Coral Surface Research Articles

Microplastic pollution in tropical coral reef ecosystems from the coastal South China Sea and their impacts on corals in situ

Coral reefs possess extremely high ecological value in tropical and subtropical waters worldwide. Microplastics as emerging and pervasive pollutants pose a great threat to the health of coral ecosystems. However, in situ studies on microplastics pollution and its impacts in coral ecosystems globally are limited. The occurrence characteristics of microplastics in the environment mediums and reef-dwelling organisms were investigated in coral reef areas from the southern Hainan Island, and the impacts of microplastics on corals in situ were evaluated in this study. Average microplastics abundance was 9.48 items L-1 in seawater, 190.00 items kg-1 in sediment, 0.36 items g-1 in coral, 1.50 items g-1 in shellfish, 0.48 items g-1 in fish gill, and 1.71 items g-1 in fish gastrointestinal tract. The prevalent microplastics in the above samples were characterized as being less than 1000 µm in size, fibrous, and transparent, with predominant polymer types as polyethylene terephthalate, polypropylene, polyethylene, and rayon. The microplastic enrichment capacity of different corals varied (Pocillopora >Acropora >Sinularia). Notably, microplastics were more abundant on the surface of corals compared to their interiors, with distinct characteristics observed, including larger-sized (>500 µm) and fiber-shaped polyethylene terephthalate microplastics on the surface and smaller-sized (20–200 µm) fragmented polyethylene microplastics within coral interiors. Furthermore, the investigation showed species-specific impacts of microplastics on corals in situ, including photosynthetic activity of photosymbionts and antioxidant and immune activities of corals. Furthermore, the ecological risks of microplastics were minor across most environmental media in the studied areas, with exceptions in the bottom seawater and surface sediment of YLW, which exhibited extreme and medium risk levels, respectively. Coral risk levels were generally medium, except for dangerous levels in DDH and high levels in LHT. The potential sources of microplastics in the marginal reefs of southern Hainan Island were primarily tourism, residential, and fishing activities.

Bioerosion and encrustation in Late Triassic reef corals from Iran

Hard substrate communities can impact coral reef growth by adding or removing calcium carbonate when they act as encrusters or bioeroders, respectively. Although such sclerobiont communities are known across the Phanerozoic, the Triassic saw a substantial increase in reef macrobioerosion. This study provides the first quantitative assessment of sclerobionts in a Late Triassic (Norian) reef coral community (from the Nayband Formation in central Iran) and establishes some post-Paleozoic trends in reef bioerosion. Sclerobionts were common on the dominant coral Pamiroseris rectilamellosa and covered between 0 and 26.4% of total coral surface area among the 145 colonies investigated. Encrustation was significantly more prevalent (1.78% of total area) than bioerosion (0.36% of total area). The underside of corals was 3.5 times more affected by sclerobionts than the upper surfaces. This suggests that the sclerobionts preferentially colonized dead parts of the corals. The main encrusting taxa were polychaete worms (76.2%), followed by bivalves (11.9%). The main bioeroders were Polychaeta (51.4%), Porifera (22.2%), and Bivalvia (20.7%). The lack of a correlation between encrustation and bioerosion intensity suggests that the number of sclerobionts is not simply a function of exposure time before final burial. Our review of the published literature suggests a declining trend in the relative importance of bivalves, polychaetes and cirripeds in reefs, whereas sponges increased in importance from the Triassic until today.

Variability of the surface boundary layer of reef-building coral species

The coral-seawater interface is an important, highly dynamic microenvironment for reef-building corals. Also known as the concentration boundary layer (CBL), it is a thin layer of seawater bordering the coral surface that dictates the biochemical exchange between the coral colony and bulk seawater. The CBL is thus a key feature that modulates coral metabolism. However, CBL variation among small-polyped coral species remains largely unknown. Therefore, we recorded over 100 profiles of dissolved O2 concentration using microsensors to characterize CBL traits (thickness, surface O2 concentration, and flux) of three small-polyped branching coral species, Acropora cytherea, Pocillopora verrucosa, and Porites cylindrica. Measurements were conducted during light and darkness combined with low or moderate water flow (2 and 6 cm s−1). We found that CBL traits differed among species. CBL thickness was lowest in A. cytherea, while P. verrucosa showed the largest depletion of surface O2 in dark and highest dark flux. In addition, we found that O2 concentration gradients in the CBL occurred with three main profile shapes: diffusive, S-shaped, and complex. While diffusive profiles were the most common profile type, S-shaped and complex profiles were more frequent in P. verrucosa and P. cylindrica, respectively, and prevailed under low flow. Furthermore, profile types differed in CBL thickness and flux. Finally, low flow thickened CBLs, enhanced changes in surface O2 concentration, and reduced flux, compared to moderate flow. Overall, our findings reveal CBL variability among small-polyped branching corals and help understand CBL dynamics in response to changes in light and water flow conditions.

