These Starfish Face Extinction. Scientists Are Helping Them Mate.

Over the past decade, the sunflower sea star has gone from a mighty predator of the Pacific Ocean floor to nearing extinction. The Nature Conservancy estimates that 5.75 billion sunflower sea stars died over the span of three years, a 94 percent global decline.

The cause, scientists say, is largely climate change and warming waters, spurred by what they now refer to as sea star wasting disease. Scientists have been racing to figure out how to bring these creatures, whose 24 limbs can stretch outward to four feet, back from the brink.

Hope is slowly on the horizon.

The Birch Aquarium at Scripps Institution of Oceanography near San Diego successfully spawned three sunflower sea stars earlier this month, the latest success story in a sprawling collaborative effort among institutions to help sea stars reproduce and eventually reintroduce them to the wild.

“There’s a lot of opportunities for good genetic diversity across the board with all of our different institutions working together,” said Melissa Torres, an aquarist at Birch Aquarium who leads the project.

Despite their “charismatic” nature, Ms. Torres said, sunflower sea stars move quickly and decisively in kelp forests, feeding on sea urchins and other types of invertebrates that feast on kelp beds. But starting in 2013, starfish started dying by the millions along the Pacific Coast from Mexico to Alaska. That was the same year that parts of the Pacific Ocean became unusually warm as part of a broader marine heat wave, nicknamed the Blob.

The starfish began to form white lesions on their limbs that would dissolve the surrounding flesh and eventually lead to their death. The disease effected scores of species of starfish, but harmed sunflower sea stars in particular, researchers found. In California and Oregon, the species is believed to be functionally extinct, meaning the population has declined to the point that it is no longer serving an ecological role or function.

Last March, federal officials recommended that sunflower sea stars should be protected under the Endangered Species Act.

Without enough sunflower sea stars, urchins have destroyed nearly all of California’s kelp forests, what marine biologists call “the lungs of the ocean.”

In the meantime, researchers at several zoos and aquariums across the United States have been working furiously to restore the population, led in part by the Aquarium of the Pacific in Long Beach, Calif., and the Henry Doorly Zoo and Aquarium in Omaha.

The Birch Aquarium, housed at the University of California, San Diego, has five sunflower stars, one of the largest populations of sunflower sea stars in captivity in California. But until recently they did not know which were female or male. To determine the sex, researchers induced spawning.

In the wild, sea stars broadcast spawn out into the water — males freely release sperm and females release eggs — where they mix and hopefully fertilize spontaneously. At the aquarium, researchers administered hormones into the legs of three sunflower stars, about 50 inches in diameter, and waited.

After several hours, the sea stars began to tent up from their flat position in what researchers described as a downward dog position like in yoga. Eggs and sperm began to slowly appear, and the researchers carefully scooped them up with glass pipettes. The genetic material was then immediately cryogenically frozen.

Ms. Torres said further testing needed to be done to determine whether offspring could survive disease or parasites. Once that’s determined, the aquarium will be able to begin the fertilization process. And because the aquarium now knows the sex of all of its sunflower sea stars, they can pair them accordingly or even share the sea stars across aquariums for breeding.

She said the aquarium was thrilled to be able to “pass these genetics to everybody else who don’t quite have the amount in the population they need or the ability to get these genetics.”

Creating multiple generations of new sunflower sea stars is the ultimate goal, one that has seen success at the Friday Harbor Laboratories in the San Juan Islands. It took three years for the lab’s juvenile sunflower sea stars to mature enough, but last February, researchers helped the sea stars complete their full life cycle.

Jason Hodin, a marine biologist who leads the Friday Harbor sunflower sea star project, described the success at Birch as “a pretty big breakthrough.”

Now Dr. Hodin is sharing with Birch his secret to success, or what he calls “our cookbook to raising a sea star.”