Hard coralsScleractinia

The reef is a limestone building and the builders are animals a few millimeters across. Each polyp sits in a cup it made itself, laying calcium carbonate under its own base and climbing as that floor rises. Everything solid on a coral reef is the stacked cups of the dead; the living animal is a skin over the top of them. Break a piece off and the color stops within a millimeter of the surface; below that is nothing but rock.

A coral cannot go looking for a partner, so it works by appointment instead. Once a year, on nights set by the moon and the water temperature, colonies across a whole reef release their eggs and sperm inside the same hour, in packets that rise and break apart at the surface. Each species uses its own night and its own hour, which is what keeps the eggs of one from meeting the sperm of another in water that crowded. In this sea it happens in the warm months, a few nights after a full moon.

The building keeps records. Massive corals lay down one dense layer and one open layer every year, so a slab sawn from a boulder head and X-rayed reads like tree rings: count the pairs and you have its age, and the chemistry locked inside each band carries the temperature of the water that year. A head two meters across has been in the same spot for centuries, longer than anyone has been measuring anything in this sea.

Most of the energy for all this construction is not caught, it is grown. Single-celled algae live inside the coral's own tissue and hand over the sugar they make, which is why reef builders stop at the depth the light stops, and why a heat-stressed colony turns white: it has thrown the algae out and left clear tissue over bare limestone. Corals in the northern 🌊 Red Sea are unusually hard to push that far. The population that founded these reefs came in through the hot southern strait and kept the tolerance it needed to get through, so the corals of the Gulf of Aqaba still sit well below the temperature that bleaches them.

Not all of them build reef. The cup corals under overhangs and inside caves carry no algae at all, stay shut through the day, and open after dark as sheets of orange or black flowers that hunt whatever the current brings.