Wound repair in Montipora capitata

We documented the microscopic morphology of tissue healing in Montipora capitata. Fragments from two healthy coral colonies were traumatized by scraping tissue and skeleton and monitored in flow-through seawater tables every 2-4 days for 40 days for gross and cellular changes. Grossly, corals appeared healed and repigmented by Day 40. Histologically, traumatized issues were undistinguishable from intact untraumatized tissues by Day 12. We suspect that the calicoblastic epidermis of basal body wall is pluripotential and can develop into surface epidermis when needed.     

Tissue loss (white syndrome) in the coral Montipora capitata is a dynamic disease with multiple host responses and potential causes

Tissue loss diseases or white syndromes (WS) are some of the most important coral diseases because they result in significant colony mortality and morbidity, threatening dominant Acroporidae in the Caribbean and Pacific. The causes of WS remain elusive in part because few have examined affected corals at the cellular level. We studied the cellular changes associated with WS over time in a dominant Hawaiian coral, Montipora capitata, and showed that: (i) WS has rapidly progressing (acute) phases mainly associated with ciliates or slowly progressing (chronic) phases mainly associated with helminths or chimeric parasites; (ii) these phases interchanged and waxed and waned; (iii) WS could be a systemic disease associated with chimeric parasitism or a localized disease associated with helminths or ciliates; (iv) corals responded to ciliates mainly with necrosis and to helminths or chimeric parasites with wound repair; (v) mixed infections were uncommon; and (vi) other than cyanobacteria, prokaryotes associated with cell death were not seen. Recognizing potential agents associated with disease at the cellular level and the host response to those agents offers a logical deductive rationale to further explore the role of such agents in the pathogenesis of WS in M. capitata and helps explain manifestation of gross lesions. This approach has broad applicability to the study of the pathogenesis of coral diseases in the field and under experimental settings.     

The distribution of the thermally tolerant symbiont lineage (Symbiodinium clade D) in corals from Hawaii: correlations with host and the history of ocean thermal stress

Spatially intimate symbioses, such as those between scleractinian corals and unicellular algae belonging to the genus Symbiodinium, can potentially adapt to changes in the environment by altering the taxonomic composition of their endosymbiont communities. We quantified the spatial relationship between the cumulative frequency of thermal stress anomalies (TSAs) and the taxonomic composition of Symbiodinium in the corals Montipora capitata, Porites lobata, and Porites compressa across the Hawaiian archipelago. Specifically, we investigated whether thermally tolerant clade D Symbiodinium was in greater abundance in corals from sites with high frequencies of TSAs. We recovered 2305 Symbiodinium ITS2 sequences from 242 coral colonies in lagoonal reef habitats at Pearl and Hermes Atoll, French Frigate Shoals, and Kaneohe Bay, Oahu in 2007. Sequences were grouped into 26 operational taxonomic units (OTUs) with 12 OTUs associated with Montipora and 21 with Porites. Both coral genera associated with Symbiodinium in clade C, and these co‐occurred with clade D in M. capitata and clade G in P. lobata. The latter represents the first report of clade G Symbiodinium in P. lobata. In M. capitata (but not Porites spp.), there was a significant correlation between the presence of Symbiodinium in clade D and a thermal history characterized by high cumulative frequency of TSAs. The endogenous community composition of Symbiodinium and an association with clade D symbionts after long‐term thermal disturbance appear strongly dependent on the taxa of the coral host.     

Vibrio owensii Induces the Tissue Loss Disease Montipora White Syndrome in the Hawaiian Reef Coral Montipora capitata

Incidences of coral disease in the Indo-Pacific are increasing at an alarming rate. In particular, Montipora white syndrome, a tissue-loss disease found on corals throughout the Hawaiian archipelago, has the potential to degrade Hawaii’s reefs. To identify the etiologic agent of Montipora white syndrome, bacteria were isolated from a diseased fragment of Montipora capitata and used in a screen for virulent strains. A single isolate, designated strain OCN002, recreated disease signs in 53% of coral fragments in laboratory infection trials when added to a final concentration of 10⁷ cells/ml of seawater. In addition to displaying similar signs of disease, diseased coral fragments from the field and those from infection trials both had a dramatic increase in the abundance of associated culturable bacteria, with those of the genus Vibiro well represented. Bacteria isolated from diseased fragments used in infection trails were shown to be descendants of the original OCN002 inocula based on both the presence of a plasmid introduced to genetically tag the strain and the sequence of a region of the OCN002 genome. In contrast, OCN002 was not re-isolated from fragments that were exposed to the strain but did not develop tissue loss. Sequencing of the rrsH gene, metabolic characterization, as well as multilocus sequence analysis indicated that OCN002 is a strain of the recently described species Vibrio owensii. This investigation of Montipora white syndrome recognizes V. owensii OCN002 as the first bacterial coral pathogen identified from Hawaii’s reefs and expands the range of bacteria known to cause disease in corals.     

