The Coral Triangle, one of the planet’s most diverse reef regions, is under siege.
The area, spanning the waters of Indonesia, Malaysia, Papua New Guinea, the Philippines, the Solomon Islands and Timor-Leste, is home to 75% of the world’s known coral species. It sustains 3,000 species of reef fish and the livelihoods of over 120 million people. But over 85% of its reefs are threatened by rising temperatures, ocean acidification, pollution and unsustainable fishing, according to the World Resources Institute.
Chiahsin Lin’s love for the ocean led to a career in marine biology. Currently working as head of the Department of Planning and Research at Taiwan’s National Museum of Marine Biology and Aquarium, he fears that future generations may never get to see living reefs.
Scientists warn that reefs worldwide could decline by 70-90% with 1.5C of global warming. In October, researchers said that rising global temperatures have pushed warm water coral reefs – found in tropical, shallow and sunny waters – into a long-term decline. This signals the Earth reaching its first climate tipping point linked to greenhouse gas emissions.
To give coral reefs a chance to survive, scientists are turning to cryopreservation – freezing coral cells, larvae and sperm to be used for future reproduction.
Lin is leading an effort to build a network of cryo-repositories in the Coral Triangle. It aims to preserve larvae from corals that could be used for reef restoration when the environment becomes suitable for their deployment. This is when conditions improve to the point where the larvae, when released, can survive and support coral growth.
“The more coral banks, the more possibilities coral can survive, and the more diversity [is preserved],” Lin says.
But the work is far from easy, and progress has been slow, with only a tiny fraction of larvae surviving the cryopreservation process.
Freezing for the future
For around two decades, scientists around the world have been freezing coral materials in liquid nitrogen at temperatures as low as -196C, notes coastal science publication Hakai. Metabolic and biological processes stop at these extreme temperatures, allowing cells and larvae to be stored for years and later thawed for reef restoration activities. This could mean raising corals that can better withstand climate stress, or to restore reefs where corals have died off.
Lin, who also teaches at the National Dong Hwa University’s Institute of Marine Biology, is maintaining a coral cryobank at the museum. He is currently leading a project to establish and maintain coral cryobanks across several countries in the Coral Triangle that would form the first coral larvae repository network in the region. It also aims to develop cryopreservation protocols, and involves scientists from Pattimura University in Indonesia, Universiti Malaysia Terengganu, University of the Philippines-Diliman, and Phuket Rajabhat University in Thailand.
The Coral Research and Development Accelerator Platform (Cordap), an initiative of the G20 nations, is supporting the project, which started in November 2024.
“It’s not just about preserving corals today; it’s about building a foundation for future research and reef restoration that can benefit generations to come,” said Maria Vanessa Baria-Rodriguez, who is leading the coral cryopreservation effort at the University of the Philippines-Diliman, in an August press release.
The project is initially focused on four species belonging to Pocilloporidae, a family of mostly reef-building corals, notes Lin. Pocilloporids are “weedy” species that grow and settle quickly on damaged reefs, making them useful for reef recovery.
Researchers collect corals from the wild in Taiwan’s Kenting National Park, Thailand’s Tang Khen Bay, Indonesia’s Ambon Island, Malaysia’s Pulau Redang, and the Philippines’ Anda Island, notes Lin.
They keep the coral colonies in tanks and set up cups fitted with fine netting under the tanks’ outlets. The tiny larvae – often less than two millimetres each and described by Lin as looking like sesame seeds – are collected in this way.
Only healthy larvae, which are swimming, are used, as freezing them at nearly -200C is an extremely biologically stressful process that can damage and kill cells. These larvae are stored in a “cryostick” that is submerged in liquid nitrogen for long-term storage. The idea is that they can then be thawed with a laser when ready to be used in reef restoration efforts or to further develop cryopreservation protocols. Lin says they can be frozen for a long time – even 10 to 20 years – “as long as the freezing protocol is optimal”.
In the Philippines, initial trials have resulted in the “successful cryopreservation of early-stage coral larvae,” the University of the Philippines-Diliman’s Marine Science Institute said in the August press release. It added that its monthly efforts continue to expand the pool of cryopreserved larvae across several different coral species.
Yet not every effort goes as planned.
The challenges of cryopreservation
In Thailand’s Phuket Rajabhat University, the project’s first attempt to culture corals in preparation for cryopreservation failed in June. “When I cultured the corals, in the first three weeks, it looked like all the corals were healthy,” says Preeyanuch Thongpoo, a molecular biologist who is leading her team’s coral cryopreservation effort. In the end, however, they did not obtain any larvae and are currently analysing the reasons for this.
Coral cryopreservation is far more complex than simply storing samples in liquid nitrogen, Lin pointed out. Many larvae die at each stage.
To protect the larvae, scientists use chemical treatments such as cryoprotectant solution formulated to help them vitrify – or solidify into a glass-like state, preventing the formation of damaging ice crystals. This increases the ability of larvae to survive the freezing process and low temperatures. The thawing process involves rapid warming using lasers.
Even with such optimised procedures, however, survival is a challenge. Lin explains that only 15-35% of the larvae survive after thawing. Out of those, just 3-24% settle successfully, depending on the species, and even fewer survive to adulthood. For now, large-scale production is still in the development stage, primarily due to technical limitations and restricted availability of larvae stemming from the highly specific conditions necessary for corals to release them, says Lin.
Cryopreservation at other stages
Beyond the larval stage, coral has also been cryopreserved and revived at other points in its lifecycle, with varying levels of success.
One example showing promise is coral sperm. It has a high survival rate after thawing, with about 50-70% remaining active, says Mary Hagedorn, a research scientist at the Smithsonian’s National Zoo and Conservation Biology Institute in the US, and a pioneer in the cryopreservation of coral sperm.
In 2021, Hagedorn and her collaborators used coral sperm from Florida and Puerto Rico, that had been frozen for up to 10 years, to fertilise fresh eggs of critically endangered Elkhorn coral collected in Curaçao, off the coast of Venezuela. The fertilised eggs were raised in Florida, where the young corals are thriving, notes Hagedorn. She adds that they are preparing to reintroduce them to degraded reef sites in Puerto Rico and the US Virgin Islands.
Cryopreserved coral sperm has also been successfully used to fertilise coral eggs collected from lab-spawned corals. In December 2024, researchers introduced the coral larvae born from this process into the Great Barrier Reef and are tracking their growth in the critical first year of life.
But the cryopreservation of other forms of coral, such as fragments, has been less successful. In 2023, scientists cryopreserved and revived adult fragments of finger coral (Porites compressa), a stony coral species endemic to Hawaii. However, they survived for only 48 hours, Hagedorn notes. “We’re not quite there with the fragments,” she says, adding that they are working on new techniques for freezing them successfully.
While coral cryopreservation is a useful way to help species, Hagedorn stresses that the oceans will still be severely impacted unless we change our behaviour. “The good thing about cryopreservation is it allows society the time to understand the impact we are having on our planet,” she says.
Coral cryopreservation is an arduous and painstaking process, with results far from guaranteed. For Lin, it is about more than just saving individual species. It is about trying to safeguard entire reef ecosystems, providing habitats for marine life and sustaining the livelihoods of millions who depend on the sea.
“It’s about giving them some opportunity to fight for tomorrow,” Lin says.
