Adriana Humanes

Marine heatwaves cause widespread coral bleaching and mortality, yet thermal responses often differ between adult and juvenile corals, with the latter frequently being more tolerant. Whether the observed enhanced tolerance of juveniles reflects intrinsic physiological differences across life stages or simply reduced environmental stress due to shaded microhabitats occupied by juveniles, or a combination of both factors, remains unclear. To test whether colony size determines thermal responses and if a threshold size exists for tolerance, we exposed 89 colonies of the reef-building coral Acropora aff. digitifera to a 6-week, 32.5 °C simulated marine heatwave under controlled light conditions, with cumulative stress reaching 20 °C-weeks. Contrary to expectations, ∼2 °C-weeks less heat stress was required to trigger bleaching and mortality in coral juveniles compared to adults. Mortality divergence across colony sizes was greatest at ∼10 °C-weeks, when the smallest corals exceeded 80% mortality, whereas the largest colonies showed <10% mortality. A breakpoint in survival odds was apparent at 13.4 cm diameter, corresponding to the size at onset of sexual maturity in the native wild population for this species. These results indicate an ontogenetic shift in physiological heat tolerance, with larger reproductively mature colonies showing enhanced tolerance. The apparent survival advantage of juveniles in the reef frequently reported during bleaching events could be more likely due to micro-environmental buffering than intrinsic physiological tolerance. Our findings suggest that juvenile corals may be more vulnerable to climate change than previously thought, particularly as declining habitat complexity could reduce microhabitat refugia on future reefs. Abstract Figure:

Sexual coral propagation is an emerging technique capable of producing large numbers of corals for coral transplantation and reef rehabilitation. In contrast to asexual coral propagation, sexual propagation increases genotypic diversity and can be used for selective breeding to enhance coral heat tolerance or other desirable traits. However, implementation at meaningful ecological scales is hindered by high mortality during early life stages, high costs associated with nursery rearing facilities, and labour-intensive outplanting methods. To overcome these issues, we developed the CoralAssist Plug (CAP), a ceramic device designed for the rapid and cost-effective outplanting of sexually propagated corals in large numbers that maximises post-outplant survivorship. CAPs combine three important functional features: 1) built-in microrefugia to protect juvenile corals from grazing, 2) a relatively small size, 3 by 1 cm, that is easy to handle and stack efficiently without compromising the survivorship of corals, and 3) a hole in the middle that facilitates handling and attachment. CAPs were settled with Acropora aff. digitifera and outplanted to a reef crest after 1 to 6 months of ex situ nursery rearing. A 3-person dive team was able to outplant ~120 CAPs in one 90-minute shallow dive (just over 2 minutes per CAP per person). With longer nursery durations of 6 months, it was possible to achieve 36 % yield (i.e., the proportion of devices with a surviving coral) 4-years post-outplant. With nursery durations shortened to 1 month, we were able to attain 24 % yield 3-years post-outplant. Microrefugia significantly enhanced post-outplant survivorship leading to an 11 % increase in yield 4 years post outplant compared to devices without microrefugia. Outplanted corals that had reached adult size, were self-attached and were reproductively mature after 4 years. Our results suggest that CAPs can play a meaningful role in reef rehabilitation by efficiently introducing sexually propagated corals into natural populations with clear applications to assisted evolution techniques, such as selective breeding.

Extreme climatic events are reshaping ecosystems worldwide as individual organisms vary markedly in their ability to withstand these disturbances. Deciphering patterns of persistence on local scales is therefore critical for predicting biodiversity trajectories under intensifying climate extremes. In this study, we examined variation in thermal stress responses among individuals of the coral Stylophora pistillata species complex during a heatwave at Heron Island Reef, Australia. More than half of the focal coral colonies died on the reef, and survival of coral fragments maintained under ex situ common thermal stress conditions was significantly correlated with the survival of their source colony. This demonstrates that survival differences result largely from biological factors rather than differential thermal exposure across reef habitats. Under common garden conditions, we observed striking differences in bleaching severity and survival times among three sympatric cryptic taxa and their highly host-specific symbiont community. Within the most locally common taxon, corals from historically warmer and more seasonally variable reef habitats seem more susceptible to bleaching, contrary to expectations. Together, these results reveal how biological differences among cryptic taxa and among individuals can shape coral responses during a heatwave and advance our understanding of coral vulnerability in a rapidly warming world.

As populations decline under climate change and other stressors, survivors become increasingly isolated. Allee effects in such fragmented populations may arise during reproduction, particularly for sessile organisms such as corals, reducing fertilisation potential and increasing the probability of recruitment failure. However, the critical distances between corals needed to maintain viable populations remains uncertain. We experimentally investigated fertilisation patterns in the broadcast spawning coral Acropora cf. hyacinthus using a manipulated patch, spacing colonies at increasing distances downstream from a central spawning aggregation. Field samples showed a steep decline in fertilisation with increasing intercolonial distance and genetic paternity analysis indicated most parents were located within 3 m of each other. Colony position relative to the current direction also influenced fertilisation success, with 81% of sequenced progeny sired by upstream colonies. Simulations projecting natural population distributions onto a virtual grid, incorporating experimental results, indicated that populations remained well-mixed at typical adult densities. Yet at reduced densities approaching 0.01 colonies m ⁻² , reproductive isolation and patchy breeding units emerged. These findings demonstrate that colony isolation substantially impairs reproductive connectivity, with additional influence from the spatial arrangement of colonies. Conservation strategies should prioritise maintaining colony density and spatial connectivity to support viable coral populations on degraded reefs.