Diazotrophic diversity in the Caribbean coral, Montastraea cavernosa

Previous research on the Caribbean coral Montastraea cavernosa reported the presence of cyanobacterial endosymbionts and nitrogen fixation in orange, but not brown, colonies. We compared the diversity of nifH gene sequences between these two color morphs at three locations in the Caribbean and found that the nifH sequences recovered from M. cavernosa were consistent with previous studies on corals where members of both the α-proteobacteria and cyanobacteria were recovered. A number of nifH operational taxonomic units (OTUs) were significantly more abundant in the orange compared to the brown morphs, and one specific OTU (OTU 17), a cyanobacterial nifH sequence similar to others from corals and sponges and related to the cyanobacterial genus Cyanothece, was found in all orange morphs of M. cavernosa at all locations. The nifH diversity reported here, from a community perspective, was not significantly different between orange and brown morphs of M. cavernosa.     

Characterization of Communities of Heterotrophic Bacteria Associated with Healthy and Diseased Corals in Nha Trang Bay (Vietnam)

A comparative investigation of the heterotrophic microflora of 11 species of healthy corals and of white-band-diseased and yellow-band-diseased corals inhabiting the reefs of Nha Trang Bay (Vietnam), which has been exposed to anthropogenic impact, was performed. Fifty-nine strains of heterotrophic bacteria isolated on Y/K and Endo media were investigated and characterized. All the isolates were identified at the genus level by consideration of the results of analysis of their phenotypic properties, determination of the molar percent of G+C bases in their DNA, and the composition of the fatty acids of lipids. In the composition of the microflora of tissues of healthy corals, γ-proteobacteria prevailed, with halomonads being dominant among them. In addition, the gram-negative bacteria included Pseudomonas and Vibrio spp., members of the Cyto-phaga-Fla-vobacterium-Bacteroides (CFB) phylogenetic cluster, and Moraxella sp. The gram-positive bacteria revealed included Bacillus, Staphylococcus, Halococcus, and Micrococcus spp., and coryneform bacteria. In the composition of the microflora of the tissues of affected corals, bacteria of the family Enterobacteriaceae and of the genera Planococcus and Arthrobacter, which were not revealed in healthy hydrobionts, were found. The anthropogenic impact is not the sole factor determining the infection of corals.     

Metabolite pools of the reef building coral Montipora capitata are unaffected by Symbiodiniaceae community composition

Some reef corals form stable, dominant or codominant associations with multiple endosymbiotic dinoflagellate species (family Symbiodiniaceae). Given the immense genetic and physiological diversity within this family, Symbiodiniaceae community composition has the potential to impact the nutritional physiology and fitness of the cnidarian host and all associated symbionts. Here we assessed the impact of the symbiont community composition on the metabolome of the coral Montipora capitata in Kāne‘ohe Bay, Hawai‘i, where different colonies can be dominated by stress-tolerant Durusdinium glynnii or stress-sensitive Cladocopium spp. Based on our existing knowledge of these symbiont taxa, we hypothesised that the metabolite profile of D. glynnii-dominated corals would be consistent with poorer nutritional support of the host relative to those corals dominated by Cladocopium spp. However, comparative metabolite profiling revealed that the metabolite pools of the host and symbiont were unaffected by differences in the abundance of the two symbionts within the community. The abundance of the individual metabolites was the same in the host and in the endosymbiont regardless of whether the host was populated with D. glynnii or Cladocopium spp. These results suggest that coral-dinoflagellate symbioses have the potential to undergo physiological adjustments over time to accommodate differences in their resident symbionts. Such mechanisms may involve host heterotrophic compensation (increasing the level of nutrition generated by feeding relative to delivery from the algae), dynamic regulation of metabolic pathways when exchange of metabolites between the organisms differs, and/or modification of both the type and quantity of metabolites that are exchanged. We discuss these adjustments and the implications for the physiology and survival of reef corals under changing environmental regimes.

