity when tested against brine shrimp larvae. In contrast to the lipophilic psammaplin A (10), the hydrophilic sulfated psammaplin A derivative (11) showed no antifouling activity even though it contains an oxime group. We therefore hypothesize that the compound needs to cross membranes (probably by diffusion) and that the target for psammaplin A lies intracellularly.

An alpha2-macroglobulin-like protein is the cue to gregarious settlement of the barnacle Balanus amphitrite

Many benthic marine invertebrates, like barnacles, have a planktonic larval stage whose primary purpose is dispersal. How these species colonize suitable substrata is fundamental to understanding their evolution, population biology, and wider community dynamics. Unlike larval dispersal, settlement occurs on a relatively small spatial scale and involves larval behavior in response to physical and chemical characteristics of the substratum. Biogenic chemical cues have been implicated in this process. Their identification, however, has proven challenging, no more so than for the chemical basis of barnacle gregariousness, which was first described >50 years ago. We now report that a biological cue to gregarious settlement, the settlement-inducing protein complex (SIPC), of the major fouling barnacle Balanus amphitrite is a previously undescribed glycoprotein. The SIPC shares a 30% sequence homology with the thioester-containing family of proteins that includes the alpha(2)-macroglobulins. The cDNA (5.2 kb) of the SIPC encodes a protein precursor comprising 1,547 aa with a 17-residue signal peptide region. A number of structural characteristics and the absence of a thioester bond in the SIPC suggest that this molecule is a previously undescribed protein that may have evolved by duplication from an ancestral alpha(2)-macroglobulin gene. Although the SIPC is regarded as an adult cue that is recognized by the cyprid at settlement, it is also expressed in the juvenile and in larvae, where it may function in larva-larva settlement interactions.

Evidence for different pharmacological targets for imidazoline compounds inhibiting settlement of the barnacleBalanus improvisus

We describe the effect of eight different imidazoline/guanidinium compounds on the settlement and metamorphosis of larvae of the barnacle Balanus improvisus. These agents were chosen on the basis of their similar pharmacological classification in vertebrates and their chemical similarity to medetomidine and clonidine, previously described as highly potent settlement inhibitors (nanomolar range). Seven of the tested compounds were found to inhibit settlement in a dose-dependent manner in concentrations ranging from 100 nM to 10 microM without any significant lethal effects. In vertebrate systems these substances have overlapping functions and interact with both alpha-adrenoceptors as well as imidazoline binding sites. Antagonizing experiments using the highly specific alpha(2)-antagonist methoxy-idazoxan or agmatine (the putative endogenous ligand at imidazoline receptors) were performed to discriminate between putative pharmacological mechanisms involved in the inhibition of cyprid settlement. Agmatine was not able to reverse the effect of any of the tested compounds. However, methoxy-idazoxan almost completely abolished the settlement inhibition mediated by guanabenz (alpha(2)-agonist, I(2) ligand), moxonidine (alpha(2)-agonist, I(1) ligand) and tetrahydrozoline (alpha-agonist, I(2) ligand). The actions of cirazoline (alpha(1)-agonist, I(2) ligand) BU 224 (I(2) ligand) and metrazoline (I(2) ligand) were not reversed by treatment with methoxy-idazoxan. These results suggest that the settlement inhibition evoked by the I(2) ligands and alpha(2)-agonists used in this study of the neurologically simple but well-organized barnacle larva is mediated through different physiological targets important in the overall settlement process.

The development of a marine natural product-based antifouling paint

Problems with tin and copper antifouling compounds have highlighted the need to develop new environmentally friendly antifouling coatings. Bacteria isolated from living surfaces in the marine environment are a promising source of natural antifouling compounds. Four isolates were used to produce extracts that were formulated into ten water-based paints. All but one of the paints showed activity against a test panel of fouling bacteria. Five of the paints were further tested for their ability to inhibit the settlement of barnacle larvae, Balanus amphitrite, and algal spores of Ulva lactuca, and for their ability to inhibit the growth of U. lactuca. Two paints caused a significant decrease in the number of settled barnacles. One paint containing extract of Pseudomonas sp. strain NUDMB50-11, showed excellent activity in all assays. The antifouling chemicals responsible for the activity of the extract were isolated, using bioassay guided fractionation, and their chemical structures determined.

Transcription and translation inhibitors permit metamorphosis up to radiole formation in the serpulid polychaete Hydroides elegans haswell

Settlement and metamorphosis in most well-studied marine invertebrates are rapid processes, triggered by external cues. How this initial environmentally mediated response is transduced into morphogenetic events that culminate in the formation of a functional juvenile is still not well understood for any marine invertebrate. The response of larvae of the serpulid polychaete Hydroides elegans to inhibitors of mRNA and protein synthesis was examined to determine if metamorphosis requires these molecular processes. Competent larvae of H. elegans were induced to metamorphose by exposing them to a bacterial film or a 3-h pulse of 10 mM CsCl in the presence of the gene-transcription inhibitor DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) or the translation inhibitor emetine. When induced to metamorphose in the presence of either inhibitor, larvae of H. elegans progressed through metamorphosis to the point at which branchial radioles start to develop. DRB and emetine inhibited the incorporation of radiolabeled uridine into RNA and radiolabeled methionine into peptides, respectively, indicating that they were effective in blocking the appropriate syntheses. Taken together, these results indicate that the induction of metamorphosis in H. elegans does not require de novo transcription or translation, and that the form of the juvenile worm is achieved in two phases. During the first phase, larvae respond to the inducer by attaching to the substratum, secreting a primary tube, resorbing the prototroch cilia, undergoing caudal elongation, and differentiating the collar; once the collar is formed, they begin secreting the secondary, calcified tube. During the second phase, the small worm develops branchial radioles and begins to grow, requiring new mRNA and protein syntheses.

