Every August or September on nights following a full moon, divers descend into the dark, warm waters off the Florida Keys to watch group sex — a fascinating reproductive eruption known as the annual coral spawn.

It's akin to an underwater snow flurry as colonies of polyps — flower-like creatures that live inside hard coral — cast small white sacs into the sea on the slim chance that sperm will drift into a genetically suitable egg, hook up, settle down and someday grow into healthy little corals of their own.

This year's spawn was particularly thrilling for scientists working to restore two important and vanishing species, staghorn and elkhorn coral, which formed the foundation of barrier reefs from Florida to the Caribbean until massive and mysterious die-offs over the last 30 years.

For the first time, stands of "farm-raised" staghorn transplanted to Molasses Reef off Key Largo were caught in the act, providing critical proof that corals cultivated in underwater nurseries can not only survive but do the wild thing.

For researchers, it has raised optimism that they might actually have a shot at slowing, maybe even reversing, decades of staggering loss. Staghorn and elkhorn — large and spectacular branching corals that once grew in sprawling forests — have declined by as much as 97 percent along a reef tract stretching from Palm Beach to the Dry Tortugas.

"This is the future of reef restoration," said marine biologist Ken Nedimyer, who helped pioneer coral cultivation in two nurseries he tends off the Upper Keys.

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