In 2019, a batch of baby elkhorn corals started growing in tanks in Florida. Their parents had never met. Their mothers lived on reefs off Curaçao, in the southern Caribbean. Their fathers lived in Florida and Puerto Rico, more than a thousand miles away. The fathers' contribution had traveled in a tank of liquid nitrogen.

The corals were the result of an experiment led partly by Mary Hagedorn, a senior research scientist at the Smithsonian's National Zoo and Conservation Biology Institute. Her team fertilized more than 150,000 eggs in Curaçao with frozen sperm, then shipped 20,000 larvae to labs in Florida to grow. When the results came out in PNAS in 2021, it was the first time frozen sperm had been used to move genes between coral populations that could never reach each other on their own.

For Hagedorn, that was proof of a much bigger idea: if reefs keep dying, a freezer might be what saves them.

A hedge against extinction

Hagedorn did not start out in coral. She trained as a marine biologist at Scripps Institution of Oceanography and spent the early part of her career studying fish physiology. She shifted to coral as it became clear how fast ocean warming was destroying reefs, and she began applying cryobiology, the science of keeping living cells alive at extremely low temperatures, to the problem.

The numbers she works against are grim. About half of the world's coral reefs have disappeared since 1950, according to Smithsonian Magazine. The Intergovernmental Panel on Climate Change has projected that reefs will decline by 70 to 90 percent if the planet warms by 1.5 degrees Celsius. "This threat is existential," Hagedorn told The Hoya in 2023. "It is cataclysmic for our lives."

Her response is a biobank. Frozen in liquid nitrogen, living coral cells can stay viable for hundreds of years. If a species disappears from the wild, the bank could hold the genetic material needed to bring it back. "Cryopreservation helps form a hedge against extinction," she told The Hoya.

The problem with sperm

For years, the bank relied mostly on frozen sperm, which Hagedorn developed a low-cost technique to preserve. Sperm has a serious limit, though. Most corals spawn only once a year, over a few nights, often in the dark hours after a full moon. Miss that window and you wait another year.

"It can be very, very challenging to get there at the right time," she told Smithsonian Magazine. "We felt like we needed to go faster."

Going faster meant freezing whole coral, which is much harder. A coral fragment is a colony of tiny animals called polyps living on a stony skeleton, with algae living inside their tissue. When water inside the cells freezes, ice crystals tear them apart.

In 2023, Hagedorn and engineers from Texas A&M University and the University of California, Berkeley, reported in Nature Communications that they had frozen and revived fragments of adult Hawaiian finger coral for the first time. Each thumbnail-sized piece held about 20 polyps. The team used a method called isochoric vitrification, which cools tissue so quickly in a sealed chamber that it turns into a glass-like solid instead of forming ice. After about two minutes in a warm water bath, the fragments went back into seawater and, over the next 24 hours, used oxygen at about the same rate as coral that had never been frozen.

"It's conceptually complicated because of thermodynamics," Hagedorn said. "But the actual process itself is really dead easy."

Scaling up

The next challenge is volume. A reef holds thousands of genetically distinct individuals, and saving a species means saving that diversity, not just one colony.

To get there, Hagedorn has worked with University of Minnesota engineers on a tool called cryomesh, a fine mesh that holds many coral larvae at once and cools them quickly. In tests with Great Barrier Reef corals at the Australian Institute of Marine Science, an abstract from the team reported that 85 percent of larvae recovered after thawing, compared with 43 percent using an older laser-warming method. Hagedorn called the approach "a real game-changer" in an interview with Voice of America.

The bank is growing. By 2023, the Coral Biobank Alliance, which the Smithsonian belongs to, had frozen sperm from 50 coral species and was holding live specimens of 200 more. Its goal is to store genetic material from every known coral species by the end of the decade.

Not a fix on its own

Hagedorn and other scientists are clear that a freezer cannot replace a healthy ocean. Very few frozen larvae have been raised all the way to adulthood, Eos reported in late 2025. Banking enough diversity for a single species could take 15 to 25 years of collection.

Rebecca Helm, a marine biologist at Georgetown University's Earth Commons Institute, put the concern plainly to The Hoya. "I think this also shouldn't be considered a reasonable response to climate change," she said. A biobank buys time. It does not cool the water.

Hagedorn has made a similar point, telling Eos that cryobanks only work alongside aquariums that grow live coral, secure storage and steady government funding. Much of her recent work is about people as much as technique. She has helped set up coral repositories in Australia, French Polynesia and Saudi Arabia, and her collaborators are now training scientists to build a network of cryobanks across Southeast Asia's Coral Triangle, home to more than 600 coral species.

"A real bottleneck for this field is there's so few people that are trained as cryobiologists," she told Eos. Then she named the reason the work has to spread beyond her lab. "No one person can cryopreserve all the species of corals in the ocean."