We observed an increase in crude mineral content in Acropora intermedia embryos during the planula-larval stage, suggesting that mineralization may start before settlement. Samples from different developmental stages, including eggs, planulae, and metamorphosed polyps, were analyzed for crude ash content using a muffle furnace. Our results indicate significant differences between stages, supporting the possibility of early mineralization, aligning with findings of Akiva et al. (2018) on larval-stage mineralization in corals.

The scleractinian family Astrocoeniidae includes three genera. Palauastrea and Stylocoeniella are widely distributed throughout the Indo-Pacific region, while species of the genus Stephanocoenia (star corals) are found only in the Atlantic. Although Astrocoeniidae is considered to be phylogenetically most closely related to Pocilloporidae, their molecular phylogenetic position remains unclear. Here we report complete mitochondrial genomes of Palaustrea ramosa and Stylocoeniella guentheri. We also compared mitochondrial genomic structure between Astrocoeniidae (P. ramosa and S. guentheri) and Pocilloporidae. This study provides evidence of the phylogenetic structure and taxonomic relationships within Astrocoeniidae.

Coral reefs possess the highest biodiversity of all marine ecosystems and zooxanthellate scleractinian corals, the keystone organisms of these reefs, are in crisis due to climate change and anthropogenic activities. Future reef conservation requires comprehensive understanding of the present status of scleractinian taxa in each region. Environmental DNA metabarcoding (eDNA-M) is a method to meet such requirements. Still, it requires optimized primers for PCR amplification of eDNA and complete genomic sequence information for bioinformatic analyses. Coral reefs of Japan reportedly host 85 scleractinian genera. Our previous study developed a primer set that can be used to amplify scleractinian mitochondrial 12S rDNA for eDNA-M analysis. However, at present, the NCBI nucleotide database contains only ~60 genera with available 12S rDNA sequences, indicating that nearly 25 genera that should be detected by this system have no sequence information. To overcome this problem and to establish a nearly complete eDNA-M system for generic level detection of Japanese scleractinians, we collected 22 scleractinian genera and sequenced their mitochondrial genomes. In addition, species of another 12 genera were re-sequenced to avoid sequence differences caused by geographic variation. Incorporation of these data into a newly constructed informatic pipeline resulted in an eDNA-M system that can detect 83 of the 85 genera. This provides a tool for comprehensive, generic level detection of scleractinian corals in Japanese waters.

Reactive oxygen species (ROS) are thought to be involved in the mechanism of coral bleaching caused by high water temperature. Corals able to acquire nutriments through heterotrophy are more resilient to bleaching. However, the detailed mechanism for the alleviation of coral bleaching is not known. Here, we investigated the effects of feeding on ROS production in the coral Galaxea fascicularis to evaluate its potential as a mitigation technique of coral bleaching. The culture experiment was conducted with G. fascicularis in a 2×2 factorial design with and without food, ambient water temperature (27°C), and high water temperature (32°C). Artemia salina was used as a food source. The results showed that SOD enzyme activity in coral and zooxanthellae increased significantly at higher water temperatures and feeding led to a decrease of this activity. This indicates that feeding can reduce ROS production, especially in the host tissue, and potentially lessen cellular damage during heat stress. However, bleaching was not completely alleviated by feeding with reduced zooxanthellae density, chlorophyll-a and zooxanthellae protein contents observed under high water temperatures. Moreover the addition of food also led to lower calcification rates, regardless of temperature. These findings highlight the potential of using food addition as a mitigation technique of coral bleaching but this will require further investigation on the long term effects of feeding to better understand the limitations of the technique.

We collected bubbles from the seafloor of coral reef areas to determine their quantity and chemical composition. The chemical composition was high in oxygen and low in carbon dioxide compared to that of the atmosphere. Oxygen fluxes to the atmosphere from coral reef waters are generally determined considering only dissolved oxygen in seawater, but accounting for bubbled oxygen increased such fluxes by about 70%. At the same time, the primary production rate in coral reefs is estimated only from the dissolved oxygen concentration in seawater, but accounting for bubbled oxygen increased it by about 20–30%. The composition of bubbled gases suggests that approximately 20% of the oxygen present may be derived from photosynthesis in the reef area. Global quantitative estimation of bubbled oxygen fluxes in tropical and subtropical coral reef waters remains critical for future research.

The major parameters of the oceanic carbonate system include pH, total dissolved inorganic carbon (C_T), total alkalinity, and fugacity of CO₂. These parameters are important for measuring the effect of CO₂ on the earth. This study developed a measurement system based on a nondispersive infrared method that can determine the C_T with high precision using an extremely small sample. This is an improvement over previously developed system. The gas flow path and flow rate, opening/closing timing of the valve that switches the flow path, purge method, and C_T calculation method were improved. The measurement only required a sample volume of 300 μL. A repeat analysis of the certified reference material (n = 8) showed that the relative standard deviation was ±0.85%. This study provides a highly accurate and precise analytical method for measuring carbonate species in coral reefs and mesocosm cultures.

