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Predicted increases in sea-surface temperatures due to climate change are likely to alter the physiology of marine organisms and ultimately influence the distribution and abundance of their populations. The consequences of increased temperatures for marine species, including decreased survival and altered rates of development, growth and settlement, are well known and often attributed to imbalances between energy supply and demand. To test this hypothesis, we calibrated the effect of temperature on rates of survival and lipid depletion for larvae of the common stony coral Acropora tenuis over a 7 °C temperature range. Temperature had a pronounced, linearly increasing effect on larval mortality, with a sixfold decrease in median survival time. Contrary to expectation, however, temperature had a quasi-parabolic effect on lipid use; rates declined as temperatures either increased above or decreased below the ambient temperature at the time of spawning. This contrasts with previous work suggesting that increased energy depletion is the cause of larval mortality at higher temperatures. Our results highlight the sensitivity of coral larvae to temperature and have implications for dispersal potential because fewer larvae will survive to disperse. Such projected declines in connectivity among coral populations are likely to undermine reef resilience.

Territories of grazing fishes in the family Pomacentridae have been documented to cover a sub- stantial proportion of shallow, exposed coral reef fronts, and these fishes can have profound effects on benthic community composition, including the recruitment and post-settlement survival of scleractinian corals. However, current studies of territorial grazer effects on corals have focused on back-reef habitats. Territorial damselfishes occur in distinct behavioural guilds ranging from indeter- minate territorial grazers with thin algal turfs and low rates of territorial aggression to intensive territorial grazers with thick turfs and high rates of aggression. To determine the impact of territorial grazers on the establishment of juve- nile corals, we surveyed the reef crest habitat of Lizard Island, Great Barrier Reef, using fixed transects to assess the effects of indeterminate and intensive territorial grazers on juvenile coral abundance and taxonomic composition. In addition, the turnover of territorial pomacentrids was monitored as well as the effects of turnover on juvenile coral assemblages. Intensive territorial grazers were asso- ciated with a significantly lower juvenile coral abundance (34 % decrease), but neither intensive nor indeterminate grazer territories impacted juvenile coral taxonomic com- position. Over the course of 1 yr, there was a high rate of territorial turnover (39.7 %). Turnover from control plots to intensive damselfish territories was accompanied by a 44 % decrease in juvenile corals; conversely, turnover from intensive damselfish territories to control plots coin- cided with a 48 % increase in juvenile corals. These find- ings reveal two main conclusions. Firstly, the association between damselfish territories and the abundance and spatial turnover of juvenile corals strongly implies that territorial grazers have a negative effect on juvenile coral populations. Secondly, the unexpectedly high temporal turnover of damselfish territories indicates that damselfish- coral-algae linkages are highly dynamic, may be exten- sively influenced by local-scale effects, and have the potential to impact the structure of coral assemblages on coral reef fronts.

Lecithotrophic, or non-feeding, marine invertebrate larvae generally have shorter pelagic larval durations (PLDs) than planktotrophic larvae. However, non-feeding larvae of scleractinian corals have PLDs far exceeding those of feeding larvae of other organisms and predictions of PLD based on energy reserves and metabolic rates, raising questions about how such longevity is achieved. Here, we measured temporal changes in metabolic rates and total lipid content of non-feeding larvae of four species of reef corals to determine whether changes in energy utilization through time contribute to extended larval durations. The temporal dynamics of both metabolic rates and lipid content were highly consistent among species. Prior to fertilization, metabolic rates were low (2.73–8.63 nmol O2 larva−1 h−1) before rapidly increasing to a peak during embryogenesis and early development 1–2 days after spawning. Metabolic rates remained high until shortly after larvae first became competent to metamorphose and then declined by up to two orders of magnitude to levels at or below rates seen in unfertilized eggs over the following week. Larvae remained in this state of low metabolic activity for up to 2 months. Consistent with temporal patterns in metabolic rates, depletion of lipids was extremely rapid during early development and then slowed dramatically from 1 week onward. Despite the very low metabolic rates in these species, larvae continued to swim and retained competence for at least 2 months. The capacity of non-feeding coral larvae to enter a state of low metabolism soon after becoming competent to metamorphose significantly extends dispersal potential, thereby accruing advantages typically associated with planktotrophy, notably enhanced population connectivity.

