Coral is an animal that cannot move, so it solves reproduction by timing. Once a year, on a handful of nights, entire reefs release their eggs and sperm within an hour of each other — millions of colonies, thousands of square kilometres, synchronised. The water fills with pink and orange bundles rising like a snowfall going the wrong way, and by morning it is over.
The trigger is a combination of sea temperature, day length and the lunar cycle. Most reefs go a few nights after a specific full moon, usually between one and four hours after sunset, and local scientists get better at predicting it every year without ever getting it exactly right. It was only described scientifically in 1981, on the Great Barrier Reef — before that nobody knew reefs did this.
You dive it at night, which means torches, and the torches are part of the problem: too much light and you are looking at plankton instead of coral. A good operator dives with red or dimmed lights and knows which colonies on their site go first. Expect one to three attempts. People who catch it describe it as the strangest thing they have seen underwater; people who miss it by a night see a normal reef and a lot of disappointed marine biologists.
Where on Earth it's best
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★ Signature spotGreat Barrier Reef — Cairns & Port DouglasAustraliaThe reef where mass spawning was first described, in 1981, and still the largest synchronised event on the planet — 400 coral species across 2,300 km releasing within the same few nights. The scale is what separates it: on the inner reefs the slick that forms on the surface next morning can be seen from the air, a pink stain kilometres long. Cairns operators run dedicated overnight spawning trips and the local research stations publish their predicted windows each year.Nowhere else combines this many species, this much reef and this much accumulated prediction data. Usually the nights following the full moon in late October, November or early December — inner reefs typically go a month before outer reefs. Water must be above about 26 °C. 2
Ningaloo ReefAustraliaA fringing reef you can swim to from the beach, which makes this the one place where a snorkeller rather than a diver can see it. The spawn also triggers the plankton bloom that brings whale sharks in behind it. Seven to nine nights after the March or April full moon. 3
Flower Garden BanksUnited StatesThe most predictable spawn on Earth. This isolated Gulf of Mexico reef, 160 km offshore, has gone seven to ten nights after the August full moon almost every year since monitoring began in 1990 — reliable enough that dive boats sell the dates a year ahead. Seven to ten nights after the August full moon, typically an hour or two after sunset. 4
Kerama Islands & OkinawaJapanNorthern-hemisphere reefs at the edge of coral’s temperature range, spawning in early summer — and unusually well studied, because Okinawan researchers have been restoring reefs with spawn-derived larvae for two decades. May to June, on nights following the full moon, once water passes about 25 °C. 5
Red Sea — Eilat & AqabaIsrael / JordanThe most-monitored reef in the world for this: the Interuniversity Institute at Eilat has recorded spawning dates for decades, and the corals here are unusual in tolerating heat that would bleach reefs elsewhere. June to July, generally a few nights after the full moon. 6
PalauPalauSpawning inside a marine sanctuary the size of Spain, on reefs healthy enough that the aggregation of fish arriving to eat the spawn is its own spectacle. March to June depending on species, on the nights around the full moon. 7
Curaçao & BonaireCuraçao / BonaireCaribbean shore diving with a research station attached — CARMABI in Curaçao publishes a spawning calendar every year, and you walk into the water from the road. September and October, three to six nights after the full moon. 8
Mesoamerican Reef — Cozumel & BelizeMexico / BelizeThe second-largest barrier reef in the world, spawning in late summer, with Belize’s protected atolls giving the healthiest coral cover in the region. August and September, in the nights after the full moon.