During the continuous monitoring of pH in environmental water, the temperature of the environmental water changes at each moment. Coral reefs are likely to be strongly affected by ocean acidification, so accurate and continuous pH monitoring is important. On the other hands, the water temperature in coral reef areas varies greatly not only due to solar radiation but also due to the mixing of open ocean water and land water. This temperature variation affects the accurate pH measurement. Preparing calibration values for the pH electrodes at all water temperatures is impossible, but the monitoring accuracy could be improved by predicting the constituent values at the on-site water temperature from the electrode calibration results at one or two temperatures. Therefore, we calibrate the pH at one or two temperatures and use the result for estimating the calibration value at another temperature. For this purpose, we estimated the pH change of the internal liquid of the electrode (assumed as a phosphate buffer) under saturated and unsaturated conditions of the coexisting KCl in the buffer.

To investigate the digestive response of corals, we detected acid phosphatase (ACPase) activity in coral tissues using Gomori's staining method. ACPase activity signals were observed under non-feeding conditions, and their intensity was reduced following antibiotic treatment. These findings suggest that ACPase activity also reflect bacterial predation by corals, indicating that antibiotic exposure is essential for detecting the digestive activity associated with a specific food source. Corals fed with Artemia exhibited such signals with pre-exposure to antibiotics. However, regardless of feeding and pre-exposure to antibiotics, the ACPase activity was strongly detected in planula larvae. Transmission electron microscopy revealed that the ACPase-positive granules were localized in lysosomes and phagolysosomes, consistent with observations in mammals. The method detecting the digestive response of corals is expected to elucidate the process in feeding response as well as in the coral-algal symbiotic process.

Compounds 1 and 2 were isolated from soft corals of the genus Lemnalia on Ishigaki Island, Okinawa Prefecture. Analysis of the spectral data obtained from nuclear magnetic resonance (NMR) analysis and comparison of the optical rotations with those in previous literature revealed that compound 2 was lemnalol, which was previously isolated from L. tenuis on Ishigaki Island. Compound 1 was identified as lemnalol acetate, in which the OH at the 4-position of lemnalol was acetylated. Compound 1 was previously synthesized artificially; however, this is the first report as a natural product. Compounds 1 and 2 exhibited moderate cytotoxicity against S1T cells, which are rampant adult T-cell leukemia cells in the Kyushu and Okinawa regions.

Boron stable isotope ratios in biogenic calcium carbonate minerals are known to reflect the decreasing of seawater pH, and thus they can be a useful tracer to track the trend of the ocean acidification. While validation of boron isotopes as a tracer for seawater pH has mainly focused on foraminiferal and coral CaCO₃, a few studies examined CaCO₃ skeletons produced by sclerosponges. In this study, we investigated stable boron and carbon isotope ratios in two sclerosponge specimens (Acanthochaetetes wellsi), collected from Okinoerabu Island, Japan. Carbon isotope ratios in both specimens showed a continuous decrease over the estimated growth periods, indicating that the Suess effect is recorded in sclerosponge skeletons. In contrast, boron isotope ratios in one specimen decreased over time, but not in the other. These findings suggest further analysis of additional specimens is necessary to determine whether boron isotope ratios in sclerosponge skeletons are reliable recorder of ocean acidification.

The molar magnesium (Mg)/calcium (Ca) ratio (mMg/Ca) of seawater has fluctuated throughout the Phanerozoic (541 Ma to the present), with three episodes favoring aragonite/high-Mg calcite formation (mMg/Ca>2) and two episodes favoring calcite formation (mMg/Ca<2), commonly referred to as aragonite and calcite seas, respectively. Although modern coral skeletons are generally composed of aragonite, seawater mMg/Ca fluctuation can lead to the formation of calcite skeletons under low mMg/Ca conditions. Temperature-dependent formation of aragonite and calcite by scleractinian corals in low-mMg/Ca seawater has also been documented, with corals producing more aragonite than in equivalent inorganic CaCO₃ precipitation experiments. Thus, aragonitic scleractinian corals appear to be capable of biologically controlling their calcification under low-mMg/Ca conditions, regardless of temperature. Large-scale transcriptomic and proteomic studies have revealed significant expression of organic matrix and extracellular matrix-like proteins during the early stages of coral skeletal development under low-mMg/Ca conditions. Notably, downregulated genes associated with the organic matrix of aragonite skeletons were vastly outnumbered by upregulated genes, suggesting that corals actively initiate aragonite skeleton construction via the skeletal organic matrix under low-mMg/Ca conditions. However, coral growth rates significantly decreased at low mMg/Ca levels under all tested temperature conditions. Despite their potential to produce calcite skeletons, scleractinian corals did not become the dominant reef builders in calcite seas; instead, rudist bivalves remained the primary carbonate producers until the end of the Cretaceous.

In the late 1990s, a debate emerged over whether coral reefs are net sources or sinks for CO₂. The prevailing consensus held that coral reefs, through calcification, act as short-term sources of CO₂. However, Kayanne et al. (1995) challenged this view by presenting observations that the Shiraho Reef on Ishigaki Island was, in fact, absorbing CO₂. Despite further surveys, research, and discussions—both directly and indirectly related to the debate—the consensus that coral reefs are CO₂ sources remained in place, and by the 2000s, the debate was considered settled. Nonetheless, the discussions significantly advanced our understanding of the relationship between coral reef community metabolism and the marine carbon cycle. Additionally, the measurement techniques and models developed during this period continue to play a key role in current ocean acidification research.

