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It is a spectacle that takes place just once a year, typically between spring and summer. On a few nights following the full moon, when water temperatures are just right, coral polyps engage in a synchronised mass spawning. Millions of tiny eggs and sperms, bundled together, are released into the water in a coordinated nocturnal event. These bundles rise to the surface under the cover of darkness in what resembles a nuptial dance. To reproduce, each bundle must meet and merge with another from the same species. By spawning en masse, corals increase their chances of successful fertilisation. The result is a planula larva that eventually settles on the seabed to begin forming a new colony.
At this precise moment, scientists from Taronga Zoo in Sydney dive into the Great Barrier Reef (GBR)—the world’s largest coral reef system. Guided by torchlight, they harvest sperm and larvae for cryopreserving (storing at extremely low temperatures) them at the Taronga CryoDiversity Bank. For over a decade, the team has been building a genetically diverse repository of coral species for future restoration. It is now the world’s largest such facility, housing samples from 30 coral species.
The fact is, the impact of bleaching, or corals expelling their symbionts when stressed by warming and other stressors, extends far beyond the loss of colour and structural integrity. It also impairs the natural reproduction of these invertebrates, particularly in genetically weaker species. This could pose a threat to their genetic diversity, essential for their long-term survival. “The cryopreservation and biobanking work have two objectives. 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,” says Jonathan Daly, aquatic cryobiology specialist at Taronga and the University of New South Wales, Australia.
Organisations around the world are racing to preserve corals before they are lost for good. The Taronga CryoDiversity Bank, in collaboration with partners such as the Smithsonian Conservation Biology Institute in the US, is collecting and preserving coral spawn. It has now joined forces with …
This article was originally published in the cover story Fading reefs in the November 1-15, 2025 print edition of Down To Earth