National Geographic

Freezing Coral Sperm to Save the Great Barrier Reef

Freezing coral sperm might sound like a dirty job, but it’s a passion for marine biologist Mary Hagedorn.

Hagedorn, of the Smithsonian Conservation Biology Institute, is creating genetic banks of coral sperm and embryonic cells that may one day help reseed the Great Barrier Reef (see pictures), a threatened, 1,800-mile-long (2,900-kilometer-long) reef along the Australian coast that’s home to the world’s largest collection of corals.

The institute is working with other partners to freeze genetic material of coral species—currently declining throughout the world—in the hopes of thawing and using the cells in ten or even a hundred years to come. Larvae produced by the sperm or embryonic cells can be raised in captivity and possibly put back into the wild if needed.

Biotechnician Ginnie Carter pours liquid nitrogen into a coral cryobank.

Photo courtesy Jim Daniels

Corals can reproduce both sexually and asexually, although most stony coral species—such as the ones Hagedorn studies—are broadcast spawners, which means they release huge numbers of reproductive cells called gametes into the water. These gametes float to the surface before the eggs and sperm join to form free-floating larvae, according to the National Oceanic and Atmospheric Administration’s Coral Reef Conservation Program. The larvae then drift about “looking” for a hard surface on which to attach.

Scientists at NOAA have already been experimenting using fresh sperm and eggs to create billions of new larvae, which have been released into the Florida Keys. However, so far “only one coral has one survived—his name is Harold,” Hagedorn said.

Yet a new coral consortium called SECORE has recently raised more than 3,000 rare coral larvae in captivity, and young coral are now growing in public zoos and aquaria worldwide. (See coral pictures.)

Christine Dell’Amore spoke with Hagedorn about what she calls an “insurance policy” for the Great Barrier Reef.

Q: Why is this repository needed?
A: It’s possible that we will lose our coral reefs in the next 30 to 50 years … we’re seeing almost 75 percent of all reefs are under siege. Both local and global causes contribute to this decline. Local causes could be pollution, sedimentation, … [boat] anchoring, dynamite [fishing]—those are all local things that can be more or less dealt with by legislation. The global causes are more difficult to deal with, and those are things like the warming of the oceans. And also as we put more CO2 into the atmosphere, it’s dissolving in the water and causing the pH of the ocean to go down. As the pH goes down, the coral are less able to absorb calcium carbonate [their main building blocks] into their skeletons.

Many of your readers may have visited a reproductive clinic—we’re applying the same techniques to coral. We’ve taken human biology and are applying it to endangered and threatened species.

Are you collecting all of the species of Great Barrier Reef coral?
No. It’ll be a very slow process. To a large extent conservation is about people, unless people think it’s important, we don’t have the money to do this. We’re going to start with one candidate species [from the Great Barrier Reef]. We will go ahead and freeze the sperm and hopefully embryonic cells from that one species and see if we can generate the enthusiasm within Australia. We are hoping this will be the case because the Great Barrier Reef is so iconic and generates so much income for Australia every year.

Do you have any interesting stories from the field on how you collect coral sperm?
First we’ll go and look at the coral and size them up. Every single coral has a unique spawning time—sometimes it’s several times in a year. In the Great Barrier Reef generally it’s once, and it goes for 45 minutes to an hour. Generally people know what time it spawns. We’ll put nets on [the surface] with divers a half hour to 45 minutes before spawning. Corals are hermaphrodites and produce egg-sperm bundles that rise to the surface [and are caught in nets]. Think of a cluster of grapes with a pocket at center with the sperm. [We then take them] into the lab and … freeze them.

How is it frozen? Describe a bit of the process of getting it ready for the repository.
It’s the same process involved in freezing human sperm—we combine the sperm and or embryonic cells with an antifreeze or cryoprotectant to help protect it during the freezing process. We test the type and strength of the cryoprotectant ahead of time. We place the … mixture into specialized cryovials, and place them on racks in a styrofoam box filled with liquid nitrogen. We cover the box and freeze the cells rapidly—28 Celsius [82 Fahrenheit] to just about -80 Celsius [-112 Fahrenheit] in five to six minutes. [We] then load the cryovials into a dry shipper, [which] stays at -200 Celsius [-328 Fahrenheit]. Currently we are storing the cells in a repository in Fort Collins, Colorado—the USDA’s Animal Germplasm Repository—as well as three other repositories around the world.


Table coral of the genus Acropora live in Australia’s Great Barrier Reef.

Photo courtesy Michael Henley, Smithsonian’s National Zoo

If it’s needed, how will you put the sperm and embryonic cells back into the wild?
Although this is something that we are interested in doing, we have not done this yet. This is because some of the science for fully rearing these larvae into adults is not fully understood just yet. … But we are starting and continuing this freezing process because there is still a lot of diversity left on the reefs, but once we lose that we’re in real trouble.

Are there similar banks for other species?
Yes. Especially at the National Zoo, and at lot of the major zoos of the world, [with species such as] giant pandas and elephants. Zoos can be thought of as small populations that are in captivity, and they’re working very hard to diversify those populations. These modern reproductive technologies are becoming very important to zoos around the world, but also to aquaria.

What’s the coolest thing about coral that readers might not know?
Number one, they’re not rocks! When you see a coral spawn, it’s one of the most magnificent things on earth. The Great Barrier Reef is the one of the only animal-made objects that you can see from space. These microorganisms have built the Great Wall of China underwater. It’s one of the seven wonders of the world.

This Q&A has been edited for length and content.

Check out more weird coverage on National Geographic News.


  1. dee
    September 26, 2012, 2:57 am

    Thanks for this….its really awesome I’ve learnt so much from this piece of information and it will enormously help me in my studies…..cheers

  2. [...] When we visited the Reef a few years ago and went diving, I was actually most impressed not by the color and or the sea life variety. I was most impressed by how much of the reef that we saw was not colorful at all, but a dusty brown, as it died. Very sensitive to changes in both ocean temperature and acidity as well as other disturbances, the Reef is well known to be threatened. So get there while you can, and join efforts to save the Reef as well. You are warned that you may have 30-50 years to make this visit. [...]

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