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How Coral Reefs Spawn
Watch coral reefs reproduce in a flurry of carefully-timed action
It's 9 p.m. and the corals are still not spawning.
Nancy Knowlton and I have been underwater for an hour, diving and snorkeling about 350 feet off the coast of Solarte Island, one of 68 islands and mangrove keys on Panama's Caribbean coast.
Neon-green glow sticks hanging from underwater buoys guide our way. Occasionally, I rise to the surface and hear the thumping bass of Latin music from a coastal town. The moon is full. Surely, this is the perfect setting for a coral love fest.
But then I recall what Knowlton had said that morning, standing over a map of her study site: "The coral are fairly predictable, but they don't send us an e-mail."
Knowlton, 60, has studied coral reefs for three decades, first while monitoring the effects of Hurricane Allen, in 1980, on reefs in Jamaica; then as founding director of the Scripps Institution of Oceanography's Center for Marine Biodiversity and Conservation in San Diego; and now as the Smithsonian's Sant Chair of Marine Science at the Natural History Museum. In that time, overfishing has allowed seaweed and algae to grow unchecked, smothering coral worldwide. Poor water quality has increased coral diseases. Deforestation and the burning of fossil fuels have burdened oceans with absorbing more carbon dioxide, which increases their acidity and makes it more difficult for corals to deposit skeletons and build reefs. Currently, a third of all coral species are reportedly at risk of extinction. "If we don't do something," says Knowlton, "we could lose coral reefs as we know them by 2050."
Such grim predictions have earned Knowlton the nickname Dr. Doom. She understands the value of coral reefs—home to one-quarter of all marine species, a source of potential biopharmaceuticals and an organic form of shoreline protection against hurricanes and tsunamis. In the Caribbean, a staggering 80 percent of corals have been destroyed in the past 30 years. Along with other marine scientists, Knowlton has been trying to help reefs survive by better understanding coral reproduction.
For decades, scientists assumed that coral colonies picked up sperm in the water and fertilized eggs internally—and some do. But in the mid-1980s, research biologists discovered that most corals are "broadcast spawners." Unable to self-fertilize, they release sacs containing both eggs and sperm, synchronizing their spawning with neighboring coral colonies. Fertilization takes place in the water. The corals appear to use three cues to begin their mass spawning: the full moon, sunset, which they sense through photoreceptors, and a chemical that allows them to "smell" each other spawning.
Since 2000, Knowlton and a team of research divers have been coming annually to Bocas del Toro, Panama. They have spotted, flagged, mapped and genetically identified more than 400 spawning coral colonies.
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