The dating scene in New York City may seem dismal, but it’s a breeze compared to what corals endure when getting randy on the reef. “As if reproduction isn’t hard enough already, they’re glued to the bottom of the ocean, and there are throngs of other species in the way,” says Kristen Marhaver, a marine biologist at the CARMABI Foundation on the southern Caribbean island of Curaçao.
Most coral species sidestep the obstacle of finding a mate altogether—they’re hermaphrodites that release large bundles of eggs and sperm, taking care of business on their own. The marine animals can also reproduce through fragmentation, which is just what it sounds like: Bits of the creature break off and take up residence nearby. But some species employ the boy-meets-girl approach to making whoopee, like the rare pillar corals that Marhaver studies in the Caribbean. Pillar corals, which grow in columns, provide shelter for fish and help weaken storm surges approaching the shore—and they’re either all female or all male. During a brief window each year, the fellas release sperm into the seawater, and the ladies release their eggs a little while later.
The trouble, from a research standpoint, is that nobody knew when sexy time was for pillar corals. And nobody has recorded any juveniles for more than a quarter-century, which could mean the species is in trouble. Figuring out when they reproduce, a colleague told Marhaver, was critical to better understanding the animals and determining whether they’re threatened. Solving the mystery would also enable scientists to bring eggs and sperm back to the lab, where they could attempt to breed and rear youngsters to study and release into the wild to bolster populations—something that’s never been done with this species. That could make coral more resilient to climate change; warming seas and ocean acidification are already causing mass coral bleaching events and slowing skeletal formation.
So Marhaver began collecting clues to when the deed takes place. Already knowing that eggs disappear from females’ tissue sometime in August, she dug up reports of a couple of divers who accidentally happened upon spawning pillar corals. She deduced that the corals most likely spawn when waters heat up in August, on the nights immediately following a full moon, soon after sunset. She and a colleague decided to test the theory. As the sun sank in the sky, they put on their scuba gear, slipped into 20-foot-deep waters on a Curaçaoan reef, and set out to catch the corals in the act.
They got lucky their first time. “It’s the kind of thing that makes you scream through your scuba regulator,” says Marhaver, who published a study on the research in the journal BMC Ecology. “It was really exciting.” They watched as milky clouds of sperm burst out of the sides of the pillars into the inky black water. Soon after, the females released their eggs, which drifted slowly in the water.
They returned the following year, armed with the knowledge of which corals were males and which were females. They placed tarps jerry-rigged with funnels over the ladies to collect the eggs and used syringes to suck up sperm from the male colonies. They mixed their catches together underwater and onshore and took them back to the lab.
A few dozen fertilized that night. “The next day we had swimming blobs,” Marhaver says of the larvae. “We put them in water tanks and offered them a nice piece of ceramic coated with a bacterial biofilm.” Within three or four days in the lab, the first larva started to attach and go through metamorphosis.
“Once we got them to fertilize, it was very hands-off,” she says. “We changed the water every so often and made sure none of them were exploding, dying, or poisoning their neighbors.”
Because the species is considered fragile, Marhaver and her colleagues kept expecting the juveniles to die. They survived for more than seven months.
Now that they know how to grow the corals in the lab, they’ll turn their attention to investigating why juveniles are so rare. They’ll also plant some of the youngsters in the wild to boost the genetic diversity of existing populations. If the juveniles survive in the wild, they could also prove more resilient to environmental changes than the adults. “When they’re younger, they have to be more flexible to adjust to wherever they choose to live,” says Marhaver. “Once they’re established, it’s harder.”
Marhaver cautions that seeding large swaths of reefs with lab-reared corals isn’t the objective. “In some small cases, it might be able to add to coral populations,” she says. “We draw the analogy to in vitro fertilization—it’s expensive, intensive, and not your first choice. It’s easier to save a reef that still exists than to grow one back from the lab.”
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