Coral larvae survive being frozen and thawed for the first time

For the first time, researchers have quick-frozen coral larvae and then — the tough part — safely thawed them.

Swathing larvae in specks of gold and then heating them with a laser warmed the frozen coral babies in milliseconds. Thawed this way, 43 percent of 2-day-old test larvae recovered well enough to start swimming again, physiologist and cryobiologist Mary Hagedorn and her colleagues report October 24 in Scientific Reports.

This success with Fungia scutaria larvae, a colorful mushroom coral species from Hawaii, opens up much-needed conservation possibilities, says Hagedorn, of the Smithsonian Conservation Biology Institute in Front Royal, Va. Biologists knew how to freeze coral sperm, but ice crystal damage from the process can kill bigger, fat-rich structures such as coral eggs and larvae. Being able to bank more than sperm may help preserve coral genetic diversity for attempts to aid reefs that falter as the climate warms.

Biologists have been freezing and storing human embryos since at least the 1980s, but the first success with any nonmammal is recent, Hagedorn says. In 2017, she, John Bischof of the University of Minnesota Twin Cities and colleagues described how warming zebrafish embryos with gold nanorods let 10 percent of the embryos survive at least 24 hours after thawing.

In a procedure loosely based on the zebrafish system, researchers enveloped larvae in an antifreeze solution with gold nanorods and froze the creatures in milliseconds with liquid nitrogen. But that wasn’t the end of the danger from ice crystals. Even during thawing, crystals can enlarge dangerously. That’s when a laser pulse quickly warmed the gold bits and thus the frozen larvae.

“It’s a bit like making a soufflé — everything has to be perfect,” Hagedorn says.