Coral sexual propagation and ex situ rearing have the potential to scale up reef restoration efforts. However, early post-settlement survivorship remains a major bottleneck, often limited by spatial competition with algae. Micrograzers can inhibit algal growth around settled corals and thereby improve their survivorship, yet optimal grazer densities and species-specific effects remain poorly understood. Here, we investigated the effects of increasing densities of the gastropod Cerithium coralium (0, 1, 2, 4, 8, 16, 32 individuals per seven settlement devices) on the survivorship and growth of juvenile Acropora aff. digitifera. Yield, defined as the presence of at least one surviving coral per device, peaked to 100 % at a density of 4 snails. At the individual level, corals in the 4- and 8-snail treatments were approximately twice as likely to survive the six-week experimental period (9 weeks post-settlement) compared to the no-grazer control. In contrast, the highest density (32 snails) reduced yield by one third (66.7 ± 7.3 %, mean ± SD) and halved survivorship (from 81 to 39 %), with elevated mortality and greater risk of crustose coralline algae overgrowth relative to controls. Coral growth did not differ among treatments. When normalized to surface area, snail numbers corresponded to an optimal density of approximately one snail per 30 cm² of grazing surface. Exceeding this threshold can reduce benefits associated with grazers and impose unintended costs on juvenile coral survival. These findings highlight the importance of carefully identifying optimal grazer densities in ex situ coral aquaculture.

Extreme climatic events are reshaping ecosystems worldwide as individual organisms vary markedly in their ability to withstand these disturbances. Deciphering patterns of persistence on local scales is therefore critical for predicting biodiversity trajectories under intensifying climate extremes. In this study, we examined variation in thermal stress responses among individuals of the coral Stylophora pistillata species complex during a heatwave at Heron Island Reef, Australia. More than half of the focal coral colonies died on the reef, and survival of coral fragments maintained under ex situ common thermal stress conditions was significantly correlated with the survival of their source colony. This demonstrates that survival differences result largely from biological factors rather than differential thermal exposure across reef habitats. Under common garden conditions, we observed striking differences in bleaching severity and survival times among three sympatric cryptic taxa and their highly host-specific symbiont community. Within the most locally common taxon, corals from historically warmer and more seasonally variable reef habitats seem more susceptible to bleaching, contrary to expectations. Together, these results reveal how biological differences among cryptic taxa and among individuals can shape coral responses during a heatwave and advance our understanding of coral vulnerability in a rapidly warming world.

Reef‐building corals face continued declines due to climate change‐amplified marine heatwaves. In addition to affecting coral heat tolerance, corals' algal endosymbionts (family Symbiodiniaceae) can reflect their prior heatwave exposure, although understanding is often limited to heatwave‐induced shifts between symbiont genera. Here, we used ITS2 metabarcoding to characterise Symbiodiniaceae assemblages in 293 individuals of the common Indo‐Pacific coral Acropora aff. digitifera in Palau (Western Pacific), between two outer‐reef regions with contrasting heatwave histories. During the strongest recorded heatwaves, southwestern ‘hotspot’ reefs have typically accrued an additional 2°C‐weeks of heat stress compared to thermal ‘refugia’ located 60 km north. In contrast to previous studies that observed declines in symbiont richness following heat stress, we found a greater diversity of symbiont taxa and low‐abundance sequence variants in ‘hotspot’ corals, predominantly within the C40 lineage in genus Cladocopium . Combining these data with experimental heatwave performance from 168 of these corals revealed that approximately 10% of heat tolerance variability at hotspot reefs was associated with hosting different symbiont taxa. Compared to other hotspot corals, those hosting symbionts with the C15h sequence variant suffered bleaching mortality at 0.8°C‐weeks lower heat stress. Despite higher variability in heat tolerance among corals from thermal refugia compared to hotspot reefs, we found no association between heat tolerance and the symbionts hosted by refugium corals. As the world's coral reefs are exposed to intensifying marine heatwaves under accelerating climate change, the low‐abundance variants that characterise symbionts within genera or lineages may become increasingly important indicators of poor heatwave tolerance.

Monitoring habitat-forming species, such as scleractinian corals, is crucial for managing ecosystems and biodiversity. Yet for assessing population health, challenges remain in reconciling conventional areal coverage surveys with individual-based demographic techniques. Here, we explore both monitoring approaches to characterize the population dynamics of the reef-building coral, Acropora cf. digitifera , on a West Pacific outer reef using photogrammetry reef mapping data spanning 5 years. Tracking 906 coral colonies showed that those exceeding approximately 10 cm diameter exhibit the fastest planar growth but also suffer excess mortality, possibly owing to increased structural vulnerability to dislodgement by wave energy or to other factors, such as disease or predation, that may be exacerbated by senescence. Area-controlled orthomosaic subsampling paired to integral projection modelling revealed a consistent decline in population density, as recruitment was insufficient to balance losses of larger colonies. Yet, total areal coverage and median colony size increased over the study, suggesting this disequilibrium population is on a recovery trajectory from past disturbance and heavily reliant on sporadic recruitment pulses not detected in our study. We find that conventional areal monitoring and demographic approaches can yield contrasting conclusions about population dynamics. Reconciling these differences for disequilibrium populations requires long-term demographic data over periods long enough to detect infrequent yet critical demographic events such as large post-disturbance recruitment events.