Intermediate 8 minutes
Coral bleaching: what it is and what it isn't
A white coral isn't dead yet, and that distinction changes everything
When a coral bleaches, it isn't painting itself white: it's turning transparent. What you're seeing is its own skeleton showing through tissue that has lost all its color. And at that point the animal is still alive, even though it has started to starve.
In this article you'll understand the mechanism behind bleaching, what causes it, under what conditions a coral can recover, and how to tell a bleached coral apart from a dead one during a dive.
Bleaching is the phenomenon that has transformed the world's reefs more than any other in recent decades. Knowing how to read it lets you understand what you're actually looking at on an affected reef, without falling into either alarmism or indifference.
What exactly happens
In the biology article you saw that coral hosts zooxanthellae in its tissue and gets most of its energy from them. Bleaching is the breakdown of that partnership.
When coral is subjected to sustained stress, above all from elevated temperature, the photosynthesis carried out by its algae starts producing harmful compounds instead of useful sugars. The relationship stops being beneficial and turns toxic. The coral's response is to expel its symbionts.
Since a coral's color comes largely from those algae, the tissue becomes almost colorless and translucent, and through it you can see the white calcium carbonate skeleton. Hence the name: there's no white pigment, there's an absence of everything else.
This is the critical point worth fixing in your mind: a bleached coral is alive. It has lost its main food source and is running on reserves and whatever little it can catch by hunting, but the animal is still there.
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