Serpulid Polychaete Hydroides Elegans Research Articles

Bacterial lipopolysaccharide induces settlement and metamorphosis in a marine larva

How larvae of the many phyla of marine invertebrates find places appropriate for settlement, metamorphosis, growth, and reproduction is an enduring question in marine science. Biofilm-induced metamorphosis has been observed in marine invertebrate larvae from nearly every major marine phylum. Despite the widespread nature of this phenomenon, the mechanism of induction remains poorly understood. The serpulid polychaete Hydroides elegans is a well established model for investigating bacteria-induced larval development. A broad range of biofilm bacterial species elicit larval metamorphosis in H. elegans via at least two mechanisms, including outer membrane vesicles (OMVs) and complexes of phage-tail bacteriocins. We investigated the interaction between larvae of H. elegans and the inductive bacterium Cellulophaga lytica, which produces an abundance of OMVs but not phage-tail bacteriocins. We asked whether the OMVs of C. lytica induce larval settlement due to cell membrane components or through delivery of specific cargo. Employing a biochemical structure–function approach with a strong ecological focus, the cells and OMVs produced by C. lytica were interrogated to determine the class of the inductive compounds. Here, we report that larvae of H. elegans are induced to metamorphose by lipopolysaccharide produced by C. lytica. The widespread prevalence of lipopolysaccharide and its associated taxonomic and structural variability suggest it may be a broadly employed cue for bacterially induced larval settlement of marine invertebrates.

Formation and Function of the Primary Tube During Settlement and Metamorphosis of the Marine Polychaete Hydroides elegans (Haswell, 1883) (Serpulidae).

AbstractThe serpulid polychaete Hydroides elegans has emerged as a major model organism for studies of marine invertebrate settlement and metamorphosis and for processes involved in marine biofouling. Rapid secretion of an enveloping, membranous, organic primary tube provides settling larvae of H. elegans firm adhesion to a surface and a refuge within which to complete metamorphosis. While this tube is never calcified, it forms the template from which the calcified tube is produced at its anterior end. Examination of scanning and transmission electron micrographs of competent and settling larvae revealed that the tube is secreted from epidermal cells of the three primary segments, with material possibly transported through the larval cuticle via abundant microvilli. The tube is composed of complexly layered fibrous material that has an abundance of the amino acids that characterize the collagenous cuticle of other polychaetes, plus associated carbohydrates. The significance of the dependence on surface bacterial biofilms for stimulating settlement in this species is revealed as a complex interaction between primary tube material, as it is secreted, and the extracellular polymeric substances abundantly produced by biofilm-residing bacteria. This association appears to provide the settling larvae with an adhesion strength similar to that of bacteria in a biofilm and significantly less when larvae settle on a clean surface.

Induction of Invertebrate Larval Settlement; Different Bacteria, Different Mechanisms?

Recruitment via settlement of pelagic larvae is critical for the persistence of benthic marine populations. For many benthic invertebrates, larval settlement occurs in response to surface microbial films. Larvae of the serpulid polychaete Hydroides elegans can be induced to settle by single bacterial species. Until now, only Pseudoalteromonas luteoviolacea had been subjected to detailed genetic and mechanistic studies. To determine if the complex structures, termed tailocins, derived from phage-tail gene assemblies and hypothesized to be the settlement cue in P. luteoviolacea were present in all inductive bacteria, genomic comparisons with inductive strains of Cellulophaga lytica, Bacillus aquimaris and Staphylococcus warneri were undertaken. They revealed that the gene assemblies for tailocins are lacking in these other bacteria. Negatively stained TEM images confirmed the absence of tailocins and revealed instead large numbers of extracellular vesicles in settlement-inductive fractions from all three bacteria. TEM imaging confirmed for C. lytica that the vesicles are budded from cell surfaces in a manner consistent with the production of outer membrane vesicles. Finding multiple bacteria settlement cues highlights the importance of further studies into the role of bacterial extracellular vesicles in eliciting settlement and metamorphosis of benthic marine larvae.