     

Differences in microbial communities between healthy and bleached coral Acropora solitaryensis from Xisha Islands,South China Sea

Serious bleaching events have been observed in Acropora solitaryensis, one of the main species of reef-building coral in the area of the Xisha islands in the South China Sea, during 2008–2011. The microbial communities of healthy and bleached coral samples were compared to explore the difference in the bacterial composition using 16S rRNA gene sequencing and denaturing gradient gel electrophoresis (DGGE). Analysis of 16S rRNA gene sequences showed that the diversity of bacteria from the corals was different between the healthy and the bleached. Albeit both the healthy coral and bleached coral displayed similar dominant bacterial species (α- and γ-proteobacteria), the ratio of Vibrio spp. increased sharply in the latter. As the bleaching developed, the diversity of the microbial community was dramatically decreased and the dominant species were replaced by γ-proteobacteria where Vibrio spp. and Escherichia spp. overwhelmed other genera. Similar results were gained by the DGGE technique though the abundance was lower. Furthermore, in the bleached tissues, Vibrio coralliilyticus was identified using both methods. These results indicated that pathogenic Vibrio spp. might be one of the factors causing A. solitaryensis bleaching in the coral reef of the Xisha islands.     

Nitrogen Fertilization Has a Stronger Effect on Soil Nitrogen-Fixing Bacterial Communities than Elevated Atmospheric CO2

Biological nitrogen fixation is the primary supply of N to most ecosystems, yet there is considerable uncertainty about how N-fixing bacteria will respond to global change factors such as increasing atmospheric CO₂ and N deposition. Using the nifH gene as a molecular marker, we studied how the community structure of N-fixing soil bacteria from temperate pine, aspen, and sweet gum stands and a brackish tidal marsh responded to multiyear elevated CO₂ conditions. We also examined how N availability, specifically, N fertilization, interacted with elevated CO₂ to affect these communities in the temperate pine forest. Based on data from Sanger sequencing and quantitative PCR, the soil nifH composition in the three forest systems was dominated by species in the Geobacteraceae and, to a lesser extent, Alphaproteobacteria. The N-fixing-bacterial-community structure was subtly altered after 10 or more years of elevated atmospheric CO₂, and the observed shifts differed in each biome. In the pine forest, N fertilization had a stronger effect on nifH community structure than elevated CO₂ and suppressed the diversity and abundance of N-fixing bacteria under elevated atmospheric CO₂ conditions. These results indicate that N-fixing bacteria have complex, interacting responses that will be important for understanding ecosystem productivity in a changing climate.     

Phylogenetically Diverse Denitrifying and Ammonia-Oxidizing Bacteria in Corals Alcyonium gracillimum and Tubastraea coccinea

To date, the association of coral–bacteria and the ecological roles of bacterial symbionts in corals remain largely unknown. In particular, little is known about the community components of bacterial symbionts of corals involved in the process of denitrification and ammonia oxidation. In this study, the nitrite reductase (nirS and nirK) and ammonia monooxygenase subunit A (amoA) genes were used as functional markers. Diverse bacteria with the potential to be active as denitrifiers and ammonia-oxidizing bacteria (AOB) were found in two East China Sea corals: stony coral Alcyonium gracillimum and soft coral Tubastraea coccinea. The 16S rRNA gene library analysis demonstrated different communities of bacterial symbionts in these two corals of the same location. Nitrite reductase nirK gene was found only in T. coccinea, while both nirK and nirS genes were detected in A. gracillimum, which might be the result of the presence of different bacterial symbionts in these two corals. AOB rather than ammonia-oxidizing archaea were detected in both corals, suggesting that AOB might play an important role in the ammonia oxidation process of the corals. This study indicates that the coral bacterial symbionts with the potential for nitrite reduction and ammonia oxidation might have multiple ecological roles in the coral holobiont, which promotes our understanding of bacteria-mediated nitrogen cycling in corals. To our knowledge, this study is the first assessment of the community structure and phylogenetic diversity of denitrifying bacteria and AOB in corals based on nirK, nirS, and amoA gene library analysis.     