Timing is everything: the effects of putative dopamine antagonists on metamorphosis vary with larval age and experimental duration in the prosobranch gastropod Crepidula fornicata

The signal transduction pathway through which excess potassium ion stimulates the larvae of many marine invertebrates to metamorphose is incompletely understood. Recent evidence suggests that dopamine plays important roles in the metamorphic pathway of Crepidula fornicata. Therefore, we asked whether blocking dopamine receptors might prevent excess potassium ion from stimulating metamorphosis in this species. Surprisingly, the effects of the three putative dopamine antagonists tested (all at 10 microM) varied with exposure duration and the age of competent larvae. Chlorpromazine, a nonspecific dopamine antagonist known to have a number of other pharmacological effects, blocked the inductive action of excess potassium ion during the initial 5-8-h exposure periods in most assays, particularly for younger or smaller competent larvae. However, chlorpromazine in the absence of excess potassium ion also stimulated metamorphosis, particularly over the next 18 h, and worked faster on older competent larvae than on younger competent larvae. The specific D(1) antagonist R(+)-Sch-23309 had similar effects, blocking potassium-stimulated metamorphosis in short-term exposures and stimulating metamorphosis in longer exposures, particularly for older competent larvae. Although the specific D(2) antagonist spiperone (SPIP) blocked the inductive effects of excess potassium ion in only 1 of 6 assays during the first 6 h of exposure, it blocked metamorphosis in 2 of the assays during 24-h exposures. Our results indicate that dopamine receptors are involved in the pathway through which excess potassium ion stimulates metamorphosis in C. fornicata. In addition, the largely latent inductive effects of chlorpromazine, an inhibitor of nitric oxide synthase, suggest that endogenous nitric oxide may play a natural role in inhibiting metamorphosis in this species. Overall, our results would then suggest that exposing larvae of C. fornicata to excess K(+) leads to a shutdown of nitric oxide synthesis via a dopaminergic pathway, a pathway that can be blocked by some dopamine antagonists. Alternatively, chlorpromazine might eventually be stimulating metamorphosis by elevating endogenous cyclic nucleotide (e.g., cAMP) concentrations, again acting downstream from the steps acted on directly by excess K(+).

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.

Identification of methyl farnesoate in the cypris larva of the barnacle, Balanus amphitrite, and its role as a juvenile hormone

Previous investigations have shown that insect juvenile hormone (JH) and its analogues induce precocious metamorphosis of barnacle cypris larvae. In the present study, methyl farnesoate (MF; structurally identical to JH III, except for the absence of an epoxide group) has been shown to have a concentration-dependent effect on the development of cyprids of the barnacle Balanus amphitrite. Analysis of cypris extracts by gas chromatography-mass spectrometry with selected ion monitoring (GC-MS-SIM) confirmed the presence of endogenous MF. These data provide evidence that MF functions as a juvenilizing hormone in barnacle cyprids, an effect that hitherto has not been noted.

Nature and perception of barnacle settlement pheromones

It is now almost 50 years since the gregarious settlement of barnacles and its chemical basis was first described. Although originally noted for Elminius modestus, mechanistic studies of gregariousness have focused on two species, Semibalanus balanoides and Balanus amphitrite. By virtue of its ease of study and its economic importance as a fouling organism, the latter species has assumed increasing importance in recent years. This paper will provide an overview of studies on settlement pheromones and their perception. An adult glycoprotein, arthropodin (now known as settlement-inducing protein complex or SIPC), was once thought to be the sole pheromone involved in the induction of cypris larval settlement. At least two other pheromones are now known to be involved, a waterborne cue originating from the adult and the cypris temporary adhesive. The latter is related, immunologically, to SIPC. In keeping with many other examples of chemical communication, the available evidence suggests that barnacle settlement induction involves receptor-ligand interactions and a signal transduction pathway(s) that translates into attachment and metamorphosis. Similar findings have been reported for some, but not all, marine invertebrate larvae examined thus far and the implications for antifoulant development are discussed.

Immunological studies on the settlement-inducing protein complex (SIPC) of the barnacle Balanus amphitrite and its possible involvement in larva-larva interactions

Immunological investigation has revealed that a settlement-inducing protein complex (SIPC), which induces cypris settlement of the barnacle Balanus amphitrite, is synthesized during larval development and accumulates in the cypris larva. We previously purified the SIPC from adult B. amphitrite, which was active when bound to a substratum. The SIPC is a glycoprotein of high molecular mass, consisting of three major subunits of 76, 88 and 98 kDa with lentil lectin (LCA)-binding sugar chains. In the present study, we prepared antiserum against each LCA-binding subunit of SIPC, and performed immunoblot analyses. Immunoblotting of adult extracts showed that anti-76-kDa antibody reacted only with the 76-kDa protein, whereas anti-88-kDa and anti-98-kDa antibodies reacted with both the 88-kDa and the 98-kDa proteins. Immunoblotting of larval extracts indicated that reactivity of the 76-kDa protein to anti-76-kDa antiserum increased during larval development and cyprid extracts reacted strongly. Moreover, by using immunostaining we found that the SIPC was contained in 'footprints' of cyprids, which have been shown to act as a settlement-inducing pheromone, and is secreted onto the antennular attachment discs. The results suggest that the SIPC (or SIPC-like proteins) is involved in both adult-larva and larva-larva interactions during settlement of the barnacle B. amphitrite.