Clockwork sex of coral reef algae.

Birds and bees do it-and we know in fairly intimate detail how they do it-but sexual reproduction among algae is often a secretive, little-studied affair. Cryptogams, they have been called, because of their hidden sex lives. Now, a chance observation by a fish ecologist sheds new light on how algae in a group important to coral reefs get their gametes together.

The insight had a lot to do with being in the right place at the right time: the coral reefs off Panama at 5 a.m. Kenneth E. Clifton and his coworkers at the Smithsonian Tropical Research Institute went snorkeling each morning to monitor fish reproduction on the reef. On one of the dives, Clifton says he "saw these algae smoking," puffing out plumes of milky green cells.

After more than a year of studying the algae and timing their eruptions, Clifton pieced together the first detailed picture, reported in the Feb. 21 Science, of how the algal community organizes sexual reproduction. "Most of these kinds of events, if they are studied at all, are only studied in the lab," says Mark Hay, a marine algae specialist at the University of North Carolina at Chapel Hill. "This is a huge step forward."

Commonly called seaweed, the algae sprawl and drape on and around coral reefs worldwide. They often show up on satellite images. As photosynthesizers, they are food for the reef organisms. Some species cut down on this predation with an internal, shell-like structure of calcium carbonate that may interfere with digestion in fish, says Clifton, now at the University of California, Santa Cruz.

Like plants, the algae can reproduce asexually. But more frequently than marine biologists had realized, these algae reproduce sexually, in a quick, tightly choreographed exchange at dawn.

Unlike most plants-but like spawning salmon-the algae's sexual exchange marks the end of the individual. Overnight, a seaweed develops grapelike clusters or other reproductive structures. Then, the organism streams into the structures, and the entire mass is discharged as gametes 24 hours later. The males of each species go first, followed within minutes by the females. Clifton noted "spawning" by different individuals within a species every few days.

"The water turns green, you can't see anything, and 15 minutes later it's over," he says. Waves break up the calcified remains, destined for a sandy beach. The fused cells settle on the reef.

As impressive as the clockwork of the sexes within a species, says Hay, is the evolutionarily important difference in timing among species. "Closely related species are partitioning out this narrow band of time in the morning so that they're not overlapping with each other" and thus not interbreeding.

What accounts for the timing? "It's a wide-open question," says Clifton, although light probably plays a part. Females kept "in solitary confinement" in a bucket still go off within a few minutes of their mates on the reef.