03 July 2019
FLORIDA KEYS — The hard corals of the Florida Keys reefs have spent eons developing a simple system to perpetuate the species, one of nature’s mysteries that works without human intervention or complete understanding.
The coral spawn is scientifically observed and documented each year. A few days after the late summer full moon phases, corals throw caution to the wind, typically for one to three nights of unabashed “sex” along Keys reefs — the continental United States’ only living coral barrier reef.
Local professional dive charters offer twilight trips to shallow reefs on or around the full moons in August and September, so divers and snorkelers can watch the action as citizen scientists of sorts.
By nature’s design, the coral “love affair” results in the release of millions of eggs and sperm, called gametes — and once they’re united, the newly formed larva or “planula” rises smoke-like through the water. The milky substance represents the future of coral life.
According to researchers, this synchronized “broadcast spawning” enables the immobile animals to spread gametes over a wide area in massive quantities to increase the probability of fertilization while overwhelming predators, the local reef fish population, who come out to feed.
The planula free-floats in surface currents and, after some time — two days to two months — settles to the bottom where it grows into a polyp. The polyp grows into a coral head by asexual budding that creates new polyps.
Different species are on different schedules and the exact hour of release is based on local time, but synchronous release is still the rule. August’s full moon, whether it occurs early or late in the month, seems to have the highest probability as the triggering event from which coral spawning predictions are made.
Scientists sleuthed this theory out by making repeated observations from year to year of the histology in the corals, recognizing there was some kind of lunar cycle in the spawning. Multiple environmental cues such as water temperature and tidal and 24-hour light cycles also contribute. 
“Sexual reproduction is the only way that corals create new genetic individuals,” said Amelia Moura, science program manager at Key Largo–based Coral Restoration Foundation. “Genetic diversity is an essential component of resilient ecosystems, as it allows them to adapt to changing conditions.”
CRF has established huge coral tree nurseries along the Keys’ reef tract that represent a unique ecosystem where hundreds of genotypes — or genetically unique individuals — of staghorn coral can spawn every year in close proximity to one another.
“These corals are ‘seeding’ surrounding reefs,” added Moura. “At these ‘living laboratories,’ researchers are also able to collect gametes, fertilize the eggs and raise coral babies in the nursery, eventually helping us to further restore the genetic diversity in wild coral populations.”
Typically, branching corals in the Florida Keys such as finger, staghorn and elkhorn corals, spawn three to five days after the full moon about two hours after sunset. Boulder corals such as brain and star corals traditionally spawn six to eight days after the August full moon about three hours after sunset.
Branching corals are expected to spawn following the Aug. 15 full moon, around Aug. 18 and 20. Though the polyp release cannot be guaranteed to happen on the exact date, the next full moon falls on Sept. 14.
Also exciting for divers is that, after twilight, several critters and creatures come out of the coral reefs to feed and move about, offering extraordinary opportunities to see marine life that normally hides during sunlight hours. Colors are especially vibrant when they are illuminated with divers’ underwater lights. 
Divers and snorkelers interested in the chance to view the underwater phenomenon of coral spawning can contact Florida Keys professional dive operators to join coral spawning charter trips. To find a dive shop offering these unique eco-excursions, visit fla-keys.com.diving.
Florida Keys visitor information: fla-keys.com or 1-800-FLA-KEYS
Florida Keys diving information: fla-keys.com.diving
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