Bioactivity responses to changes in mucus-associated bacterial composition between healthy and bleached Porites lobata corals

This study aims to investigate how bioactivities of the coral surface mucus layer (SML) respond to changes in mucus-associated bacterial communities between bleached and healthy Porites lobata corals in Nha Trang Bay, Vietnam. The findings suggested that significant shifts in the mucus-associated bacterial communities were related to changes in coral health states from bleached to healthy P. lobata colonies (p < 0.05), while bacterial compositions were not significantly different across seasons and locations (p > 0.05). Of which 8 genera, Shewanella, Fusibacter, Halodesulfovibrio, Marinifilum, Endozoicomonas, Litoribacillus, Algicola, and Vibrio were present only in the SML of bleached coral while absent in the SML of the healthy one. As compared with the bleached SML, the healthy SML demonstrated stronger antibacterial activity against a coral bleaching pathogen, V. coralliilyticus, higher antitumor activity against HCT116 cell accompanied with increased induction of cleaved PARP and accelerated cell nucleic apoptosis and cycle arrest at S and G2/M phases exhibiting several typical characteristics, cell shrinkage, lost cell contact, and apoptotic body formation. Moreover, putative compounds detected at 280 nm in the healthy SML were obviously higher than those in the bleached one, probably they could be bioactive molecules responsible for competitively exclusion of pathogens, Algicola and Vibrio, from the healthy SML.

Pre-Bleaching Coral Microbiome Is Enriched in Beneficial Taxa and Functions.

Coral reef health is tightly connected to the coral holobiont, which is the association between the coral animal and a diverse microbiome functioning as a unit. The coral holobiont depends on key services such as nitrogen and sulfur cycling mediated by the associated bacteria. However, these microbial services may be impaired in response to environmental changes, such as thermal stress. A perturbed microbiome may lead to coral bleaching and disease outbreaks, which have caused an unprecedented loss in coral cover worldwide, particularly correlated to a warming ocean. The response mechanisms of the coral holobiont under high temperatures are not completely understood, but the associated microbial community is a potential source of acquired heat-tolerance. Here we investigate the effects of increased temperature on the taxonomic and functional profiles of coral surface mucous layer (SML) microbiomes in relationship to coral-algal physiology. We used shotgun metagenomics in an experimental setting to understand the dynamics of microbial taxa and genes in the SML microbiome of the coral Pseudodiploria strigosa under heat treatment. The metagenomes of corals exposed to heat showed high similarity at the level of bacterial genera and functional genes related to nitrogen and sulfur metabolism and stress response. The coral SML microbiome responded to heat with an increase in the relative abundance of taxa with probiotic potential, and functional genes for nitrogen and sulfur acquisition. Coral-algal physiology significantly explained the variation in the microbiome at taxonomic and functional levels. These consistent and specific microbial taxa and gene functions that significantly increased in proportional abundance in corals exposed to heat are potentially beneficial to coral health and thermal resistance.

Niche-dependent sponge hologenome expression profiles and the host-microbes interplay: a case of the hawaiian demosponge Mycale Grandis.

