To Help Corals Fight Back, Scientists Are Breeding Populations Separated by Hundreds of Miles

A new study demonstrates that assisted reproduction using cryopreserved sperm leads to offspring that might be more resilient in the face of climate change

Elkhorn coral
The Smithsonian's Mary Hagedorn and hundreds of colleagues collaborated on the project, which used cryopreserved elkhorn coral sperm to fertilize live eggs to create larvae. SCBI, photo by Stephani Gordon

What was in many ways a gamble has paid off for a dedicated group of biologists—and possibly for other scientists looking for a novel way to preserve threatened species.

Using frozen sperm to fertilize live eggs, the researchers were able to breed elkhorn coral (Acropora palmata) from one geographic area with elkhorn from a different area, paving the way to potentially rescue not just that endangered species and making it more resilient to climate change, but also providing an avenue of hope to restore other species.

“No one believed we could do this,” says Mary Hagedorn, senior research scientist at the Smithsonian Conservation Biology Institute and co-lead author on a paper describing the work that was presented at the Reef Futures Conference in Florida last week.

“It was Herculean that we got this done. The work was so hard on every single level,” she says.

Hagedorn and hundreds of colleagues collaborated on the project, which used cryopreserved elkhorn sperm from Florida, Puerto Rico and Curaçao in the Carribean to fertilize live eggs collected from elkhorn coral in Curaçao to create larvae which were then reared in isolation tanks at nurseries in Florida. So far, the one-month-old baby corals (or recruits) make up one of the largest living populations created from cryopreserved material—with the exception of human beings, says Hagedorn.

Colleagues who study corals had disdained the idea of using cryopreservation as an assisted reproduction technique, she says. “It was very difficult to get people to understand how important this scientific process was not only for maintaining biodiversity but with helping with restoration processes, especially where we’re trying to create better, more robust corals for the future,” says Hagedorn.

coral reef
“It’s extra special that we made this breakthrough in elkhorn corals, because they’re so important for building coral reefs and protecting shorelines all around the Caribbean,” says co-author Kristen Marhaver. SCBI, photo by Stephani Gordon

“Being able to cryopreserve coral sperm allows us to grow corals the same way humans have been growing crops for centuries—by saving genetic diversity in seed banks and breeding the strongest individuals from local populations to help the species do better overall,” says the study’s co-lead author Kristen Marhaver in a Smithsonian press release.

The researchers chose the elkhorn—a quick-growing large, branching coral with thick antler-like branches—because it acts as an essential protector for islands facing destruction from rising seas, and because of its importance in providing fish habitat throughout the Caribbean. It has also been listed since 2006 as a threatened species by the National Marine Fisheries Service.

Elkhorn have declined by more than 90 percent in the Caribbean—due to disease, and warming waters.

“It’s extra special that we made this breakthrough in elkhorn corals, because they’re so important for building coral reefs and protecting shorelines all around the Caribbean,” says Marhaver, an associate scientist at CARMABI Research Station in Curaçao.

The idea behind the work was to see if the elkhorn living in the warm waters off the coast of Curaçao could confer some type of special attributes on elkhorn in coastal Florida and Puerto Rico, now experiencing warmer temperatures.

Instead of moving an elkhorn from Curaçao to the other locations—which, even though they are the same species, could potentially introduce new fungi or bacteria into those non-native waters—they decided to crossbreed the sperm and the eggs of the elkhorn from different locations.

But the researchers first had to capture both sperm and eggs—which involved timing, and, a bit of luck. Elkhorns, like all corals, are hermaphrodites, and thus release both eggs and sperm simultaneously.

Smithsonian & Partners Pioneer Method to Boost Endangered Coral Populations

It is known that elkhorn spawn once a year in August in the Caribbean, usually around a full moon. To get the sperm, divers normally descend to a reef, and look for the signal that the elkhorn is “setting,” or holding the sperm-egg bundles. They only hold those bundles for about a half-hour, so divers have to quickly throw an artificial silk net over the bundles—which look almost like acne—and wait for them to release, says Hagedorn. The net funnels the floating bundles into a collection device, and scientists can later separate out the sperm, a milky fluid, from the eggs.

Both are only viable for about five to seven hours—which makes the idea of breeding a Curaçao elkhorn with a Florida elkhorn a tricky proposition. But in 2007 Hagedorn and her colleagues developed a technique to freeze sperm. They started a sperm bank in Puerto Rico in 2008 and another one soon after in Florida. That frozen sperm was used to fertilize the live eggs they gathered in Curaçao.

Still, the collection process did not go smoothly in Curaçao. The reefs were close to shore, which made access easy—divers just waded in with their gear. But Hagedorn and her colleagues weren’t entirely sure when elkhorn in Curaçao spawn. With a full moon in late July and another in August, they didn’t want to take any chances. So, for six weeks, the scientists prepared for collection, and the divers went to the reef—every night—until, finally they witnessed a large spawn over two days in early September, almost two weeks after the August full moon.

The researchers used fresh sperm from Curaçao to fertilize those fresh eggs. They also created three other batches of crosses. They used frozen sperm of elkhorn from Curaçao, Florida and Puerto Rico each to fertilize the eggs of elkhorn from Curaçao.

Not surprisingly, the fresh sperm did better than the frozen—a 91 percent fertilization rate, compared to 37 to 82 percent for the frozen Curaçao sperm, 3 to 19 percent for the frozen Florida sperm, and 0 to 24 percent for the frozen Puerto Rico sperm. Hagedorn says it often depended on egg quality, which varied over the two nights. And, the frozen sperm in some cases had been on ice for 10 years; today, cryopreservation techniques are better, and would likely lead to higher success rates, she says.

And though the corals are the same species, they have some genetic distinctions, so biological incompatibility is also an issue, Hagedorn says. “But it worked, that was the major thing,” she points out.

The researchers had hoped to gather and send 100,000 fertilized larvae, but instead were only able to create 20,000. At one month, 55 percent of the juveniles created from frozen sperm, and 81 percent of those created with fresh sperm, were still alive at the two institutions, the Mote Marine Laboratory and the Florida Aquarium Center for Conservation, where they are being monitored. That’s a huge success, says Hagedorn.

“I think the best I’ve ever done is four percent,” she says. “The grow out has just been phenomenal.”

The next step is to show that they can mature. And the hope is to speed up that maturation—using a technique developed by Mote—to get them to spawn at three years of age, instead of seven, says Hagedorn.

And eventually, she would like to find a way to get the new, possibly more-resilient elkhorns into the ocean. “As we try to create better tools for restoration, we have to be able to try them in the wild,” says Hagedorn. “Otherwise, why create them?”

Because they cross-bred, it’s proof they are the same species—though isolated, geographically-distant species. Despite the similarities, introduction into the wild is not to be taken lightly, says Hagedorn. “We have to be very cautious in how we do that.”

Funding for this project was provided by Paul G. Allen Philanthropies, the Paul M. Angell Family Foundation and the Volgenau-Fitzgerald Family Fund.

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