Several groups of animals genuinely harness sunlight to produce energy, though none of them do it quite the way a plant does. Some steal the photosynthetic machinery outright from algae, some host living algae inside their own tissues, and at least one insect appears to use pigments borrowed from fungi to run a stripped-down, light-driven energy system. The strategies are wildly different from one another, and the biology behind each is stranger than a simple label like “solar-powered animal” suggests.
Sea Slugs That Steal Chloroplasts
The most famous photosynthetic animals are a group of sea slugs in the order Sacoglossa. These small, often brilliantly green molluscs feed on algae and do something no other animals are known to do on their own: they strip the chloroplasts out of the algal cells they eat and keep those chloroplasts alive inside their own bodies. The stolen chloroplasts, called kleptoplasts, are tucked into cells lining the slugs’ branching digestive system, where they continue photosynthesizing for weeks or months.
The best-studied species is Elysia chlorotica, a bright green slug found along the Atlantic coast of North America. It feeds on the alga Vaucheria litorea, and after a period of eating, it can survive on nothing but light and air, essentially living like a plant.
1PubMed Central. The making of a photosynthetic animal Another well-studied species, Plakobranchus ocellatus, collects chloroplasts from multiple species of green algae in tropical reefs. In starvation experiments, individuals kept in total darkness died within two months, while those given light survived up to five months, confirming that the stolen chloroplasts were providing real nutritional support.2PubMed Central. Seasonality and Longevity of the Functional Chloroplasts Retained by the Sacoglossan Sea Slug Plakobranchus ocellatus van Hasselt, 1824 Inhabiting A Subtropical Back Reef Off Okinawa-jima Island, Japan
This phenomenon, called kleptoplasty, is unique among photosymbiotic relationships because it involves an animal and a stolen organelle rather than two whole organisms living together.3Current Biology. Quick guide Kleptoplasty The slugs are not hosting algal cells; they are running algal hardware inside their own cells. Some species manage this for only days, while others maintain functional kleptoplasts for months. Elysia viridis, a European species, even adjusts its behavior based on the state of its stolen chloroplasts: slugs whose kleptoplasts are acclimated to bright light seek out brighter spots, and they fold their wing-like body flaps (parapodia) to shield the chloroplasts from too much sun.4PubMed Central. Kleptoplast photoacclimation state modulates the photobehaviour of the solar-powered sea slug Elysia viridis
Why Stolen Chloroplasts Should Not Work
Here is what makes kleptoplasty so puzzling. In a plant cell, the chloroplast’s own genome encodes only a fraction of the proteins needed for photosynthesis. Most of those proteins are encoded in the plant’s nuclear DNA, manufactured in the cell’s cytoplasm, and shipped into the chloroplast. When a sea slug steals a chloroplast and puts it inside its own animal cell, the nuclear support system is gone. The chloroplast should break down quickly. Yet in species like E. chlorotica, it keeps working for months.
For years, the leading explanation was horizontal gene transfer: perhaps the slugs had incorporated key algal genes into their own genomes over evolutionary time, and those genes supplied the missing proteins. Early work reported finding an algal photosynthesis gene, psbO, integrated into the slug’s DNA and expressed in its tissues.5PubMed Central. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica Transcriptome studies went further, identifying dozens of algal gene sequences in slug tissues, many involved in photosynthesis-related processes.6Molecular Biology and Evolution. Transcriptomic Evidence for the Expression of Horizontally Transferred Algal Nuclear Genes in the Photosynthetic Sea Slug, Elysia chlorotica
But subsequent genome-level analyses told a different story. When researchers sequenced the E. chlorotica genome at the DNA level rather than just looking at RNA, they found no convincing evidence that algal genes had been stably integrated into the slug’s germline.7PubMed Central. Genome Analysis of Elysia chlorotica Egg DNA Provides No Evidence for Horizontal Gene Transfer into the Germ Line of This Kleptoplastic Mollusc The same result appeared in P. ocellatus: its genome showed no algae-derived photosynthetic genes whatsoever, even though its chloroplasts functioned normally for months.8PubMed Central. Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus The earlier transcript findings may have reflected contamination from algal material rather than genuine gene transfer. This debate is not fully resolved, but the current consensus leans toward the slugs managing without wholesale gene theft.
