Ecology of fouling bryozoans in Cochin waters

An extensive study was conducted on the settlement and seasonal succession of fouling bryozoans in and around Cochin Harbour during the period 1965 to 1966; it was the first study of its kind performed in Indian waters. A total of 14 species was recorded from localities which were ecologically dissimilar. A distinct succession of species, mainly of a seasonal character, was observed. Seasonal succession usually occurs in localities where seasons are well marked, and where different species of animals with short life spans settle during particular times of the year. The study has shown that a distinct seasonal succession of bryozoans occurs in this locality; out of the 14 species which appeared on the panels not one species settled throughout the year. This was due to seasonal differences in the ecological conditions which prevailed in the area investigated. The significant seasonal influences on the incidence of bryozoans led to a categorisation of the species into the following patterns (a) typical brackish-water forms not encountered in purely marine localities; (b) typical brackish-water forms which may occur in marine localities; (c) typical marine forms never found in localities where low salinity conditions prevail.Part of a thesis submitted by the first-named author for his degree of Doctor of Philosophy of the University of Kerala, India.     

Association of bacteria with bryozoan larvae

Seven species of marine bryozoans were examined for the presence of bacteria associated with the larvae. In three species (Bugula neritina, B. pacifica, and B. simplex), rod-shaped bacteria were consistently observed in the pallial sinus of the larvae, independent of geographic location, year, or season. Larvae of closely related bryozoans (B. stolonifera, B. turrita, Scrupocellaria bertholetti, and Tricellaria occidentalis) occurring sympatrically with those containing bacteria lacked this apparent symbiosis. Those bryozoans examined that possessed the bacteria-larva association are among the most frequently encountered fouling bryozoans.     

Waterborne chemical compounds in tropical macroalgae: positive and negative cues for larval settlement

Settlement sites of marine invertebrate larvae are frequently influenced by positive or negative cues, many of which are chemical in nature. Following from the observation that many shallow-water, Hawai'ian marine macroalgae are free of fouling by sessile invertebrates, we predicted that the algae are chemically protected and dependent on either surface-bound or continuously released soluble compounds to deter settling invertebrate larvae. To address the importance of waterborne algal compounds, we experimentally determined whether larvae of two of Hawai'i's dominant hard-surface fouling organisms, the polychaete tube worm Hydroides elegans and the bryozoan Bugula neritina, would settle in the presence of waters conditioned by 12 species of common Hawai'ian macroalgae (representing the Phaeophyta, Chlorophyta, Rhodophyta and Cyanophyta). The results included a full spectrum of biological responses by each larval species to waterborne algal compounds. Larval responses to conditioned water were consistent for each algal species, but the outcomes were not predictable based on the taxonomic relationships of the algae. For example, among the species of Phaeophyta examined, different conditioned waters were: (1) toxic, (2) inhibited settlement, (3) simulated settlement, or (4) had no effect, compared to larvae in control dishes containing filtered seawater. Additionally, larval responses to aged (24 h) conditioned waters could not be predicted from the results of assays run with conditioned waters utilized immediately after preparation. Finally, settlement by larvae of one species did not predict outcomes of tests for the other species. Four of 12 shallow-reef Hawai'ian macroalgae tested released compounds into surrounding waters that immediately killed or inhibited settlement by both H. elegans and B. neritina (toxic: Dictyota sandvicensis; inhibitory: Halimeda discoidea, Sphacelaria tribuloides, Ulva reticulata); the remaining 8 algal species prevented settlement by one of these fouling organisms but for the other had no effect or, in some cases, even stimulated settlement     

On predation of pelagic larvae and early juveniles of marine bottom invertebrates by adult benthic invertebrates and their passing alive through their predators

