Encounter · Coral Bleaching

Recognising Coral Bleaching

What the diver who has seen it didn't know they were looking at

You may have photographed it without knowing what it was. A coral that had turned white — completely, startlingly white, the skeleton visible through tissue so pale it was nearly translucent. Or perhaps a coral that had gone a colour you had never seen on a reef before: vivid purple, electric pink, a yellow so saturated it looked artificial. You thought it was unusual. You thought it was beautiful. You took the photograph and moved on.

Both of those corals were in the process of dying.

Coral bleaching is the most visible sign of thermal stress in a reef system, and it is the event that, more than any other, is reshaping what reefs look like worldwide. The diver who has been underwater in the last decade has almost certainly seen it. Most have not known what they were looking at. This article changes that.

The mechanism

What is actually happening inside the coral

Coral is an animal. That fact surprises many divers who have thought of reef structure as something closer to rock or plant. Each coral colony is composed of thousands of individual polyps — tiny, soft-bodied animals — that secrete the calcium carbonate skeleton beneath them and live within it.

But the coral polyp cannot survive on its own. It lives in partnership with microscopic algae called zooxanthellae — single-celled organisms that live within the coral's tissue. These algae photosynthesise, converting sunlight into energy, and share that energy with the coral. In return, the coral provides the algae with shelter, carbon dioxide, and the nutrients they need. The relationship sustains both. The zooxanthellae also give the coral most of its colour — the rich browns, greens, and golds of a healthy reef.

When ocean temperatures rise even slightly above the coral's tolerance threshold — typically just 1–2°C above the seasonal maximum, sustained for several weeks — the relationship breaks down. The elevated temperature disrupts the zooxanthellae's photosynthetic process, causing them to produce harmful reactive oxygen molecules. The coral's response is to expel the algae entirely — ejecting the very organisms that give it energy and colour.

The white coral is not dead. It is a coral that has expelled its algae and is now surviving on its own reserves. It is under extreme stress. It is transparent. What you are seeing through the tissue is the white calcium carbonate skeleton beneath.

Without its zooxanthellae, the coral loses up to 90% of its energy supply. It can survive for several weeks in this state — if the temperature drops and conditions return to normal, the algae can recolonise and the coral can recover. If the stress continues, the coral starves and dies, and its skeleton is colonised by algae of a different kind, turning it the dark brown or green of a dead reef.

The threshold at which this happens is not dramatic. A global average temperature increase that sounds small in conversation — one degree, two degrees — translates directly into bleaching events that affect entire ocean regions. The Great Barrier Reef experienced mass bleaching in 2016, 2017, 2020, 2022, and 2024. What were once rare events are becoming annual ones.

The reframe

The most beautiful thing on the reef
may be the most stressed

Before the stages, one thing needs to be named directly — because it is the thing most likely to confuse a diver who encounters bleaching without this knowledge.

Some bleaching corals do not turn white. They turn vivid, saturated, extraordinary colours — purple, pink, yellow, cyan — colours that look nothing like stress and everything like spectacle. Divers photograph these corals with genuine delight. They are among the most visually striking things you can see on a reef. And they are corals in crisis.

What is happening is this: when the zooxanthellae are expelled and the coral loses its normal pigmentation, some coral species respond by producing their own fluorescent pigments. These pigments act as a kind of sunscreen — the coral is attempting to protect itself from the UV radiation that the loss of its algal partner has made suddenly dangerous. The colours are real and vivid precisely because the coral is producing them in quantities it would never need under normal conditions.

The fluorescent phase is a last attempt at self-protection. It is also a brief one. If the thermal stress does not relent, the coral will exhaust this response and proceed to death. The photograph you took of the most extraordinary-looking coral on the reef may be the last record of it alive.

Fluorescent colours and what they indicate

The colours of a coral under thermal stress

Purple and violet — among the most commonly seen fluorescent stress responses. Often appear at the tips and edges of branching corals first, before spreading to the entire colony.

Pink and magenta — particularly striking on table corals and brain corals. The saturation is often more intense than anything seen on a healthy reef.

Yellow and gold — less common but unmistakable. Often appears on massive Porites corals and may persist for several weeks before the coral either recovers or dies.

Cyan and blue-green — the rarest of the bleaching colours, associated with specific coral genera. Exceptionally photogenic and exceptionally concerning.

The field guide

The four stages of bleaching

What to look for at each stage — and what it means for the coral's survival.

Pale

Early thermal stress — zooxanthellae density declining

The coral's colour has faded but not disappeared. Browns have become tans, greens have become yellows, the overall impression is of a reef that has been washed out. The tissue is still present and the zooxanthellae are still there, but their population is declining. This is the stage at which intervention — a reduction in local stressors, a drop in water temperature — has the best chance of reversing the process. Most divers do not recognise this stage because the change is gradual and easy to attribute to light conditions or depth.

