Reef-building Scleractinian Corals Research Articles

Distinct coral environments shape the dynamic of planktonic Vibrio spp.

BackgroundCoral reefs are one of the most biodiverse and productive ecosystems, providing habitat for a vast of species. Reef-building scleractinian corals with a symbiotic microbiome, including bacteria, archaea, viruses and eukaryotic microbes, are referred to coral holobionts. Among them, coral diseases, mainly caused by Vibrio spp., have significantly contributed to the loss of coral cover and diversity. Habitat filtering across the globe has led to a variety structure of marine bacterial communities. Coral species, quantity and characteristics are significant differences between the Xisha Islands and Daya Bay (Guangdong Province). Thus, the Vibrio communities may be distinct between coral rich and poor areas.ResultsThrough comparison of Vibrio dynamics between coral-rich (Xisha Islands) and coral-poor (Daya Bay) locations, we uncovered differences in Vibrio abundance, diversity, community composition and assembly mechanisms associated with corals. The higher abundance of Vibrio in coral rich areas may indicate a strong interaction between vibrios and corals. V. campbellii, Paraphotobacterium marinum and V. caribbeanicus were widely distributed in both coral rich and poor areas, likely indicating weak species specificity in the coral-stimulated growth of Vibrio. Random-forest prediction revealed Vibrio species and Photobacterium species as potential microbial indicators in the coral rich and coral poor areas, respectively. Ecological drift rather than selection governed the Vibrio community assembly in the Xisha Islands. Comparatively, homogenizing selection was more important for the Daya Bay community, which may reflect a role of habitat filtration.ConclusionThis study revealed the different distribution pattern and assembly mechanism of Vibrio spp. between coral rich and poor areas, providing the background data for the research of Vibrio community in coral reef areas and may help the protection of coral reef at the biological level. The main reasons for the difference were different number and species of corals, environmental (e.g., temperature) and spatial factors. It reflected the strong interaction between Vibrio and corals, and provided a new perspective for the investigation of Vibrio in coral reef ecosystem.

Fast-growing species shape the evolution of reef corals

Ecological interactions are ubiquitous on tropical coral reefs, where sessile organisms coexist in limited space. Within these high-diversity systems, reef-building scleractinian corals form an intricate interaction network. The role of biotic interactions among reef corals is well established on ecological timescales. However, its potential effect on macroevolutionary patterns remains unclear. By analysing the rich fossil record of Scleractinia, we show that reef coral biodiversity experienced marked evolutionary rate shifts in the last 3 million years, possibly driven by biotic interactions. Our models suggest that there was an overwhelming effect of staghorn corals (family Acroporidae) on the fossil diversity trajectories of other coral groups. Staghorn corals showed an unparalleled spike in diversification during the Pleistocene. But surprisingly, their expansion was linked with increases in both extinction and speciation rates in other coral families, driving a nine-fold increase in lineage turnover. These results reveal a double-edged effect of diversity dependency on reef evolution. Given their fast growth, staghorn corals may have increased extinction rates via competitive interactions, while promoting speciation through their role as ecosystem engineers. This suggests that recent widespread human-mediated reductions in staghorn coral cover, may be disrupting the key macroevolutionary processes that established modern coral reef ecosystems.

Its What’s on the Inside That Counts: An Effective, Efficient, and Streamlined Method for Quantification of Octocoral Symbiodiniaceae and Chlorophyll

