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Night Diving for Coral Colonies with SECORE International

This story was originally published on January 22, 2018.

Off the coast of Curaçao, a few nights after the August full moon, something is happening with elkhorn and staghorn corals. Slowly, egg-and-sperm bundles bubble up on the stalks of coral. All at once, the Caribbean waters fill with pink clouds of these gametes, released en masse by millions of corals across the reef. The bundles float toward the surface to catch the ocean current, dispersing across the water to create new coral colonies. This synchronous spawning by elkhorn and staghorn corals happens only one night a year, and Shedd is there to witness it.

Coral colonies provide food and shelter for thousands of fish and invertebrate species in the Caribbean. Their presence is vital, and Shedd—as a leader in conservation and research—is committed to helping restore these crucial yet dwindling populations.

Shipping crates filled with tetrapods, four-pronged clay structures covered in grooves for young coral polyps to settle into. Each tetrapod is about the size of the palm of a hand.

Since 2006, Shedd Aquarium and SECORE International (SExcual COral REproduction) have monitored the mass-spawning event in the Caribbean to better understand coral reproduction and to develop strategies for restoring corals in the wild. The coral crusaders who make up SECORE International use a unique method: Dive in the midst of the mass spawning, collect the eggs and sperm, commingle bundles to fertilize the eggs, provide the resulting larvae with structures to begin growing on in a protected environment and eventually place the young corals on reefs to develop fully.

SECORE International is an international organization that started in 2002 based on the coral reproduction research of Dr. Dirk Petersen at the Rotterdam Zoo. It has grown to work with more than 60 supporting partners like Shedd all over the world.

At the project’s inception, the SECORE International team focused its efforts on two of the most important reef-building corals in the Caribbean: staghorn and elkhorn corals (Acropora cervicornis and A. palmata). Both species are listed under the U.S. Endangered Species Act, and they were the first critically endangered coral species on the IUCN Red List, an internationally recognized inventory of species at risk of extinction.

After much experimentation with coral-rearing techniques, the group celebrated a big victory in 2016: One of the elkhorn corals that was outplanted to a reef spawned for the first time, confirming that SECORE International’s method could help restore reefs.

Over the last decade the SECORE International team has gathered enough knowledge about elkhorn coral to determine within a few days when these wild corals will spawn. This enables the team to head to the island a week in advance to prepare. As the predicted spawn time approaches, the group begins diving on the reefs and waiting for the big event. In the wake of a strange warming/cooling period that caused some corals to bleach, the team waited longer than anticipated in 2017.

The coral finally started spawning on night nine after the full moon. Shedd expert Keoki Burtonwas in Curaçao with the SECORE International team, ready to swim into action under the waves. “What we’re doing is scanning the reef,” Keoki says, “seeing who’s ‘staging,’ which means the bundles start to bubble up on the coral columns. Once you see that, you cover the coral in a big bag that feeds up to a collection bottle and zip it shut with a bungee cord. All of those sperm-and-egg bundles float up to the top of the bag and collect right into the bottle. Then you just cap it off, put it on your belt and move on to the next colony.” The SECORE International team only collects a portion of the total spawn, allowing some of the corals to spawn naturally.

The swimming is tough going, given the amount of gear each team member is carrying. “It’s a lot of swimming against currents, trying to navigate your way in the dark underwater,” Keoki says. “We’re carrying all this equipment, dragging these giant sacks of nets that you always want to take off because of the current, plus two flashlights. It’s definitely a labor of love.”

“We’re carrying all this equipment, dragging these giant sacks of nets that you always want to take off because of the current, plus two flashlights. It’s definitely a labor of love.”

Keoki Burton, Shedd expert

Out of the two target species of coral, only elkhorn spawned. Staghorn corals didn’t go off as the team had hoped. Dive teams at other sites around the Caribbean were successful in collecting staghorn bundles.

With the elkhorn coral, the Curaçao team achieved a 95 percent fertilization rate. The team tried a new method to settle the larvae this year. As in past years, the collected bundles were rushed to a lab for fertilization, but this year the team also attempted to fertilize some of them right on the beach.

Shedd experts snorkle in the warm waters of the Caribbean as they wait and prepare for the annual elkhorn and staghorn coral spawning.
Tiny coral polyps can barely be seen growing on submerged ceramic tetrapods, designed to provide a good place for spawning corals to attach themselves.

Keoki continues, “A brilliant idea that Shedd’s coral expert, Mark Schick, and the rest of the SECORE International team had was to set up pools right there on the beach and allow the larvae to settle in their natural water chemistry instead of moving them to an offshore laboratory. So we were right there on the shore in our wetsuits and gear, and we fertilized the bundles on the beach and immediately threw those newly developing embryos into the pools to settle on the tiles.”

The tiles are tetrapod structures that are covered in grooves for the larvae to settle in. Once the larvae do settle, the tiles are thrown back out over the reef. “We can throw them out like sowing a field,” Keoki says. “These tiles are still a topic of discussion, regarding their effectiveness, but we’ve had some trials that did really well and some where they never hook into the reef. So far, though, it looks like it worked fantastically.”

The team produced more than 600 tetrapod tiles with about 23 settling larvae on each. The tiles were outplanted across 10 healthy reef sites, and preliminary observations indicate high survival rates. Better yet, some tiles have already developed small colonies, which are the first steps to building a healthier reef.

―Katie Antonsson