Coral bleaching and disease have often been hypothesized to be mutually reinforcing or co-occurring, but much of the research supporting this has only drawn an implicit connection through common environmental predictors. In this study, we examine whether an explicit relationship between white syndrome and bleaching exists using assemblage-level monitoring data from up to 112 sites on reef slopes spread throughout the Great Barrier Reef over 11 years of monitoring. None of the temperature metrics commonly used to predict mass bleaching performed strongly when applied to these data. Furthermore, the inclusion of bleaching as a predictor did not improve model skill over baseline models for predicting white syndrome. Similarly, the inclusion of white syndrome as a predictor did not improve models of bleaching. Evidence for spatial co-occurrence of bleaching and white syndrome at the assemblage level in this data set was also very weak. These results suggest the hypothesized relationship between bleaching and disease events may be weaker than previously thought, and more likely to be driven by common responses to environmental stressors, rather than directly facilitating one another.

Bleaching in corals due to environmental stress represents a loss of energy intake often leading to an increase in mortality risk. Successful coral recovery from severe bleaching events may depend on the rate of replenishment of algal symbiont populations following the period of thermal stress, the supply of an alternative food source, or both. Here, we explore the role of food availability in promoting the survival and recovery of a common coral (Acropora intermedia) following acute experimentally induced thermal stress. Fed corals were provided with live rotifers daily, to maintain densities of zooplankton in tanks that are typical of coral reefs. After a 6-week acclimation phase, heated corals were subjected to a +4 °C thermal anomaly for a 7-day period (bleaching phase) then temperatures were returned to normal for a further 2 weeks (recovery phase). Results demonstrated that heated corals had higher survival when they were provided with heterotrophic food. Fed corals experienced reduced loss of chlorophyll a, relative to unfed corals. During the recovery phase, both fed and unfed corals recovered within a few days; however, fed corals recovered to pre-bleaching phase levels of chlorophyll a, whereas unfed corals stabilized approximately one-third below this level. Protein levels of fed corals declined markedly during the bleaching phase, but recovered all of their losses by the end of the recovery phase. In contrast, unfed corals had low protein levels that were maintained throughout the experiment. To the extent that these results are representative of corals’ responses to thermal anomalies in nature, the findings imply that availability of particulate food matter has the potential to increase corals’ capacity to survive thermally induced bleaching and to ameliorate its sub-lethal effects. They also support the hypothesis that different rates of heterotrophy are an important determinant of variation in resilience to thermal stress among reef environments.

Estimating larval retention at individual reefs by local scale three-dimensional flows is a significant problem for understanding, and predicting, larval dispersal. Determining larval dispersal commonly involves the use of computationally demanding and expensively calibrated/validated hydrodynamic models that resolve reef wake eddies. This study models variation in larval retention times for a range of reef shapes and circulation regimes, using a reef-scale three-dimensional hydrodynamic model. It also explores how well larval retention time can be estimated based on the “Island Wake Parameter”, a measure of the degree of flow turbulence in the wake of reefs that is a simple function of flow speed, reef dimension, and vertical diffusion. The mean residence times found in the present study (0.48–5.64 days) indicate substantial potential for self-recruitment of species whose larvae are passive, or weak swimmers, for the first several days after release. Results also reveal strong and significant relationships between the Island Wake Parameter and mean residence time, explaining 81–92% of the variability in retention among reefs across a range of unidirectional flow speeds and tidal regimes. These findings suggest that good estimates of larval retention may be obtained from relatively coarse-scale characteristics of the flow, and basic features of reef geomorphology. Such approximations may be a valuable tool for modeling connectivity and meta-population dynamics over large spatial scales, where explicitly characterizing fine-scale flows around reef requires a prohibitive amount of computation and extensive model calibration.