Ocean warming driven bleaching is one of the greatest threats to zooxanthellate cnidarians in the Anthropocene. Bleaching is the loss of Symbiodiniaceae, chlorophyll, or both from zooxanthellate animals. To quantify bleaching and recovery, standardised methods for quantification of Symbiodiniaceae and chlorophyll concentrations have been developed for reef-building scleractinian corals, but no such standard method has been developed for octocorals. For stony corals, quantification of Symbiodiniaceae and chlorophyll concentrations often relies on normalisation to skeletal surface area or unit of biomass [i.e., protein, ash-free dry weight (AFDW)]. Stiff octocorals do not change their volume, as such studies have used volume and surface area to standardise densities, but soft-bodied octocorals can alter their size using water movement within the animal; therefore, Symbiodiniaceae and chlorophyll cannot accurately be measured per unit of surface area and are instead measured in units of Symbiodiniaceae and chlorophyll per μg of host protein or AFDW. Though AFDW is more representative of the full biomass composition than host protein, AFDW is more time and resource intensive. Here, we provide a streamlined methodology to quantify Symbiodiniaceae density, chlorophyll concentration, and protein content in soft-bodied octocorals. This technique uses minimal equipment, does not require freeze-drying or burning samples to obtain ash weight, and is effective for down to 0.2 g wet tissue. Bulk samples can be centrifuged, the Symbiodiniaceae pellet washed, and the supernatant saved for protein analysis. This efficient technique allows for clean, easy to count samples of Symbiodiniaceae with minimal animal protein contamination. Chlorophyll a and c2 extractions occurs at different rates, with chlorophyll a taking 24 h to extract completely at 4°C and chlorophyll c2 taking 48 h. Finally, we found that where necessary, wet weight may be used as a proxy for protein content, but the correlation of protein and wet weight varies by species and protein should be used when possible. Overall, we have created a rapid and accurate method for quantification of bleaching markers in octocorals.

Temporal Variation in the Microbiome of Tropical and Temperate Octocorals.

Bacterial members of the coral holobiont play an important role in determining coral fitness. However, most knowledge of the coral microbiome has come from reef-building scleractinian corals, with far less known about the nature and importance of the microbiome of octocorals (subclass Octocorallia), which contribute significantly to reef biodiversity and functional complexity. We examined the diversity and structure of the bacterial component of octocoral microbiomes over summer and winter, with a focus on two temperate (Erythropodium hicksoni, Capnella gaboensis; Sydney Harbour) and two tropical (Sinularia sp., Sarcophyton sp.; Heron Island) species common to reefs in eastern Australia. Bacterial communities associated with these octocorals were also compared to common temperate (Plesiastrea versipora) and tropical (Acropora aspera) hard corals from the same reefs. Using 16S rRNA amplicon sequencing, bacterial diversity was found to be heterogeneous among octocorals, but we observed changes in composition between summer and winter for some species (C. gaboensis and Sinularia sp.), but not for others (E. hicksoni and Sarcophyton sp.). Bacterial community structure differed significantly between all octocoral species within both the temperate and tropical environments. However, on a seasonal basis, those differences were less pronounced. The microbiomes of C. gaboensis and Sinularia sp. were dominated by bacteria belonging to the genus Endozoicomonas, which were a key conserved feature of their core microbiomes. In contrast to previous studies, our analysis revealed that Endozoicomonas phylotypes are shared across different octocoral species, inhabiting different environments. Together, our data demonstrates that octocorals harbour a broad diversity of bacterial partners, some of which comprise 'core microbiomes' that potentially impart important functional roles to their hosts.

Gametogenic development and synchronous spawning of the acroporid corals Acropora cytherea and Acropora tenuis in the Red Sea

Sexual reproduction of corals is one of the essential processes for the stability and integrity of coral populations. Hence, this study was conducted to assess the reproductive and spawning patterns of two reef-building scleractinian corals belonging to genus Acropora, namely Acropora cytherea and Acropora tenuis inhabiting Hurghada, northern Red Sea. Histological and in situ examinations were carried out on 16 tagged colonies from the two coral species (8 for each species) in two selected shallow reef sites located in the north and south of Hurghada. Coral fragments were collected monthly from March 2018 to June 2018 and from October 2018 till May 2019 for dissection and histological analyses. The present results indicated that A. cytherea and A. tenuis were hermaphroditic broadcast spawners with a single annual gametogenic cycle (7 months) that began in October 2018 and terminated in April 2019, and both oogenesis and spermatogenesis were occurring for 7 and 6 months, respectively. Both species exhibited conspicuous spawning synchrony amongst colonies of the same species, and the time of spawning was mostly correlated with lunar cycle and photoperiod (day length). They spawned around the full moon in April 2018 and 2019. A. Cytherea spawned two days before the full moon (30 Apr) in April 2018 and one day before the full moon (19 Apr) in April 2019. A. tenuis spawned one day before the April full moon in 2018, while in 2019 it spawned in the full moon day (19 Apr) and one day after the full moon. This study provided information that will aid in future coral reef restoration and management. Further research on the reproduction of additional coral species is required in order to provide a better perspective of reproductive patterns in Red Sea scleractinian corals.

