On Valentine’s Day, Melissa Torres strung up red tinsel hearts around a shallow pool at her workplace, the Birch Aquarium at Scripps Institution of Oceanography in La Jolla, Calif. She and her colleagues were arranging a romantic encounter of sorts, and the stakes were high. The happy couple, a pair of sunflower sea stars, belonged to a species that has nearly vanished because of climate change.

Sunflower sea stars are a far cry from their smaller pink cousins that you might know from Finding Nemo and SpongeBob SquarePants. They have up to 24 arms and can grow to diameters of more than three feet. They’re also voracious hunters that prey upon sea urchins living among the 100-foot-tall stalks of algae that make up the kelp forests of the Pacific Northwest.

“In a perfect, climate-change-free world, they would be keeping the kelp forest ecology at a perfect eco-balance,” Ms. Torres said. But in 2013, a 1,000-mile-wide mass of warm water nicknamed the Blob formed in the North Pacific. As a consequence of the heat, a strange wasting condition began spreading among the sunflower sea star population. Since then, an estimated 90 percent of all sunflower sea stars have perished. They have been declared functionally extinct in California and Oregon.

Without sea stars to keep urchin populations in check, the urchins are eating too much of the kelp forests’ giant algae. It’s a big problem, Ms. Torres said, because the algae “provide not only homes and food for animals, but food and carbon sequestration for humans.”

Since 2019, the Birch Aquarium has been part of a wide network of aquariums and research centers focusing on sunflower sea star conservation, starting with efforts to breed healthy, genetically diverse sea stars in captivity. In this leg of the project, Ms. Torres and her team wanted to see if they could fertilize sea star eggs using both fresh and frozen sperm.

When sea stars reproduce, it’s an impersonal affair: They release clouds of eggs and sperm into the water. Ms. Torres injected a male and a female with an enzyme that caused them to release eggs and sperm, and then she and her colleagues waited — two hours for the male, and four hours for the female.

Once both animals had successfully spawned, “we were jumping and screaming and hugging each other and freaking out,” she said. The team collected the sea stars’ output, and then used it to fertilize the eggs.

In addition to the fresh sperm, the team repeated the procedure with frozen sperm from the same male that had been preserved at the Frozen Zoo, a cryogenic facility at the San Diego Zoo. The team experimented with sperm samples stored at both –112 and –320 degrees Fahrenheit; both trials, as well as the fresh sperm, proved capable of fertilizing the fresh eggs.

The experiments resulted in millions of fertilized eggs, which the team distributed to the Birch Aquarium and several of its partners, including the Aquarium of the Pacific and the California Academy of Sciences.

Even with this successful fertilization project, though, the future of sunflower sea stars is in jeopardy because of the warming climate. Still, this recent milestone provides a proof of concept of captive breeding methods, said Doug Pace, an associate professor at California State University, Long Beach. Dr. Pace studies sunflower sea stars’ ability to survive in different temperatures, and he works with researchers examining the genetic blueprints of different populations. It’s possible that this work may reveal the genes necessary for “a sunflower star that can handle the challenging conditions that the future may hold,” he said.

As of last week, the fertilized eggs at the Birch Aquarium had progressed to the larval stage of development, as evidenced by their ability to eat the algae they had been fed. “You can see pink stomachs in every little larva,” Ms. Torres said. They don’t yet have arms or other recognizable sea star features, “but they’re growing.”

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