Connectivity of larvae among metapopulations in open marine systems can be a double-edged sword, allowing for the colonization and replenishment of both desirable and undesirable elements of interacting species-rich assemblages. This article studies the effect of recruitment by coral and macroalgae on the resilience of grazed reef ecosystems. In particular, we focus on how larval connectivity affects regime shifts between alternative assemblages that are dominated either by corals or by macroalgae. Using a model with bistability dynamics, we show that recruitment of coral larvae erodes the resilience of a macroalgae-dominated ecosystem when grazing is high, but has negligible effect when grazing is low. Conversely, recruitment by macroalgae erodes the resilience of a coral-dominated ecosystem when grazing is low, leading to a regime shift to macroalgae. Thus, spillover of coral recruits from highly protected areas will not restore coral cover or prevent flips to macroalgae in the surrounding seascape if grazing levels in these areas are depleted, but may be pivotal for re-building coral populations if grazing is high. Fishing restrictions and the re-introduction of herbivores should therefore be a prime conservation objective for preventing undesirable regime shifts. Connectivity by some components of coral reef assemblages (e.g., macroalgae, pathogens, crown-of-thorns starfish) may be detrimental to sustaining reefs, especially where overfishing and other drivers have eroded their resilience, making them more vulnerable to a regime shift.

Survival of pelagic marine larvae is an important determinant of dispersal potential. Despite this, few estimates of larval survival are available. For scleractinian corals, few studies of larval survival are long enough to provide accurate estimates of longevity. Moreover, changes in mortality rates during larval life, expected on theoretical grounds, have implications for the degree of connectivity among reefs and have not been quantified for any coral species. This study quantified the survival of larvae from five broadcast-spawning scleractinian corals (Acropora latistella, Favia pallida, Pectinia paeonia, Goniastrea aspera, and Montastraea magnistellata) to estimate larval longevity, and to test for changes in mortality rates as larvae age. Maximum lifespans ranged from 195 to 244 d. These longevities substantially exceed those documented previously for coral larvae that lack zooxanthellae, and they exceed predictions based on metabolic rates prevailing early in larval life. In addition, larval mortality rates exhibited strong patterns of variation throughout the larval stage. Three periods were identified in four species: high initial rates of mortality; followed by a low, approximately constant rate of mortality; and finally, progressively increasing mortality after approximately 100 d. The lifetimes observed in this study suggest that the potential for long-distance dispersal may be substantially greater than previously thought. Indeed, detection of increasing mortality rates late in life suggests that energy reserves do not reach critically low levels until approximately 100 d after spawning. Conversely, increased mortality rates early in life decrease the likelihood that larvae transported away from their natal reef will survive to reach nearby reefs, and thus decrease connectivity at regional scales. These results show how variation in larval survivorship with age may help to explain the seeming paradox of high genetic structure at metapopulation scales, coupled with the maintenance of extensive geographic ranges observed in many coral species.

To live successfully in wave-swept habitats, plants and animals must be able to survive, consume resources, and reproduce in the presence of incessant, variable and often unpredictable levels of water motion at a range of scales. However, there is a relatively poor understanding of water motion in natural habitats at the scales necessary to determine its potential physiological and ecological consequences. Using an historic record of hourly wind conditions, a depth profile and two rigorously tested oceanographic models, 37-years of hourly wave driven water motion were hindcast spatially on a typical subtidal coral reef platform (maximum horizontal displacement, velocity and acceleration per wave cycle). For larger waves, those likely to constitute ecological disturbances, around 95% of the wave’s height that is lost over the whole reef occurs within the first 50 m of the crest. The field-validated model of spatiotemporal variation in water motion provided a framework for quantitatively predicting several physiological and ecological effects of wave motion, such as nutrient and gas fluxes and mortality rates from hydrodynamic disturbances. It also suggested a sharp ecological transition between a crest habitat in which disturbance-mediated coexistence mechanisms are important, and a flat habitat in which they are much less important.