India’s Reefs Tell a Surprising Story of Survival in the Fourth Global Coral Bleaching Event

When the fourth global coral bleaching event swept across the tropics between 2023 and 2025, more than 80 percent of the world’s tropical coral reefs experienced extreme levels of heat stress, driven in large part by the powerful El Niño of 2023. For scientists watching the Northern Indian Ocean, one question loomed large: how had India’s scattered and understudied reefs fared? A new nationwide study, published in the journal Coral Reefs, offers the most comprehensive answer yet, and its findings upend several long-held assumptions about which corals can withstand a warming ocean. Drawing on a coordinated collaboration of professional researchers and citizen observers, the study assessed bleaching responses across five major reef regions of India, from remote oceanic atolls to mainland fringing and patchy reef formations, revealing a patchwork of vulnerability and resilience that defies simple prediction.

The scale of the effort was itself remarkable. India’s reefs are dispersed across vastly different oceanographic settings: the coral atolls of Lakshadweep in the Arabian Sea, the fringing and patch reefs of the Gulf of Mannar, Palk Bay and Goa on the mainland, and the islands of the Maldives-adjacent Andaman archipelago in the Bay of Bengal, including the Mahatma Gandhi Marine National Park (MGMNP). Data on mass bleaching impacts across South Asia have historically been sparse, leaving the region largely invisible in global assessments. By mobilizing a network of scientists, dive operators and trained citizen observers, the research team assembled standardized observations of bleaching and mortality across most of the subcontinent’s major reef areas during a single, globally significant thermal anomaly. The result is a rare, region-wide snapshot of how a mass bleaching event unfolds across an entire nation’s reef estate.

The study’s central metric was degree heating weeks, or DHW, the standard satellite-derived measure of accumulated heat stress that underpins global bleaching forecasting systems such as NOAA’s Coral Reef Watch. As expected, bleaching intensity generally increased with accumulated heat stress. But the strength of that relationship varied dramatically from region to region, exposing the limits of DHW as a universal predictor of reef damage. Two reefs exposed to similar thermal loads could emerge with profoundly different outcomes, a finding that echoes a growing body of evidence that global forecast models need regional and temporal calibration if they are to serve as reliable early-warning tools for reef managers.

Nowhere was this regional variability starker than in the contrast between Lakshadweep and the Mahatma Gandhi Marine National Park. Lakshadweep emerged as the hardest-hit region in the country, with up to 37 percent of coral cover bleached or dead even at intermediate levels of heat stress. The oceanic atolls, which sit in the open Arabian Sea with little local protection from sustained warming, appear to have offered their corals no thermal escape. In sharp contrast, MGMNP in the Andaman Islands recorded only 11.5 percent bleaching or mortality, despite experiencing significant heat stress. The authors attribute this striking difference to regional oceanographic processes capable of creating mesoscale thermal refugia, localized zones where currents, internal waves or other physical mechanisms buffer reefs from the worst of the heat. Previous work in the Andaman Sea has shown that large-amplitude internal waves can deliver pulses of cooler water to reef surfaces during thermal stress, and the new findings lend further weight to the idea that such hidden refugia may be decisive in determining which reefs survive the coming decades.

The taxonomic story proved equally compelling. Across all regions, the branching and plating genera Acropora, Pocillopora, Galaxea and Montipora emerged as the most affected, consistent with the canonical hierarchy of bleaching susceptibility established in reefs worldwide. These fast-growing, structurally complex corals are the architectural engineers of Indo-Pacific reefs, and their disproportionate losses carry cascading consequences for fish habitat, carbonate production and reef growth. At the other end of the spectrum, the massive and encrusting genera Pavona, Platygyra, Goniastrea and Favites were among the least affected, their thick tissues and stress-tolerant symbionts once again proving their worth under thermal duress. For reef managers, this hierarchy has long served as a rough rule of thumb for anticipating post-bleaching community composition.

