Assisted evolution breeds corals and their Symbiodinium partners for heat tolerance: selective crossing, symbiont shuffling, fluorescent tagging of recruits. Almost all of it comes down to one or two nights a year, when the broodstock tanks spawn.

The look is that night. Black-blue seawater, red head-lamps because corals cannot see red, pink-orange bundles rising to a slick on the surface. The next morning the same recruits sit under blue excitation and fluoresce green. Nothing here is decorative — every colour is a signal someone recorded.