Diversity in skeletal architecture influences biological heterogeneity and Symbiodinium habitat in corals

Scleractinian corals vary in response to rapid shifts in the marine environment and changes in reef community structure post-disturbance reveal a clear relationship between coral performance and morphology. With exceptions, massive corals are thought to be more tolerant and branching corals more vulnerable to changing environmental conditions, notably thermal stress. The typical responses of massive and branching coral taxa, respectively, are well documented; however, the biological and functional characteristics that underpin this variation are not well understood. We address this gap by comparing multiple biological attributes that are correlated with skeletal architecture in two perforate (having porous skeletal matrices with intercalating tissues) and two imperforate coral species (Montipora aequituberculata, Porites lobata, Pocillopora damicornis, and Seriatopora hystrix) representing three morphotypes. Our results reveal inherent biological heterogeneity among corals and the potential for perforate skeletons to create complex, three-dimensional internal habitats that impact the dynamics of the symbiosis. Patterns of tissue thickness are correlated with the concentration of symbionts within narrow regions of tissue in imperforate corals versus broad distribution throughout the larger tissue area in perforate corals. Attributes of the perforate and environmentally tolerant P. lobata were notable, with tissues ～5 times thicker than in the sensitive, imperforate species P. damicornis and S. hystrix. Additionally, P. lobata had the lowest baseline levels of superoxide and Symbiodinium that provisioned high levels of energy. Given our observations, we hypothesize that the complexity of the visually obscured internal environment has an impact on host–symbiont dynamics and ultimately on survival, warranting further scientific investigation.     

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.     

Food selection of a corallivorous butterflyfish Chaetodon austriacus in the Red Sea, with particular emphasis on size and fish symbionts of corals

The exquisite butterflyfish Chaetodon austriacus feeds mainly on Acropora , Pocillopora , Montipora and Stylophora in the northern Red Sea. Large colonies of Acropora are preferred over smaller colonies and other coral genera, whereas avoidance is indicated for corals that are occupied by certain coral-associated gobiid fishes of the genus Gobiodon . It is suggested that, apart from coral identity, colony size and potential repellent effects of particular coral symbionts are both important sources of variation in the food selectivity of corallivorous chaetodotids.     

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.     

Dynamics of lipid and fatty acid composition of shallow-water corals under thermal stress: an experimental approach

Coral bleaching induces changes in lipid and fatty acid composition that result in low lipid content, reducing the likelihood of coral survival. Species-specific differences in the metabolism of lipid reserves may contribute to the differential resistance of corals under acute heat exposures. Here, we examined the dynamics of lipids and fatty acid abundance in corals subjected to short-term heat stress. The stony corals Acropora intermedia, Montipora digitata, and the soft coral Sinularia capitalis all showed a 60–75% decline in both storage and structural lipids. However, S. capitalis and M. digitata exhibited no significant change in the percentages of structural lipids (i.e., polar lipids and sterols) until they had lost 90–95% of their endosymbionts, whereas A. intermedia showed a rapid decline in structural lipids after a 50% loss of symbionts. After a 90–95% loss of symbionts under heat stress, all three corals showed a relative depletion of polyunsaturated fatty acids that had symbiont biomarkers, suggesting that polyunsaturated fatty acids were translocated from the symbiont to the coral host tissue.     

Establishment of Coral–Algal Symbiosis Requires Attraction and Selection

Coral reef ecosystems are based on coral–zooxanthellae symbiosis. During the initiation of symbiosis, majority of corals acquire their own zooxanthellae (specifically from the dinoflagellate genus Symbiodinium) from surrounding environments. The mechanisms underlying the initial establishment of symbiosis have attracted much interest, and numerous field and laboratory experiments have been conducted to elucidate this establishment. However, it is still unclear whether the host corals selectively or randomly acquire their symbionts from surrounding environments. To address this issue, we initially compared genetic compositions of Symbiodinium within naturally settled about 2-week-old Acropora coral juveniles (recruits) and those in the adjacent seawater as the potential symbiont source. We then performed infection tests using several types of Symbiodinium culture strains and apo-symbiotic (does not have Symbiodinium cells yet) Acropora coral larvae. Our field observations indicated apparent preference toward specific Symbiodinium genotypes (A1 and D1-4) within the recruits, despite a rich abundance of other Symbiodinium in the environmental population pool. Laboratory experiments were in accordance with this field observation: Symbiodinium strains of type A1 and D1-4 showed higher infection rates for Acropora larvae than other genotype strains, even when supplied at lower cell densities. Subsequent attraction tests revealed that three Symbiodinium strains were attracted toward Acropora larvae, and within them, only A1 and D1-4 strains were acquired by the larvae. Another three strains did not intrinsically approach to the larvae. These findings suggest the initial establishment of corals–Symbiodinium symbiosis is not random, and the infection mechanism appeared to comprise two steps: initial attraction step and subsequent selective uptake by the coral.     

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.     

