Species-specific interactions between algal endosymbionts and coral hosts define their bleaching response to heat and light stress

The impacts of warming seas on the frequency and severity of bleaching events are well documented, but the potential for different Symbiodinium types to enhance the physiological tolerance of reef corals is not well understood. Here we compare the functionality and physiological properties of juvenile corals when experimentally infected with one of two homologous Symbiodinium types and exposed to combined heat and light stress. A suite of physiological indicators including chlorophyll a fluorescence, oxygen production and respiration, as well as pigment concentration consistently demonstrated lower metabolic costs and enhanced physiological tolerance of Acropora tenuis juveniles when hosting Symbiodinium type C1 compared with type D. In other studies, the same D-type has been shown to confer higher thermal tolerance than both C2 in adults and C1 in juveniles of the closely related species Acropora millepora. Our results challenge speculations that associations with type D are universally most robust to thermal stress. Although the heat tolerance of corals may be contingent on the Symbiodinium strain in hospite, our results highlight the complexity of interactions between symbiotic partners and a potential role for host factors in determining the physiological performance of reef corals.     

Investigating the causes and consequences of symbiont shuffling in a multi-partner reef coral symbiosis under environmental change

Dynamic symbioses may critically mediate impacts of climate change on diverse organisms, with repercussions for ecosystem persistence in some cases. On coral reefs, increases in heat-tolerant symbionts after thermal bleaching can reduce coral susceptibility to future stress. However, the relevance of this adaptive response is equivocal owing to conflicting reports of symbiont stability and change. We help reconcile this conflict by showing that change in symbiont community composition (symbiont shuffling) in Orbicella faveolata depends on the disturbance severity and recovery environment. The proportion of heat-tolerant symbionts dramatically increased following severe experimental bleaching, especially in a warmer recovery environment, but tended to decrease if bleaching was less severe. These patterns can be explained by variation in symbiont performance in the changing microenvironments created by differentially bleached host tissues. Furthermore, higher proportions of heat-tolerant symbionts linearly increased bleaching resistance but reduced photochemical efficiency, suggesting that any change in community structure oppositely impacts performance and stress tolerance. Therefore, even minor symbiont shuffling can adaptively benefit corals, although fitness effects of resulting trade-offs are difficult to predict. This work helps elucidate causes and consequences of dynamism in symbiosis, which is critical to predicting responses of multi-partner symbioses such as O. faveolata to environmental change.     

Resilience and acclimation to bleaching stressors in the scleractinian coral Porites cylindrica

‘Resilience’, the capacity of the coral symbiosis with dinoflagellate algal symbionts (‘zooxanthellae’) to recover after bleaching, is a little-studied but crucial aspect of coral responses to bleaching stressors. This study investigated the response of the zooxanthella population in the coral Porites cylindrica after bleaching either naturally on a shallow subtidal reef or experimentally in response to elevated temperature and darkness. Coral resilience was influenced by the nature and duration of the stressor. Corals strongly bleached by natural stressors were less resilient than those that had been partially bleached; and a similar recovery profile was obtained for corals experimentally bleached by exposure to elevated temperature, in which recovery was slower for corals thermally-stressed 96 h than for 72 h. The opposite trend was evident for corals exposed to darkness, indicating that the bleaching trigger had a strong impact on coral resilience. When P. cylindrica recently recovered from bleaching was subjected to a repetition of bleaching stressors, it did not display acclimation, i.e. experience-mediated acquisition of resistance to bleaching stressors. The zooxanthella populations in all corals tested throughout the experiments were typed by PCR-RFLP as clade C, indicating that coral responses were not accompanied by any substantial change in zooxanthella composition at the cladal level.     

Endosymbiotic flexibility associates with environmental sensitivity in scleractinian corals

Flexibility in biological systems is seen as an important driver of macro-ecosystem function and stability. Spatially constrained endosymbiotic settings, however, are less studied, although environmental thresholds of symbiotic corals are linked to the function of their endosymbiotic dinoflagellate communities. Symbiotic flexibility is a hypothesized mechanism that corals may exploit to adapt to climate change. This study explores the flexibility of the coral–Symbiodinium symbiosis through quantification of Symbiodinium ITS2 sequence assemblages in a range of coral species and genera. Sequence assemblages are expressed as an index of flexibility incorporating phylogenetic divergence and relative abundance of Symbiodinium sequences recovered from the host. This comparative analysis reveals profound differences in the flexibility of corals for Symbiodinium, thereby classifying corals as generalists or specifists. Generalists such as Acropora and Pocillopora exhibit high intra- and inter-species flexibility in their Symbiodinium assemblages and are some of the most environmentally sensitive corals. Conversely, specifists such as massive Porites colonies exhibit low flexibility, harbour taxonomically narrow Symbiodinium assemblages, and are environmentally resistant corals. Collectively, these findings challenge the paradigm that symbiotic flexibility enhances holobiont resilience. This underscores the need for a deeper examination of the extent and duration of the functional benefits associated with endosymbiotic diversity and flexibility under environmental stress.     

Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event

Cross‐ecosystem nutrient subsidies play a key role in the structure and dynamics of recipient communities, but human activities are disrupting these links. Because nutrient subsidies may also enhance community stability, the effects of losing these inputs may be exacerbated in the face of increasing climate‐related disturbances. Nutrients from seabirds nesting on oceanic islands enhance the productivity and functioning of adjacent coral reefs, but it is unknown whether these subsidies affect the response of coral reefs to mass bleaching events or whether the benefits of these nutrients persist following bleaching. To answer these questions, we surveyed benthic organisms and fishes around islands with seabirds and nearby islands without seabirds due to the presence of invasive rats. Surveys were conducted in the Chagos Archipelago, Indian Ocean, immediately before the 2015–2016 mass bleaching event and, in 2018, two years following the bleaching event. Regardless of the presence of seabirds, relative coral cover declined by 32%. However, there was a post‐bleaching shift in benthic community structure around islands with seabirds, which did not occur around islands with invasive rats, characterized by increases in two types of calcareous algae (crustose coralline algae [CCA] and Halimeda spp.). All feeding groups of fishes were positively affected by seabirds, but only herbivores and piscivores were unaffected by the bleaching event and sustained the greatest difference in biomass between islands with seabirds versus those with invasive rats. By contrast, corallivores and planktivores, both of which are coral‐dependent, experienced the greatest losses following bleaching. Even though seabird nutrients did not enhance community‐wide resistance to bleaching, they may still promote recovery of these reefs through their positive influence on CCA and herbivorous fishes. More broadly, the maintenance of nutrient subsidies, via strategies including eradication of invasive predators, may be important in shaping the response of ecological communities to global climate change.     

The dynamics of microbial partnerships in the coral Isopora palifera

Both bacteria and algal symbionts (genus Symbiodinium), the two major microbial partners in the coral holobiont, respond to fluctuations in the environment, according to current reports; however, little evidence yet indicates that both populations have any direct interaction with each other in seasonal fluctuation. In this study, we present field observations of a compositional change in bacteria and Symbiodinium in the coral Isopora palifera in three separate coral colonies following monthly sampling from February to November in 2008. Using massively parallel pyrosequencing, over 200 000 bacterial V6 sequences were classified to build the bacterial community profile; in addition, the relative composition and quantity of Symbiodinium clades C and D were determined by real-time PCR. The results showed that coral-associated bacterial and Symbiodinium communities were highly dynamic and dissimilar among the tagged coral colonies, suggesting that the effect of host specificity was insignificant. The coral-associated bacterial community was more diverse (Shannon index up to 6.71) than previous estimates in other corals and showed rapid seasonal changes. The population ratios between clade C and D groups of Symbiodinium varied in the tagged coral colonies through the different seasons; clade D dominated in most of the samples. Although significant association between bacteria and symbiont was not detected, this study presents a more detailed picture of changes in these two major microbial associates of the coral at the same time, using the latest molecular approaches.     

Nitrate competition in a coral symbiosis varies with temperature among Symbiodinium clades

Many reef-building corals form symbioses with dinoflagellates from the diverse genus Symbiodinium. There is increasing evidence of functional significance to Symbiodinium diversity, which affects the coral holobiont''s response to changing environmental conditions. For example, corals hosting Symbiodinium from the clade D taxon exhibit greater resistance to heat-induced coral bleaching than conspecifics hosting the more common clade C. Yet, the relatively low prevalence of clade D suggests that this trait is not advantageous in non-stressful environments. Thus, clade D may only be able to out-compete other Symbiodinium types within the host habitat when conditions are chronically stressful. Previous studies have observed enhanced photosynthesis and fitness by clade C holobionts at non-stressful temperatures, relative to clade D. Yet, carbon-centered metrics cannot account for enhanced growth rates and patterns of symbiont succession to other genetic types when nitrogen often limits reef productivity. To investigate the metabolic costs of hosting thermally tolerant symbionts, we examined the assimilation and translocation of inorganic ¹⁵N and ¹³C in the coral Acropora tenuis experimentally infected with either clade C (sub-type C1) or D Symbiodinium at 28 and 30 °C. We show that at 28 °C, C1 holobionts acquired 22% more ¹⁵N than clade D. However, at 30 °C, C1 symbionts acquired equivalent nitrogen and 16% less carbon than D. We hypothesize that C1 competitively excludes clade D in hospite via enhanced nitrogen acquisition and thus dominates coral populations despite warming oceans.     