A 2025 study may have found part of the answer. Researchers discovered that sea slugs create a specialized organelle, which they named the “kleptosome,” to house stolen chloroplasts. These kleptosomes appear to be modified versions of the digestive compartments cells normally use to break down food, arrested at a stage just before the chloroplast would be destroyed. In other words, the slug’s cells have evolved to pause their own digestion at exactly the right moment, keeping the chloroplast intact and functional instead of dissolving it.9Cell. A host organelle integrates stolen chloroplasts for animal photosynthesis
Corals and Giant Clams Farm Their Algae
Reef-building corals are arguably the most ecologically important photosynthetic animals on the planet, even though they outsource the actual photosynthesis to tiny algae living inside their tissues. These symbiotic algae (dinoflagellates in the family Symbiodiniaceae) live inside coral cells and produce sugar-rich photosynthates, which can fully cover the coral’s energy needs.10PubMed Central. Reef-building corals farm and feed on their photosynthetic symbionts Recent work has reframed this relationship: corals do not just passively receive leaked sugars. They actively “farm” their symbionts by limiting the nitrogen and phosphorus supply, forcing the algae to overproduce carbon-rich compounds. The coral then harvests both the excess photosynthates and, when nutrients allow, digests surplus algal cells outright. It is less a partnership and more a managed garden.
Giant clams take a different architectural approach to the same general strategy. Species like Tridacna harbor dense populations of photosynthetic algae deep in their fleshy mantle tissue. To feed those algae enough light, the clams have evolved iridescent cells called iridocytes. These cells use tiny internal structures that act like mirrors and lenses: they back-reflect wavelengths of light that are not useful for photosynthesis while scattering useful wavelengths forward and laterally into the tissue, distributing light evenly to algae arranged in vertical columns.11PubMed Central. Photosymbiotic giant clams are transformers of solar flux The result is something like a biological fiber-optic system, allowing algae that are buried millimeters deep in clam tissue to receive the light they need.12PubMed Central. Wavelength-specific forward scattering of light by Bragg-reflective iridocytes in giant clams
The difference between corals and sea slugs is worth noting. Corals and clams keep entire living algal cells and regulate them. Sea slugs rip out the chloroplasts and discard the rest. Both are “photosynthetic animals” in common usage, but the mechanisms have almost nothing in common.
Flatworms That Die Without Sunlight
Symsagittifera roscoffensis is a tiny acoel flatworm found on Atlantic tidal beaches that has taken photosymbiosis to an extreme. It harbors the green alga Tetraselmis convolutae inside its body, and this relationship is not optional: without the algal partner, the flatworm invariably dies.13PubMed Central. The chimerical and multifaceted marine acoel Symsagittifera roscoffensis: from photosymbiosis to brain regeneration The worms congregate in dense mats on sandy beaches at low tide, angling their bodies toward the sun. When the tide returns, they burrow down to avoid being washed away, then resurface when the water recedes.
These worms have a reduced or absent digestive system in adult form, meaning they are so dependent on their algae that they have essentially given up eating on their own. The algal cells are not inherited, though. Each generation of worms must acquire fresh algae from the environment early in life, making this a remarkably tight but still externally sourced dependency.14PubMed. Behaviour of the plathelminth Symsagittifera roscoffensis under different light conditions and the consequences for the symbiotic algae Tetraselmis convolutae
The Only Vertebrate With Algae Inside Its Cells
Vertebrates were long assumed to be incompatible with photosymbiosis. Their adaptive immune systems are very good at recognizing and destroying foreign cells. So it came as a surprise when researchers in 2011 discovered that the spotted salamander, Ambystoma maculatum, harbors green algae not just around its eggs but inside its own embryonic cells.15PubMed Central. Intracellular invasion of green algae in a salamander host
The relationship between spotted salamander embryos and the alga Oophila amblystomatis had been known for over a century, but it was assumed to be strictly external: the algae live in the jelly surrounding the eggs, producing oxygen that benefits the embryos and consuming the embryos’ waste carbon dioxide. The discovery of algae physically inside salamander cells upended that picture. Imaging and DNA analysis confirmed intracellular algal invasion of embryonic tissues.16PubMed Central. Heterotrophic Carbon Fixation in a Salamander-Alga Symbiosis
Transcriptome analysis of salamander cells hosting these intracellular algae revealed something intriguing about how the salamander tolerates them. Several genes associated with dampening the immune response, particularly those that dial down a key inflammatory signaling pathway, were upregulated in cells containing algae.17PubMed Central. Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis The salamander appears to partially suppress its own immune attack in the cells where algae are present. Whether this is something the alga actively manipulates or the salamander has evolved to tolerate is still unknown. Regardless, it remains the only documented case of photosynthetic algae living inside vertebrate cells.
Aphids That Make Their Own Pigments
Pea aphids (Acyrthosiphon pisum) are the only animals known to synthesize carotenoid pigments on their own. Most animals get carotenoids from their diet, but aphids acquired the genes to make them through horizontal gene transfer from fungi millions of years ago. These pigments are responsible for the green, orange, and reddish color variations seen in different aphid populations.
A provocative 2012 study reported that these carotenoids do more than provide color. In orange-colored aphids, which produce the most carotenoids, ATP levels responded to light exposure. The proposed mechanism is a rudimentary light-harvesting system: carotenoids in the aphid’s body absorb photons and release electrons, which are funneled into the mitochondria to drive ATP production.18Scientific Reports. Light- induced electron transfer and ATP synthesis in a carotene synthesizing insect This is not photosynthesis in the plant sense because no carbon is being fixed, but it is genuine light-driven energy production.