Most researches on sponge holobionts focus primarily on symbiotic microbes, yet data at the level of the sponge hologenome are still relatively scarce. Understanding of the sponge host and its microbial gene expression profiles and the host-microbes interplay in different niches represents a key aspect of sponge hologenome. Using the Hawaiian demosponge Mycale grandis in different niches as a model, i.e. on rocks, on the surface of coral Porites compressa, under alga Gracilaria salicornia, we compared the bacterial and fungal community structure, functional gene diversity, expression pattern and the host transcriptome by integrating open-format (deep sequencing) and closed-format (GeoChip microarray) high-throughput techniques. Little inter-niche variation in bacterial and fungal phylogenetic diversity was detected for M. grandis in different niches, but a clear niche-dependent variability in the functional gene diversity and expression pattern of M. grandis host and its symbiotic microbiota was uncovered by GeoChip microarray and transcriptome analyses. Particularly, sponge host genes related to innate immunity and microbial recognition showed a strong correlation with the microbial symbionts' functional gene diversity and transcriptional richness in different niches. The cross-niche variability with respect to the symbiont functional gene diversity and the transcriptional richness of M. grandis holobiont putatively reflects the interplay of niche-specific selective pressure and the symbiont functional diversity. Niche-dependent gene expression profiles of M. grandis hologenome and the host-microbes interplay were suggested though little inter-niche variation in bacterial and fungal diversity was detected, particularly the sponge innate immunity was found to be closely related to the symbiotic microbes. Altogether, these findings provide novel insights into the black box of one sponge holobiont in different niches at the hologenome level.

The effect of coral colony morphology, coral surface condition, particle size, and seeding point on the trapping and deposition of microplastics

Coral reefs are increasingly identified as microplastic sinks. Understanding the trapping and deposition effects on microplastics among coral colonies of different morphologies can help identify which corals and coral reefs are at higher risk of microplastic exposure. Here, we used a current-generating saltwater flume to explore microplastic trapping and deposition among branching coral, Pocillopora acuta, colonies with contrasting morphologies (open and compact), together with varying coral surface conditions (live, dead, and waxed), microplastic sizes (400 to 500 μm and 900 to 1000 μm), and seeding points (above-colony and mid-colony). Results revealed that more microplastics were trapped by, and deposited nearer to, compact colonies compared to those with a more open morphology—likely due to differences in flow dynamics. More of the larger microplastics were trapped, as were those introduced at the mid seeding point, but coral surface condition had no significant effect. These findings add to the growing evidence that corals are effective at trapping and facilitating deposition of microplastics. Branching corals with compact structures are potentially at high risk of microplastic pollution impact. We posit that coral composition, i.e. the relative abundance of compact branching colonies, will affect microplastic accumulation in natural reef environments. SynopsisThis study demonstrates the effects of coral morphology on microplastic trapping and deposition, providing mechanistic insights into the factors that contribute to coral reefs acting as microplastic sinks.

Effects of bleaching on oxygen dynamics and energy metabolism of two Caribbean coral species

As mass coral bleaching events become more frequent, it is increasingly important to elucidate the factors underlying coral susceptibility and survival. We measured photosynthesis, respiration, and O2 concentration at the coral tissue surface, Symbiodiniaceae genotypes, and energy metabolic enzyme activities in Agaricia agaricites and Orbicella franksi throughout experimentally-induced thermal bleaching (+3 °C). A. agaricites colonies started to bleach two days into the thermal treatment and were fully bleached between Days 19–31. In contrast, O. franksi colonies only started to bleach on Day 12 and five colonies fully bleached between Days 24–38 while the remining three colonies took up 55 days. Both species experienced decreased photosynthesis and respiration rates as bleaching progressed. As a result, daytime O2 concentration at the coral surface shifted from hyperoxia in unbleached corals to normoxia in partially bleached corals, and to near hypoxia in fully bleached corals. Additionally, nighttime tissue surface O2 concentration shifted from hypoxia to normoxia, likely resulting from decreased symbiotic algae density, respiration, and photosynthates that fuel coral aerobic respiration. Genetic profiling of internal transcribed spacer 2 (ITS2) revealed differences in Symbiodiniaceae clade proportions between control and bleached colonies. Activity levels of energy metabolic enzymes did not significantly vary between control and bleached A. agaricites, but malate dehydrogenase and strombine dehydrogenase activities were significantly higher in bleached O. franksi colonies compared to controls. These differences were driven by the three O. franksi colonies that took the longest to bleach and contained >98 % Durusdinium sp. D1. The shifts in O2 dynamics within the microhabitat of bleached corals may have important implications for the metabolism of the coral holobiont while the changes in Symbiodiniaceae ITS2 profile and the upregulation of energy metabolic enzymes identify a potential factor contributing to bleaching dynamics.

