They look like crawling leaves, but these sea slugs are anything but ordinary. MCB Professor Nick Bellono calls them “the weirdest animal we’ve ever studied”—a bold claim from a researcher whose lab specializes in strange creatures. But there’s good reason for the fascination: these marine slugs can perform photosynthesis, a feat no animal should be able to accomplish.
A new paper from the Bellono lab, published this week in Cell, reveals the surprising cellular machinery that enables Sacoglossan sea slugs to “steal” chloroplasts from algae and harness sunlight for months at a time. The team discovered that the slugs store the stolen chloroplasts inside a previously unknown animal organelle called the kleptosome—a compartment that maintains photosynthetic activity and, during starvation, even serves as an internal food reserve.
An Evolutionary Echo
Photosynthesis in animals is not supposed to happen. Chloroplasts, the green organelles that convert sunlight into energy, are found in plants and algae, not animals. Yet Sacoglossan slugs upend that rule. They pierce algae with a specialized tooth-like structure called a radula, slurp out the contents, and somehow retain only the chloroplasts in their own cells. These hijacked organelles—known as kleptoplasts—continue to perform photosynthesis for up to nine months inside the slugs’ bodies.
“How do they keep something functional that doesn’t belong to them?” Bellono asked. “That’s the mystery we wanted to solve.” The team ultimately determined how the stolen chloroplast is maintained and how it is utilized (as a stored food source) to mediate starvation resistance.
The phenomenon resembles a biological throwback to early eukaryotic evolution, when ancient cells engulfed free-living bacteria, giving rise to the mitochondria and chloroplasts that power life today. “This feels like a modern, more transient version of endosymbiosis—it happens within a single animal’s lifetime,” Bellono explained.
Mucus and Missteps
The project began with a deceptively simple question: How do slugs maintain chloroplasts without access to the algal nucleus, which normally provides essential photosynthetic enzymes?
To find out, the team attempted to isolate the chloroplasts from slugs and label newly synthesized proteins to determine whether any originated from the animal itself. But there was a problem.
“We spent the first six to nine months just trying to get rid of slug mucus,” Bellono said, laughing. “We followed every established protocol for pelleting chloroplasts from plants, and nothing worked. Slugs are basically mucus machines.”
With essential contributions from Amy Si-Ying Lee of the Dana-Farber Cancer Institute, the team succeeded in cleaning the samples and made a surprising discovery: the new proteins were indeed coming from the slugs—but they weren’t the enzymes they expected. Instead, they resembled endosomal proteins, which are typically involved in cellular trafficking and nutrient uptake.
That observation led to a critical realization: the chloroplasts weren’t floating freely in slug cells. Under the microscope, the team saw a thin membrane enveloping each chloroplast—an animal-made compartment now known as the kleptosome.
Meet the Kleptosome
Kleptosomes are a novel type of animal organelle, distinguished by their unique function and structure. They house and stabilize the stolen chloroplasts, using ATP-sensitive ion channels to create a favorable internal environment for photosynthesis. When conditions are favorable, the kleptosomes maintain the chloroplasts’ health and functionality. But under starvation, they shift roles—breaking down the chloroplasts to provide critical nutrients.
“We saw the slugs change color, from green to orange, like autumn leaves,” Bellono said. “That’s when they’re digesting the chloroplasts inside their kleptosomes. It’s an incredible survival strategy.”
The discovery of kleptosomes not only clarifies how these animals maintain photosynthesis but also demonstrates the flexibility of animal cells when subjected to evolutionary pressure. Bellono’s team further found signs of similar organellar strategies in corals and anemones—other animals known for housing photosynthetic symbionts.
“This mechanism—using host-derived organelles to house foreign ones—may be more widespread than we thought,” Bellono said. “It’s a beautiful example of how unrelated lineages converge on similar solutions when biology needs them to.”
Slugs, Stingers, and Serendipity
Like many discoveries, this one began with a wrong turn. Bellono originally became interested in sea slugs after hearing from an aquarist that certain slugs, when fed coral, turned fluorescent—suggesting they were somehow stealing glowing proteins from their prey. Although those specific slugs eluded capture, the search led him to the algae-eating Sacoglossans and their peculiar photosynthetic habit.
“We pivoted to chloroplast-stealing slugs because they were a bit easier to find,” Bellono recalled. “But they opened up this whole other world.”
And they weren’t alone. Bellono mentioned other species of slugs that can ingest sea anemones and repurpose their stinging cells for defense. “There’s a broad theme here—of animals stealing specialized parts from other organisms and integrating them into their own biology. We’re only scratching the surface.”
The lead author on the study, ,Corey Allard, now a faculty member at Harvard Medical School, is building a research program around this very theme: how animals acquire and repurpose organelles and structures from their prey to gain new functions.
The implications of the study reach far beyond sea slugs. It sheds light on the mechanics of endosymbiosis, the evolutionary process by which organisms permanently integrate others into their cells. In doing so, it provides a real-time model of how entirely new organelles can emerge.
“We often think of evolution as something that happened long ago or occurs slowly,” Bellono said. “But here, we’re watching similar processes unfold in modern animals, in a way we can dissect and study.”
And thanks to the slugs’ unusual habits, scientists now have a new window into how foreign parts can become functional pieces of animal physiology—and, perhaps someday, lead to synthetic biological innovations inspired by these natural kleptomaniacs.