Reproductive traits and Microstructure of Acropora digitifera and Acropora gemmifera (Scleractinia, Anthozoa) inhabiting the Northern Red Sea (Hurghada, Egypt).

Reef-building scleractinian corals proliferate and maintain their populations, in part, through sexual reproduction. They typically reproduce sexually as either gonochoric (separate male and female) or hermaphroditic (producing both eggs and sperm) colonies. Both types of sexual reproduction achieved as spawning processes that take place once a year and this process affected by environmental conditions such as temperature, photoperiod and pH level. In relation, the current study was designed to explore the reproduction patterns of two Acroporoid coral species; Acropora digitifera and Acropora gemmifera which inhabiting the northern Red Sea (Hurghada, Egypt). A scanning electron microscope (SEM) was used to identify both coral species and to obtain their microstructure details. For reproduction study purpose, gonads maturity status and spawning timing of both studied species were noticed, either by direct field observations, during the study period including two consecutive spawning seasons, or observed in aquaria, by taking live coral colony samples for watching their specific spawning release. Results indicated that both investigated species are hermaphroditic broadcasting spawner (release buoyant egg-sperm bundles) and showed gradual gonads maturity development begin from October until their spawning time in late April. All polyps became empty from egg sperm bundles in early May. There was a clear relationship between the spawning time of the two coral species and the lunar cycle, temperature, and photoperiod. Spawning of study species occurred on nights (nearly 3-4 hours after sunset) during the new moon and full moon phases. The present study concluded that A. digitifera and A. gemmifera both spawned before April full moon within two days.

Integrated evidence reveals a new species in the ancient blue coral genus Heliopora (Octocorallia)

Maintaining the accretion potential and three dimensional structure of coral reefs is a priority but reef-building scleractinian corals are highly threatened and retreating. Hence future reefs are predicted to be dominated by non-constructional taxa. Since the Late Triassic however, other non-scleractinian anthozoans such as Heliopora have contributed to tropical and subtropical reef-building. Heliopora is an ancient and highly conserved reef building octocoral genus within the monospecific Family Helioporidae, represented by a single extant species – H. coerulea, Pallas, 1766. Here we show integrated morphological, genomic and reproductive evidence to substantiate the existence of a second species within the genus Heliopora. Importantly, some individuals of the new species herein described as Heliopora hiberniana sp. nov. feature a white skeleton indicating that the most diagnostic and conserved Heliopora character (the blue skeleton) can be displaced. The new species is currently known only from offshore areas in north Western Australia, which is a part of the world where coral bleaching events have severely impacted the scleractinian community over the last two decades. Field observations indicate individuals of both H. coerulea and H. hiberniana sp. nov. were intact after the 2016 Scott Reef thermal stress event, and we discuss the possibility that bleaching resistant non-scleractinian reef builders such as Heliopora could provide new ecological opportunities for the reconfiguration of future reefs by filling empty niches and functional roles left open by the regression of scleractinian corals.

The influence of different deep-sea coral habitats on sediment macrofaunal community structure and function.