Effects of surface geometry on light exposure, photoacclimation and photosynthetic energy acquisition in zooxanthellate corals.

Symbiotic corals display a great array of morphologies, each of which has unique effects on light interception and the photosynthetic performance of in hospite zooxanthellae. Changes in light availability elicit photoacclimation responses to optimize the energy balances in primary producers, extensively documented for corals exposed to contrasting light regimes along depth gradients. Yet, response variation driven by coral colony geometry and its energetic implications on colonies with contrasting morphologies remain largely unknown. In this study, we assessed the effect of the inclination angle of coral surface on light availability, short- and long-term photoacclimation responses, and potential photosynthetic usable energy. Increasing surface inclination angle resulted in an order of magnitude reduction of light availability, following a linear relationship explained by the cosine law and relative changes in the direct and diffuse components of irradiance. The light gradient induced by surface geometry triggered photoacclimation responses comparable to those observed along depth gradients: changes in the quantum yield of photosystem II, photosynthetic parameters, and optical properties and pigmentation of the coral tissue. Differences in light availability and photoacclimation driven by surface inclination led to contrasting energetic performance. Horizontally and vertically oriented coral surfaces experienced the largest reductions in photosynthetic usable energy as a result of excessive irradiance and light-limiting conditions, respectively. This pattern is predicted to change with depth or local water optical properties. Our study concludes that colony geometry plays an essential role in shaping the energy balance and determining the light niche of zooxanthellate corals.

Escaping the benthos with Coral Reef Arks: effects on coral translocation and fish biomass.

Anthropogenic stressors like overfishing, land based runoff, and increasing temperatures cause the degradation of coral reefs, leading to the loss of corals and other calcifiers, increases in competitive fleshy algae, and increases in microbial pathogen abundance and hypoxia. To test the hypothesis that corals would be healthier by moving them off the benthos, a common garden experiment was conducted in which corals were translocated to midwater geodesic spheres (hereafter called Coral Reef Arks or Arks). Coral fragments translocated to the Arks survived significantly longer than equivalent coral fragments translocated to Control sites (i.e., benthos at the same depth). Over time, average living coral surface area and volume were higher on the Arks than the Control sites. The abundance and biomass of fish were also generally higher on the Arks compared to the Control sites, with more piscivorous fish on the Arks. The addition of Autonomous Reef Monitoring Structures (ARMS), which served as habitat for sessile and motile reef-associated organisms, also generally significantly increased fish associated with the Arks. Overall, the Arks increased translocated coral survivorship and growth, and exhibited knock-on effects such as higher fish abundance.

Microbial source tracking of fecal pollution to coral reef lagoons of Norfolk Island, Australia

Fecal pollution contributes to global degradation of water quality and requires identification of the source(s) for predicting human health risk, tracking disease, and developing management strategies. While fecal indicator bacteria are commonly used to detect fecal pollution, they cannot identify sources. Novel approaches, such as microbial source tracking (MST), can be applied to evaluate the origin of fecal pollution. This study examined fecal pollution in the coral reef lagoons of Norfolk Island, Australia where reef health decline has been related to nutrient input. The primary objective of this study was to evaluate the host sensitivity and specificity of two human wastewater-associated marker genes (Bacteroides HF183 (HF183) and cross-assembly phage (crAssphage)) and four animal feces associated marker genes targeting avian, ruminant, dog, and pig (Helicobacter-associated GFD (GFD), Bacteroides BacR (BacR), Bacteroides DogBact (DogBact), and Bacteroides Pig-2-Bac (Pig-2-Bac)) in wastewater and animal fecal samples collected from Norfolk Island. The prevalence and concentrations of these marker genes along with enterococci genetic marker (ENT 23S rRNA) of general fecal pollution and human adenovirus (HAdV), which is considered predominantly a pathogen but also a human-wastewater associated marker gene, were determined in surface, ground, and marine water resources. A secondary objective of this study was to assess the sources and pathways of fecal pollution to a sensitive marine environment under rainfall events. HF183, crAssphage, HAdV, and BacR demonstrated absolute host sensitivity values of 1.00, while GFD and Pig-2-Bac had host sensitivity values of 0.60, and 0.20, respectively. Host specificity values were > 0.94 for all marker genes. Human and animal (avian, ruminant, dog) fecal sources were present in the coral reef lagoons and surface water whereas groundwater was polluted by human wastewater markers. This study provides understanding of fecal pollution in water resources on Norfolk Island, Australia after precipitation events. The results may aid in effective water quality management, mitigating potential adverse effects on both human and environmental health.