Feeding behaviour of a serpulid polychaete: Turning a nuisance species into a natural resource to counter algal blooms?

Occurrence of algal blooms in coastal waters is predicted to be more prevalent in future. To minimize their occurrence, manipulating the grazing pressure by suspension feeders is a potential management strategy, but its effectiveness may depend on their feeding preference. Therefore, we assessed the clearance rate of a widespread serpulid polychaete Hydroides elegans in larval and adult stages on various coastal phytoplankton. Additionally, the growth and development of H. elegans after consuming these phytoplankton were determined to reflect its sustainability to counter algal blooms. Results showed that H. elegans can consume and utilize different phytoplankton, except diatom Thalassiosira pseudonana, for growth and development in both life stages. Given the fast-colonizing ability which allows easy manipulation of abundance, H. elegans is considered practically and biologically ideal for tackling algal blooms. Other suspension feeders with different feeding niches could be used in combination to maximize the versatility of the top-down control.

Draft Genome Sequence of Pseudoalteromonas luteoviolacea HI1, Determined Using Roche 454 and PacBio Single-Molecule Real-Time Hybrid Sequencing.

We report here the 6.0-Mb draft genome assembly of Pseudoalteromonas luteoviolacea strain HI1 using Roche 454 and PacBio single-molecule real-time hybrid-sequencing analysis. This strain is of biological importance since it has the capacity to induce the settlement and metamorphosis of the serpulid polychaete Hydroides elegans and the coral Pocillopora damicornis.

Physiological and behavioural responses of different life stages of a serpulid polychaete to hypoxia

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 477:135-145 (2013) - DOI: https://doi.org/10.3354/meps10175 Physiological and behavioural responses of different life stages of a serpulid polychaete to hypoxia Y. S. Leung1, P. K. S. Shin1, J. W. Qiu2, P. O. Ang3, J. M. Y. Chiu4, V. Thiyagarajan4, S. G. Cheung1,* 1Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, SAR 2Department of Biology, Hong Kong Baptist University, Hong Kong, SAR 3School of Life Sciences, Chinese University of Hong Kong, New Territories, Hong Kong, SAR 4School of Biological Sciences, The University of Hong Kong, Pokfulam, Hong Kong, SAR *Corresponding author. Email: bhsgche@cityu.edu.hk ABSTRACT: Hypoxia has become a major threat to coastal marine ecosystems worldwide. Among various groups of marine invertebrates, sessile ones are more susceptible to hypoxia since they cannot move away from hypoxic waters. The serpulid polychaete Hydroides elegans Haswell, 1883 is a fast-proliferating, dominant, sessile-fouling species in the tropical Pacific, which includes Hong Kong. We studied the effects of hypoxia on the fertilization success, embryogenesis and physiological and behavioural responses in both larval and adult stages of H. elegans. Results showed that fertilization success was reduced and embryogenesis was retarded at a dissolved oxygen (DO) level of 2 mg O2 l-1. At this DO level, the swimming velocity of the 2 d old larvae significantly increased, which may serve as an escape response. H. elegans showed a very high tolerance to hypoxia in both larval and adult stages, as indicated by the 24 and 48 h lethal DO concentation (LC50). However, the respiration rate and clearance rate of adult H. elegans were significantly reduced at 2 mg O2 l-1. Partial recovery was observed 2 d after normoxia was resumed. Previous studies have demonstrated that adult H. elegans suffer from high mortality and that there are no new recruits in summer when the temperature is high and DO concentration low. The results are interpreted as the effect of low salinity. Our study has strengthened the notion that low salinity contributes to adult mortality in summer; however, the lack of summer recruitment is possibly a result of the combined effect of low salinity and hypoxia. KEY WORDS: Hypoxia · Hydroides elegans · LC50 · Embryogenesis · Respiration rate · Clearance rate Full text in pdf format PreviousNextCite this article as: Leung YS, Shin PKS, Qiu JW, Ang PO, Chiu JMY, Thiyagarajan V, Cheung SG (2013) Physiological and behavioural responses of different life stages of a serpulid polychaete to hypoxia. Mar Ecol Prog Ser 477:135-145. https://doi.org/10.3354/meps10175 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 477. Online publication date: March 12, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Copper affects biofilm inductiveness to larval settlement of the serpulid polychaete Hydroides elegans (Haswell)