Deep-sea corals can create a highly complex, three-dimensional structure that facilitates sediment accumulation and influences adjacent sediment environments through altered hydrodynamic regimes. Infaunal communities adjacent to different coral types, including reef-building scleractinian corals and individual colonies of octocorals, are known to exhibit higher macrofaunal densities and distinct community structure when compared to non-coral soft-sediment communities. However, the coral types have different morphologies, which may modify the adjacent sediment communities in discrete ways. Here we address: (1) how infaunal communities and their associated sediment geochemistry compare among deep-sea coral types (Lophelia pertusa, Madrepora oculata, and octocorals) and (2) do infaunal communities adjacent to coral habitats exhibit typical regional and depth-related patterns observed in the Gulf of Mexico (GOM). Sediment push cores were collected to assess diversity, composition, numerical abundance, and functional traits of macrofauna (>300 µm) across 450 kilometers in the GOM at depths ranging from 263–1,095 m. Macrofaunal density was highest in L. pertusa habitats, but similar between M. oculata and octocorals habitats. Density overall exhibited a unimodal relationship with depth, with maximum densities between 600 and 800 m. Diversity and evenness were highest in octocoral habitats; however, there was no relationship between diversity and depth. Infaunal assemblages and functional traits differed among coral habitats, with L. pertusa habitats the most distinct from both M. oculata and octocorals. These patterns could relate to differences in sediment geochemistry as L. pertusa habitats contained high organic carbon content but low proportions of mud compared to both M. oculata and octocoral habitats. Distance-based linear modeling revealed depth, mud content, and organic carbon as the primary factors in driving coral infaunal community structure, while geographic location (longitude) was the primary factor in functional trait composition, highlighting both the location and ecological differences of L. pertusa habitats from other coral habitats. Enhanced habitat structural complexity associated with L. pertusa and differences in localized hydrodynamic flow may contribute to the dissimilarities in the communities found among the coral types. Our results suggest a decoupling for infaunal coral communities from the typical depth-related density and diversity patterns present throughout soft-sediment habitats in the GOM, highlighting the importance of deep-sea corals in structuring unique communities in the nearby benthos.

Caribbean massive corals not recovering from repeated thermal stress events during 2005-2013.

Massive coral bleaching events associated with high sea surface temperatures are forecast to become more frequent and severe in the future due to climate change. Monitoring colony recovery from bleaching disturbances over multiyear time frames is important for improving predictions of future coral community changes. However, there are currently few multiyear studies describing long‐term outcomes for coral colonies following acute bleaching events. We recorded colony pigmentation and size for bleached and unbleached groups of co‐located conspecifics of three major reef‐building scleractinian corals (Orbicella franksi, Siderastrea siderea, and Stephanocoenia michelini; n = 198 total) in Bocas del Toro, Panama, during the major 2005 bleaching event and then monitored pigmentation status and changes live tissue colony size for 8 years (2005–2013). Corals that were bleached in 2005 demonstrated markedly different response trajectories compared to unbleached colony groups, with extensive live tissue loss for bleached corals of all species following bleaching, with mean live tissue losses per colony 9 months postbleaching of 26.2% (±5.4 SE) for O. franksi, 35.7% (±4.7 SE) for S. michelini, and 11.2% (±3.9 SE) for S. siderea. Two species, O. franksi and S. michelini, later recovered to net positive growth, which continued until a second thermal stress event in 2010. Following this event, all species again lost tissue, with previously unbleached colony species groups experiencing greater declines than conspecific sample groups, which were previously bleached, indicating a possible positive acclimative response. However, despite this beneficial effect for previously bleached corals, all groups experienced substantial net tissue loss between 2005 and 2013, indicating that many important Caribbean reef‐building corals will likely suffer continued tissue loss and may be unable to maintain current benthic coverage when faced with future thermal stress forecast for the region, even with potential benefits from bleaching‐related acclimation.