The initial motivation for this debate was rooted in an engineering perspective, exploring whether the hypothesis of coral reefs as CO₂ sinks could be applied for carbon sequestration. This contrasted with the established scientific view that, at a global scale, coral reefs function as CO₂ sources within the natural carbon cycle. In recent years, as emission reductions alone have proven insufficient to mitigate global warming, geoengineering approaches that enhance CO₂ fixation have gained interest. The expansive photosynthetic areas of coral reefs, created by calcification, warrant re-examination for their potential use in CO₂ fixation via photosynthesis, not only by corals but also by seaweeds and seagrasses. Additionally, the capacity of coral reefs to buffer against ocean acidification through the dissolution of calcium carbonate presents another area for exploration.

To fully understand this potential, it is essential to analyze the spatial and temporal dynamics of coral reefs, situating the fate of organic matter and nutrient cycling within the coral reef food web. This requires the development of 3D models that integrate physical processes and carbonate systems, ultimately proposing engineering solutions that combine CO₂ absorption with sustainable fishery resources. This review revisits the CO₂ sink/source debate in coral reefs, reevaluates the current understanding, and discusses potential future research directions and geoengineering applications.

The components of sunscreen products released into the oceans have raised concerns over their possible adverse effects on corals in nearshore areas, leading to the ban of some sunscreen products and the use of alternative components. The aim of this study was to examine the impacts of seven major sunscreen components; ethylhexyl methoxycinnamate (EHMC), octocrylene (OC), oxybenzon-3 (BP-3) , butyl methoxydibenzoylmethane (BMDBM), terephthalylidene dicamphor sulfonic acid (TDSA), zinc oxide (ZnO) and hydrophobically-coated ZnO on the survival of a scleractinian coral species Acropora digitifera along with the assessment of photochemical efficiency of symbiotic dinoflagellates within the host tissues. No significant effects on survival and photochemical efficiency were observed within 5 days of high dose (1 mgL^-1) exposure to the 6 sunscreen ingredients. However, acute mortality was observed under ZnO exposure in a concentration-dependent manner (0.01-1.0 mgL^-1), whereas hydrophobically-coated ZnO didn't cause such mortality. The results of this study suggest that the direct and acute impact of ZnO on corals can be mitigated with an appropriate hydrophobic coating while maintaining similar sunscreen efficacy.

Coral reef biodiversity relies on a dynamic balance between destruction and renewal. In this dynamic, the titan triggerfish (Balistoides viridescens) plays a paradoxical role—visibly damaging reefs by breaking coral rock, yet enhancing structural integrity by preying on bioeroding bivalves that undermine reef frameworks. This investigation evaluates B. viridescens as an ecological engineer in a feedback loop, examining how its foraging influences reef structure. Titan feeding and the resulting bioerosion were observed in various locations, including Lakshadweep, Indonesia, and many locations in the Maldives. An exploratory study was conducted on Kunfunadhoo Reef in Baa Atoll, Maldives. Surveys quantified coral cover, bivalve density, triggerfish abundance, and bioerosion rates. Coral mortality in 1998 produced abundant substrate for boring bivalve invasion, and by 2004, erosion of dead branching and tabulate corals increased the exposure of coral rock colonized by boring bivalves—a preferred prey of B. viridescens. While titan bioerosion rivaled that of individual scarids, parrotfish collectively eroded far more substrate. Although titan bioerosion may facilitate reef renewal by clearing weakened substrate, its benefits depend on CaCO₃ accretion exceeding erosion—a balance increasingly threatened by ocean warming and acidification. This study explores the titan's dual role in reef erosion and construction, and indicates the need for broader spatial and temporal assessments.

Pleurocorallium konojoi (white precious coral), belonging to family Coralliidae, is harvested mainly off Kochi, Japan, but because it inhabits waters deeper than 100 m, fundamental information on its biological traits, including its reproduction, remains scarce. In particular, no oocyte data have been available for the period after June, the presumed spawning season, when gametes reach their final stage of development. In this study, we applied non-destructive micro-computed tomography (micro-CT) to a P. konojoi collected by ROV in July. A 99% ethanol-preserved branch was scanned using micro-CT. Besides externally visible oocyte, multiple siphonozooids contained spherical structures interpreted as oocytes. Twenty-five oocytes were measured at their largest sections; the mean diameter was 235.6±4.0 µm, significantly exceeding Stage III values reported previous study. Therefore, the oocytes observed here were classified as Stage IV or at the Stage III–IV transition. However, photographs taken before fixation indicated original diameters of 518.9 µm, indicating that the oocytes imaged after scanning had likely shrunk because of the preservative process, the passage of time, or partial dehydration during CT scanning. Although absolute oocyte diameters must be interpreted with caution, our findings show that micro-CT is a valuable, non-invasive tool for visualizing Coralliidae oocytes and reliably inferring their developmental stage.