Photosynthetic circadian rhythmicity patterns of Symbiodium,the coral endosymbiotic algae

Biological clocks are self-sustained endogenous timers that enable organisms (from cyanobacteria to humans) to anticipate daily environmental rhythms, and adjust their physiology and behaviour accordingly. Symbiotic corals play a central role in the creation of biologically rich ecosystems based on mutualistic symbioses between the invertebrate coral and dinoflagellate protists from the genus Symbiodinium. In this study, we experimentally establish that Symbiodinium photosynthesis, both as a free-living unicellular algae and as part of the symbiotic association with the coral Stylophora pistillata, is ‘wired’ to the circadian clock mechanism with a ‘free-run’ cycle close to 24 h. Associated photosynthetic pigments also showed rhythmicity under light/dark conditions and under constant light conditions, while the expression of the oxygen-evolving enhancer 1 gene (within photosystem II) coincided with photosynthetically evolved oxygen in Symbiodinium cultures. Thus, circadian regulation of the Symbiodinium photosynthesis is, however, complicated as being linked to the coral/host that have probably profound physiochemical influence on the intracellular environment. The temporal patterns of photosynthesis demonstrated here highlight the physiological complexity and interdependence of the algae circadian clock associated in this symbiosis and the plasticity of algae regulatory mechanisms downstream of the circadian clock.     

Apoptosis and autophagy as mechanisms of dinoflagellate symbiont release during cnidarian bleaching: every which way you lose

Cnidarian bleaching results from the breakdown in the symbiosis between the host cnidarian and its dinoflagellate symbiont. Coral bleaching in recent years has increasingly caused degradation and mortality of coral reefs on a global scale. Although much is understood about the environmental causes of bleaching, the underlying cellular mechanisms of symbiont release that drive the process are just beginning to be described. In this study, we investigated the roles of two cellular pathways, host cell apoptosis and autophagy, in the bleaching process of the symbiotic anemone Aiptasia pallida. Host cell apoptosis was experimentally manipulated using gene knockdown of an anemone caspase by RNA interference, chemical inhibition of caspase using ZVAD-fmk and an apoptosis-inducer wortmannin. Autophagy was manipulated by chemical inhibition using wortmannin or induction using rapamycin. The applications of multiple single treatments resulted in some increased bleaching in anemones under control conditions but no significant drop in bleaching in individuals subjected to a hyperthermic stress. These results indicated that no single pathway is responsible for symbiont release during bleaching. However, when multiple inhibitors were applied simultaneously to block both apoptosis and autophagy, there was a significant reduction in bleaching in heat-stressed anemones. Our results allow us to formulate a model for cellular processes involved in the control of cnidarian bleaching where apoptosis and autophagy act together in a see-saw mechanism such that if one is inhibited the other is induced. Similar interconnectivity between apoptosis and autophagy has previously been shown in vertebrates including involvement in an innate immune response to pathogens and parasites. This suggests that the bleaching response could be a modified immune response that recognizes and removes dysfunctional symbionts.     

A comparison of the thermal bleaching responses of the zoanthid Palythoa caribaeorum from three geographically different regions in south Florida

Coral bleaching involves the loss of symbiotic algae (zooxanthellae) from reef corals and other cnidarians during periods of environmental stress, particularly elevated temperature. In this study we compared the thermal bleaching responses of the zoanthid Palythoa caribaeorum from three populations along the southeast coast of Florida. Winter (2002-2003) and summer (2003) samples from three geographically separate sites were experimentally exposed to increased temperatures and the loss of zooxanthellae was measured. Population densities of zooxanthellae were analyzed and their genetic identity determined using PCR-DGGE analysis of the internal transcribed spacer region 2. The results showed that samples of P. caribaeorum from reefs that experienced the smallest range in annual seawater temperature released the most zooxanthellae. Seasonal comparisons revealed that winter samples lost more zooxanthellae than summer samples. P. caribaeorum harbored two genetic types of zooxanthellae, C1 and D1a. Individual colonies contained populations of only C1 or D1a, or combinations of C1 and D1a. However, these genotypic patterns did not relate latitudinal distribution nor to differences in experimental thermal tolerance.     