Copper (Cu) contamination is a potential threat to the marine environment due to the use of Cu-based antifouling paints. Cu stress on larval settlement of the polychaete Hydroides elegans was investigated, and this was linked to Cu stress on biofilms and on the biofilm development process. The inductiveness of young biofilms was more easily altered by Cu stress than that of old biofilms, indicating the relative vulnerability of young biofilms. This might result from changes in bacterial survival, the bacterial community composition and the chemical profiles of young biofilms. Cu also affected biofilm development and the chemical high performance liquid chromatograph fingerprint profile. The results indicate that Cu affected larval settlement mainly through its effect on the process of biofilm development in the marine environment, and the chemical profile was crucial to biofilm inductiveness. It is strongly recommended that the effects of environmentally toxic substances on biofilms are evaluated in ecotoxicity bioassays using larval settlement of invertebrates as the end point.

Life history trade-offs and response to selection on egg size in the polychaete worm Hydroides elegans

In order to examine the genetic relationships among life-history traits in a hermaphroditic species we used artificial selection for increased egg size and measured correlated responses across the life cycle of the serpulid polychaete Hydroides elegans, a protandrous sequential hermaphrodite. We recorded sex ratios across generations, and measured egg size, egg energy, larval volume at two time points, juvenile tube length, adult dry weight and fecundity after selection. Selection for larger eggs produced positive correlated responses in egg energy, fecundity and larval size at competence. Selection for increased egg size was also manifested by earlier sex change and this resulted in selected individuals spending less time as males relative to controls. We propose that egg size is negatively correlated with duration of andromorphy, that is, that female fitness trades off with male fitness.

Induction of Larval Settlement in the Serpulid Polychaete Hydroides elegans (Haswell): Role of Bacterial Extracellular Polymers

Induction of Larval Settlement in the Serpulid Polychaete Hydroides elegans (Haswell): Role of Bacterial Extracellular Polymers

Induction of larval settlement in the serpulid polychaete Hydroides elegans (Haswell): Role of bacterial extracellular polymers

Larval settlement in the marine polychaete Hydroides elegans is effectively mediated upon contact with the surface of marine bacterial films. Using the bacterium Roseobacter litoralis as a model strain, the effect of bacterial extracellular polymers (exopolymers) on larval settlement of H. elegans was investigated. Bioassays with exopolymer fractions dissociated from bacterial films evoked the initial stages of the larval settlement process, i.e. larvae slowed down, secreted a mucous thread and crawled over the surface. This response is typical of larvae that encounter an attractive bacterial film. In contrast, bioassays with exopolymers in association with UV‐irradiated, metabolically inactive bacterial films evoked complete settlement. However, the percentage of responding larvae was negatively correlated with the magnitude of UV‐dosage. Since UV energy crosslinks both intra‐ and extracellular proteinaceous components, it could not be distinguished whether the decrease in larval settlement was due to a modification of proteinaceous components of exopolymers or due the elimination of cellular activity. Nevertheless, the results ascribe bacterial exopolymers the role of an indicator of substratum suitability and provide evidence that the polysaccharide moiety of exopolymers does not complement this effect.