Possible beneficial interactions of ciliated protozoans with coral health and resilience.

Microbial interactions contribute significantly to coral health in the marine environment. Most beneficial associations have been described with their bacterial communities, but knowledge of beneficial associations between protozoan ciliates and corals is still lacking. Ciliates are important bacterial predators and provide nutrition to higher trophic-level organisms. The mucus secreted by corals and the microenvironment of the coral surface layer attract ciliates based on their food preferences. The mixotrophic and heterotrophic ciliates play a major role in nutrient cycling by increasing nitrogen, phosphorus, and extractable sulfur, which can enhance the proliferation of coral beneficial microbe. Besides, bacterial predator ciliates reduce the pathogenic bacterial population that infects the coral and also act as bioindicators for assessing the toxicity of the reef ecosystem. Thus, these ciliates can be used as a beneficial partner in influencing coral health and resilience under various stress conditions. Herein, we explore the urgent need to understand the complex beneficial interactions of ciliates that may occur in the coral reef ecosystem.

Constructing highly stable and biomimetic supercapacitor materials of 2D MoS2 coupled with NiCoLDH @chitosan carbon microsphere

Due to the growing demand for clean and renewable energy, the effective utilization of renewable resources in energy storage and has been concerned. And high specific capacitance and stable electrochemistry for energy storage devices are challenging to develop. Learning from unique structure of goniastrea coral, we design to construct three-dimensional layered conductive electrodes material made of MoS2@chitosan carbon spheres (MNCCS) composite. The NiCoLDH nanosheets construct on the surface of conductive chitosan carbon spheres to effectively form open permeable channels and rich conductive networks, improves the efficient energy storage and electrochemical stability. MoS2 is strongly coupled with NiCoLDH nanosheets through heterointerface, which resembles tentacles on the surface of coral, exposing more active sites to efficiently trap ions. Besides, the obtained material possesses favorable heterojunction interface and defects, which provide sufficient available electroactive sites and numerous storage charge area. Hence, The MNCCS presents an outstanding specific capacitance of 2605.9 F g−1 at 1 A g−1. Notably, a supercapacitor fitted with MNCCS electrode and CCS electrode achieves high specific capacitance (234.4 F g−1 at 1 A g−1), the maximum energy density of 77.0W h kg−1 at highest power density of 667.4W kg−1, along with the excellent cycling stability of 90.8% retention over 10,000 cycles. This work provides a new perspective for fabricating high-performance and stable supercapacitors electrode material for clean energy storage.

Distinct patterns of coral lesion composition from national/quasi-national marine parks of Kerama and Yoron Islands, Ryukyu Archipelago

A wide variety of coral communities are found in the Ryukyu Archipelago in southern Japan. Although reports of coral diseases and lesions in this region have increased since the 2000's, the variations of coral diseases among islands are somewhat limited. In this study, we conducted surveys to reveal the coral lesion assemblage and patterns in the marine national park area of Kerama Island and the quasi-national park on Yoron Island. Both sites are located 150 km away apart in the Ryukyu Archipelago. In total, 12 gross lesions were found in 30 coral genera. Corals in Yoron Island had 22% greater gross lesion abundance than Kerama Island (13.40 ± 1.36%) with an overall mean lesion abundance of 16.83 ± 1.66% (mean ± SE). Each Island shows a distinct pattern and group of coral lesion compositions. The disease lesion group of 7.21 ± 1.37% was dominated by White Syndrome (WS) and Black Band Disease (BBD), which severely infected various coral genera in Yoron Island. Corals in Kerama Island were predominantly affected by non-disease group lesion (9.76 ± 1.33%), such as Chalinula nematifera (CN) sponge overgrowing on the coral surface. Corals in the genus Pachyseris were highly susceptible (17.79%) to the diseases on Yoron Island, while Acroporid corals were the most susceptible (8.32%) to the diseases on Kerama Island. In addition, Porites were affected by disease and non-disease lesions from all sites in both Kerama and Yoron. These findings highlight the emergence of potential threats to coral communities in these national/quasi-national marine parks in Kerama and Yoron Island, which may be a factor limiting the survival and growth of key Scleractinian coral taxa. In addition, immediate consideration of environmental monitoring may be required to prevent coral disease outbreaks in the future.

