Sydney / Great Barrier Reef— Once a year, usually between spring and summer, the Great Barrier Reef undergoes a rare and spectacular event. In the nights following the full moon, when water temperatures are ideal, millions of coral polyps release eggs and sperm in a synchronised mass spawning. The tiny reproductive bundles drift upward like a glowing cloud, increasing the chances of fertilisation and the birth of new coral colonies.
During this brief window, scientists from Sydney’s Taronga Zoo dive into the reef by torchlight to collect coral sperm and larvae. These samples are taken to the Taronga CryoDiversity Bank, where they are stored at extremely low temperatures. Over the past decade, the bank has become the largest coral cryopreservation facility in the world, safeguarding genetic material from 30 different coral species.
Bleaching Is Undermining Coral Reproduction
Coral bleaching caused mainly by ocean warming forces corals to expel the symbiotic algae that provide them with food and colour. Beyond the visual damage, bleaching has serious long-term effects, including weakening corals’ natural reproductive abilities. Some genetically fragile species are finding it increasingly difficult to produce viable offspring, threatening the reef’s genetic diversity and resilience.
“The cryopreservation and biobanking work have two objectives,” explained Jonathan Daly, aquatic cryobiology specialist at Taronga and the University of New South Wales. “One is to secure as much genetic diversity as possible while those populations still exist on the reef. The second is to use those samples to support the development of resilient corals and aid aquaculture production.”
A Global Race to Preserve What Remains
International collaborations are accelerating as climate change pushes coral ecosystems toward repeated mass bleaching events. The Taronga CryoDiversity Bank works with global partners, including the Smithsonian Conservation Biology Institute, to expand genetic repositories and research into coral resilience.
Researchers hope that the stored coral embryos and cells will eventually support large-scale restoration efforts, potentially enabling scientists to breed corals that are better able to withstand warming waters and ocean acidification.
But the window for action is narrowing. As marine heatwaves intensify, natural spawning events could weaken further, making it harder for coral populations to regenerate on their own.
For now, each spawning season is a critical opportunity a moment when science works urgently in the dark, collecting and preserving life that may help rebuild reefs in the future.