Plasticity of shallow reef corals across a depth gradient.

Global warming harms coral reefs. Mesophotic coral reef ecosystems (MCEs) have been suggested to serve as refugia for shallow reefs. Information on the adaptation potential of shallow corals at MCEs is a prerequisite for understanding the refuge potential of MCEs. In this study, we investigated the photoacclimation potential of four shallow coral species transplanted at different depths over 1 year. The results showed that the corals-Pocillopora damicornis, Porites cylindrica, and Turbinaria reniformis-survived and acclimated to a wide range of light regimes at the depths of 5, 20, and 40 m. However, Acropora tenuis survived only at 5 and 20 m depth and showed significant morphological alteration at 20 m depth. Our results indicate that shallow corals have substantial plasticity with respect to depth changes. Changes in photosynthetic performance and phenotypic plasticity within these coral species may act as a buffer for depth-related changes and as modulators of evolutionary responses.

Response of resistant larvae of the coral Acropora tenuis to future thermal stress.

Seawater temperatures are rising rapidly and severely due to climate change, negatively affecting coral reef communities. The persistence of coral populations depends on their success during the early life stages. Thermal conditioning during the larval stage can increase coral larvae's ability to tolerate high temperatures in subsequent stages. We studied the response of resistant larvae of Acropora tenuis to thermal stress to increase their thermal tolerance during the juvenile stage. Larvae were exposed to ambient (∼26 °C) and thermal stress (∼31 °C) temperatures. Then, settlement success on preconditioned tiles was determined. After 28 days at ambient temperature, the juveniles were exposed to thermal stress for 14 days, and their survival was assessed. Our results showed that thermal stress in the larval stage did not alter the thermal tolerance of juveniles, and they could not acclimate to heat stress. As a result, the summer's heat waves could potentially threaten their resilience.

Acclimation potential of Acropora to mesophotic environment.

Mesophotic coral ecosystems may serve as a refuge for reef-building corals to survive the ongoing climate change. Distribution of coral species changes during larval dispersal. However, the acclimation potential in the early life stages of corals at different depths is unknown. This study investigated the acclimation potential of four shallow Acropora species at different depths via the transplantation of larvae and early polyps settled on tiles to 5, 10, 20, and 40 m depths. We then examined physiological parameters, such as size, survival, growth rate, and morphological characteristics. The survival and size of juveniles of A. tenuis and A. valida at 40 m depth were significantly higher than those at other depths. In contrast, A. digitifera and A. hyacinthus showed higher survival rates at shallow depths. The morphology (i.e., size of the corallites) also varied among the depths. Collectively, the shallow coral larvae and juveniles displayed substantial plasticity at depth.

Reactions of juvenile coral to three years of consecutive thermal stress.

As global temperatures continue to rise, corals are being exposed to increasing heat stress throughout their early life stages; however, the impact of this phenomenon is poorly understood. We exposed the reef-building coral Acropora tenuis juveniles to ∼26-28 °C (control) and ∼ 31 °C (heat stress) for one week per year over three consecutive years. In the first year of heat stress, >96 % of juveniles survived despite symbiotic algal densities in juvenile corals declining. In comparison, survival rates in the third year of heat stress declined to 50 %. Survival rates under natural conditions after stress also gradually decreased in the stressed groups. The rate in the reduction of survivorship was prominent in the consecutive thermally stressed groups (juveniles stressed twice in two years). Symbiotic algal density and photosynthetic activity (Fv/Fm) also declined in stressed juvenile groups. However, heat stress did not significantly affect the growth of juveniles. In the third year of heat stress, temperature negatively affected the physiology of juveniles in terms of survivorship, brightness (an indicator of bleaching), symbiotic algal density, and photosynthetic efficiency. Stress across consecutive years appeared to cause the survivorship of juvenile corals to decline, with three years of stress contributing to the severe decline of a reef. In conclusion, A. tenuis juveniles are not able to acclimatize to heat stress, with successive heat waves of <7 days in the summer potentially negatively affecting resilience.

Adaptations by the coral Acropora tenuis confer resilience to future thermal stress.

Elevated temperatures cause coral bleaching and reef degradation. However, coral may have strategies to survive by reproducing more heat-tolerable larvae. We examine the direct and carryover effects of thermal stress on fecundity and fitness in the reef-building coral Acropora tenuis. Fragments from the same colony are subjected to control temperature (~27.5 °C) or heat stress (~31 °C) for ten days. We then examine the fecundity of adults (egg number and size) and the thermal tolerance of larvae and recruits (survival rates, growth, and size). The stressed fragments show a trade-off in egg production, an increase in egg number but a decrease in size. In addition, larvae and recruits from the stressed colony show marginally higher survival rates in the higher water temperature but do not differ in the control condition. Therefore, corals produce more heat-resistant larvae and recruits after they experience heat stress, which may improve coral reef resilience.

Genetic species identification and population structure of grouper Epinephelus coioides (Hamilton, 1822) collected from fish markets along the Persian Gulf and the Oman Sea.

Many ecologically important and valuable fisheries marine species have been misidentified in terms of both the statistical data and market demand. Correct identification at the species level and the population genetic structure of the orange-spotted grouper (Epinephelus coioides), a precious fish in the Persian Gulf and the Oman Sea, was tested using mitochondrial cytochrome oxidase subunit I (DNA barcoding) and D-loop sequencing. The results revealed that the Epinephelus species found in the region, including E. coioides, E. bleekeri, E. polylepis, and E. chlorostigma were all mistakenly grouped together and identified as only E. coioides. Moreover, the analysis of molecular variance (AMOVA) of E. coioides samples using the D-loop showed a significantly unique genetic structure (ΦST = 0.068, p < 0.001) within the E. coioides population throughout the Persian Gulf and the Oman Sea, with the pairwise genetic difference between sampling locations in UAE and the Iranian coast. Moreover, D-loop sequences analysis showed two distinct haplotype groups scattered among the sampling locations, which did not correlate with the geographic distance between the sampling locations. These findings indicate that the issue of misidentification should be highlighted in the management and conservation of E. coioides. As this type of misidentification is likely to happen to other threatened marine species as well, the efficacy of using genetic markers for the correct identification, both at the species and the population level, is vital.

Baseline assessment of coral diseases in an environmentally extreme environment of the northern Persian Gulf.

The coral diseases are recognized as serious threats to coral reef ecosystems and a major contributor to the reduction of zooxanthellate scleractinian populations worldwide. Quantitative disease surveys can provide important information on the status and trends of coral reef health over the time. In this study, coral disease surveys were carried out during August 2014 and 2015 at designated sites located in the northern Persian Gulf. Seven different diseases affecting eight scleractinian genera were documented: Black Band Disease (BBD), Skeletal Growth Anomaly (SGA), Arabian Yellow Band Disease (AYBD), White Mat Disease (WMD), White Syndrome (WS), Porites Pink Spot (PPS), and Porites Bleached Patches (PBP). Additionally, it should be noted that this is the first report of SGA in Acropora sp. found in the Persian Gulf. The results of this study generated a baseline dataset of various related diseases in the northern Persian Gulf which can be utilized for future studies.