Induction of Larval Settlement in the Serpulid Polychaete Hydroides elegans (Haswell): Role of Bacterial Extracellular Polymers

Induction of Larval Settlement in the Serpulid Polychaete Hydroides elegans (Haswell): Role of Bacterial Extracellular Polymers

Metamorphic Competence, a Major Adaptive Convergence in Marine Invertebrate Larvae1

Larvae from diverse marine-invertebrate phyla are able to respond rapidly to environmental cues to settlement and to undergo very rapid metamorphic morphogenesis because they share the developmental trait of metamorphic competence. The competent state, characteristic of larvae as diverse as those of cnidarian planulae, molluscan veligers, and barnacle cyprids, is one in which nearly all requisite juvenile characters are present in the larva prior to settlement. Thus metamorphosis, in response to more or less specific environmental cues (inducers), is mainly restricted to loss of larva-specific structures and physiological processes. Competent larvae of two “model marine invertebrates” studied in the authors' laboratory, the serpulid polychaete Hydroides elegans and the nudibranch Phestilla sibogae, complete metamorphosis in about 12 and 20 hr, respectively. Furthermore, little or no de novo gene action appears to be required during the metamorphic induction process in these species. Contrasting greatly with the slow, hormonally regulated metamorphic transitions of vertebrates and insects, competence and consequent rapid metamorphosis in marine invertebrate larvae are conjectured to have arisen in diverse phylogenetic clades because they allow larvae to continue to swim and feed in the planktonic realm while simultaneously permitting extremely fast morphological transition from larval locomotory and feeding modes to a different set of such modes that are adaptive to life on the sea bottom.

Metamorphic Competence, a Major Adaptive Convergence in Marine Invertebrate Larvae

Larvae from diverse marine-invertebrate phyla are able to respond rapidly to environmental cues to settlement and to undergo very rapid metamorphic morphogenesis because they share the developmental trait of metamorphic competence. The competent state, characteristic of larvae as diverse as those of cnidarian planulae, molluscan veligers, and barnacle cyprids, is one in which nearly all requisite juvenile characters are present in the larva prior to settlement. Thus metamorphosis, in response to more or less specific environmental cues (inducers), is mainly restricted to loss of larva-specific structures and physiological processes. Competent larvae of two “model marine invertebrates” studied in the authors' laboratory, the serpulid polychaete Hydroides elegans and the nudibranch Phestilla sibogae, complete metamorphosis in about 12 and 20 hr, respectively. Furthermore, little or no de novo gene action appears to be required during the metamorphic induction process in these species. Contrasting greatly with the slow, hormonally regulated metamorphic transitions of vertebrates and insects, competence and consequent rapid metamorphosis in marine invertebrate larvae are conjectured to have arisen in diverse phylogenetic clades because they allow larvae to continue to swim and feed in the planktonic realm while simultaneously permitting extremely fast morphological transition from larval locomotory and feeding modes to a different set of such modes that are adaptive to life on the sea bottom.

Larval settlement in the serpulid polychaete Hydroides elegans in response to bacterial films: an investigation of the nature of putative larval settlement cue

Larval settlement in the marine polychaete Hydroides elegans (Haswell) is induced by certain bacteria in marine biofilms. The exact nature of the settlement cue that larvae of H. elegans receive from these bacteria remains unknown. In this study, we revealed some properties of the bacterially derived larval settlement cue by investigating the larval settlement inductive activity of two bacterial strains after various treatments. These two bacterial strains, Roseobacter sp. and an α-subclass Proteobacteria, are highly inductive to larval settlement of H. elegans. The larvae responded similarly to Roseobacter and Proteobacteria in all the larval settlement bioassays, suggesting that the larval settlement-inducing substances produced by these bacteria may share common characteristics. First of all, the larvae did not settle in the seawater conditioned by the bacteria attached as a film or by the bacteria that were freely suspended in seawater. The results suggest that the putative larval settlement cue is not released into seawater and, therefore, should be associated with the surface of the bacteria. Secondly, formaldehyde treatment entirely eliminated the larval settlement induction activity of the bacterial films, and streptomycin treatment reduced the percentage of larval settlement on the bacterial films in a concentration-dependent manner. Since both treatments can kill bacteria with little damage to the surface chemistry of bacterial cells, the decline in larval settlement is suggested be due to a reduction of the viable bacterial population in the bacterial films. In fact, the reduction of larval settlement in the streptomycin treatments coincided with the decrease in viable bacterial populations in broth cultures containing respective concentrations of streptomycin. These results suggest that the viability of Roseobacter and Proteobacteria is important to their settlement induction effect. Since the larval settlement induction activity of the bacterial strains appears to correlate with their viability, we suggest that the putative larval settlement cue is derived from a metabolic pathway in the bacteria and that the cue is exported to and concentrated at the extracellular polymer matrix of the bacterial cell, at which the larvae establish contact with the bacteria. The larval settlement cue may be highly susceptible to degradation so that a metabolically active bacterial film is needed to maintain the putative cue at a concentration that surpasses the threshold for induction of larval settlement.

