Bleached coral looks stark white, as if the skeleton has been dipped in bone-white paint while the coral is still alive. Healthy coral gets its rich color from microscopic algae living inside its tissue, so when those algae are lost, the transparent tissue reveals the bright white calcium carbonate skeleton underneath. But pure white isn’t the only visual sign. Some corals turn vivid neon purple, blue, or yellow before going white, and others fade to a pale, washed-out version of their original color. What you see depends on the species, how far the bleaching has progressed, and whether the coral is mounting a last-ditch defense.
Healthy Coral vs. Bleached Coral
A healthy reef is a patchwork of browns, greens, purples, and muted golds. Those colors come primarily from symbiotic algae called zooxanthellae that live inside the coral’s tissue by the millions. The algae photosynthesize and share nutrients with the coral, and their pigments give each colony its characteristic hue. A thriving reef looks dense and richly toned, sometimes even dull, because those living browns and olive greens dominate.
When a reef bleaches, it transforms dramatically. Individual colonies turn ghostly white, and across a large area, the effect is unmistakable: stretches of reef that once looked like an underwater forest suddenly resemble fields of white rubble. During mass bleaching events on the Great Barrier Reef, trained observers can identify the damage from aircraft flying overhead. Aerial surveys categorize reefs by how much white or fluorescent color is visible in shallow water, and during severe events more than 90% of the coral cover in a given area can appear bleached. That level of whitening is visible even from hundreds of feet in the air.
The Neon Phase
Not all bleaching corals turn white right away. Some produce an explosion of neon color, glowing in vivid purples, blues, pinks, and yellows that look almost artificially bright. These colors come not from the algae but from proteins the coral itself produces, part of the same family as green fluorescent protein (GFP). In some species, these proteins can make up as much as 14% of the coral’s total soluble protein content.
Scientists believe this fluorescent display is a protective response. The proteins act like a chemical sunscreen, scattering and absorbing excess light to shield whatever symbiotic algae remain inside the tissue. In a sense, the coral is trying to make conditions tolerable enough that the algae might return or hang on. The result is striking: a reef in early bleaching can look like a neon art installation, with colonies glowing in colors far more saturated than anything you’d see on a healthy reef. If conditions don’t improve, though, even these colorful corals eventually lose the battle and fade to white.
Why the Color Disappears
The underlying process starts when water temperatures stay too high for too long. NOAA tracks cumulative heat stress using a metric called degree heating weeks, which combines how far temperatures exceed the local norm and how many weeks that stress persists. When heat stress reaches about 4 degree heating weeks, bleaching risk becomes significant. At 8 degree heating weeks, widespread bleaching with mortality of heat-sensitive species is likely.
At the cellular level, the heat damages the algae’s ability to photosynthesize. Their internal chemistry goes haywire, and they start producing reactive oxygen species, essentially toxic byproducts that damage both the algae and the coral tissue around them. The coral’s body responds by treating its once-beneficial partner as a threat. It expels the algae, digests them, or sheds entire cells that contain them. Researchers have identified at least six different cellular pathways the coral uses to rid itself of the algae, including direct expulsion, digestion, and detachment of host cells. Once enough algae are gone, there’s nothing left to color the tissue, and the white skeleton shows through.
Which Corals Show It First
Bleaching doesn’t hit all corals equally, and the visual patchwork on a stressed reef reflects real biological differences. Fast-growing branching corals, particularly staghorn and similar species, are among the first to turn white. Their delicate, finger-like branches have a high surface-area-to-volume ratio that makes them especially sensitive to heat, and their skeletons are structurally weaker. On a reef in the early stages of a bleaching event, you’ll often see branching colonies already bright white while nearby boulder-shaped corals still hold some color.
Massive, dome-shaped corals tend to be more resistant. They bleach too, but they hold on longer and typically show a more gradual color loss, fading to pale tan or cream before going fully white. This means a partially bleached reef has a distinctive look: clusters of white branches interspersed with larger, still-colored mounds. The contrast is one of the clearest visual cues that heat stress is affecting a reef unevenly.
What Dead Coral Looks Like
Bleached coral is stressed but still alive. The tissue is intact, and if conditions improve within a few weeks, the coral can reabsorb algae and recover. But if heat stress continues, the coral starves and dies. Dead coral looks different from bleached coral in important ways.
At first, a recently dead colony may still be white, making it hard to distinguish from a living bleached one without close inspection. Within days to weeks, though, algae from the surrounding water begin colonizing the exposed skeleton. The white surface turns fuzzy green or brown as filamentous algae take hold. Over months, the skeleton becomes increasingly overgrown, losing its defined shape under layers of turf algae and sediment. Eventually, what was once a recognizable coral colony becomes an algae-covered lump, sometimes further eroded by boring organisms that tunnel into the weakened skeleton. On aerial surveys of severely damaged reefs, observers can identify recent mortality by the brownish-green patches among the white, a sign that some colonies have already crossed the line from bleached to dead.
What Recovery Looks Like
When water temperatures drop and conditions stabilize, surviving corals can regain their algae and return to their original color. Recovery is visually the reverse of bleaching but slower and less dramatic. A recovering coral gradually shifts from white to pale brown as algae repopulate the tissue. The brown color, while less visually impressive than the neon phase or even the pre-bleaching hues, is actually the clearest sign of health. It means the algae are back and photosynthesizing.
During recovery from a 2024 bleaching event at Lord Howe Island, marine scientists documented corals darkening back to brown as they reabsorbed their symbiotic algae over a period of weeks. Full recovery of color typically takes several weeks to a few months, depending on the severity of the bleaching and the species involved. But a coral that recovers its color isn’t necessarily back to full health. Bleaching depletes energy reserves, slows growth, and reduces reproductive output for months or even years afterward. A reef that looks colorful again may still be fragile.
The Scale of What’s Happening Now
The world is currently in its fourth global mass bleaching event, confirmed by NOAA in April 2024. Since January 2023, bleaching-level heat stress has affected roughly 84.4% of the world’s coral reef area, and mass bleaching has been documented in at least 83 countries and territories. That means the visual transformation described here isn’t happening in isolated patches. It’s playing out across entire ocean basins, from the Caribbean to the Indian Ocean to the Pacific, turning thousands of square miles of once-colorful reef into fields of white, neon, and, in the worst cases, algae-covered rubble.