What bleaching actually is
A CORAL POLYP is an animal, a tiny relative of the ANEMONE sitting in a stony cup it built itself. But it cannot feed itself properly, so it keeps a crop: microscopic ALGAE living inside its own tissue, catching sunlight and handing over most of the sugar they make. The algae are also what give the REEF its colour. The coral animal is nearly transparent.
When the water stays too warm, that arrangement breaks down and the polyp expels its algae. The white you see in a bleached reef is not dead coral — it is the bare limestone skeleton showing through clear, empty tissue. The animal is still alive at that point and can recover if the heat passes quickly enough. If it does not, it starves. That is why a degree or two of ocean warming matters so much more than it sounds.
A healthy reef is deafening
Put a microphone underwater on a living reef and it sounds like frying bacon. Snapping shrimp fire their claws hard enough to make a bubble collapse with a crack, fish grunt and pop, PARROTFISH scrape audibly at the rock. A dead reef, by contrast, is close to silent.
That turns out to matter, because baby fish and coral larvae drifting in the PLANKTON use the noise to find somewhere to settle. Researchers have tried the obvious next step: loudspeakers on damaged reefs playing recordings of healthy ones. Twice as many young fish moved in as on the silent control patches. It does not rebuild a reef by itself, but it is a rare piece of good news in this subject.
The starfish that eats reefs
Not every threat is the temperature. The crown-of-thorns is a STARFISH the size of a dustbin lid, covered in venomous spines, and it eats coral — pushing its stomach out through its mouth and digesting the POLYP off the skeleton where it sits. One animal can strip several square metres a year.
A few of them are a normal part of a reef. The trouble is the outbreaks, when numbers explode into the millions and strip whole sections bare, and they appear to be made worse by fertiliser running off the land and feeding the PLANKTON their larvae eat. Divers have spent decades killing them one at a time by hand, and Australia now sends down a robot that recognises the shape and gives each one a lethal injection by itself.
The busiest address in the ocean
Reefs cover well under one per cent of the OCEAN floor and support something like a quarter of all sea life — which is the reason for the rainforest comparison. The Great Barrier Reef runs for over 2,000 kilometres and is the largest structure ever built by living things, assembled cup by cup over thousands of years.
The residents have some strange arrangements. Every CLOWNFISH hatches male; the largest fish in an ANEMONE becomes the female, and if she dies her partner changes sex and takes over. A SEAHORSE is the one fish where the male carries the pregnancy. And the whole grid lives in the same warm TROPICAL LAGOON: a SHOAL flickering past a SHARK, a TURTLE grazing, an EEL in a crevice, a CRAB, a STARFISH, a SPONGE, and a DIVER with a SNORKEL floating over the top of the most COLORFUL neighbourhood on Earth.