Animal Forecast

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 worm in Carrie 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.

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 Fogarty diving 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.

One after another, the crew members sat on the gunnels, held their masks, and fell backward into the dark water. I splashed in with considerably less grace and followed Brian Nelson of the National Aquarium. Underwater, our lights illuminated schools of tiny, silvery fish and clouds of floating plankton. The water was rough, and I dropped close to a patch of stinging fire coral. I scuttled awkwardly into deeper water.

Sponge coral in Carrie Bow Cay, Belize, during the 2012 spawning season.
Sponge in Carrie Bow Cay, Belize, during the 2012 spawning season

Courtesy of Abby Wood

Nelson shone his light on a broad yellow coral arm, like a doctor examining an especially interesting rash. He looked into the tiny mouths of the coral polyps, trying to spot the pearly-pink egg-sperm bundles as they “set,” or prepared to emerge. He moved on to another coral and then another, finding nothing. The spawn would not happen tonight, at least not on this reef.

Rising to the surface, Nelson spat and shook his head. “There’s a lot of life down there—there’s just a vibe, like something’s going to happen,” he said. “But the more you know about coral, the less …”

Hagedorn shouted from the boat. “See anything?”


“You’re kidding.” Her voice was flat.

Nelson sank underwater again, continuing his search. After another half-hour, we returned to the boat. No one had seen any signs of spawning.

Before climbing the ladder back into the boat, I dropped under the surface for a final look. Two reef squid, each as long as a forearm and nearly transparent, lingered under the hull. They startled and paused, and then one bluff-charged into the beam of my flashlight, its glassy fins flared. I felt taunted.

The next day, everyone was tense, aware that if the spawn didn’t happen that night, it was unlikely to happen at all. They kept up a studied cheerfulness, organizing and re-organizing equipment. The day was hot, humid, and still.

Just after sunset, a light rain began, and wind shook the palm fronds. That was a bad sign, everyone agreed. Or maybe it was a good sign. The aquarists traded stories of successful and unsuccessful spawns, like anxious relatives awaiting a birth.

Carrie Bow Cay, Belize, during the 2012 coral spawning season.
Dr. Nicole Fogarty arranging vials in Carrie Bow Cay, Belize, during the 2012 coral spawning season

Courtesy of Abby Wood

The group gathered on the research-station porch, drumming fingers and jiggling knees. Nelson had turned his baseball cap inside out, rally-cap style. Andrew McLeod, who had a worsening ear infection and a fierce set of mosquito bites, scratched. One of the station researchers had plugged an iPod into a small set of speakers. As the group sat in silence, Johnny Cash began to sing “Flushed Down the Bathroom of Your Love.”  

Departure time arrived, and I followed McLeod into the shallow water just off the southern end of the island, swimming out to a reef thick with elkhorn and staghorn coral. I held a set of nets; if the coral looked set to spawn, I would hand the nets to McLeod to attach.

McLeod slipped under the surface and finned toward his territory. He stopped at the first elkhorn coral, sinking to the bottom for a good look. After a long minute, he gestured for a net. He moved on to the next coral, and gestured again.

A half-hour later, McLeod and the other researchers had tied down almost two-dozen nets, each topped by a clear plastic collecting vial. In the beams of our flashlights, the nets glowed and swayed like ghosts. Fish darted busily among them. McLeod kept swimming, checking exposed branches of coral.

Then he pointed at the end of one vial, where a few tiny red globes hung in the water. I looked at a nearby branch of coral and realized that what I had thought was an especially thick cloud of plankton was, in fact, a cloud of sperm and eggs, each bundle floating out of a coral polyp mouth and rising to the surface.

Abruptly, it seemed, we were surrounded by a storm of spawn—below us, beside us, even collecting in swirls on the dark surface above us.

Sponge and fish in Carrie Bow Cay, Belize, during the 2012 coral spawning season.
Sponge and fish in Carrie Bow Cay, Belize, during the 2012 coral spawning season

Courtesy of Abby Wood

Another diver passed, giving a thumbs-up. I surfaced, leaned back to look at the Milky Way, and realized that the air stank—a dense, salty, unmistakable funk. The aquarists had told me that they could smell coral spawning, but I’d thought they were joking.

Within minutes, each vial was filled with thousands of bundles, each containing both sperm and eggs. McLeod capped one and handed it to me, moving slowly so as not to break up the bundles and start fertilization before the samples reached the laboratory.

As I swam back toward the boat to deliver the vial, my light, which had begun to dim during the excitement of the spawn, failed. I swam the final stretch in the dark, holding the vial in front of me, skirting silhouettes of coral. I was suddenly aware of the silent, hidden nature of the spawn; only artificial light had turned it into a show.

We returned to land and the aquarists set to work, stirring the sperm and eggs in bowls of seawater to encourage fertilization. It was close to midnight, but the aquarists joked with one another as they bounced up and down the stairs. “Look at all those happy scientists!” bellowed Scott Jones, the science coordinator of the station, as he passed the laboratory door on the way to bed.

Hagedorn laughed. “Really, it takes so little to make us happy,” she said. She has recently developed techniques to freeze and store coral sperm and embryonic cells, a bank that could preserve endangered corals’ genetic material long-term. She held up a plastic vial, which contained about a teaspoon of sperm and eggs. “But just think,” she said to no one in particular. “This is enough for an entire reef.”

Counted larvae in Carrie Bow Cay, Belize, during the 2012 coral spawning season.
Counted coral larvae in Carrie Bow Cay, Belize, during the 2012 spawning season

Courtesy of Abby Wood

When the group went to bed, well after midnight, they were still happy. The elkhorn coral eggs had been fertilized, and soon the aquarists would move tens of thousands of coral larvae into incubation tanks of warm, circulating seawater.

In the morning, the news was not so good. A tropical depression was headed straight for Belize. By noon, the depression had become a tropical storm, and the sky was chalk-white and heavy. It was clear that everyone on Carrie Bow would have to evacuate to the mainland.

The larvae couldn’t be moved at this early stage of their development, so the aquarists attached battery-powered pumps to the tanks and left the larvae to ride out the storm alone.

The group spent a night in the mountains near the Guatemalan border, waiting glumly for Harvey to blow over. The younger researchers passed the evening over poker and coconut rum. They finally returned to Carrie Bow the next afternoon, fully expecting the larvae to have suffocated. When they peeked into the tanks, however, they saw that about half the poppy-seed-sized larvae remained pink and healthy.

On the heels of that storm, however, came another, and with it travel delays and confusion. When the aquarists finally arrived home and opened their sealed boxes of coral larvae, they found that the stress of the overnight evacuation or of extended travel had somehow interfered with development. Not one coral larva was still alive.

Heroic measures to save endangered species, it turns out, are just like any other kind of heroism—difficult, often expensive, likely to fail. It’s a lot cheaper and easier to protect animals and plants before they’re in mortal danger, but for many species—including the staghorn and elkhorn corals—that chance is gone. Marine protected areas and other traditional conservation measures may still do some good, but in the long term, their survival may depend on the sperm and eggs in Mary Hagedorn’s freezers, the handful of healthy captive-grown coral colonies in aquaria around the world, and the small crew of people willing to tangle with hurricanes, fate, and epic frustration on the Caribbean reefs.

This summer, the aquarists will try again. If the weather is good and the coral cooperative, they’ll return with hundreds of thousands of coral larvae. And if not … well, they’re trying not to think about that possibility. When it comes to corals, they know they can’t count on much.