High zooxanthella density shortens the survival time of coral cell aggregates under thermal stress

To investigate bleaching mechanisms in coral-zooxanthella symbiotic systems, it is important to study the cellular- or tissue-level responses of corals to stress. We established an experimental system to study the stress responses of coral cells using coral cell aggregates. Dissociated coral cells aggregate to form spherical bodies, which rotate by ciliary movement. These spherical bodies (tissue balls) stop rotating and disintegrate when exposed to a thermal stress. Tissue balls prepared from dissociated cells of Fungia sp. and Pavona divaricata were exposed to either elevated temperature (31 °C, with 25 °C as the control) or elevated temperature in the presence of exogenous antioxidants (ascorbic acid and catalase, or mannitol). The survival curves of tissue balls were markedly different between 31 and 25 °C. At 31 °C, most tissue balls disintegrated within 24 h, whereas at 25 °C, most tissue balls survived for more than 24 h. There was a negative correlation between survival time and the zooxanthella density of tissue balls at 31 °C, but no significant relationship was found at 25 °C. Antioxidants extended the survival time of tissue balls at high temperature, suggesting that zooxanthellae produce reactive oxygen species under stress. These results indicate that zooxanthellae produce harmful substances and damage coral cells under high-temperature stress. Tissue balls provide a good experimental system with which to study the effects of stress and various chemical reagents on corals cells.     

Signaling-mediated cross-talk modulates swarming and biofilm formation in a coral pathogen Serratia marcescens

Interactions within microbial communities associated with marine holobionts contribute importantly to the health of these symbiotic organisms formed by invertebrates, dinoflagellates and bacteria. However, mechanisms that control invertebrate-associated microbiota are not yet fully understood. Hydrophobic compounds that were isolated from surfaces of asymptomatic corals inhibited biofilm formation by the white pox pathogen Serratia marcescens PDL100, indicating that signals capable of affecting the associated microbiota are produced in situ. However, neither the origin nor structures of these signals are currently known. A functional survey of bacteria recovered from coral mucus and from cultures of the dinoflagellate Symbiodinium spp. revealed that they could alter swarming and biofilm formation in S. marcescens. As swarming and biofilm formation are inversely regulated, the ability of some native α-proteobacteria to affect both behaviors suggests that the α-proteobacterial signal(s) target a global regulatory switch controlling the behaviors in the pathogen. Isolates of Marinobacter sp. inhibited both biofilm formation and swarming in S. marcescens PDL100, without affecting growth of the coral pathogen, indicative of the production of multiple inhibitors, likely targeting lower level regulatory genes or functions. A multi-species cocktail containing these strains inhibited progression of a disease caused by S. marcescens in a model polyp Aiptasia pallida. An α-proteobacterial isolate 44B9 had a similar effect. Even though ∼4% of native holobiont-associated bacteria produced compounds capable of triggering responses in well-characterized N-acyl homoserine lactone (AHL) biosensors, there was no strong correlation between the production of AHL-like signals and disruption of biofilms or swarming in S. marcescens.     

The influence of protected area management on the status of coral reefs at Misali Island,Tanzania following the 1998 bleaching event in the western Indian Ocean

The reefs of Misali Island, Tanzania suffered serious coral mortality as a result of the 1998 coral bleaching event in the western Indian Ocean. Four years after this event, Frontier‐Tanzania assessed reef substrate in Misali Island Marine Conservation Area (MIMCA) using scuba surveys. Substrate cover was compared between sites within and outside the nonextraction zone (NEZ) of MIMCA and recovery rates were estimated through reference to previous studies. Coral recovery at Misali Island following the bleaching event has generally been slow, but nonextractive management of MIMCA has enhanced coral recovery within the NEZ. The absence of fishing activities in the NEZ has sustained herbivorous fish populations and thus reduced algal overgrowth; however, outside the NEZ, diving and anchoring activities have caused further coral damage and led to increased levels of debris. Awareness‐raising programmes could lessen these problems and the NEZ appeared to be well‐placed to capitalize on nonmanageable factors that are thought to mitigate the effects of coral bleaching. In addition, the implementation of flexible zoning schemes could improve recovery rates after future bleaching events.     