Lowered temperature and reduced salinity retarded development of early life history stages of Acropora valida from the marginal environment

To better understand the possible consequences of the co-occurrence of salinity and temperature stresses on reef building corals in subtropical marginal environment, laboratory based experiments were conducted on early embryonic developmental success of the newly fertilized eggs of Acropora valida from Hong Kong under different interactive temperature [24, 27 (ambient), 30 °C] and salinity [33 psu (ambient), 30, 28, 26 psu] treatments. Our results showed the effect of temperature to be comparatively stronger than that of salinity in affecting rate of embryonic development. There was a 4 h delay in embryonic development at lowered (24 °C) and slightly accelerated development at elevated (30 °C) temperatures. In addition, the interaction of temperature and salinity significantly affected the proportion of abnormal embryos. At 27 °C and 30 °C, the percentage of developmental abnormalities was generally low (<25 %) in salinities that ranged from 33 to 28 psu. A significant increase was observed at 26 psu treatment. Similar low level of abnormal embryonic development was observed at 24 °C except that increased abnormalities were already observed at a higher salinity of 28 psu. These results demonstrated that lowered temperature and reduced salinity (24 °C and 26 psu) have the potential to negatively impact the reproductive success of this coral species, and would ultimately lead to substantial decline in sexual recruitment. Such negative impact, however, could be alleviated under a warmer ocean, therefore allowing the persistence of this coral species in the long-term. These results are important for a better understanding of the possible consequences of climate change on reef building scleractinian corals in a subtropical marginal environment like Hong Kong.

Colony-specific investigations reveal highly variable responses among individual corals to ocean acidification and warming

As anthropogenic climate change is an ongoing concern, scientific investigations on its impacts on coral reefs are increasing. Although impacts of combined ocean acidification (OA) and temperature stress (T) on reef-building scleractinian corals have been studied at the genus, species and population levels, there are little data available on how individual corals respond to combined OA and anomalous temperatures. In this study, we exposed individual colonies of Acropora digitifera, Montipora digitata and Porites cylindrica to four pCO2-temperature treatments including 400 μatm-28 °C, 400 μatm-31 °C, 1000 μatm-28 °C and 1000 μatm-31 °C for 26 days. Physiological parameters including calcification, protein content, maximum photosynthetic efficiency, Symbiodinium density, and chlorophyll content along with Symbiodinium type of each colony were examined. Along with intercolonial responses, responses of individual colonies versus pooled data to the treatments were investigated. The main results were: 1) responses to either OA or T or their combination were different between individual colonies when considering physiological functions; 2) tolerance to either OA or T was not synonymous with tolerance to the other parameter; 3) tolerance to both OA and T did not necessarily lead to tolerance of OA and T combined (OAT) at the same time; 4) OAT had negative, positive or no impacts on physiological functions of coral colonies; and 5) pooled data were not representative of responses of all individual colonies. Indeed, the pooled data obscured actual responses of individual colonies or presented a response that was not observed in any individual. From the results of this study we recommend improving experimental designs of studies investigating physiological responses of corals to climate change by complementing them with colony-specific examinations.

Elevated temperature enhances normal early embryonic development in the coral Platygyra acuta under low salinity conditions

To better understand the possible consequences of climate change on reef building scleractinian corals in a marginal environment, laboratory experiments were conducted to examine the interactive effects of changes in salinity and temperature on percent fertilization success and early embryonic development of the coral Platygyra acuta. In the present study, a salinity of 24 psu (ambient 32 psu) reduced fertilization success by 60 %. Normal embryonic development was reduced by >80 % at 26 psu (ambient 33 psu) with 100 % abnormal development at 22 psu under ambient temperature. Elevated temperature (+3 °C) above the ambient spawning temperature did not show any negative effects on fertilization success. However, there was a trend for more abnormal embryos to develop at elevated temperature in the 2 d of the spawning event. The interactive effects between salinity and temperature are statistically significant only on normal embryonic development of P. acuta, but not on its fertilization success. Salinity was revealed to be the main factor affecting both fertilization success and normal embryonic development. Interestingly, the much lower fertilization success (76 %) observed in the second day of spawning (Trial 2) under ambient temperature recovered to 99 % success under elevated (+3 °C) temperature conditions. Moreover, elevated temperature enhanced normal early embryonic development under lowered salinity (26 psu). This antagonistic interactive effect was consistently observed in two successive nights of spawning. Overall, our results indicate that, in terms of its fertilization success and embryonic development, P. acuta is the most tolerant coral species to reduced salinity thus far reported in the literature. Elevated temperature, at least that within the tolerable range of the corals, could apparently alleviate the potential negative effects from salinity stresses. This mitigating role of elevated temperature appears not to have been reported on corals before.

