Rising water temperatures, pollution and other adverse conditions caused primarily by humans are pushing the Caribbean’s magnificent elkhorn coral (Acropora palmate) to the brink of extinction. It is the first coral listed under the Endangered Species Act as threatened. Since the 1980s, populations of this once pervasive marine invertebrate have declined by nearly 99 percent.

At the Smithsonian’s National Zoological Park, Reproductive Scientist Mary Hagedorn and Invertebrates Keeper Mike Henley are working to protect elkhorn coral by establishing genetically diverse reproducing colonies of this marine animal in laboratory aquariums in Washington, D.C.

In August, the scientists netted some 12,000 spawning elkhorn gametes in Puerto Rico during nighttime dives. The gametes were transferred to a laboratory on the beach for artificial insemination and then transported to the National Zoo.

At the Zoo, Henley has been coaxing the coral larvae to settle onto special tiles in a 90-gallon, saltwater tank outfitted with high-wattage lights and a custom-built surge device that mimics the movement of the surf. From the original 12,000 gametes, only 158 larvae have developed to settle on the tiles, forming millimeter-sized polyps that may eventually grow into thriving colonies. The small percentage of elkhorn taking hold as polyps underscores the difficulty of rearing such species in captivity

“We achieved some important milestones this year, including learning more about the larvae-rearing process, and we were able to cryopreserve—freeze, store and thaw—coral sperm,” Hagedorn says.
Hagedorn, a pioneer in the cyropreservation of coral sperm and eggs, is working with the Sexual Coral Reproduction Program to create a genome resource bank to help preserve the elkhorn’s genetic diversity. Preserving the elkhorn’s existing genetic diversity is essential to giving this coral the best possible chance of survival following reintroduction to its natural habitat in the future.