Scubaism

Every year, patrons of Florida Keys dive and snorkel shops play a guessing game. Locals and visitors alike try to guess the date and time of the annual coral spawn — sometimes nicknamed “sex on the reef” — on North America’s only living coral barrier reef, which parallels the island chain.

This year, however, guessing the correct date and time is complicated by two full moons 30 days apart that are likely to create two spawns instead of one.

Corals have evolved a method of reproduction called “broadcast spawning.” The immobile animals send their eggs and sperm into the water in massive quantities. When egg and sperm unite, the resulting larval-stage “planula” swims to the surface to drift in the current and grow. After some time — two days to two months — the planula settles to the bottom where it grows into a polyp. The polyp grows into a coral head by asexual budding that creates new polyps.

Key to the successful creation of the next generation is corals in a given location broadcasting their eggs and sperm cells simultaneously. So the great mystery that has challenged researchers is: How do brainless, stationary, uncommunicative corals know exactly when to broadcast their spawn?

Thanks to the pioneering research of Dr. Alina Szmant of the University of North Carolina Wilmington, and ongoing studies by Dr. Margaret Miller of the National Oceanographic and Atmospheric Administration’s National Marine Fisheries Service, the mystery surrounding the coral spawn gradually is being solved. Szmant’s observations have shown that Caribbean corals spawn around the August full moon.

The generally accepted schedule is for branching corals (e.g., finger, staghorn and elkhorn corals) in the Florida Keys to spawn three to five days after the full moon, about two hours after sunset. Star and boulder corals spawn six to eight days after the August full moon, about three hours after sunset.

But this summer, the show is expected to hit the currents twice. There are full moons, on July 29 and Aug. 28. The last time there was a similar schedule of full moons, the corals broke ranks.

In 2004 the full moons were on July 31 and Aug. 29. Acroporidae, the family that includes elkhorn, staghorn and finger corals, spawned early in August. Montastrea, the family that includes star and boulder corals, spawned in September, about 30 days later.

Miller’s team expects to see branching corals spawn between Aug. 1 and 3. Star corals are expected to spawn between Sept. 3 and 5. But Miller added, “It’s impossible to tell.”