The dynamic quantitative balance between prey and predator invertebrate species inhabiting the same shallow-shelf (sublittoral level bottom) benthic communities was first discussed by Thorson (1953). Thorson considered the exact timing of larval settlement of prey and predator species possessing pelagic development and temporal supression of the adult predators' feeding activities during reproduction at the time of the preys' settlement to constitute the major factors which facilitate survival of the prey species in such communities. However, information obtained demonstrates that Thorson's “mechanism of balance between predator and prey species of benthic communities” is not always effective in securing survival from predation not only of the prey's spat but even sometimes of the predator's spat also. Because of this, the “mechanism” can not be rated as universally effective in all situations. Analysis of the data so far published demonstrates that, in marine benthic communities, especially in shallow-shelf waters, it is not uncommon for gametes, larvae, or early juveniles of different prey species to pass alive through suspension (filter)-feeding and deposit-feeding adult invertebrates preying on them. Sometimes development can even continue after excretion by predators. The hypothesis of Voskresensky (1948) and Goycher (1949) of the importance of this phenomenon for the maintenance and recruitment of the mussel Mytilus edulis and other filter-feeding lamellibranchs of nearshore waters preying on their own and other lamellibranch pelagic larvae must be rejected on the basis of accumulated data on their feeding and general biology and on the adverse influence of the mucous of their faecal pellets and pseudofaeces on the larvae excreted by them alive. The data considered here demonstrate that, although the passing alive of larvae and spat of benthic invertebrates through benthic predators is not uncommon in shallow-shelf bottom-communities, it plays no important role in the processes of maintenance and recruitment of the species and communities involved nor of the marine benthos as a whole. The actual ecological significance of predation on pelagic larvae and bottom spat of benthic invertebrate prey species by all three main trophic groups of marine benthos (suspension or filter-feeders, deposit-feeders, carnivores) and its importance to predator-prey dynamics in marine benthic communities remains open to debate until more reliable quantitative data become available.     

Seaweed secondary metabolites as antifoulants: effects of Dictyota spp. diterpenes on survivorship, settlement, and development of marine invertebrate larvae

Summary. A recent investigation showed that the brown seaweed Dictyota menstrualis was unfouled relative to co-occurring seaweeds, and that larvae of fouling invertebrates avoided settling on D. menstrualis due to chemicals on its surface. The secondary metabolites dictyol E and pachydictyol A are among the compounds found on this alga's surface. In the present study, we tested the effects of specific diterpenes from Dictyota on the survivorship, growth, and development of invertebrate larvae and developing juveniles that could foul seaweeds. Exposure to dictyol E, dictyol B acetate, pachydictyol A, and dictyodial from Dictyota menstrualis and D. ciliolata caused significant larval mortality, abnormal development, and reduce growth rates for three species of co-occurring invertebrates when their larvae were forced into contact with these metabolites. Larvae were damaged at metabolite concentrations as low as 5% of maximum possible surface concentrations of these compounds for the populations of Dictyota we studied. The negative effects of these secondary metabolites on potential foulers, in conjunction with data demonstrating larval avoidance of dictyol-covered surfaces, suggest that these compounds could function as chemical defenses against fouling, and could select for larvae that avoid hosts producing these metabolites. Received 25 May 1998; accepted 22 June 1998.     

The adaptive value of larval behavior of a colonial ascidian

Didemnum candidum Savigny, 1816 is an abundant member of the fouling community encrusting docks in Pearl Harbor, Hawaii. In the laboratory, larvae of D. candidum are positively phototactic upon release, but become indifferent to the direction of light prior to settlement. In both the laboratory and the field, settlement is most abundant on shaded, downward-facing surfaces, the location where juvenile survival is greatest. The colonial ascidians Diplosoma listerianum Milne-Edwards, 1842 and Diplosoma sp. also settle abundantly on shaded undersurfaces in the field. Juvenile survival of the Diplosoma species is not affected by exposure to direct light, while survival is greatest on downward-facing surfaces. For Didemnum candidum, light is an important environmental cue used by larvae to locate settlement sites on shaded, downward-facing surfaces where juvenile survival is enhanced.     

Origin,composition, and function of secretions from pyriform organs and internal sacs of four settling cheilo-ctenostome bryozoan larvae

Settling larvae of the bryozoans Flustrellida hispida, Flustra foliacea, Alcyonidium polycum, and Bugula neritina exhibit a common behavioral pattern. A settlement site is selected after initial exploration, and temporary attachment is accomplished with a sticky acid mucopolysaccharide secretion exuded from the pyriform organ. Permanent attachment is accomplished by acid mucopolysaccharide and protein secretions of the everted internal sac.     

