These Sea Slugs Can ‘Eat’ Sunlight—but They’re No Astrophage. Here’s How the ‘Project Hail Mary’ Antagonist Has a Real-Life Analog in Earth’s Oceans

By snatching chloroplasts from algae, animals called sacoglossans produce their own energy through photosynthesis

Niamh Ordner

March 18, 2026 7:30 a.m.

The sea slug is bright green, shaped like a leaf and doing something very few animals can: harnessing the power of the sun.

Certain species of sacoglossan sea slugs can conduct photosynthesis—an exceptionally rare ability in the animal kingdom—earning them the nickname “crawling leaves.” They manage this feat by slurping chloroplasts from algae and storing them in specialized cells through a process known as “kleptoplasty,” which literally translates to “stealing plastids.”

“The sea slugs are a remarkable example of this [phenomenon], because they’re a multicellular organism—or, in particular, an animal—that’s able to do this,” says Joshua Widhalm, a horticulturist at Purdue University.

From robbing algae to feasting on sunlight, this behavior sounds like science fiction. In fact, it’s not too far from the premise of Andy Weir’s 2021 novel, Project Hail Mary, and its film adaptation, which will arrive in theaters March 20. In the story, humanity faces the threat of widespread death from Astrophage—alien microbes that are growing on the surface of the sun and feeding on its energy. The presence of Astrophage reduces the sunlight that reaches Earth and risks sending the planet into another ice age.

In reality, nothing quite like Astrophage exists. But these strange sea slugs and other extreme organisms on our planet have real-life versions of their extraordinary traits, suggesting that evolution’s possibilities may be limited only by our imaginations.

A remarkably rare ability

When sacoglossan sea slugs feed, they pierce algal cells and suck out the contents. Most of the organelles and other bits are digested immediately, but specialized cells engulf chloroplasts, which contain photosynthetic pigment—and in some slugs, these are later digested as well.

Others sequester the chloroplasts within their bodies, tapping into their photosynthetic abilities to produce energy for weeks—or sometimes months—like a biological battery pack. The emerald-green slug Elysia chlorotica offers one of the most extreme examples. “They are able to feed once, and then they can complete their adult life cycle—which is about eight or nine months—without having to feed again,” says Michael Middlebrooks, an invertebrate zoologist at the University of Tampa.

Perhaps even more remarkable is that the animals can counteract the accumulation of toxic byproducts from photosynthesis, which are known to damage cells.

“When you think about how an organism acquires photosynthesis, you have to think about how it copes with the bad parts of it,” says Patrick Keeling, a biologist in the botany department at the University of British Columbia in Canada. The process makes energy, but it also produces highly reactive chemicals that can break down cell structures. “It’s basically like you’re lighting the fuse of a bomb inside yourself.”

One potential explanation is that by sequestering chloroplasts in isolated compartments, the sea slugs can keep toxic byproducts away from other cells. A September study published in Cell supports this idea, finding that these structures, called “kleptosomes,” maintain the chemical environment necessary for photosynthesis while containing the resulting chemicals.

This helps explain why kleptoplasty is so rare. Most organisms that may have tried to acquire photosynthetic abilities likely didn’t survive the attempt, because they failed to evolve strategies to accommodate the buildup of toxic byproducts. “Anything might try, but … you’d have to have all these characteristics lined up to survive,” says Keeling. “We don’t know about the ones who didn’t survive, because they’re just gone. They’re extinct.”

Kleptoplasty evolved independently at least twice in sea slugs, and it has also been documented in flatworms and one-celled protozoans. Nevertheless, it remains an extraordinarily uncommon ability among multicellular organisms. The sea slug “is unique, as far as an animal doing it and how good they are at doing this,” says Middlebrooks.

Scientists have proposed several explanations as to why sea slugs developed kleptoplasty. Perhaps it’s a form of camouflage, as the stolen chloroplasts’ green pigment helps the animals blend in among the algae they feed on. Or the slugs might have evolved it to actively photosynthesize, creating sugars that they can access later if food becomes scarce. “Maybe it’s multifunctional—helping to camouflage the slug but also acting as a storage pool of food,” Widhalm says.

