Between 2023 and 2025, over 80% of tropical coral reefs experienced extreme levels of heat stress coinciding with the El Ni & ntilde;o of 2023, and resulting in the 4th global mass coral bleaching event. Despite having multiple reef regions, data on the impacts of mass bleaching events on coral reefs across South Asia is sparse. Through a coordinated, nationwide collaboration of scientists and citizen observers, we assessed bleaching responses across five reef regions in India, spanning most of the subcontinent's major reef areas, from oceanic atolls to mainland fringing and patchy reef formations. While bleaching intensity increased with accumulated heat stress (DHW) as expected, it showed distinct regional variability, revealing the limits of DHW as a universal predictor. Among the regions, Lakshadweep was the most affected with up to 37% coral cover bleached or dead at intermediate DHW. In contrast, MGMNP faced only 11.5% bleaching or mortality, likely reflecting regional oceanic processes that could create mesoscale thermal refugia. Across regions, Acropora, Pocillopora, Galaxea, and Montipora, were the most affected genera, while Pavona, Platygyra, Goniastrea, and Favites were among the least affected. Palk Bay displayed a reversal of canonical genus-level susceptibilities, with only modest bleaching or mortality of 13.6% in the ordinarily vulnerable Acropora colonies, but 61.8% mortality in the ordinarily resistant Porites colonies. These contrasting regional and taxonomic responses indicate how context-specific resistance patterns, and local oceanography may mediate thermal stress and underscore the need to integrate these local geographical contexts in our understanding of bleaching impacts on tropical reefs.
The Amphistegina bleaching index (ABI) provides a simple, low-cost method for assessing photo-oxidative stress in reef-dwelling foraminifera. The ABI, developed for Florida reefs, has been applied to assess the extent of such stress in several regions globally, including the Maldives, and now in Australia. Here, we assess the application of the ABI for capturing variability in photo-inhibitory stress across contrasting reef settings and temporal scales. We applied the ABI at four sites in the central Maldivian archipelago, adding a new assessment point to previous surveys conducted in 2015 and 2018, and at two sites at Lizard Island on the northern Great Barrier Reef, comparing austral winter (2022) and summer (2024) conditions. In the Maldives, comparisons with previously published data from the same sites collected in 2015 and 2018 indicate very low levels of bleaching in 2025, with the geometric mean of bleached specimens at 2
Coral reef ecosystems are impacted by a myriad of human activities and influenced by complex interactions between habitat characteristics and environmental stressors, yet the relative influence of these factors on reef fish diversity remains unclear. Here, we examine the effects of coral species richness (SR), structural complexity, fishing pressure, human disturbance, marine protection and wave exposure on reef fish taxonomic and functional diversity in a multi-use marine protected area (MPA) in Raja Ampat, Indonesia. Using a structural equation modeling approach, we found that fish SR was strongly predicted by both coral SR and structural complexity, while fish functional dispersion (FDis) showed no significant relationship with these habitat variables and was elevated in reefs with higher relative dead coral cover. Fishing pressure, estimated based on accessibility and population density, was associated with lower fish FDis, lower reef structural complexity, and higher dead coral cover, and was indirectly associated with lower fish SR through pathways mediated by habitat variables. Human disturbance, as estimated via direct proximity to settlements, showed comparatively weak and largely non-significant associations with habitat and fish diversity variables. We found that marine protection mitigated the impact of fishing pressure on fish SR and supported more structurally complex reefs. We emphasize the need to understand the interactions between environmental stressors and habitat features when designing conservation strategies for complex and biodiverse coral reef ecosystems, particularly as human activities continue to impact coral reefs worldwide.
Quantifying reef-scale metabolic fluxes is essential for understanding coral reef health and resilience. We measured Net Community Production (NCP) and Net Community Calcification (NCC) at multiple sites across a shallow forereef in Palau for three summers (2021–2023) using gradient-flux (GF) and eddy-covariance (EC) methods paired with structure-from-motion (SfM) photogrammetry. Diel-integrated fluxes of NCP ranged from − 408 to 569 mmol O₂ m−2 day−1 and NCC from − 218 to 503 mmol CaCO₃ m−2 day−1 across sites and years. Spatial differences among sites reached 173 mmol O₂ m−2 day−1, while temporal changes reached 226 mmol O₂ m⁻2 day⁻1 between years. Relationships between fluxes and environmental parameters were dominated by light, with weaker contributions from flow speed, temperature, and depth. Most sites were net heterotrophic and calcifying. Sites adjacent to Tabkukau channel, with strong tidal flows and dominated by tabular corals, were less heterotrophic, whereas low-flow, branching-dominated sites exhibited stronger heterotrophy, highlighting the coupling between benthic composition, hydrodynamics, and metabolism. Across years, increasing coral cover and colony calcification rates from SfM confirmed reef accretion. Stormy weather associated with Typhoon Khanun to the north of Palau in 2023 coincided with increased productivity and an episode of dissolution, presumably driven by nutrient inputs from land runoff and sedimentation. At the longest time series site, NCP increased from heterotrophy (− 155 mmol O₂ m−2 day−1 in 2021) to autotrophy (+ 71 mmol O₂ m−2 day−1 in 2023), but this trend was driven by a few extreme days of high NCP; these differences underscore the need for longer-term monitoring.
Corallivory (the consumption of live coral tissue) threatens reef health and can undermine restoration efforts, particularly as coral cover declines. Although predators may suppress corallivore populations through direct consumption, their non-consumptive effects remain poorly understood. We tested whether chemical cues from two spiny lobsters, Panulirus argus and P. guttatus, elicit avoidance responses in two Caribbean corallivores: the snail Coralliophila galea and the fireworm Hermodice carunculata. Using chemical choice trials with lobster-conditioned versus control seawater, we found that fireworms consistently avoided P. argus chemical cues, confirmed by both discrete choice outcomes and time spent data. C. galea, in contrast, showed a significant initial avoidance response to P. argus cues that weakened over the trial, with snails increasingly occupying the mixing zone rather than sustaining their initial position on the control side. By final choice, the pattern of significant differences changed; snails no longer avoided the P. argus side over the control side specifically. Instead, they were significantly more likely to occupy the mixing zone than either the control or the P. argus side, which did not differ significantly from one another. This pattern is consistent with a reduced movement, sheltering in place strategy, but could also reflect habituation or reduced chemosensory responsiveness. In contrast, fireworms made rapid, sustained avoidance responses to P. argus, consistent with active flight behavior. Contrary to our expectation, both corallivores showed weaker or no significant avoidance of the reef-obligate, P. guttatus than of P. argus, possibly reflecting differences in chemical cue composition, concentration, or perceived threat. These results indicate a previously undocumented risk induced behavioral response in which P. argus lobsters may contribute to reduced coral tissue loss without direct predation, although this study measured avoidance behavior rather than corallivory itself. Our findings highlight the importance of invertebrate predators in reef systems and suggest that incorporating predator presence into coral restoration planning merits further investigation as a means of leveraging natural predator–prey dynamics.
