Once a year, typically after sunset in the late summer, the coral baby-making process begins.
Large colonies of coral spawn, spewing out sperm and eggs, often in pea-size bundles, that drift around until they encounter the spawn of other corals. Fertilized eggs turn into coral larvae — tiny and squishy free-swimming organisms — which eventually settle on the seafloor. There they metamorphose, like a caterpillar to a butterfly, into a coral polyp. Those polyps clone themselves over and over again, eventually forming larger coral colonies that build reefs.
That’s how typical reef-building corals, like the iconic elkhorn and staghorn species, with their antler-like appearance, produce offspring. That’s how they’ve been doing it for millions of years.
But in Florida, this process is broken.
Most species of hard corals that form the reef’s complex structure and help safeguard coastal communities from storm surge are not having babies in the wild anymore, top marine ecologists told me. For at least a decade, and likely longer, researchers have barely found any new coral babies or juveniles of most hard corals during surveys in South Florida and the Florida Keys, home to the largest coral reef in the continental US. “We’re just not seeing new babies,” said Scott Winters, CEO of Coral Restoration Foundation, a conservation group.
While many of these corals are still spawning in the wild — the part where they release sperm and eggs — something is preventing that spawn from eventually growing into polyps, or babies, Winters, and other experts said.
This is a serious problem for a long embattled reef.
Florida’s reef ecosystem has been facing a near-constant barrage of disturbances, from marine heatwaves and hurricanes to disease and pollution. These threats have killed off most of the live hard coral in the Florida Keys. And should the reef’s reproductive woes persist, it may never recover or even survive on its own. While corals can reproduce asexually — by cloning themselves — sexual reproduction is incredibly important because it introduces new genetic diversity that helps corals adapt to the increasingly hostile ocean conditions.
Now, to keep Florida’s reef alive, scientists have to breed corals almost entirely in labs on land, where the water is clean and comfortably warm. Margaret Miller, a coral reproductive biologist, says it’s a sign that reef conservation is in a new era — one in which saving corals requires managing them under human care.
Why aren’t corals making babies?
“We just don’t know,” Winters said.
There’s no one explanation for the baby bust, marine scientists told me. Rather, each step of the reproductive process has likely been compromised to some degree.
Over the last few decades, coral reefs in Florida and across much of the Caribbean have lost an enormous amount of their large, adult coral colonies that can spawn. Around 2014, a nasty affliction called Stony Coral Tissue Loss Disease began spreading in Florida, killing tens of millions of colonies and nearly exterminating the local population of some species. A marine heatwave in 2023, the worst on record, dealt another powerful blow, causing many more colonies to bleach and die.
That means there simply aren’t many corals left to reproduce. And even those that have survived might not spawn because they’ve been stressed out by bleaching, disease, or other threats. Instead, they just put energy towards staying alive.
Corals that do spawn run into another problem: With so few colonies left, corals are often too far away from each other for their spawn to meet and produce fertilized eggs. They’re like a bunch of sexually active singles that don’t live close enough to hook up.
“For many species, we suspect that they’re probably too far apart to successfully fertilize,” said Nick Jones, a research scientist at Nova Southeastern University. “They’re just not meeting.”
New research suggests that corals need to be within roughly 30 feet of each other in order for the sperm of one to fertilize the eggs of another. (This is a problem impacting other sea creatures in Florida, as well: Individuals are too isolated to find each other and mate.)
But even if coral eggs do become fertilized and grow into larvae, they face additional challenges.
In the first few days of their lives, coral larvae need to attach themselves to a bare patch of seafloor, in a process called “settling.” And once they find a place to settle, they metamorphose into a baby coral. In Florida and much of the Caribbean, however, there’s hardly any free space. It’s all covered in seaweed, or macroalgae. In recent decades, seaweed has grown out of control thanks to a die-off of algae-eating animals, such as large sea urchins, and pollution.
“Even if you have adequate larval supply, the habitat where you want them to settle and grow up is not particularly healthy,” said Miller, research director at SECORE International, a coral restoration organization.
