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Nature. The Symbiotic Sea Slug Elysia chlorotica

A host organelle integrates stolen chloroplasts for animal photosynthesis

Abstract. Eukaryotic life evolved over a billion years ago when ancient cells engulfed and integrated prokaryotes to become modern mitochondria and chloroplasts. Sacoglossan “solar-powered” sea slugs possess the ability to acquire organelles within a single lifetime by selectively retaining consumed chloroplasts that remain photosynthetically active for nearly a year. The mechanism for this “animal photosynthesis” remains unknown. Here, we discovered that foreign chloroplasts are housed within novel, host-derived organelles we term “kleptosomes.” Kleptosomes use ATP-sensitive ion channels to maintain a luminal environment that supports chloroplast photosynthesis and longevity. Upon slug starvation, kleptosomes digest stored chloroplasts for additional nutrients, thereby serving as a food source. We leveraged this discovery to find that organellar retention and digestion of photosynthetic cargo has convergently evolved in other photosynthetic animals, including corals and anemones. Thus, our study reveals mechanisms underlying the long-term acquisition and evolutionary incorporation of intracellular symbionts into organelles that support complex cellular function.

A draft genome assembly of the solar-powered sea slug Elysia chlorotica

Abstract Elysia chlorotica, a sacoglossan sea slug found off the East Coast of the United States, is well-known for its ability to sequester chloroplasts from its algal prey and survive by photosynthesis for up to 12 months in the absence of food supply. Here we present a draft genome assembly of E. chlorotica that was generated using a hybrid assembly strategy with Illumina short reads and PacBio long reads. The genome assembly comprised 9,989 scaffolds, with a total length of 557 Mb and a scaffold N50 of 442 kb. BUSCO assessment indicated that 93.3% of the expected metazoan genes were completely present in the genome assembly. Annotation of the E. chlorotica genome assembly identified 176 Mb (32.6%) of repetitive sequences and a total of 24,980 protein-coding genes. We anticipate that the annotated draft genome assembly of the E. chlorotica sea slug will promote the investigation of sacoglossan genetics, evolution, and particularly, the genetic signatures accounting for the long-term functioning of algal chloroplasts in an animal.

Active Host Response to Algal Symbionts in the Sea Slug Elysia chlorotica

Abstract. Sacoglossan sea slugs offer fascinating systems to study the onset and persistence of algal-plastid symbioses. Elysia chlorotica is particularly noteworthy because it can survive for months, relying solely on energy produced by ingested plastids of the stramenopile alga Vaucheria litorea that are sequestered in cells lining its digestive diverticula. How this animal can maintain the actively photosynthesizing organelles without replenishment of proteins from the lost algal nucleus remains unknown. Here, we used RNA-Seq analysis to test the idea that plastid sequestration leaves a significant signature on host gene expression during E. chlorotica development. Our results support this hypothesis and show that upon exposure to and ingestion of V. litorea plastids, genes involved in microbe-associated molecular patterns and oxidative stress-response mechanisms are significantly up-regulated. Interestingly, our results with E. chlorotica mirror those found with corals that maintain dinoflagellates as intact cells in symbiosomes, suggesting parallels between these animal–algal symbiotic interactions.

The Sea Slug Defying Biological Orthodoxy

Symbiosis may be more important to evolution than scientists once thought.