Role of bacteria in larval settlement and metamorphosis of the polychaete Hydroides elegans

The serpulid polychaete Hydroides elegans Haswell, 1883 is an early colonist of new substrata in Pearl Harbor, Oahu, Hawaii. When metamorphically competent, larvae of H. elegans will settle rapidly upon an acceptably biofilmed surface, but not on a clean surface. In this study we found the ability of larvae to respond selectively to inductive surfaces to be retained for at least 3 wk. Of a series of bacterial strains isolated from Hawaiian marine biofilms, 13 induced larval settlement, 11 gave moderate or mixed results, and 10 others did not stimulate the settlement of H. elegans. The amount of settlement induced by monospecific strains was rarely as great as with natural, multispecies films. Most of the isolated bacteria were motile Gram-negative rods, but Gram-positive strains were also present, and diverse metabolic types were represented in the study. Biofilms killed by treatment with heat, ultraviolet radiation or chemical fixatives were no longer inductive. Soluble, dialyzable, heat-stable bacterial products induced settlement and metamorphosis more slowly. The range of bacteria producing an inductive signal suggests either that there are multiple cues, or that the cue is common to many bacteria. Chemical signals characteristic of early microbial biofilms may indicate freshly available substrata with optimal potential for the growth and survival of H. elegans.

Induction of larval attachment and metamorphosis in the serpulid polychaete Hydroides elegans by dissolved free amino acids:mode of action in laboratory bioassays

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 190:167-178 (1999) - doi:10.3354/meps190167 Induction of larval attachment and metamorphosis in the serpulid polychaete Hydroides elegans by dissolved free amino acids: mode of action in laboratory bioassays Manfred Beckmann*, Tilman Harder, Pei-Yuan Qian** Department of Biology, Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, Hong Kong *Present address: Cardiff School of Biosciences, Cardiff University, PO Box 915, Cardiff CF10 3TL, Wales, UK **Corresponding author. E-mail: boqianpy@ust.hk ABSTRACT: Larval settlement in the serpulid polychaete Hydroides elegans (Haswell) reportedly occurs in response to both an attractive bacterial film and polar, aliphatic dissolved free amino acids (DFAA). In this study, we tested whether the larval metamorphic response was caused by direct DFAA perception or by induction of DFAA-utilizing bacteria in assay dishes. Still-water laboratory assays were performed with sterile measures in the presence and absence of an antibiotic mixture (2.5 x 10-4 M streptomycin, 1.0 x 10-4 M penicillin). In the absence of these antibiotics our results revealed a significant correlation between DFAA decrease rates, bacterial film development and larval metamorphosis within a 24 h assay period. After 12 h, DFAA concentrations decreased below the detection limit of 0.013 µM, the attached bacterial density in assay dishes was approx. 2 x 104 cells mm-2, and larval metamorphosis was insignificantly low (20%). After 24 h, DFAA were no longer detectable, the attached bacterial density was unchanged, and larval metamorphosis was significantly high (50%). In contrast, naturally biofilmed dishes showed a significant metamorphic response of 50% after 12 h. In analogous experiments in the presence of antibiotics, the percentage of larval metamorphosis varied between 3 and 34% after 24 h while the number of attached bacteria was low (0.5 to 5 x 103 cells mm-2). We assume that the metamorphic response in these treatments was caused by inductive, antibiotic-resistant bacteria. Our results suggest that larval metamorphosis was exclusively triggered by an inductive bacterial film rather than by direct larval perception of DFAA. Despite most sterile measures at the set-up of bioassays, bacteria were inevitably inoculated into test dishes as they were found associated with the larva and suspended in the larval culture water. Therefore, our results point to a systematic error in this established assay procedure. If putative signaling compounds serve as a nutrition source for larval settlement inducing marine bacteria, we conclude that an explicit investigation of a chemical metamorphic cue's efficacy is unreliable. The ecological significance of the transformation of DFAA into an attractive bacterial film for larval attachment and metamorphosis in H. elegans is discussed. KEY WORDS: Hydroides elegans · Polychaete · Amino acids · Antibiotics · Biofilm · Bacteria · Metamorphosis · Settlement induction Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 190. Publication date: December 14, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Induction of larval attachment and metamorphosis in the serpulid polychaete Hydroides elegans by dissolved free amino acids:isolation and identification