Characterisation of coral explants: a model organism for cnidarian–dinoflagellate studies

Coral cell cultures made from reef-building scleractinian corals have the potential to aid in the pursuit of understanding of the cnidarian–dinoflagellate symbiosis. Various methods have previously been described for the production of cell cultures in vitro with a range of success and longevity. In this study, viable tissue spheroids containing host tissue and symbionts (coral explants) were grown from the tissues of Fungia granulosa. The cultured explants remained viable for over 2 months and showed morphological similarities in tissue structure and internal microenvironment to reef-building scleractinian corals. The photophysiology of the explants (1 week old) closely matched that of the parent coral F. granulosa. This study provides the first empirical basis for supporting the use of coral explants as laboratory models for studying coral symbioses. In particular, it highlights how these small, self-sustaining, skeleton-free models can be useful for a number of molecular, genetic and physiological analyses necessary for investigating host–symbiont interactions at the microscale.

Calcification responses of symbiotic and aposymbiotic corals to near-future levels of ocean acidification

Abstract. Increasing the acidity of ocean waters will directly threaten calcifying marine organisms such as reef-building scleractinian corals, and the myriad of species that rely on corals for protection and sustenance. Ocean pH has already decreased by around 0.1 pH units since the beginning of the industrial revolution, and is expected to decrease by another 0.2–0.4 pH units by 2100. This study mimicked the pre-industrial, present, and near-future levels of pCO2 using a precise control system (± 5% pCO2), to assess the impact of ocean acidification on the calcification of recently settled primary polyps of Acropora digitifera, both with and without symbionts, and adult fragments with symbionts. The increase in pCO2 of ~100 μatm between the pre-industrial period and the present had more effect on the calcification rate of adult A. digitifera than the anticipated future increases of several hundreds of micro-atmospheres of pCO2. The primary polyps with symbionts showed higher calcification rates than primary polyps without symbionts, suggesting that: (i) primary polyps housing symbionts are more tolerant to near-future ocean acidification than organisms without symbionts, and (ii) corals acquiring symbionts from the environment (i.e., broadcasting species) will be more vulnerable to ocean acidification than corals that maternally acquire symbionts.

Scleractinian coral cell proliferation is reduced in primary culture of suspended multicellular aggregates compared to polyps

Cell cultures from reef-building scleractinian corals are being developed to study the response of these ecologically important organisms to environmental stress and diseases. Despite the importance of cell division to support propagation, cell proliferation in polyps and in vitro is under-investigated. In this study, suspended multicellular aggregates (tissue balls) were obtained after collagenase dissociation of Pocillopora damicornis coral, with varying yields between enzyme types and brands. Ultrastructure and cell type distribution were characterized in the tissue balls (TBs) compared to the polyp. Morphological evidence of cellular metabolic activity in their ciliated cortex and autophagy in their central mass suggests involvement of active tissue reorganization processes. DNA synthesis was evaluated in the forming multicellular aggregates and in the four cell layers of the polyp, using BrdU labeling of nuclei over a 24h period. The distribution of BrdU-labeled coral cells was spatially heterogeneous and their proportion was very low in tissue balls (0.2±0.1%), indicating that suspended multicellular aggregate formation does not involve significant cell division. In polyps, DNA synthesis was significantly lower in the calicoderm (<1%) compared to both oral and aboral gastroderm (about 10%) and to the pseudostratified oral epithelium (15-25% at tip of tentacle). DNA synthesis in the endosymbiotic dinoflagellates dropped in the forming tissue balls (2.7±1.2%) compared to the polyp (14±3.4%) where it was not different from the host gastroderm (10.3±1.2%). A transient (24h) increase was observed in the cell-specific density of dinoflagellates in individually dissociated coral cell cultures. These results suggest disruption of coral cell proliferation processes upon establishment in primary culture.