Making things worse, coral larvae that come from stressed out parents are likely weak and may run out of energy before they can find a suitable place to settle, said Nicole Fogarty, a marine biologist at University of North Carolina Wilmington. Even the larvae that do settle and transform into baby corals have to deal with the same disturbances that threaten adults, including pollution, heatwaves, and disease.
Together, these challenges have upended coral sexual reproduction for most hard, reef-building corals in Florida. That’s why when researchers go looking for babies, they hardly find any. For one study, published in 2023, scientists, including Fogarty, put out more than 5,000 terracotta tiles across the Florida Keys, on which baby corals are known to grow. Over three years, they didn’t detect a single baby coral from the genus Orbicella, a key reef-building variety that produces large, lumpy mounds. They found just one baby from the genus Acropora, which includes staghorn and elkhorn corals. Recent research in Southeast Florida found a similarly small number of reef-building babies.
There are exceptions: A handful of hard corals are still producing offspring, including a reef-building species called the massive starlet coral. Yet most of them, besides the starlet, are considered “weedy” — meaning, they’re common and more resistant to threats like warming and disease. And while the starlet coral is making babies, those youngins are still struggling to grow due to the sheer frequency of disturbances, Jones said.
A new era for reef conservation
Coral reefs can, to an extent, survive without making babies. Polyps can clone themselves. If you break off a small piece of coral — containing a polyp or more — from a colony, it will continue to grow and build its own colony. This is similar to how a houseplant cutting can eventually turn into a full-size plant.
But this reproductive strategy has limitations. Different pieces of the same coral colony are, for the most part, genetically identical. And that can leave them vulnerable to threats like warming and disease. A coral’s ability to tolerate stress is, to an extent, determined by its genetics. The more genetic variety you have, the better chance that some individuals will survive. Resilience is rooted in diversity.
Coupled with rising temperatures, a lack of sexual reproduction threatens the survival of Florida’s coral reef. Reef-building species simply can’t make it on their own. To keep coral populations alive, scientists have to intervene on an enormous scale. They bring coral populations out of the ocean and plop them into tanks on land, where they can ensure that water conditions are stable and survival is more certain. And there, they manually breed the colonies — triggering them to spawn with water temperature and light, and mixing the sperm and eggs together to produce larvae.
Contemporary coral conservation in Florida has become similar to efforts to save giant pandas, black-footed ferrets, and other incredibly rare species, Miller said: There are so few coral colonies left that scientists must bring them into captivity and carefully manage breeding, to avoid inbreeding and a loss of diversity. Gone are the days where simply protecting coral in the wild with a marine park was sufficient. Now conservation must happen in labs.
“That’s where the Florida corals are now,” Miller said. “It’s literally a Noah’s Ark approach.”
It’s not just Florida. Reef-building corals throughout much of the Caribbean are heading in the same direction, Miller said, especially after the 2023 bleaching event. Caribbean corals, too, are struggling to produce babies.
“I think Florida is a harbinger of things to come,” Winters told me.
The good news is that lab-based coral breeding has been incredibly successful. Scientists can get corals to produce babies in tanks, and those babies tend to have a high survival rate. In captivity — on land, ironically — coral is healthy. What’s more is that researchers can, to an extent, breed corals that are more tolerant to heat or other stresses and then “out-plant” them, or adhere them to the seafloor to help restore the reef.
But this isn’t enough. The wild ocean is increasingly unfriendly to reef-building corals, as waters warm and storms become more powerful. Unless climate change is slowed, by limiting the carbon we release into the atmosphere, even the best coral breeding programs won’t hold off extinction.
“We’re raising corals in nice conditions on land, but when we out-plant them they don’t tend to live particularly well because we haven’t addressed any of the causes that have led them to get to this position,” Jones said. “It is pretty depressing.”
“The restoration that most of us have been doing in Florida has not suddenly led to a thriving reef system,” Jones said. “It can’t be just conservation without government action.”
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