Complementary (secondary) metabolites in an octocoral competing with a scleractinian coral: effects of varying nutrient regimes

Competitive interactions between two sessile, epibenthic species were investigated on the Great Barrier Reef (GBR) in the presence and absence of added nutrients, as part of the Enrichment of Nutrients on Coral Reefs Experiment (ENCORE). Sarcophyton ehrenbergi Marenzeller (Octocorallia: Alcyonacea), an alcyonacean soft coral, and Pocillopora damicornis (Linnaeus), a scleractinian coral, were relocated and placed in contact with each other on large plastic grids on each of 12 micro-atolls within the One Tree Island (OTI) lagoon (23°30′S, 152°96′E, GBR). These micro-atolls were allocated in equal-sized groups to three enrichment treatments (addition of nitrogen, N; addition of phosphorus, P; addition of both nitrogen and phosphorus, N+P) and one control. Non-relocated (NR) and relocated colonies were also monitored as controls. After relocation and 1 year of nutrient enrichment, concentrations of a terpenoid complementary metabolite—sarcophytoxide—and wax esters were analyzed in colonies of S. ehrenbergi that had been exposed to elevated concentrations of N, P, N+P and compared with colonies on the non-nutrient-enriched control. Non-relocated control colonies from the natural environment were monitored over a period of 1 year and compared to colonies relocated to the control micro-atolls to assess handling effects. Analyses were performed on non-interacting S. ehrenbergi colonies, S. ehrenbergi colonies in experimental contact with P. damicornis colonies, and on non-interacting S. ehrenbergi colonies from the site of initial collection. Significant differences were found between sarcophytoxide levels in colonies of S. ehrenbergi in contact with P. damicornis vs. control/non-contact colonies; contact colonies had higher levels of this metabolite. Non-relocated control colonies of S. ehrenbergi exhibited significantly higher levels of sarcophytoxide than relocated control colonies. Augmentation of nutrient levels in micro-atolls significantly increased sarcophytoxide levels in S. ehrenbergi colonies relative to colonies on the control micro-atolls, although this response was not strong. Concentrations of fatty esters increased significantly through time in S. ehrenbergi colonies in their natural setting (non-relocated controls). This variability was not observed in relocated colonies in the treatment and control micro-atolls, irrespective of contact with P. damicornis. Concentrations of fatty esters in colonies of S. ehrenbergi in contact with P. damicornis were significantly lower than control/non-contact colonies, indicating that there is a cost in terms of stored energy reserves for the production of additional complementary metabolites when involved in competition for space. Augmentation of P levels in micro-atolls induced significant increases in fatty ester levels within S. ehrenbergi colonies vs. colonies in control micro-atolls, or in micro-atolls treated with added N or N+P together. These findings indicate that interspecific competition for space between a scleractinian coral and an alcyonacean soft coral and/or changes in the environmental nutrient regime can influence concentrations of complementary/secondary metabolites in the alcyonacean coral and the organism's stored energy reserves.     

Experimental antibiotic treatment identifies potential pathogens of white band disease in the endangered Caribbean coral Acropora cervicornis

Coral diseases have been increasingly reported over the past few decades and are a major contributor to coral decline worldwide. The Caribbean, in particular, has been noted as a hotspot for coral disease, and the aptly named white syndromes have caused the decline of the dominant reef building corals throughout their range. White band disease (WBD) has been implicated in the dramatic loss of Acropora cervicornis and Acropora palmata since the 1970s, resulting in both species being listed as critically endangered on the International Union for Conservation of Nature Red list. The causal agent of WBD remains unknown, although recent studies based on challenge experiments with filtrate from infected hosts concluded that the disease is probably caused by bacteria. Here, we report an experiment using four different antibiotic treatments, targeting different members of the disease-associated microbial community. Two antibiotics, ampicillin and paromomycin, arrested the disease completely, and by comparing with community shifts brought about by treatments that did not arrest the disease, we have identified the likely candidate causal agent or agents of WBD. Our interpretation of the experimental treatments is that one or a combination of up to three specific bacterial types, detected consistently in diseased corals but not detectable in healthy corals, are likely causal agents of WBD. In addition, a histophagous ciliate (Philaster lucinda) identical to that found consistently in association with white syndrome in Indo-Pacific acroporas was also consistently detected in all WBD samples and absent in healthy coral. Treatment with metronidazole reduced it to below detection limits, but did not arrest the disease. However, the microscopic disease signs changed, suggesting a secondary role in disease causation for this ciliate. In future studies to identify a causal agent of WBD via tests of Henle–Koch''s postulates, it will be vital to experimentally control for populations of the other potential pathogens identified in this study.     

