Increasing comparability among coral bleaching experiments

Coral bleaching is the single largest global threat to coral reefs worldwide. Integrating the diverse body of work on coral bleaching is critical to understanding and combating this global problem. Yet investigating the drivers, patterns, and processes of coral bleaching poses a major challenge. A recent review of published experiments revealed a wide range of experimental variables used across studies. Such a wide range of approaches enhances discovery, but without full transparency in the experimental and analytical methods used, can also make comparisons among studies challenging. To increase comparability but not stifle innovation, we propose a common framework for coral bleaching experiments that includes consideration of coral provenance, experimental conditions, and husbandry. For example, reporting the number of genets used, collection site conditions, the experimental temperature offset(s) from the maximum monthly mean (MMM) of the collection site, experimental light conditions, flow, and the feeding regime will greatly facilitate comparability across studies. Similarly, quantifying common response variables of endosymbiont (Symbiodiniaceae) and holobiont phenotypes (i.e., color, chlorophyll, endosymbiont cell density, mortality, and skeletal growth) could further facilitate cross-study comparisons. While no single bleaching experiment can provide the data necessary to determine global coral responses of all corals to current and future ocean warming, linking studies through a common framework as outlined here, would help increase comparability among experiments, facilitate synthetic insights into the causes and underlying mechanisms of coral bleaching, and reveal unique bleaching responses among genets, species, and regions. Such a collaborative framework that fosters transparency in methods used would strengthen comparisons among studies that can help inform coral reef management and facilitate conservation strategies to mitigate coral bleaching worldwide

Fluorescence of coral larvae predicts their settlement response to crustose coralline algae and reflects stress

Multi-coloured homologues of the green fluorescent protein generate some of the most striking visual phenomena in the ocean. Despite their natural prominence in reef-building corals and widespread use in biotechnology, their biological role remains obscure. Here, we experimented with larvae of Acropora millepora to determine what can be learned about a coral larva or recruit from its fluorescent colour. We performed 12 crosses between seven A. millepora colonies representing differing fluorescence phenotypes, the larvae of which were exposed to a natural settlement cue (crustose coralline algae) and heat–light stress. Parental effects explained 18 per cent of variation in colour and 47 per cent of variation in settlement. The colour of the larval family emerged as a predictor of the settlement success: redder families were significantly less responsive to the provided settlement cue (p = 0.006). This relationship was owing to a correlation between parental effects on settlement and colour (r2 = 0.587, p = 0.045). We also observed pronounced (16%) decline in settlement rate, as well as subtle (2%), but a statistically significant decrease in red fluorescence, as a consequence of heat–light stress exposure. Variation in settlement propensity in A. millepora is largely owing to additive genetic effects, and is thought to reflect variation in dispersal potential. Our results suggest an optical signature to discriminate between long- and short-range dispersing genotypes, as well as to evaluate stress. Further research in this direction may lead to the development of field applications to trace changes in coral life history and physiology caused by global warming

Physiological effects of heat and cold exposure in the common reef coral Acropora millepora

Reef-forming corals are under threat globally from climate change, leading to changes in sea temperatures with both hot and cold events recorded and projected to increase in frequency and severity in the future. Tolerance to heat and cold exposure has been found to be mutually exclusive in other marine invertebrates, but it is currently unclear whether a trade-off exists between hot and cold thermal tolerance in tropical corals. This study quantified the changes in physiology in Acropora millepora from the central Great Barrier Reef subjected to three temperature treatments; sub-lethal cold, ambient and sub-lethal heat (23.0 degrees C, 27.0 degrees C and 29.5 degrees C, respectively). After 10 weeks, pigment content and Symbiodiniaceae density increased in cold-treated corals but decreased in heat-treated corals relative to corals at ambient conditions. Heat-treated corals gained less mass relative to both ambient and cold-treated corals. These results indicate that the physiological condition of A. millepora corals examined here improved in response to mild cold exposure compared to ambient exposure and decreased under mild heat exposure despite both these temperatures occurring in situ around 15% of the year. The energetic condition of corals in the hotter treatment was reduced compared to both ambient and cooler groups, indicating that corals may be more resilient to mild cold exposure relative to mild heat exposure. The results indicate that the corals shifted their resource allocation in response to temperature treatment, investing more energy into skeletal extension rather than maintenance. No evidence of thermal tolerance trade-offs was found, and cold thermal tolerance was not lost in more heat-tolerant individuals. An enhanced understanding of physiological responses of corals at both ends of the thermal spectrum is important for predicting the resilience of corals under projected climate change conditions

Increasing comparability among coral bleaching experiments

Coral bleaching is the single largest global threat to coral reefs worldwide. Integrating the diverse body of work on coral bleaching is critical to understanding and combating this global problem. Yet investigating the drivers, patterns, and processes of coral bleaching poses a major challenge. A recent review of published experiments revealed a wide range of experimental variables used across studies. Such a wide range of approaches enhances discovery, but without full transparency in the experimental and analytical methods used, can also make comparisons among studies challenging. To increase comparability but not stifle innovation, we propose a common framework for coral bleaching experiments that includes consideration of coral provenance, experimental conditions, and husbandry. For example, reporting the number of genets used, collection site conditions, the experimental temperature offset(s) from the maximum monthly mean (MMM) of the collection site, experimental light conditions, flow, and the feeding regime will greatly facilitate comparability across studies. Similarly, quantifying common response variables of endosymbiont (Symbiodiniaceae) and holobiont phenotypes (i.e., color, chlorophyll, endosymbiont cell density, mortality, and skeletal growth) could further facilitate cross-study comparisons. While no single bleaching experiment can provide the data necessary to determine global coral responses of all corals to current and future ocean warming, linking studies through a common framework as outlined here, would help increase comparability among experiments, facilitate synthetic insights into the causes and underlying mechanisms of coral bleaching, and reveal unique bleaching responses among genets, species, and regions. Such a collaborative framework that fosters transparency in methods used would strengthen comparisons among studies that can help inform coral reef management and facilitate conservation strategies to mitigate coral bleaching worldwide