A team member captures the moment a coral releases its eggs. — Photo baokhanhhoa.vn
This milestone marks a significant advancement in the study of coral reproduction and underscores the arduous journey of scientists battling to preserve marine ecosystems.
Historically, Vietnam has relied on asexual reproduction, known as coral fragmentation, to propagate corals. In this method, scientists extract branches from parent coral colonies and transplant them to degraded reef areas to restore coverage.
Each fragment, typically one to three centimetres in size, can increase coral coverage by three to six times in a short period. While simple to implement, this technique lacks long-term sustainability.
"Not all coral colonies can be fragmented. We can only select certain types suited for this method," said Võ Sỹ Tuấn, a marine ecology expert.
He noted that asexual reproduction does not enhance genetic diversity, as each fragment retains the exact genetic makeup of its parent colony. This uniformity leaves reefs vulnerable to mass die-offs during environmental crises such as rising sea temperatures.
"Transporting fragments between distant regions is risky. If optimal conditions are not maintained, transplanted corals can die from shock," said Nguyễn Văn Long, head of the Aquatic Resources Department at the Institute of Oceanography.
By comparison, sexual reproduction, where sperm and eggs from different coral colonies combine to produce genetically diverse offspring, offers a more resilient solution.
This method, successfully implemented in Australia, Germany, Japan, and China, is now the focus of global coral restoration efforts.
Coral sexual reproduction involves a synchronised release of eggs and sperm into the water, typically triggered by water temperature or lunar cycles.
"If the release is not simultaneous, the chances of fertilisation drop sharply as currents sweep gametes away. Corals have evolved to maximise fertilisation through mass spawning," explained Đặng Đỗ Hùng Việt, a member of the team.
Once fertilised, zygotes develop into floating larvae for one to three weeks before settling on suitable substrates to grow into juvenile corals.
This process boosts genetic diversity and adaptability, enabling future generations to withstand rising sea temperatures, ocean acidification and diseases - key threats amid intensifying climate change.
However, coral spawning lasts mere minutes and occurs just once annually per colony, forcing the team to conduct hundreds of night dives to capture these fleeting moments.
"Eggs are mostly pink, though some are yellow or white, ranging from pearl-sized to dust-like particles. After release, they float to the surface, where sperm fertilise them," Việt said.
However, timing remains unpredictable. "This year, corals spawned at a certain time, but next year could be different. Once, a colleague waited underwater for an hour with no spawning, only for it to happen right after he surfaced," Long recounted.
Visible bulges on coral colonies signal imminent spawning, guiding researchers to prime observation spots. "At night, with low tides, we can not use boats. We swim long distances to reach the corals, facing unpredictable currents that make it dangerous," Việt added.
The team, including researcher Hoàng Ngọc Lâm, endured 10-day monitoring stints from 6pm to 10pm, under immense pressure to not miss the brief spawning window. "Missing it by minutes means waiting another year," Lâm said.
Their perseverance paid off on April 30, 2024, when they recorded coral spawning in Nha Trang Bay, the first such documentation in Vietnam.
"While the nation celebrated a holiday, we were diving. The reward was gametes and footage of this magical process," Việt said. "We were ecstatic after sleepless nights watching it unfold before our eyes," Lâm added.
Collected eggs and sperm were separated and cross-fertilised in labs, avoiding inbreeding through natural genetic barriers. "In March, when the next spawning season begins, we hope to replicate this success," said team member Đinh Thị Phương Thúy.
Nguyễn Như Hưng, coastal branch director at the Vietnam-Russia Tropical Centre, emphasised the broader goal: "We want people to recognise the value of reef restoration and the effort it demands, fostering greater awareness and marine conservation."
With initial gamete samples secured, the team aims to refine breeding techniques to supply coral stock for large-scale restoration projects.
"If corals vanish, the ocean floor becomes barren and all life disappears. That’s what drives me to protect these reefs," Việt said.