Host specificity and adaptive evolution in settlement behaviour of coral-associated barnacle larvae (Cirripedia: Pyrgomatidae)

Coral-associated organisms often exhibit a continuum of host specificities. We do not know whether the variation in host specificity is related to the settlement organs or preferential settlement behaviours of the larvae. We examined the morphology of attachment discs, the settlement and metamorphosis of coral barnacles—Pyrgoma cancellatum (lives in a single coral species), Nobia grandis (two families of corals), and Armatobalanus allium (six families of corals). Our results revealed that the attachment organ of all three species are spear-shaped with sparse villi, indicating that the morphology of the attachment organs does not vary among species with different host specificities. Larvae of P. cancellatum and N. grandis only settle on their specific hosts, suggesting that chemical cues are involved in the settlement. Cyprids of N. grandis display close searching behaviour before settlement. Cyprids of P. cancellatum settle immediately on their specific host corals, without any exploratory behaviour. The host specificity and exploratory behaviours of coral barnacle cyprids are results of adaptive evolution. We argue that there is a trade-off between exploration and energy conservation for metamorphosis processes. Coral barnacle metamorphosis is longer when compared to free-living species, likely because it involves the development of a tube-shaped base on the coral surface.

Open science resources from the Tara Pacific expedition across coral reef and surface ocean ecosystems

The Tara Pacific expedition (2016–2018) sampled coral ecosystems around 32 islands in the Pacific Ocean and the ocean surface waters at 249 locations, resulting in the collection of nearly 58 000 samples. The expedition was designed to systematically study warm-water coral reefs and included the collection of corals, fish, plankton, and seawater samples for advanced biogeochemical, molecular, and imaging analysis. Here we provide a complete description of the sampling methodology, and we explain how to explore and access the different datasets generated by the expedition. Environmental context data were obtained from taxonomic registries, gazetteers, almanacs, climatologies, operational biogeochemical models, and satellite observations. The quality of the different environmental measures has been validated not only by various quality control steps, but also through a global analysis allowing the comparison with known environmental large-scale structures. Such publicly released datasets open the perspective to address a wide range of scientific questions.

Cold-water coral framework architecture is selectively shaped by bottom current flow

The three-dimensional (3D) structure of habitat-forming corals has profound impacts on reef ecosystem processes. Elucidating coral structural responses to the environment is therefore crucial to understand changes in these ecosystems. However, little is known of how environmental factors shape coral structure in deep and dark waters, where cold-water coral (CWC) reefs thrive. Here, we attempt to infer the influence of current flow on CWC framework architecture, using 3D scanning to quantify colony shape traits (volume compactness and surface complexity) in the reef-building CWC Desmophyllum pertusum from adjacent fjord and offshore habitats with contrasting flow regimes. We find substantial architectural variability both between and within habitats. We show that corals are generally more compact in the fjord habitat, reflecting the prevailing higher current speeds, although differences in volume compactness between fjord and offshore corals are more subtle when comparing the fjord with the more exposed side of the offshore setting, probably due to locally intensified currents. Conversely, we observe no clear disparity in coral surface complexity between habitats (despite its positive correlation with volume compactness), suggesting it is not affected by current speed. Unlike volume compactness, surface complexity is similarly variable within a single colony as it is between colonies within the same habitat or between habitats and is therefore perhaps more dependent than volume compactness on microenvironmental conditions. These findings suggest a highly plastic, trait-specific and functionally relevant structural response of CWCs to current flow and underscore the importance of multiple concurrent sources of hydrodynamic forcing on CWC growth.