Effect of microbial films on settlement of bryozoan larvae (Bugula simplex,B. stolonifera and B. turrita)

The influence of microbial films on larval settlement was studied for sympatric species of cellularioid cheilostome bryozoans (Bugula simplex, B. stolonifera, and B. turrita). Settlement experiments were conducted in the laboratory in 1981 with and without a choice of filmed or unfilmed substrates. When given a choice of substrates, the larvae of all three bryozoan species significantly preferred the filmed substrate. When not given a choice of substrates, both B. simplex and B. turrita larvae tended not to settle on the unfilmed substrate, while the filmed and unfilmed substrates were equally attractive to B. stolonifera. Microbial films enhanced the settlement of all three species.     

Larval recruitment in response to manipulated field flows

Settlement responses to boundary-layer flow of several invertebrate taxa, including the hydroid Tubularia crocea, the bryozoans Bugula turrita and Schizoporella unicornis, and the tube-building polychaete Hydroides dianthus were studied in manipulated field flows in Great Harbor, Massachusetts, USA. During three experiments in 1989 and two in 1990, densities of newly-recruited larvae were measured on flat plates, whose flow regimes had been manipulated by altering the leading-edge configurations. Settlement responses to flow were strongly species-specific, with T. crocea preferring regions of both high turbulence and strong shear stress, and S. unicornis settling a most exclusively in regions of high shear stress. B. turrita settled most prominently in regions of reduced shear stress, exhibiting settlement patterns that closely approximated predictions from a model of passive particle contact. H. dianthus showed a moderate avoidance of regions with high shear stress. These results indicate that boundary-layer flows affect settlement of several common encrusting species, a probable consequence of larval behaviors such as substrate rejection or exploration in response to flow. These responses are likely to generate patchiness during initial colonization of natural habitats, and certainly affect colonization of settlement panels commonly used in marine ecological studies.     

Mortality of newly metamorphosed eastern oysters (Crassostrea virginica) in mesohaline Chesapeake Bay

 Stocks of eastern oysters, Crassostrea virginica (Gmelin), in mesohaline Chesapeake Bay, USA, exhibit a high degree of inter-annual and spatial variability in recruitment. We found that cumulative oyster spatfall on off-bottom collector plates, measured throughout the summer in 14 years over a span of three decades, was highly positively correlated (r ² = 0.8) with juvenile oyster recruitment on adjacent oyster bars. Total abundances of juvenile oysters on these bars were, however, generally 99.7% lower than predicted from cumulative seasonal larval settlement on collector plates. We propose that although the number of larvae metamorphosing was the key factor in determining the gross annual pattern of recruitment to these mesohaline oyster bars, the actual magnitude of recruitment was governed by post-settlement processes, such as competition for limited resources and predation. We tested the hypothesis that predation may be partly responsible for high post-settlement juvenile oyster mortality. We performed a series of 3-d field investigations over two summers (1989, 1990) at a mesohaline site, employing cages of various mesh sizes (400, 800, 1500 μm) to protect hatchery-reared spat of 0.5 to 4.0 mm shell height. Mortality rates for spat held for 3 d in the estuary (17.8%) were significantly higher (P = 0.0001) for the smallest spat (0.5 to 2.0 mm) compared with those of 2.01 to 4.0 mm (4.2%). In 1990, but not in 1989, enclosure within 400 and 800 μm mesh cages significantly (P = 0.004) increased survival during 3-d deployments (9.4 and 10.1%, respectively) compared with spat unprotected by mesh cages (21.9%). In a series of laboratory predation studies that used the entire community of invertebrates that could penetrate the cages, microscopic juvenile polyclad flatworms, Stylochus ellipticus, were the only organisms that we observed crawling into living oysters and feeding on oyster tissue. Large flatworms (50 to 200 mm²) are known to be important predators on oysters, but this ability of flatworms that were so small (<ca. 5 mm²) and translucent as to be almost invisible without magnification to feed on immediate post-metamorphic oysters has not been documented previously. Our results suggest that the rate of mortality due to predation in mesohaline Chesapeake Bay is much reduced once spat survive for 2 to 3 weeks post-metamorphosis. Thus, it is likely that predation in the 1 to 2 week period immediately after settlement may be a crucial factor in the structuring of eastern oyster populations. Received: 21 December 1998 / Accepted: 2 December 1999     

