Scientists found 90% of young corals surveyed remained healthy compared with 25% of older corals, following a massive bleaching event in the Caribbean.
Young corals grown using in vitro fertilization (IVF) and planted on reefs around the U.S., Mexico and the Caribbean have surprised scientists, having largely survived last year’s record marine heatwave, while older corals struggled.
One study found that 90% of young IVF corals surveyed remained healthy and brightly colored, retaining the algae that live inside them and supplying them with nutrients. In contrast, only about a quarter of older, non-IVF corals remained healthy.
The rest, including large colonies that may have been alive for centuries, are either experiencing heat bleaching – drawing algae out of their tissues and turning white – or fading, shedding some of the algae. Some died in the heat wave before the survey was conducted.
Dr Margaret Miller, lead author and director of research at Secore International, a coral reef conservation organisation, said: “[Gelombang panas] is a terrible time. But I was impressed and surprised that the data showed such an extreme pattern.”
The young corals have been raised for the past five years using a version of IVF developed by Secore. Divers collect coral sperm, which is used to fertilize eggs in a lab. The resulting baby corals are then planted on reefs throughout the Caribbean to form colonies.
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Most coral restoration efforts have historically focused on fragmentation techniques—where corals are broken into small pieces and moved to new locations. Rather than producing exact clones, as fragmentation does, propagating corals through IVF increases genetic diversity, giving them a better chance of adapting to heat over time.
“Natural selection in a reef environment will select for the best,” Miller said.
The 771 young corals in the study—a fraction of the thousands raised each year by Secore and partner institutions—live on restored reefs off the coast of Mexico, the Dominican Republic, the U.S. Virgin Islands, and the Dutch Caribbean territories of Bonaire and Curaçao.
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Researchers at the Sombrero Reef in the Dominican Republic compare young and old elkhorn corals (Acropora palmata). The reef was once teeming with large, branching species, but most of them died during a white band disease outbreak in the 1980s. Now the reef is home to a scattering of old corals that survived the disease and many young elkhorns that are being raised to restore the reef.
When Maria Villalpando, a researcher at the Dominican Foundation for Marine Studies (Fundemar), surveyed corals after the peak of heat stress last fall, she found that young elkhorn corals were healthy. “They weren’t even fading,” she said.
However, older elkhorn corals did not fare so well. “Unfortunately, we lost most of them after this bleaching event.”
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Why young corals are more heat-tolerant is not fully understood. There are several hypotheses, but Miller suspects that they may be better able to acquire heat-tolerant symbiotic algae, and may try out several types of algae, some of which are better able to cope with heat than others.
“They’re pretty exploratory in the early stages,” Miller said. Eventually, young corals settle on a type of symbiont that works for them, he added.
Previous research has suggested that if they live long enough, young corals may become less tolerant of heat stress as they age, making them increasingly vulnerable as global temperatures rise.
Miller said in Australia, there have been bleaching events every two years for the past six years. In the Caribbean, they occur about every five years. He added that after a bleaching event, even if a colony survives, its ability to reproduce is affected for the next few years.
“So now, with the intervals between these heat waves getting so short, coral propagation alone is unlikely to change the fate of these populations,” Miller said.
“We need to address the underlying causes of global climate change. But I think it’s important to strengthen coral populations while we wait, because that might buy us a little bit of time.” (The Guardian/Z-3)
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