Functional significance of dinitrogen fixation in sustaining coral productivity under oligotrophic conditions

Functional traits define species by their ecological role in the ecosystem. Animals themselves are host–microbe ecosystems (holobionts), and the application of ecophysiological approaches can help to understand their functioning. In hard coral holobionts, communities of dinitrogen (N₂)-fixing prokaryotes (diazotrophs) may contribute a functional trait by providing bioavailable nitrogen (N) that could sustain coral productivity under oligotrophic conditions. This study quantified N₂ fixation by diazotrophs associated with four genera of hermatypic corals on a northern Red Sea fringing reef exposed to high seasonality. We found N₂ fixation activity to be 5- to 10-fold higher in summer, when inorganic nutrient concentrations were lowest and water temperature and light availability highest. Concurrently, coral gross primary productivity remained stable despite lower Symbiodinium densities and tissue chlorophyll a contents. In contrast, chlorophyll a content per Symbiodinium cell increased from spring to summer, suggesting that algal cells overcame limitation of N, an essential element for chlorophyll synthesis. In fact, N₂ fixation was positively correlated with coral productivity in summer, when its contribution was estimated to meet 11% of the Symbiodinium N requirements. These results provide evidence of an important functional role of diazotrophs in sustaining coral productivity when alternative external N sources are scarce.     

Genomic and proteomic analyses of the coral pathogen Vibrio coralliilyticus reveal a diverse virulence repertoire

Vibrio coralliilyticus has been implicated as an important pathogen of coral species worldwide. In this study, the nearly complete genome of Vibrio coralliilyticus strain P1 (LMG23696) was sequenced and proteases implicated in virulence of the strain were specifically investigated. The genome sequence of P1 (5 513 256 bp in size) consisted of 5222 coding sequences and 58 RNA genes (53 tRNAs and at least 5 rRNAs). Seventeen metalloprotease and effector (vgrG, hlyA and hcp) genes were identified in the genome and expressed proteases were also detected in the secretome of P1. As the VcpA zinc-metalloprotease has been considered an important virulence factor of V. coralliilyticus, a vcpA deletion mutant was constructed to evaluate the effect of this gene in animal pathogenesis. Both wild-type and mutant (ΔvcpA) strains exhibited similar virulence characteristics that resulted in high mortality in Artemia and Drosophila pathogenicity bioassays and strong photosystem II inactivation of the coral dinoflagellate endosymbiont (Symbiodinium). In contrast, the ΔvcpA mutant demonstrated higher hemolytic activity and secreted 18 proteins not secreted by the wild type. These proteins included four types of metalloproteases, a chitinase, a hemolysin-related protein RbmC, the Hcp protein and 12 hypothetical proteins. Overall, the results of this study indicate that V. coralliilyticus strain P1 has a diverse virulence repertoire that possibly enables this bacterium to be an efficient animal pathogen.     

Diversity and dynamics of bacterial communities in early life stages of the Caribbean coral Porites astreoides

In this study, we examine microbial communities of early developmental stages of the coral Porites astreoides by sequence analysis of cloned 16S rRNA genes, terminal restriction fragment length polymorphism (TRFLP), and fluorescence in situ hybridization (FISH) imaging. Bacteria are associated with the ectoderm layer in newly released planula larvae, in 4-day-old planulae, and on the newly forming mesenteries surrounding developing septa in juvenile polyps after settlement. Roseobacter clade-associated (RCA) bacteria and Marinobacter sp. are consistently detected in specimens of P. astreoides spanning three early developmental stages, two locations in the Caribbean and 3 years of collection. Multi-response permutation procedures analysis on the TRFLP results do not support significant variation in the bacterial communities associated with P. astreoides larvae across collection location, collection year or developmental stage. The results are the first evidence of vertical transmission (from parent to offspring) of bacteria in corals. The results also show that at least two groups of bacterial taxa, the RCA bacteria and Marinobacter, are consistently associated with juvenile P. astreoides against a complex background of microbial associations, indicating that some components of the microbial community are long-term associates of the corals and may impact host health and survival.