Ubiquitous associations and a peak fall prevalence between apicomplexan symbionts and reef corals in Florida and the Bahamas

Although apicomplexans are a widely recog- nized and important parasitic group, little is known about those associated with invertebrates, such as reef-building scleractinian corals. To resolve the potential impact of api- complexans on coral health, it is first necessary to further describe this group of putative parasites and determine their prevalence among host species. Here, it was hypothesized that apicomplexan prevalence would vary seasonally, simi- lar to what occurs in other marine apicomplexans as well as some coral symbionts. To test this, Caribbean scleractinian species Porites astreoides, Montastraea (=Orbicella) annu- laris, M.( =O.) faveolata ,a ndSiderastrea siderea were sampled seasonally from two reefs each in the Florida Keys and the Bahamas for 9- and 5.5-year periods, respectively. Utilizing a PCR-based screening assay, apicomplexan DNA was detected from most Floridian (80.1 %: n = 555/693) and Bahamian (90.7 %: n = 311/343) coral tissue samples collected over these multi-year periods. Furthermore, api- complexan DNA was detected from nearly all (98.7 %: n = 78/79) single polyps sampled at multiple locations within six M. faveolata colonies, indicating little to no in- tracolonial variation in the screening assay. Mixed-model logistic regression was utilized to determine the effects of season, host species, and reef on apicomplexan prevalence. The model identified a significant seasonal effect, with the highest apicomplexan prevalence occurring during fall. There also was a large effect of host species, with apicom- plexan prevalence significantly lower among S. siderea colonies relative to the other species. While reef did not have a significant effect in the full model, there was a sig- nificant difference in apicomplexan prevalence between Floridian and Bahamian reefs for S. siderea, implying regional differences in this host species. Despite seasonal and species-specific differences in prevalence, apicomplex- ans are ubiquitous constituents of these particular sclerac- tinian coral species from Florida and the Bahamas.

Effects of delayed settlement on post-settlement growth and survival of scleractinian coral larvae

Demographic connectivity requires both the dispersal of individuals between sub-populations, and their subsequent contribution to population dynamics. For planktonic, non-feeding marine larvae, the capacity to delay settlement enables greater dispersal distances, but the energetic cost of delayed settlement has been shown to adversely impact post-settlement fitness in several taxa. Here, we assess whether delayed settlement influences mortality rates or growth rates for the first 6 weeks following settlement of the scleractinian coral, Acropora tenuis. Coral larvae that were settled at 2, 4, and 6 weeks after spawning, and then deployed in the field, showed negligible effects of delayed settlement on post-settlement survival and time to initial budding for colony formation. Between-cohort differences in budding rate appeared to be explained by temporal variation in the post-settlement acquisition of zooxanthellae. The potential for coral larvae to remain in the pelagic zone for increased periods of time with little to no effect on post-settlement survival and growth suggests that the capacity for delayed settlement is likely to have meaningful demographic consequences for broadcast-spawning reef-building corals, and that the predicted trade-off between delayed settlement and post-settlement fitness is less applicable to reef-building scleractinian corals than other taxa with non-feeding larvae.