By Zoë Schlanger

“The sea slug Elysia chlorotica is a small, unassuming creature that performs a remarkable feat: It eats algae and steals its chloroplasts, then incorporates them into its own body,” the horoscope explained. Years ago I had incorporated this fact into my own view of the world, and it had changed my understanding of the rules of biology.
This particular slug starts life a brownish color with a few red dots. Then it begins to eat from the hairlike strands of the green algae Vaucheria litorea: It uses specialized teeth to puncture the alga’s wall, and then it slurps out its cells like one might slurp bubble tea, each bright-green cellular boba moving up the algal straw. The next part remains partially unexplained by science. The slug digests the rest of the cell but keeps the chloroplasts—the plant organelles responsible for photosynthesis—and distributes these green orbs through its branched gut. Somehow, the slug is able to run the chloroplasts itself and, after sucking up enough of them, turns a brilliant green. It appears to get all the food it needs for the rest of its life by way of photosynthesis, transforming light, water, and air into sugar, like a leaf.
Elysia chlorotica is a bewitching example of symbiosis. It is flat, heart-shaped, and pointed at the tail, and angles itself toward the sun. Its broad surface is grooved by a web of veins, like a leaf’s is. Ignore its goatish head, and you might assume this slug was a leaf, if a particularly gelatinous one. Sidney Pierce, a marine biologist retired from the University of South Florida, remembers his surprise when a grad student brought a specimen into his office in the Marine Biological Laboratory at Woods Hole, on Cape Cod, more than two decades ago. Photosynthesis requires specialized equipment and chemistry, which animals simply do not have—“yet here was an animal that’s figured out how to do it,” he told me. He spent the next 20-odd years trying to find the mechanism. “Unfortunately, I didn’t get all the way to the end,” he said.
No one has, as my colleague Katherine J. Wu has written. The algae and the slug may have managed some kind of gene transfer, and over time, produced a new way of living, thanks not to slow, stepwise evolution—the random mutation within a body—but by the wholesale transfer of a piece of code. A biological skill leaked out of one creature into another.
All of us are likely leakier than we might assume. After all, every cell with a nucleus, meaning all animal and plant cells, has a multigenetic heritage. Mitochondria—the organelles in our cells responsible for generating energy—are likely the product of an ancient symbiosis with a distant ancestor and a microbe, and have their own separate DNA. So we are walking around with the genetic material of some other ancient life form suffused in every cell. And the earliest ancestor of all plants was likely the product of a fusion between a microbe and a cyanobacterium; plants’ photosynthesizing organelles, too, have distinct DNA. Lynn Margulis, the biologist who made the modern case for this idea, was doubted for years until new genetic techniques proved her correct.
Her conviction about the symbiotic origins of mitochondria and chloroplasts was a monumental contribution to cell biology. But Margulis took her theory further; in her view, symbiosis was the driving force of evolution, and many entities were likely composites. Evolution, then, could be traced not only through random mutation, but by combination. “Life did not take over the globe by combat, but by networking. Life forms multiplied and complexified by co-opting others, not just by killing one another,” she wrote, with her son, in 1986. This remains pure conjecture, and an exaggeration of the role of symbiosis beyond what mainstream evolutionary theory would support; random mutation is still considered the main driver of speciation.
Yet more scientists now wonder if symbiosis may have played a larger role in the heritage of many species than we presently understand. Phillip Cleves, a geneticist at the Carnegie Institution for Science who studies the symbiotic relationship between corals and their algae symbionts, told me how, as an undergraduate, he was blown away by the fact that corals’ alliance with algae made possible ecosystems—coral reefs—that support a quarter of all known marine life. The algae cells live, whole, inside coral cells, and photosynthesize as normal, sustaining the coral in nutrient-poor tropical waters. “I realize now that that type of interaction between organisms is pervasive across the tree of life,” he said.
It’s probable that the ancestors of all eukaryotes were more influenced by bacteria in their environments than modern evolutionary theory has accounted for. “All animals and plants likely require interactions with microbes, often in strong, persistent symbiotic associations,” Margaret McFall-Ngai, a leading researcher of the role of microbes in animal development, wrote in 2024. These interactions, she argued, are so fundamental to life that the animal immune system should perhaps be thought of as a sort of management system for our many microbial symbionts. Although biology has been slow to recognize symbiosis’s significance, she thinks this line of research should now take center stage, and could alter how all stripes of biologists think about their work.
Cleves, too, sees himself as working to build a new field of science, by training people on how to ask genetic questions about symbiotic relationships in nature: When I called him, he was preparing to teach a four-week course at the Marine Biological Laboratory in Woods Hole on exactly that. Genomic research has only relatively recently been cheap enough to apply it routinely and broadly to all sorts of creatures, but now scientists can more easily ask: How do animals’ interactions with microbes shape the evolution of individual species? And how does that change dynamics in an ecosystem more broadly?
Elysia chlorotica is also a lesson in how easily the boundaries between an organism and its environment can be traversed. “Every time an organism eats, a whole wad of DNA from whatever it’s eating passes through the animal. So DNA gets transferred all the time from species to species,” Pierce told me. Most times it doesn’t stick, but on the rare occasions when it does, it can reroute the fate of a species. “I think it happens more than it’s recognized, but a lot of times it’s hard to recognize because you don’t know what you’re looking for. But in these slugs, it’s pretty obvious,” he said. They’re bright green.

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