Responses of marine fouling communities to sewage abatement in Kaneohe Bay,Oahu, Hawaii

The development of marine fouling communities was followed as part of a sewage abatement study in Kaneohe Bay (Oahu, Hawaii). Panels immersed near the outfall during sewage discharge were dominated by dense mats of blue-green algae and diatoms. This mat was significantly lighter at this station when sewage did not flow into the bay, and it never developed on panels within the same sector of the bay but away from the influence of the outfall. Taxonomic diversity, and the frequency of filamentous algae and the serpulidHydroides elegans were reduced near the outfall during discharge. It is suggested that this reduction in diversity and abundance of certain key foulers is due to competitive exclusion by the dense blue-green and diatom mats, and is not a direct effect of the sewage effluent.     

Larvae of a colonial ascidian use a non-contact mode of substratum selection on a coral reef

The rate at which larvae successfully recruit into communities of marine benthic invertebrates is partially dependent upon how well larvae avoid benthic predators and settle on appropriate substrata. Therefore, to be able to predict recruitment success, information is needed on how larvae search for settlement sites, whether larvae preferentially settle on certain substrata, and the extent to which there are adequate cues for larvae to find these substrata. This article describes how larvae of the colonial ascidian Diplosoma similis find settlement sites on a coral reef. Direct field observations of larval settlement were made on a fringing reef in Kaneohe Bay, Oahu, Hawaii, between September 1985 and April 1986. A comparison of the substrata that larvae contacted prior to settlement relative to the percentage cover of these substrata on the study reef suggests that larvae are using a non-contact mode of substratum identification to locate suitable settlement sites. This mode of substratum identification allowed 74% of larvae to evade predation by benthic organisms who would otherwise have eaten larvae if they had been contacted. Of those larvae that evaded predation, 88% subsequently settled on the same two substrata upon which most adults are found (dead coral or the green alga Dictyosphaeria cavernosa). This pattern of settlement was probably a result of active selection, since the two substrata cover only 14.4% of the reef's surface and currents had little effect on the direction in which larvae swam. An important contributing factor to the high success rate of larval settlement on suitable substrata was the lack of any temporal decay in substratum preference. It is concluded that for Diplosoma similis larval supply is a sufficient predictor of larval settlement rate. However, for marine invertebrates whose larvae are passively dispersed and exhibit a greater temporal decay in substratum preference, larval settlement should generally have a greater dependency on spatial variation in the abundance of benthic predators and suitable substrata.     

Inhibition and facilitation of bryozoan and ascidian settlement by natural multi-species biofilms: effects of film age and the roles of active and passive larval attachment

Sessile marine invertebrate larvae can recognize suitable settlement substrata by using various environmental cues, including organic/microbial biofilms. In laboratory choice assays, the effect of biofilms of varying ages on the settlement behaviour of two fouling organisms was assessed. The species included the arborescent cheilostome bryozoan Bugula flabellata (Thompson) and the solitary ascidian Ciona intestinalis (L.), both of which are characteristic of temperate sublittoral hard substratum assemblages in northwest Europe. Experiments were carried out using polystyrene petri dish substrata preconditioned with multispecies biofilms from natural laboratory-aquarium seawater for 1, 3, 6 or 12 d. Unfilmed (new, initially sterile) dishes were used as control substrata. Whereas the coronate larvae of B. flabellata generally were inhibited by biofilming, irrespective of film age, the settlement of tadpole larvae of C. intestinalis was facilitated on biofilmed substrata, and numbers of settled larvae generally increased with biofilm age: the highest mean numbers were counted on 12 d-old biofilms. In C. intestinalis, settlement and metamorphosis are processes which can be temporally separate and are possibly induced by different environmental cues. This study therefore distinguished between C. intestinalis larvae which were attached to the biofilm surface by the anterior, and those larvae entrapped by the biofilm but not settled in the conventional meaning of the term. As reported in previous studies, we did observe that such entrapped larvae could subsequently attach and develop successfully into sessile juveniles. Both the numbers of “attached” and “trapped” tadpoles increased with biofilm age. Assuming that “settlement” is essentially a process involving the active behavioural response of larvae to environmental cues, it seems that the facilitated attachment of C. intestinalis onto biofilmed substrata is due to the combined effect of active habitat selection and passive deposition/“entrapment” of larvae onto the “sticky” substratum. Received: 21 August 1996 / Accepted: 21 November 1996     

