Young corals bred using in vitro fertilization (IVF) and planted in reefs around the US, Mexico and the Caribbean surprised scientists after most survived last year’s record. marine heat wavewhile older corals struggled.
A study found that 90% of the young IVF-created corals surveyed remained healthy and colorful, clinging to the algae that live in them and provide them with nutrition. In contrast, only about a quarter of older non-IVF corals remained healthy.
The rest, including large colonies that may have lived for centuries, have either been bleached by the heat – which drives the algae out of their tissue and turn white – or pale, which expels some of the algae. Some died in the heat wave before the survey was done.
Dr Margaret Miller, lead author and research director at Secore International, a reef conservation organisation, said: “[The heatwave] was a horrible time. But I was impressed and surprised that the data came out with such an extreme pattern.”
The young corals have been bred over the past five years using a version of IVF developed by Secore. Divers collected coral spawn, which was used to fertilize eggs in the laboratory. The resulting baby corals were then planted on reefs across the Caribbean to form colonies.
Most coral restoration efforts have historically focused on fragmentation techniques – where corals are broken into smaller pieces and transplanted to a new location. Rather than producing exact clones, as fragmentation does, breeding corals through IVF increased genetic diversity, giving them a better chance of adapting to heat over time. “Natural selection back in the reef environment will select the best,” Miller said.
The 771 young corals in the study – a fraction of the thousands bred each year by Secore and partner institutions – live in restored reefs off Mexico, the Dominican Republic, the US Virgin Islands and the Dutch Caribbean territories of Bonaire and Curaçao.
Researchers at Sombrero Reef in the Dominican Republic compared young and old elkhorn corals (Acropora palmata). The reef was once filled with the large branching species, but most died during an outbreak of white band disease in the 1980s. The reef is now home to scattered older corals spared by the disease and numerous young elk corals bred to restore the reef.
When Maria Villalpando, researcher at the Dominican Foundation for Marine Studies (Fundemar), examined the corals after peak heat stress last fall, she found the young elkhorn corals to be healthy. “They weren’t even pale,” she said. However, the older elk antler corals did not do well. “Unfortunately, we lost most of them after this bleaching event.”
Why the young corals are more heat tolerant is not well understood. There are several hypotheses, but Miller suspects they can get better symbiotic algae that tolerate heat, and can try out different types of algae, some of which handle heat better than others. “They’re quite investigative in those early stages,” Miller said. Eventually, the young corals settle on types of symbionts that will work for them, she said.
Previous research suggests that if they live long enough, the young corals will probably become less tolerant to heat stress as they agemaking them increasingly vulnerable as global temperatures rise.
Miller said that in Australia there has been a bleaching event every other year for the past six years, while in the Caribbean it happens about every five years. She added that after a bleaching event, even if a colony survives, its ability to reproduce is compromised for a number of subsequent years.
“So now that the intervals between these heat waves have become so short, it is unlikely that coral reproduction alone will change the fate of these populations,” Miller said. “We must address the underlying causes of global climate change. But I think it’s important that we replenish coral populations in the meantime, because that might buy us some time.”
