Coral Spawning: The Night Reefs Release Millions of Eggs

On the night of August 13, 1990, divers anchored over the Flower Garden Banks, two underwater mountains rising from the Gulf of Mexico floor more than 100 miles off Texas, watched pale pink dots begin lifting off the reef an hour after sunset. The dots were coral eggs. Colonies that had sat separate and silent all year were releasing their entire reproductive output in the same hour, and the water above the reef filled with drifting bundles. The crew of the dive boat Fling had documented the first mass spawning ever recorded in the Atlantic, and NOAA’s sanctuary researchers have timed the display nearly every year since.

The event is coral spawning, the reef’s annual reproduction compressed into a few hours. Its choreography is a two-scale problem: every colony on the reef must agree on the night, and every species must agree on its own hour. How corals solve that problem, and what the drifting larvae face afterward, explains why a single bad summer can quiet a reef for years.

How Do Corals Reproduce: Bundles, Brooders, and Broadcast

Most reef-building corals are hermaphrodites. Each polyp carries both sexes, and in the weeks before spawning it packages eggs and sperm into buoyant bundles roughly the size of BBs, visible for days as beads beneath the polyp tissue. On the night, polyps release simultaneously. The bundles rise through the water column, break apart at the surface, and fertilization happens in open water. NOAA’s education page for the sanctuary notes the aftermath plainly: floating gametes leave a sheen on the surface and a distinctive odor.

The broadcast method has variants. Great star coral keeps the sexes on separate colonies, so male colonies release milky clouds of sperm while female colonies hold their eggs roughly half an hour longer and release them already fertilized. A smaller set of species skips the water column entirely and broods larvae internally.

The scale of the synchrony is recent science. In 1984, a team led by Peter Harrison published the first documented mass spawning in Science, based on observations across the Great Barrier Reef. Follow-up work two years later recorded 105 coral species spawning within the same narrow window. Fertilized eggs develop into planulae: flattened, ciliated coral larvae smaller than a grain of rice that drift for days to weeks before testing the reef surface for a place to cement themselves and begin secreting skeleton.

The Cue Stack: Moonlight, Sunset, and Warm Water

The full recipe remains open. After more than three decades of monitoring, NOAA researchers at Flower Garden Banks still describe the exact cues as undetermined, most likely some combination of lunar cycle, solar cycle, water temperature, and chemical or light signals. The observable clockwork is precise:

• Spawning typically runs 7 to 10 nights after the August full moon.
• Release begins about an hour after sunset and continues for roughly four hours.
• Species peaks arrive on the eighth and tenth evenings in a consistent order, which keeps the eggs of one species from meeting the sperm of another.

That last detail points at the harder half of the problem. A reef of millions of polyps runs no central clock; whatever the cue turns out to be, every colony reads it the same way.

The calendar decides which month the window lands in. In 2026 the August full moon fell on August 27, which pushed the 7-to-10-night window into early September, around the 3rd through the 6th. NOAA sanctuary research ecologist Michelle Johnston has observed that in years when August holds two full moons, the split lunar calendar can produce a second spawning event in early September, an extra larval delivery for the reef. The next full moon, the Harvest Moon on September 26, is the one divers in Florida and the Caribbean are counting from now.

Inside a Mass Coral Spawning Event

Divers describe an upside-down snowstorm. Bundles lift off in sheets from every colony, the water fills with rising specks, and fish feed in the drift until they are sated. The sanctuary’s video record even catches ruby brittle stars climbing the colonies to intercept bundles mid-release. The volume is the strategy. NOAA attributes the event’s function to sheer number: enough gametes that fertilization succeeds despite dilution and predation, with the surplus designed to disperse genetically mixed offspring across large distances. At Flower Garden Banks the display is among the densest in the Caribbean because the reef carries an unusually high cover of broadcast-spawning species.

Coral Spawning Season Florida: The 2026 Windows

Florida and the wider Caribbean run on the same lunar logic, keyed to late-summer full moons. Applying the 7-to-10-night rule to the 2026 calendar gives two windows: the August 27 full moon set this year’s primary window around September 3 to 6, and the September 26 full moon sets up the next around October 3 to 6. The sanctuary’s decades of logs translate into a short planning list: be in the water at sunset, plan several consecutive evenings because species peak on different nights, and check the sanctuary’s spawning observations page for local timing before booking a trip.

Stressed Reefs Skip the Snowstorm

Spawning is expensive, and a stressed colony economizes. NOAA notes that stressed corals lack the energy to reproduce, so a predictable, vibrant spawning event doubles as a health indicator for the reef. The converse is measurable. After back-to-back bleaching killed much of the adult broodstock on the Great Barrier Reef in 2016 and 2017, larval recruitment in 2018 fell 89 percent below historical levels, according to a study led by Terry Hughes at James Cook University. The moon kept its schedule; the adults were gone.

The larvae that are produced swim into a crowded field of stressors. Heatwaves that bleach adults before the night even arrives shrink the broodstock, the mechanism behind marine heatwave bleaching forecasts. Oxybenzone, the sunscreen filter examined in the oxybenzone breakdown, is most destructive at exactly this stage: the 2015 study led by Craig Downs that anchored the first sunscreen bans recorded a laboratory LC50 of 139 micrograms per liter in light, along with DNA damage and skeleton deformities in planulae at lower doses. And a larva that settles must lay down its first skeleton in water roughly 30 percent more acidic than the preindustrial ocean, the calcification squeeze covered in the acidification explainer.

The counting rule is short: 7 to 10 nights after the full moon, in the water at sunset, for as many evenings as the schedule allows. The number worth carrying is 89 percent, what a reef’s larval supply becomes the year after its adults cook. The upside-down snowstorm happens once a year when enough of the reef survives to throw it, and every colony on the wall you swim past is a lottery winner: one fertilized bundle in millions that found hard bottom and started building.