Coral Sex Is Spectacular

Which species will adapt to climate change—and which ones won’t.
Feb. 26 2013 3:02 PM

Swimming in Sperm and Eggs

Coral sex is explosive, exquisitely timed, and funky.

Sea anemone in Carrie Bow Cay, Belize, during the 2012 spawning season.
A tube worminCarrie Bow Cay, Belize, during the 2012 spawning season

Courtesy of Abby Wood

The Mesoamerican Barrier Reef, a narrow band of coral stretching from the Yucatan to northern Honduras, hugs the Belizean coastline like a giant parenthesis. In a few places, the main spine of the reef rises above the surface, forming low islands exposed to the wind and waves of the open sea.

One of these islands, 13 miles offshore, houses the Smithsonian Institution’s Carrie Bow Cay Marine Field Station. When I stepped ashore one sweaty evening, the station had an air of cheerful dereliction. Researchers in bikinis and half-zipped wetsuits circled in and out of the bare-bones laboratories. A hand-lettered wooden sign near the station house entry read “FREE BEER TOMORROW.”

The evening’s task would be delicate, however, and tension was building. It was three days after the full moon, and some of the corals near Carrie Bow were expected to begin their annual spawn once night fell. A team of aquarists and marine scientists had gathered on the island in hopes of collecting sperm and eggs released into the water by endangered coral species.

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If all went well, the scientists would each return home with a supply of coral larvae ready to be raised in captivity—and, perhaps, serve as an insurance policy for the Caribbean’s fast-declining reefs. If not … well, they were trying not to think about that possibility. When it comes to coral, they know they can’t count on much.

On the sandy back steps of the research station, Mary Hagedorn of the Smithsonian sat in front of a picnic table piled with equipment. “OK, everyone, let’s rehearse,” she said. She turned to Abby Wood, a professional actor who volunteers in the invertebrate department at the National Zoo. “So I’m a coral,” Hagedorn said, sticking her hands over her head and laughing. “You’re going to slip the bag over me …”

Wood, a dark-haired 30-year-old with a big, deep voice—she played Tybalt in an all-female production of Romeo and Juliet—pantomimed what Hagedorn and the aquarists hoped would soon happen in the water.

Divers would hitch the silk bags over a few branches of spawning coral, catching the sperm and egg bundles as they floated upward and collecting them in plastic vials attached to the tops of the bags.

Underwater in Carrie Bow Cay, Belize, during the 2012 spawning season
Dr. Nicole Fogartydiving in Carrie Bow Cay, Belize, during the 2012 spawning season

Courtesy of Abby Wood

When the spawn petered out—spawns last only a few minutes—the divers would carefully cap the vials and hand them to a snorkeler, who would ferry them back to the research boat. Wood mugged her way through the demonstration. It was part practice, part ritual.

Coral sex is more complicated than one might imagine. Corals can reproduce asexually—that is, coral fragments can grow into clones of their parent. But corals can also reproduce through the fertilization of eggs by sperm. Sexual reproduction preserves genetic diversity, and with it a species’ ability to withstand and adapt to change.

But coral sex probably doesn’t happen as much as it used to. In the Caribbean, warming water, disease, overfishing, and other problems have killed 80 percent of the region’s coral, turning many reefs into rocks and seaweed. Similar foes are killing coral in the Pacific, where the extent of living coral is thought to have shrunk by half in recent decades. These smaller, weaker, and more diffuse populations seem to be less likely to spawn—and when they do, their sperm and eggs are less likely to meet in the water.

In 2006, a group of European and U.S. aquarists, experts in aquatic plant and animal husbandry, were concerned about the ongoing declines in coral worldwide and decided to try raising sexually reproduced coral in captivity. They knew it would be a challenge: They would have to collect coral sperm and eggs in the wild during infrequent, never-quite-predictable coral spawns, fertilize the eggs in the laboratory, and, once they had young, living corals, figure out how to keep them alive through adolescence. They hoped that sexually reproduced captive coral could be used to revitalize or restore wild coral populations damaged by overfishing, climate change, or other forces.

The aquarists managed to bring some endangered staghorn coral larvae back to their aquaria, where they varied water temperatures, water chemistry, flow rates and feeding regimes, trying to find the optimal conditions for each species. “We were basically trying to re-create the ocean in a box,” says Michael Henley, an invertebrate curator at the National Zoo.

Each year, as the techniques improved, a few more coral larvae survived and grew. The largest captive-grown staghorn coral colony, which lives at a research station in southern Florida, is now as broad as a dinner plate. In the summer of 2010, the aquarists began releasing young captive-grown corals on a reef near Curaçao, and many are still alive—an encouraging sign for larger-scale restoration efforts.

Now, on the coast of Belize, the aquarists wanted to try their techniques with elkhorn coral, another gravely endangered coral species in the Caribbean.

Just after sunset, the crew loaded a boat with gear and set off for a half-submerged atoll near Carrie Bow with some large, healthy-looking specimens of coral. Lightning flashed in the distance, over the open sea, and the moon began to rise, huge and orange.

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