What actually lives inside a coral polyp?

Single-celled algae called zooxanthellae, living inside the coral's own tissues. They photosynthesise and hand over most of the sugar they make - up to ninety per cent of the coral's energy - and in return get shelter and a supply of the nitrogen and carbon dioxide the animal produces. The coral is an animal running on a captive plant.

That is why reefs exist where they do. Tropical surface water is nutrient-poor - a marine desert by any ordinary measure - and a reef is one of the most productive ecosystems on Earth sitting in the middle of it. The partnership is what makes that possible, and it is also why corals need shallow clear water: the algae need light.

It is also the failure mode. When water is too warm for too long the relationship breaks down and the coral expels the algae, losing both its colour and its food supply - which is what bleaching is. A bleached coral is not dead and can recover if conditions ease, but it is starving, and the Great Barrier Reef has now had mass bleaching events in 1998, 2002, 2016, 2017, 2020, 2022 and 2024. The intervals have become shorter than the ten or so years a damaged reef needs to rebuild.