Comparative analysis of symbiont ratios and gene expression in natural populations of two Bathymodiolus mussel species

Bathymodiolus mussels associated with deep-sea hydrothermal vents and cold seeps harbor chemosynthetic endosymbiotic bacteria in bacteriocytes located in the gill epithelium. Two distinct morphotypes of γ-proteobacteria, sulfur- and methane-oxidizing, have been identified and form a dual symbiosis in B. azoricus mussels from the Mid-Atlantic Ridge and in B. aff. boomerang from cold seeps in the Gulf of Guinea. Thiotrophic bacteria (SOX) are capable of fixing CO2 in the presence of sulfide or thiosulfate and methanotrophic bacteria (MOX) use methane both as a carbon and an energy source. In this study we used quantitative real-time PCR to test whether symbiont abundance and gene expression varied between the two mussel species. Results showed that B. azoricus from two hydrothermal sites had similar ratios and gene expression pattern for both symbiont types. In B. aff. boomerang, SOX ratio and ATP sulfurylase gene expression show differences between specimens collected on the different sites. Analysis of symbiont ratios in both species indicated a clear dominance of MOX symbionts in B. aff. boomerang and SOX symbionts in B. azoricus. We also evidenced that the species from the deeper sites (B. aff. boomerang) and mussels collected from sulfur and methane rich habitats showed higher symbiont ratio suggesting that environmental parameters may have significant impacts on the symbiont ratios in Bathymodiolus mussels.     

Spatio-Temporal Analyses of Symbiodinium Physiology of the Coral Pocillopora verrucosa along Large-Scale Nutrient and Temperature Gradients in the Red Sea

Algal symbionts (zooxanthellae, genus Symbiodinium) of scleractinian corals respond strongly to temperature, nutrient and light changes. These factors vary greatly along the north-south gradient in the Red Sea and include conditions, which are outside of those typically considered optimal for coral growth. Nevertheless, coral communities thrive throughout the Red Sea, suggesting that zooxanthellae have successfully acclimatized or adapted to the harsh conditions they experience particularly in the south (high temperatures and high nutrient supply). As such, the Red Sea is a region, which may help to better understand how zooxanthellae and their coral hosts successfully acclimatize or adapt to environmental change (e.g. increased temperatures and localized eutrophication). To gain further insight into the physiology of coral symbionts in the Red Sea, we examined the abundance of dominant Symbiodinium types associated with the coral Pocillopora verrucosa, and measured Symbiodinium physiological characteristics (i.e. photosynthetic processes, cell density, pigmentation, and protein composition) along the latitudinal gradient of the Red Sea in summer and winter. Despite the strong environmental gradients from north to south, our results demonstrate that Symbiodinium microadriaticum (type A1) was the predominant species in P. verrucosa along the latitudinal gradient. Furthermore, measured physiological characteristics were found to vary more with prevailing seasonal environmental conditions than with region-specific differences, although the measured environmental parameters displayed much higher spatial than temporal variability. We conclude that our findings might present the result of long-term acclimatization or adaptation of S. microadriaticum to regionally specific conditions within the Red Sea. Of additional note, high nutrients in the South correlated with high zooxanthellae density indicating a compensation for a temperature-driven loss of photosynthetic performance, which may prove promising for the resilience of these corals under increase of temperature increase and eutrophication.     

Do clades of symbiotic dinoflagellates in scleractinian corals of the Gulf of Eilat (Red Sea) differ from those of other coral reefs?

The symbiotic association between corals and zooxanthellae has been a major contributing factor in the success of reef-building corals. Most of these endocellular microalgal symbionts belong to the dinoflagellate genus Symbiodinium. However, considerable genetic diversity was revealed within this taxon, as is evident in the several clades of Symbiodinium found in association with hermatypic corals all over the world. The coral reefs of Eilat (Aqaba), where winter temperature minima of 21 °C are close to threshold values that prevent reef development, are among the northernmost reefs in the world. Furthermore, due to the circulation patterns of the Gulf, the extremely high evaporation, and lack of any riverine inputs, the Gulf's waters are highly saline (40.5‰). In spite of the extreme location, a high diversity of coral species has been reported in this area. In this study, using PCR, we specifically amplified zooxanthellae 18S ribosomal DNA from symbionts of 11 coral species, and analyzed it with respect to RFLP and DNA sequence.Of the several clades described from the same coral hosts in other localities, only A and C were found in the present study. Symbiodinium populations in the host examined from Eilat were different relative to other parts of the world. This distribution is discussed in relation to reproduction strategy: broadcasting versus brooding. Based on our results, we suggest that clade A is transferred through a closed system. As mass bleaching in the Gulf has never been observed, we suggest that the adaptive mechanisms presumably favoring clade diversity were not yet significant in our relatively cool area.