Coral reefs occupy less than one tenth of one percent of the ocean floor, yet they support roughly a quarter of all known marine species. This astonishing density of life is built, quite literally, by animals: stony corals secrete calcium carbonate skeletons that accumulate over centuries into the vast, wave-resistant structures we call reefs.
The corals themselves survive through one of nature’s most celebrated partnerships. Embedded in their tissues live single-celled algae known as zooxanthellae, which photosynthesise and pass most of the resulting sugars to their hosts. In return, the algae receive shelter and a steady supply of nutrients. It is these algae that give corals their colour — and it is their departure that gives bleaching its name. When water temperatures remain elevated for weeks at a time, the partnership breaks down: corals expel their algae, turn bone white, and begin to starve.
Less famous than the corals, but scarcely less important, are the reef’s grazers. Parrotfish spend their days scraping algae from the reef surface with beak-like teeth, preventing fast-growing seaweeds from smothering young corals. In the process they grind up coral rock and excrete it as fine white sediment; a single large parrotfish can produce hundreds of kilograms of sand each year, much of which eventually settles on the beaches that tourists admire.
Scientists increasingly argue that reef conservation must look beyond the corals themselves. Marine reserves that protect herbivorous fish from overfishing consistently show faster recovery after bleaching events, because grazed surfaces give coral larvae the bare rock they need to settle. The lesson is one ecologists have learned in forests and grasslands alike: protect the system’s engineers, and the architecture tends to look after itself.