The calcareous tube-building polychaete Hydroides elegans (Haswell) is a major fouling organism in tropical waters around the world. In the marine environment, larvae of H. elegans rapidly settle and metamorphose in response to acceptable surface-bound bio-organic films. In addition to biological inducers, previous investigations by our group have indicated chemical inducers for larval settlement and metamorphosis in H. elegans. Crude samples derived from adult H. elegans and colonies of the bryozoan Bugula neritina induced a high percentage of larval metamorphosis in laboratory assays. Here, we describe a bioassay-guided isolation and purification of larval metamorphic inducers originating from these samples. Biologically active HPLC-purified fractions of both samples were composed of free amino acids. The entire free amino acid composition was determined by quantitation with ortho-phthaldialdehyde (OPA) and was found to be comprised of aspartic acid, glutamic acid, serine. histidine, glycine, arginine, alanine, asparagine, glutamine and taurine in concentrations ranging from 0.2 μM (histidine) to 5.6 μM (taurine). In the laboratory assay, the larval metamorphic response to an artificially prepared sample in identical concentrations of these amino acids was very similar to the natural isolates.

Metamorphic-Signal Transduction in Hydroides elegans (Polychaeta: Serpulidae) Is Not Mediated by a G Protein.

Evidence from larvae of hydrozoans, gastropods, and barnacles suggests that G protein-coupled receptors mediate induction of settlement and metamorphosis in response to environmental cues. We examined responses of larvae of the serpulid polychaete Hydroides elegans to neuropharmacological agents to determine if G protein-coupled receptors or their associated signal-transduction pathways regulated induction of metamorphosis by bacterial cues. Larvae of Hydroides elegans metamorphose rapidly and in high proportions when exposed to bacterial biofilms. Neither the G-protein activator Gpp[NH]p nor the inhibitor GDP-{beta}-S affected metamorphosis. Although the nonspecific phosphodiesterase inhibitors IBMX, theophylline, and papaverine induced larvae to metamorphose, RO-20-1724 (an inhibitor selective for cAMP-specific phosphodiesterase IV) and the cyclic nucleotide analogs db-cAMP and db-cGMP had no effect on metamorphosis. The adenylate cyclase activator forskolin inhibited responses of larvae to inductive bacterial biofilms. These apparently conflicting results may be due to side effects of IBMX, theophylline, papaverine, and forskolin on ion transport. The phorbol ester TPA, an activator of protein kinase C, also had no effect on larval metamorphosis. These experiments indicate that G protein-coupled receptors and signal transduction by the adenylate cyclase/cyclic AMP or phosphatidyl-inositol/ diacylglycerol/protein kinase C pathways are not components of the morphogenetic pathway that is directly responsible for processing metamorphic cues in H. elegans.