Responses to High Seawater Temperatures in Zooxanthellate Octocorals

Increases in Sea Surface Temperatures (SSTs) as a result of global warming have caused reef-building scleractinian corals to bleach worldwide, a result of the loss of obligate endosymbiotic zooxanthellae. Since the 1980’s, bleaching severity and frequency has increased, in some cases causing mass mortality of corals. Earlier experiments have demonstrated that zooxanthellae in scleractinian corals from three families from the Great Barrier Reef, Australia (Faviidae, Poritidae, and Acroporidae) are more sensitive to heat stress than their hosts, exhibiting differential symptoms of programmed cell death – apoptosis and necrosis. Most zooxanthellar phylotypes are dying during expulsion upon release from the host. The host corals appear to be adapted or exapted to the heat increases. We attempt to determine whether this adaptation/exaptation occurs in octocorals by examining the heat-sensitivities of zooxanthellae and their host octocoral alcyonacean soft corals – Sarcophyton ehrenbergi (Alcyoniidae), Sinularia lochmodes (Alcyoniidae), and Xenia elongata (Xeniidae), species from two different families. The soft coral holobionts were subjected to experimental seawater temperatures of 28, 30, 32, 34, and 36°C for 48 hrs. Host and zooxanthellar cells were examined for viability, apoptosis, and necrosis (in hospite and expelled) using transmission electron microscopy (TEM), fluorescent microscopy (FM), and flow cytometry (FC). As experimental temperatures increased, zooxanthellae generally exhibited apoptotic and necrotic symptoms at lower temperatures than host cells and were expelled. Responses varied species-specifically. Soft coral hosts were adapted/exapted to higher seawater temperatures than their zooxanthellae. As with the scleractinians, the zooxanthellae appear to be the limiting factor for survival of the holobiont in the groups tested, in this region. These limits have now been shown to operate in six species within five families and two orders of the Cnidaria in the western Pacific. We hypothesize that this relationship may have taxonomic implications for other obligate zooxanthellate cnidarians subject to bleaching.

Scleractinian Corals and Reefs of Vietnam as a Part of the Pacific Reef Ecosystem

The paper analyzes both published and unpublished results of the investigations of Vietnamese reef building corals and reefs performed in the last decades of the twentieth century and first decades twenty-first. The state of the art in the study of reef-building scleractinian corals and reefs is presented. The scleractinian fauna of Vietnam is shown to match in species diversity (350 species of 80 genera) the tropical coral fauna of the Indonesian–Malacca fertile center, from which Indo-Pacific reef-building corals originated. The whole Vietnam coast from the Gulf of Tonkin to the Gulf of Siam is a biogeographically single whole and is a part of the Indo-Polynesian Province of the Indo-Pacific Area.

Gene expression profiles of cytosolic heat shock proteins Hsp70 and Hsp90 from symbiotic dinoflagellates in response to thermal stress: possible implications for coral bleaching

Unicellular photosynthetic dinoflagellates of the genus Symbiodinium are the most common endosymbionts of reef-building scleractinian corals, living in a symbiotic partnership known to be highly susceptible to environmental changes such as hyperthermic stress. In this study, we identified members of two major heat shock proteins (HSPs) families, Hsp70 and Hsp90, in Symbiodinium sp. (clade C) with full-length sequences that showed the highest similarity and evolutionary relationship with other known HSPs from dinoflagellate protists. Regulation of HSPs gene expression was examined in samples of the scleractinian coral Acropora millepora subjected to elevated temperatures progressively over 18h (fast) and 120h (gradual thermal stress). Moderate to severe heat stress at 26°C and 29°C (+3°C and +6°C above average sea temperature) resulted in an increase in algal Hsp70 gene expression from 39% to 57%, while extreme heat stress (+9°C) reduced Hsp70 transcript abundance by 60% (after 18h) and 70% (after 120h). Elevated temperatures decreased an Hsp90 expression under both rapid and gradual heat stress scenarios. Comparable Hsp70 and Hsp90 gene expression patterns were observed in Symbiodinium cultures and in hospite, indicating their independent regulation from the host. Differential gene expression profiles observed for Hsp70 and Hsp90 suggests diverse roles of these molecular chaperones during heat stress response. Reduced expression of the Hsp90 gene under heat stress can indicate a reduced role in inhibiting the heat shock transcription factor which may lead to activation of heat-inducible genes and heat acclimation.