White

Full bleaching — zooxanthellae expelled, skeleton visible

The coral is fully bleached. The zooxanthellae have been expelled and the tissue, now transparent, reveals the white calcium carbonate skeleton beneath. The coral is alive — the polyps are still present and can still feed by capturing zooplankton at night. A fully bleached coral can survive for several weeks, but is surviving on reserves alone. Recovery at this stage requires the water temperature to drop and the zooxanthellae to recolonise before those reserves are exhausted. The window is narrow.

Fluorescent

Stress response — fluorescent pigments produced as UV protection

The coral is producing fluorescent pigments as a protective response to the UV radiation it is now exposed to without its algal partner. The colours — purple, pink, yellow, cyan — are real and vivid. They are also a signal of acute stress. The more saturated the colour, the more desperately the coral is protecting itself. This stage may precede, follow, or occur simultaneously with full whitening, depending on the coral species. It is not a sign of health. It is the most misread visual signal in bleaching events.

Dead

Tissue loss — skeleton colonised by turf algae

The coral polyps have died and the skeleton is now being colonised by turf algae, which gives it a dark brown, green, or grey appearance. This is often mistaken for a healthy coral by divers unfamiliar with the difference between live coral tissue and algae-covered skeleton. Dead coral can look textured and alive from a distance. Up close, the absence of polyp tissue and the presence of fine filamentous algae over the skeleton surface are the distinguishing features. A dead coral's structural contribution to the reef may persist for years — until physical damage, bioerosion, or further bleaching reduces it to rubble.

The prognosis

Bleached is not dead. What determines whether a coral recovers.

The single most important thing to understand about bleaching is that a bleached coral is not necessarily a dead one. The coral has expelled its algae and is under extreme stress, but the animal itself is still alive. Recovery is possible — and understanding what determines it changes how the diver reads the reef after a bleaching event.

Duration of thermal stress. A brief spike — one or two weeks — gives the coral a genuine chance of recovery. Sustained elevated temperatures over months, which is increasingly the pattern, reduce it dramatically.

Severity of bleaching. A coral that has paled but not fully whitened has retained some of its zooxanthellae and has a better prognosis than one that is fully bleached. Partial bleaching — affecting some branches or sections but not others — is often survivable.

Local water quality. Reefs under additional pressure from pollution, sedimentation, or nutrient runoff have significantly lower recovery rates. Reducing local stressors before and after a bleaching event measurably improves outcomes.

Coral species. Some species are more thermally tolerant than others. Massive Porites corals, for example, tend to bleach later and recover more reliably than branching Acropora, which are among the first to bleach and the least likely to survive prolonged stress.

Previous bleaching history. A coral that has bleached and recovered before may have some tolerance advantage — or may be so depleted that its reserves are insufficient for another recovery. The cumulative stress of repeated events is the defining challenge of the current era.

Recovery, when it happens, is slow and visible. Within weeks of temperatures dropping, zooxanthellae begin to recolonise bleached tissue, and the coral gradually regains its colour — starting pale and deepening over months. A diver who saw a reef during a bleaching event and returns six months later may see the same corals restored, or may see them dark and algae-covered. The reef will tell you which it is, if you know what to look for.

The recolonisation process is not guaranteed. A coral that regains its zooxanthellae in the weeks after bleaching has survived. A coral that remains white for more than two months has very likely not survived — though a small number of slow-recovering species, such as massive Porites, may take longer before a final prognosis can be read.

What to do with the observation

The diver as witness and recorder

A diver who descends on a reef during or after a bleaching event and can identify what stage the coral is in, which species have bleached and which have not, and whether recovery is beginning — that diver is carrying scientific data. The question is what to do with it.

CoralWatch was built precisely for this observation. Its Coral Health Chart documents the colour of individual corals across a standardised six-point scale — the same scale that maps directly onto the pale-to-white bleaching progression described in this article. A diver who completes a CoralWatch survey during a bleaching event is adding ground-truth data to a global database that tracks bleaching in near real-time across 130 countries. The data is openly available and used by reef scientists worldwide.

Reef Check's monitoring protocol captures bleaching at a more structured level — trained EcoDivers conduct transect surveys that record bleaching percentage across coral cover, enabling before-and-after comparisons at permanent monitoring sites. If you are diving a site that has been bleached, and the site is within Reef Check's network, your survey becomes part of the long-term record of that reef's recovery or decline.

The photograph you took of the fluorescent coral is not just a photograph. It is a record of a specific coral, at a specific location, at a specific point in a bleaching event. With a date and GPS coordinates, it becomes data. With a CoralWatch submission, it becomes science.

You now know what you were looking at. The next time you descend on a reef that has been through a bleaching event — or is in the middle of one — you will not surface uncertain. You will know what stage the coral is in, what its prognosis is, and what your observation is worth. The Citizen Science Directory at Dive Conservation lists the programmes built to receive it.

What is actually happening inside the coral

The most beautiful thing on the reefmay be the most stressed

The four stages of bleaching

Bleached is not dead. What determines whether a coral recovers.

The diver as witness and recorder

The most beautiful thing on the reef
may be the most stressed