Stimulation of Metamorphosis in the Polychaete Hydroides elegans Haswell (Serpulidae).

The serpulid polychaete Hydroides elegans is a common, cosmopolitan warm-water biofouling organism. Competent larvae of H. elegans metamorphose rapidly after induction by marine biofilms. Only 15 min after coming in contact with the metamorphic cue, larvae have completed secretion of the primary tube; secretion of the secondary, calcareous tube begins 1.5 h after the primary tube has been deposited. Metamorphosis is characterized by disappearance of the prototroch and differentiation of the tentacular crown in the head region, the collar and thoracic membrane in the thoracic region, and the pygidium at the tip of the abdomen. These morphogenetic events were used to gauge the responses of larvae to biofilms, as well as to the artificial inducers Cs+ and K+. A maximal metamorphic response to the two ions requires exposure to different concentrations and durations, i.e., a 3-h pulse of 10 mM CsCl, or a 24-h continuous exposure to 50 mM excess KCl. The metamorphic response to Cs+ or K+ is much slower than the response to biofilms, demonstrating that the tissues respond differently to artificial inducers. The differences in the kinetics of the responses to the natural and cationic inducers suggest that the induction mechanisms are not the same. When these artificial inducers were used, some, but not all, of the metamorphosed juveniles never attached to the substratum or secreted a primary tube, probably as a result of secondary effects of the ions on processes of tube formation. The exact mechanisms by which Cs+ and excess K+ induce metamorphosis are still unclear, although we assume, as do others, that these agents act by depolarizing the membranes of excitable sensory cells and not by interacting with specific receptors.

Settlement of the serpulid polychaete Hydroides elegans (Haswell) on the arborescent bryozoan Bugula neritina (L.): evidence of a chemically mediated relationship

The tube building polychaete Hydroides elegans Haswell was found living attached to colonies of the arborescent bryozoan Bugula neritina (L.) in Port Shelter, Hong Kong. Field data collected during the period of January through May 1996, showed that H. elegans density reached 77.6 individuals of H. elegans per g wet weight of B. neritina. Density of H. elegans on B. neritina at depths from the surface to 0.5 m was lower than that at depths below 1 m. In January–March, when there were no H. elegans settling on PVC plates or found on natural substrata, numbers on B. neritina were ca. 5 per g wet weight. H. elegans settled on B. neritina and grew rapidly as mean diameter of tubes increased from 605 μm in February to 936 μm in March. In laboratory experiments, larvae of H. elegans settled and metamorphosed on branches of B. neritina and on the bottom of dishes containing B. neritina leachate. Compounds extracted from the leachate of B. neritina induced 74% of H. elegans larvae to metamorphose at a concentration of 16 μg/ml seawater, compared to 5% in dishes containing only filtered seawater (controls). Metabolites from the leachate of B. neritina which were bound to amberlite XAD-2, indicating they are lipophilic in nature, induced over 70% metamorphosis in H. elegans larvae at 56 μg/ml seawater. A biofilm from one of four strains of bacterial isolates associated with the surface of B. neritina induced low levels of metamorphosis in H. elegans larvae, while other bacterial isolates were detrimental to the survival of juvenile H. elegans. Field experiments further demonstrated that H. elegans settled preferentially on Phytagel discs embedded with whole extracts of B. neritina over control Phytagel discs. Metabolites from B. neritina deterred feeding on alginate pellets by assemblages of local fishes in field assays. Metabolites originating from B. neritina, bacteria colonizing B. neritina, and the complex structure of B. neritina contributed to the recruitment of H. elegans to B. neritina surfaces. Hydroides elegans may gain a refuge from predation by associating with B. neritina colonies both from its structural and chemical attributes.