But Palk Bay broke the rule in spectacular fashion. In this shallow, turbid bay on India’s southeastern coast, the study documented a near-complete reversal of canonical genus-level susceptibilities. The ordinarily vulnerable Acropora colonies showed only modest bleaching or mortality, at 13.6 percent, while the ordinarily resistant Porites colonies suffered a staggering 61.8 percent mortality. Such a reversal is rare and scientifically provocative. It suggests that local conditions in Palk Bay, possibly including prior exposure to recurrent heat stress, the presence of thermally tolerant Symbiodiniaceae symbionts such as Durusdinium, or the moderating effects of turbidity, have reshaped the thermal tolerances of its coral communities in ways that global generalizations cannot capture. It also serves as a cautionary tale: resistance traits are context-specific, and a genus that survives in one reef region may collapse in another.

The mechanistic implications reach deep into coral biology. Bleaching occurs when heat-stressed corals expel or lose the photosynthetic symbiotic algae living in their tissues, starving the coral animal of its primary energy source. Whether a coral bleaches, recovers or dies depends on an interplay of factors: the symbiont types it hosts, its history of thermal exposure, the energy reserves it carries into the event, and the physical environment surrounding it. High-frequency temperature variability, for instance, is known to reduce bleaching risk by priming coral physiological responses, while chronic local stressors can erode resilience. The Indian study’s regional contrasts, Lakshadweep’s open-ocean exposure versus MGMNP’s wave-buffered refugia, Palk Bay’s turbid, heat-conditioned waters versus the clearer reefs of the Gulf of Mannar, provide a natural experiment in how these mechanisms play out at landscape scale.

The findings arrive at a sobering moment. The fourth global bleaching event has confirmed what many reef scientists feared: that warming-driven bleaching is no longer episodic but is ushering in an era of near-annual thermal stress, with global warming tripling the persistence of marine heatwaves and intensifying them by roughly a degree Celsius. For India, the stakes are concrete. Lakshadweep’s atolls support island communities whose shorelines depend on reef-derived sediment, and studies have warned that most atolls may become increasingly uninhabitable by mid-century as sea-level rise exacerbates wave-driven flooding. The loss of up to 37 percent of coral cover in a single event, at only intermediate heat stress, signals that these reefs may have less thermal headroom than their Maldivian and Chagos neighbors, and that recovery between now and the next event is far from guaranteed.

Yet the study is not simply a eulogy. The survival of MGMNP’s reefs, the unexpected toughness of Palk Bay’s Acropora, and the resilience of massive genera across the country collectively point to genuine, mappable refugia and resistance hotspots that could anchor conservation planning. The authors argue that local oceanography and context-specific resistance patterns must be integrated into national and global assessments of bleaching impacts, rather than relying on heat-stress metrics alone. Protecting the oceanographic processes that cool reefs, identifying and safeguarding thermal refugia, and monitoring the survivors of this event as seed populations for recovery are strategies that emerge directly from the data. The study’s underlying dataset has been made openly available through Zenodo, an act of transparency that should accelerate comparative analyses across the wider Indo-Pacific.

As the ocean continues to warm, the fate of India’s reefs will depend on how quickly science can move from global averages to local realities. This nationwide collaboration has shown that the answers lie not in a single number on a satellite map, but in the interplay of currents, waves, symbionts and history that makes every reef region unique. In the ruins of Palk Bay’s Porites and the survivors of the Andaman Sea, Indian reef science has found both a warning and a roadmap.

Subject of Research: Regional and genus-specific coral bleaching responses across India's reef regions during the fourth global coral bleaching event

Article Title: Regional and genus-specific factors underpin bleaching variation across India’s corals reefs during the fourth global coral bleaching event

Article References: Pinto, W., Deshpande, K., Lobo, A. S., Jamalabad, A., Hussain, A., Paul, A., Dutta, A., Arjunwadkar, C., Patel, F. D., Thareja, H., Nangia, I., Josh, J., Goenka, K., Manikandan, B., Namboothri, N., Kuwalekar, P., Nambiar, S., Jaishankar, S., Mahesh, S., … Arthur, R. (2026). Regional and genus-specific factors underpin bleaching variation across India’s corals reefs during the fourth global coral bleaching event. Coral Reefs. https://doi.org/10.1007/s00338-026-02919-7

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02919-7

Keywords: coral bleaching, fourth global bleaching event, degree heating weeks, Lakshadweep, Andaman Islands, thermal refugia, Acropora, Porites, Palk Bay, thermal stress, Indian Ocean reefs, bleaching susceptibility