Inhibitory effects of red algal extracts on larval settlement of the barnacle Balanus improvisus

We examined the chemical antifouling properties of four sublittoral red algae, Chondrus crispus, Delesseria sanguinea, Osmundea ramosissima, and Polyides rotundus, which are all rarely fouled in the field. Two different approaches were used. Firstly, we tested the effects of lipophilic crude extracts on the settlement behaviour of cyprid larvae of the co-existing barnacle Balanus improvisus. Secondly, in a settlement preference experiment, we tested whether B. improvisus cyprid larvae settle on living algae when given a choice between natural algal surfaces and control surfaces. With this procedure, we were able to test both if the algae inhibit recruitment of cyprids, and if this inhibition is a result of chemistry. The settlement of B. improvisus larvae was strongly inhibited at concentrations estimated to be potentially ecologically relevant for all of the tested extracts. However, only C. crispus significantly inhibited settlement in the preference experiment, even though there was also a tendency for settlement inhibition on P. rotundus and O. ramosissima. In contrast, D. sanguinea seemed to stimulate settlement. This contradiction probably resulted from an extraction of metabolites that naturally occur only inside the alga. However, as this study shows, a combination of settlement assays with whole-cell extracts and preference tests of ecologically relevant fouling organisms on natural algal and control surfaces may be a useful procedure to avoid erroneous conclusions regarding natural antifouling roles of compounds based on settlement assays with only whole-cell extracts. Furthermore, this study also shows that production of inhibitory metabolites may explain the low degree of fouling, especially by B. improvisus, on C. crispus.Communicated by L. Hagerman, Helsingør     

Alkaloids and ascidian chemical defense: Evidence for the ecological role of natural products fromEudistoma olivaceum

The surface of the colonial ascidianEudistoma olivaceum (Van Name) is almost completely free of fouling organisms. I provide evidence that this epibiont-free surface is maintained by the alkaloids, Eudistomins G and H. These alkaloids were extracted from colonies collected in the shallow subtidal of Indian River Lagoon, Florida. USA, in November 1987. Concentrations of less than one-fifth of those found in the living ascidian inhibited the settlement of the larvae of five invertebrate species relative to appropriate controls in laboratory and field trials. Standard pharmacological assays have revealed that Eudistomins G and H were not the most biologically active of the eudistomins, indicating that these standard assays are not necessarily good predictors of antifouling activity. In an examination of possible alternate roles for these eudistomins, they proved ineffective as a fish antifeedant. PinfishLagadon rhomboides consumed agar pellets to which Eudistomins G and H had been added at concentrations 10- to 100-fold higher than those effective against settling larvae. These findings indicate that biologically active marine natural products may serve specific ecological roles.     