Researchers note that kleptoplasty resembles an early stage of endosymbiosis—an evolutionary process in which one organism lives inside the cell or body of another in a symbiotic relationship—because the host benefits from a foreign organelle without integrating it into its own genome.

The sea slugs’ kleptoplasty, in this way, might shed light on how plant cells evolved. Endosymbiotic theory suggests that about two billion years ago, a cell engulfed a photosynthetic bacterium and, rather than digesting it, kept it alive. This act gave rise to the chloroplasts in the plants and algae of today—and a similar process led to the mitochondria in animal cells. Continued research into how sacoglossans engulf chloroplasts could offer a model of the initial phases of endosymbiosis and, therefore, the origin of multicellular life. “By studying kleptoplasty, we may gain some insight into what those early steps were,” says Widhalm.

Understanding how these sea slugs steal and utilize foreign organelles could also lead to a range of technological advances, with applications in agriculture, pharmaceuticals, medicine and combating climate change. Widhalm described one idea already circulating in the medical field: that yeast engineered to perform kleptoplasty might be used in drug production. As the yeast works to create new and potentially useful molecules, it could then fuel itself with nothing more than water, light and carbon dioxide.

“There’s lots of possibilities that we could dream up,” says Widhalm. Some proposed ideas range from designer cells engineered to snatch and store carbon dioxide from the atmosphere to new clinical treatments that could replace faulty organelles in individuals with mitochondrial diseases, which have no cure. Still, he notes that these applications remain speculative.

Snatching sunlight and resisting radiation

In Project Hail Mary, Astrophage absorb electromagnetic energy—a type of radiation that includes visible light—store it internally and use it to move through space to infect other stars or reproduce on planets.

According to Betül Kaçar, an astrobiologist at the University of Wisconsin-Madison, organisms developing the ability to “eat” radiation isn’t out of the realm of possibility—in fact, it has happened already on Earth.

“Life found a way to eat pretty much anything it can on this planet. It’s quite remarkable,” she says. “If you think about it, the fact that life can capture photons [or particles of light] is ‘eating radiation.’ … For many microbes, the photons are great resources of energy. So phototrophs are an example of radiation-eating organisms on this planet.” Plants are well-known phototrophs, but sea slugs are unexpected ones, making their ability to gain fuel from radiation just as remarkable as that of the Astrophage microbes.

One major question for biologists regarding the feasibility of Astrophage is how anything could live on the surface of the sun, which sits at a scorching 5,500 degrees Celsius (about 10,000 degrees Fahrenheit)—roughly 80 times the hottest naturally occurring temperature ever recorded on Earth. “It’s like growing life on a hydrogen bomb,” says Kaçar.

At these temperatures, proteins and DNA—the very building blocks of life—begin to fall apart. The intense levels of electromagnetic radiation emitted by the sun are also a matter of concern, as they could lead to mutations and cancerous growths. For the fictional Astrophage, this isn’t a problem—the microbes are resistant to radiation. In fact, the Hail Mary, the spaceship used by the protagonist Ryland Grace, has a protective layer of Astrophage microbes that shield the crew from radiation exposure.

In the real world, organisms here on Earth have also evolved impressive radiation defenses, according to Kaçar. Deinococcus radiodurans, a bacterium that’s one of the hardiest known organisms, can survive high levels of radiation by repairing its DNA with extraordinary speed and accuracy. Protected by the pigment melanin, some fungi at the Chernobyl nuclear disaster site grow better in high-radiation environments than they do without radiation.

These fantastic abilities are merely the solutions that evolution has come up with on one world, in one corner of the universe. Our visions of alien life are inherently biased by the only examples of living things we’ve ever encountered, and Weir’s Astrophage is just one notion of what life beyond Earth could look like. But chances are, if evolution has surprised us so thoroughly on our own planet, the universe could hold stranger and more wondrous beings than we can even begin to dream up.

“The more I’ve learned, the more I’ve realized we know almost nothing,” says Middlebrooks. “Every time we answer a question, it creates a dozen more.”