Distribution, dietary preference and larval settlement preference of three scleractinain-coral-eating nudibranchs Phestilla spp. (Nudibranchia: Trinchesiidae) from Hong Kong waters

Phestilla is a small genus of scleractinain-coral-eating nudibranchs that comprises 11 valid species distributed throughout the Indo-Pacific region. We conducted a qualitative survey in Hong Kong waters and found Phestilla fuscostriata, P. goniophaga and P. melanobrachia in low densities. Phestilla fuscostriata and P. goniophaga mainly occurred in the northeast to east region while P. melanobrachia mainly occurred in the east to southeast region. They exhibit different levels of host specificity, with Phestilla fuscostriata associated with Pavona decussata, P. goniophaga associated with Goniopora spp., and P. melanobrachia associated with dendrophylliids. Laboratory experiments showed that they fed on their host corals only and were killed when placed on the surface of non-host corals. The feeding rate was size-dependent, with the bigger P. goniophaga and P. melanobrachia on average removed 1.44–8.33 polyps/day/individual and 0.10–2.75​ polyps/day/individual from the coral respectively, and the smaller P. fuscostriata removed only 0.031polyp/day/individual from the coral. Chemical cues released from the hosts had significant inducing effect on the larval metamorphosis of P. goniophaga and P. melanobrachia, but in P. fuscostriata such effect was insignificant. Their larvae were able to disperse to location nearby Hong Kong waters due a shorter period of 1–3 days required for metamorphic competence.

Epipsammic Diatom &lt;I&gt;Cocconesis&lt;/I&gt; sp. as New Bioeroder in Scleractinian Coral

Highlight Research The endolithic bioeroders were identified. Cocconeis sp. was new for the science of euendolithic bioeroder of Scleractinian coral. SEM revealed Cocconeis sp. etch and bore of coral skeleton. Pennate diatom was present on coral cavities. Abstract Laminar coral, Montipora, contributes to the coral reef ecosystem. The laminar life form is usually used by juvenile reef biota to shelter and prey. In an intertidal area, such as Pantai Kondang Merak, these corals are susceptible to erosion caused by mechanical and biological forces. Strong current or other anthropogenic activity may break coral colonies into pieces. Also, some grazers from reef fish and bioeroder potentially weaken coral structures. This study aimed to find the effect of biological agents from Bacillariophyceae, such as Cocconeis diatom, on the bioerosion process in laminar coral, e.g., Montipora. Ten montiporid corals from Pantai Kondang Merak were observed to find bio-eroding activities. Each coral colony was divided into 12 parts and photographed to record signs of bio-erosion on coral surface. While observing microborers, a 2x2 cm of the coral surface was observed using Scanning Electron Microscopy (SEM). SEM revealed that a frustule of Cocconeis sp. was found inside aragonite laminar coral from Pantai Kondang Merak. Cocconeis naturally grows on the coasts of tropical oceans as benthic organisms. They are reported as living attached to many substrates’ surfaces and within the substrate as euendolith. This study revealed that Cocconeis sp. lived inside the coral skeleton as micro bioeroder by attaching their valve in coral aragonite. Further work needs more observations of another potential euendolith diatom living inside the coral and to build new information on their mechanism of bio-eroding process in more detail.

Non-invasive estimation of coral polyp volume and surface area using optical coherence tomography

The surface area (SA) and three-dimensional (3D) morphology of reef-building corals are central to their physiology. A challenge for the estimation of coral SA has been to meet the required spatial resolution as well as the capability to preserve the soft tissue in its native state during measurements. Optical Coherence Tomography (OCT) has been used to quantify the 3D microstructure of coral tissues and skeletons with nearly micron-scale resolution. Here, we develop a non-invasive method to quantify surface area and volume of single coral polyps. A coral fragment with several coral polyps as well as calibration targets of known areal extent are scanned with an OCT system. This produces a 3D matrix of optical backscatter that is analyzed with computer algorithms to detect refractive index mismatches between physical boundaries between the coral and the immersed water. The algorithms make use of a normalization of the depth dependent scatter intensity and signal attenuation as well as region filling to depict the interface between the coral soft tissue and the water. Feasibility of results is judged by inspection as well as by applying algorithms to hard spheres and fish eggs whose volume and SA can be estimated analytically. The method produces surface area estimates in calibrated targets that are consistent with analytic estimates within 93%. The appearance of the coral polyp surfaces is consistent with visual inspection that permits standard programs to visualize both point clouds and 3-D meshes. The method produces the 3-D definition of coral tissue and skeleton at a resolution close to 10 µm, enabling robust quantification of polyp volume to surface area ratios.