Epibiotic community of the horseshoe crab Tachypleus gigas

Horseshoe crabs act as moving substrata for simple to complex communities of small marine organisms. Amplexed adult pairs migrate for breeding once every 2 weeks from deep waters towards nearshore waters during highest high tide. Female horseshoe crabs bury themselves to the level of the lateral eyes to deposit eggs while the male crabs fertilize them. Subsequently eggs are buried by the female. Tachypleus gigas (Müller) is the most abundant horseshoe crab species above available along the Orissa coast (India). Adults reach terminal anecdysis once sexually mature and live with their carapace for 4 to 9 years. In spite of this, epibiosis is limited. In the current investigation, differences in the epibiotic community (diatoms and macro-epibionts) present on horseshoe crabs, according to gender, were evaluated, and the macro-epibiont population from different regions of the carapace was mapped. In general, female horseshoe crabs harbored fewer epibionts than the males. Among the diatoms, Navicula spp., Nitzschia spp. and Skeletonema sp. were dominant in both sexes. However, the abundance and diversity of diatoms was greater on the carapaces of male crabs. Among the macro-epibionts, the acorn barnacle (Balanus amphitrite Darwin) and encrusting bryozoan (Membranipora sp.) were the most dominant forms. Barnacles and bryozoans were greater in abundance in the “rough” zone (cardiopthalmic region and anterior region of the opisthosoma). Mapping of the macro-epibionts from different regions of the carapace revealed differential distribution in males and females. Such differentiated distribution of the macro-epibionts can be related to factors such as changing habitat by the horseshoe crabs during breeding, mechanical abrasion and surface availability during mating and nesting periods, requirements of epizootic larvae and surface properties of the carapace (wettability and roughness). In the case of females, mechanical abrasion and surface availability played an important role in the epibiotic community structure and distribution patterns. The surface wettability measurements indicated male carapace to be slightly more hydrophobic than the female carapace. Scanning electron microscopy revealed that the male carapace was comparatively rough compared to the smooth carapace of females. A comparison of surface properties of the carapace indicated that the male carapace is more conducive for epibiosis. Received: 23 August 1999 / Accepted: 25 January 2000     

Effect of zinc on the settlement of the oyster Crassostrea gigas

The influence of zinc upon the pattern and success of settlement was examined in the oyster Crassostrea gigas. Late larvae were more tolerant of zinc than embryos. A delay in settlement was recorded in treatments containing zinc as low as 125 μg/l, and numbers of larvae settling was reduced in the presence of zinc. Larvae subjected to zinc immediately prior to settlement showed evidence of slowing in behavioural development. Those larvae which settled in the presence of zinc, however, when ongrown in clean water were as viable as controls. Zinc at concentrations of 250 and 500 μg/l suppressed spat growth, but recovery was rapid upon subsequent on-growing in clean water conditions.     

Induction of metamorphosis in larvae of the oyster Crassostrea gigas using neuroactive compounds

Chemical (neuroactive compounds at different concentrations and exposure times) and physical (water agitation, light) factors with potential effects on the metamorphosis of larvae of the oyster Crassostrea gigas (Thunberg) larvae have been studied. The neurotransmitters l-dihydroxyphenylalanine (DO), epinephrine (EP), norepinephrine (NE), and acetylcholine (AC) have been identified as very active inducers of metamorphosis, whilst serotonin (SE), dopamine (DA) and potassium (K) were less effective inducers. The -aminobutyric acid (GA) and ammonium (AM) were found ineffective at the concentrations tested. Exposure to 10^-4 M EP for 15 min was sufficient to promote >80% metamorphosis within 48 h, whereas NE required 2 h to exert comparable induction. Maximum induction by DO (>50%) was achieved after 2 h exposure to 10^-4 M. However, unlike EP and NE, DO was lethal at that concentration in the long term. Maximum induction by AC (30% metamorphosis) was achieved at a concentration of 10^-4 M. In contrast to other neurotransmitters, AC induced settlement behaviour, cementation and eventual metamorphosis, yielding postlarvae which were all attached to the substratum. EP and NE triggered the morphogenetic changes, by-passing settlement and leading to postlarvae not cemented to the substratum. DO induced mostly attached spat at low concentrations (10^-5 M) and unattached spat at high concentrations (10^-4 M), and a similar pattern was apparent for the weaker inducers SE and DA. Regarding physical factors, a highly reflectant surface significantly increased the percentage of attached spat obtained, compared to a dark bottom. No consistent effect of water current or light was detected on the production of unattached spat. The three different forms of induction are discussed in relation to different regulatory pathways of settlement and metamorphosis.     

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. Received: 14 October 1998 / Accepted: 5 September 2000     

Energy metabolism of newly settledOstrea edulis spat during metamorphosis

An energy budget for metamorphosis in oyster spatOstrea edulis L., collected in the Instituto Español de Oceanografía during March–April 1988, was determined. The energy cost of metamorphosis was calculated from measurements of biochemical components which form the principal reserves of energy and which are used significantly during metamorphosis. The results indicated that protein supplied most of the energy inO. edulis spat. The idea that neutral lipids were used selectively as an energy reserve during the metamorphosis of this bivalve marine larvae was questioned.