More recent work has added nuance. Under cold stress, the light-to-ATP effect becomes much stronger: at 12°C, ATP content under bright light was about 2.4 times higher than under dim light. This temperature-gated response suggests the system specifically kicks in as a survival mechanism during cold conditions, not as a general energy source.19PubMed Central. Horizontally Transferred Carotenoid Genes Associated with Light-Driven ATP Synthesis to Promote Cold Adaptation in Pea Aphid, Acyrthosiphon pisum The findings remain somewhat controversial because the effect sizes are modest and the exact biophysical pathway has not been fully mapped. But the basic observation, that aphids with more carotenoids produce more ATP when exposed to light, has been replicated.
Sponges and Cyanobacteria
Many marine sponges harbor dense communities of cyanobacteria or microalgae in their tissues. These relationships can be significant: in some tropical species, the photosynthetic symbionts generate enough organic carbon to supplement what the sponge gets from filter-feeding. The relationship is clearly mutualistic, with the sponge providing a stable habitat and the microbe providing sugars.
In experiments with the blue aquarium sponge Lendenfeldia chondrodes, growth tracked closely with light availability. Sponges given full ambient light grew roughly 79% over the experimental period, while those given four times as much light grew 106%. Sponges kept in reduced light grew substantially less.20PubMed Central. Light Availability Affects the Symbiosis of Sponge Specific Cyanobacteria and the Common Blue Aquarium Sponge (Lendenfeldia chondrodes) This direct link between light and sponge growth demonstrates that the photosynthetic contribution is not trivial: the sponge literally grows faster when its internal microbes get more sun.
How Much Energy Do Animals Actually Get From Light?
Radiotracer experiments measuring carbon flow in photosymbiotic animals show that algal symbionts can release up to half of the carbon they fix through photosynthesis to their animal host.21Oxford Academic (Journal of Experimental Botany). Photosynthetic symbioses in animals The actual numbers are probably higher, because some of the labeled carbon gets lost to respiration and then refixed by the algae before it can be measured. The release is selective, too. Algae do not simply leak everything they make; they export specific compounds while retaining others for their own growth.
For corals in well-lit shallow water, this photosynthetic subsidy can cover the animal’s entire carbon budget. For deep-water corals or sponges in dim environments, the contribution drops and the animal relies more on filter-feeding or other food sources. The benefit is always context-dependent: light intensity, water clarity, temperature, and the density of symbionts all influence how much energy an animal extracts from its photosynthetic partners.
Why This Keeps Evolving Independently
Photosynthetic partnerships between animals and algae have arisen independently many times across the animal kingdom, in sponges, corals, flatworms, molluscs, and at least one vertebrate.22The Royal Society. Animal–chlorophyte photosymbioses: evolutionary origins and ecological diversity The fact that such different lineages have converged on similar strategies suggests that the selective advantages are strong wherever sunlight is abundant and nutrients are scarce. A reef is a perfect example: plenty of light, very little dissolved nitrogen or phosphorus. An animal that can tap into photosynthesis gains a huge competitive edge.
Green algae from the core chlorophyte clade show up as symbionts in particularly distantly related animal hosts, meaning this is not a case of one ancestral partnership branching out. These are separate events, separate algal lineages partnering with separate animal lineages, sometimes producing strikingly similar outcomes. A flatworm lying on a beach in Brittany and a giant clam on a Pacific reef have arrived at the same basic solution through entirely different evolutionary paths.
Putting Chloroplasts in Mammalian Cells
If sea slugs can run stolen chloroplasts inside animal cells, could the same trick work in mammals? A Japanese research team tested this by isolating chloroplasts from a primitive red alga and introducing them into cultured mammalian cells. The chloroplasts were taken up by the cells, settled into the cytoplasm near the nucleus surrounded by mitochondria, and maintained their internal membrane structure. They retained active photosynthetic electron transport for at least two days after incorporation.23Proceedings of the Japan Academy, Series B. Incorporation of photosynthetically active algal chloroplasts in cultured mammalian cells towards photosynthesis in animals
Two days is a long way from the months-long retention seen in sea slugs, and these were cells in a dish, not cells in a living animal. But the result shows that mammalian cells do not immediately destroy foreign chloroplasts, which was not obvious beforehand. The immune system of a whole organism would present a far greater challenge, as the spotted salamander story illustrates: even a salamander has to partially suppress its immune response to coexist with intracellular algae. Still, the work opens a line of inquiry into whether synthetic biology could eventually engineer light-harvesting capability into animal tissues, for biomedical applications if not for nutrition. The researchers framed it as a proof of concept for “artificially photosynthetic animal cells,” a goal that is still far off but no longer purely hypothetical.