Photosynthesis is the fundamental biological process that converts light energy into chemical energy, creating the sugars that fuel life on Earth. This solar power system is conventionally understood to be the exclusive domain of plants, algae, and some bacteria. However, the strict line separating the animal kingdom from this plant-like ability is blurred in a few extraordinary cases. Certain animals have evolved specialized, temporary, or co-dependent methods to harness solar energy. These exceptions do not represent true, inherited animal photosynthesis but rather ingenious biological workarounds utilizing the machinery of other life forms.
Partnerships: Hosting Photosynthetic Organisms
The most widespread method by which animals gain a photosynthetic advantage is through mutualistic symbiosis, involving the animal housing an entire photosynthetic organism (phototroph) within its tissues. The phototroph, typically a single-celled alga, receives a protected environment and a steady supply of metabolic waste products from the host.
In return, the animal receives a significant portion of the sugars, glycerol, and amino acids produced by the symbiont’s photosynthesis. Reef-building corals are the most famous examples, hosting microscopic dinoflagellate algae known as zooxanthellae within their endodermal cells. These algae can supply up to 90% of the coral’s energy needs, which is essential for constructing their massive calcium carbonate skeletons in nutrient-poor tropical waters.
Giant clams (Tridacnidae) employ a similar strategy, cultivating zooxanthellae in a specialized, highly branched tubular system throughout their fleshy mantle tissue. The clam opens its shell during the day, exposing the algae to sunlight to maximize energy production. This arrangement allows the clams to grow to immense sizes, relying on the photosynthetic output of their internal garden for sustenance.
A unique example exists in the spotted salamander (Ambystoma maculatum), which forms a mutualism with the green alga Oophila amblystomatis. The algae colonize the salamander’s egg casings, and remarkably, the algal cells penetrate and live inside the salamander’s embryonic cells. The developing embryo benefits from supplemental oxygen and carbohydrates, while the algae use the nitrogenous waste and carbon dioxide generated by the embryo’s metabolism.
Kleptoplasty: The Art of Stealing Chloroplasts
A more direct method of solar energy capture is kleptoplasty, meaning “stolen plastic.” This phenomenon is best demonstrated by sacoglossan sea slugs, such as the eastern emerald sea slug (Elysia chlorotica). The slug feeds exclusively on the alga Vaucheria litorea, using a specialized rasping mouthpart to puncture the algal cell wall and suck out the contents.
During feeding, the slug digests everything except the chloroplasts, which are sequestered into the slug’s digestive cells via phagocytosis. Once integrated, these stolen organelles, called kleptoplasts, continue to function inside the animal’s cells for up to ten months. This allows the slug to survive long periods of starvation simply by basking in the sun.
The ability of these kleptoplasts to remain active without the alga’s nucleus, which contains genes for most maintenance proteins, is a significant biological puzzle. Early research suggested the slug had integrated some algal genes into its own nucleus via horizontal gene transfer to produce the necessary proteins. However, more recent genomic studies have largely failed to find widespread evidence for this functional transfer in the slug’s germline.
The current understanding suggests the slug’s own genes, particularly those related to immunity and preventing oxidative stress, are activated to protect the stolen organelles from degradation. The kleptoplasts’ ability to function for months without the full genetic support system is attributed to their inherent stability. This phenomenon is a temporary solar subsidy, allowing the slug to function as a plant for a portion of its life cycle before needing to feed again.
Why True Animal Photosynthesis Remains Rare
The reliance of these solar-powered animals on external organisms, either whole or as organelles, highlights a fundamental constraint on the evolution of true, permanent animal photosynthesis. A fully functional photosynthetic system requires a massive genetic and structural investment that far exceeds the capacity of the chloroplast’s small genome. A complete chloroplast needs approximately 2,500 to 3,500 proteins for its construction, maintenance, and repair.
Over 95% of these essential proteins are encoded by genes located in the cell’s main nucleus, not the chloroplast itself. For a complex animal to evolve true photosynthesis, it would need to incorporate and permanently manage thousands of foreign genes. This level of genetic integration has only occurred once in the evolutionary history of plants and algae, and this immense genetic hurdle is compounded by the issue of energy efficiency.
Photosynthesis provides a low-density energy source, which is well-suited for sessile organisms like plants that have low metabolic demands. Active, mobile animals have a high metabolic rate and a volume-to-surface area ratio that is energetically unfavorable for light capture. The energy a large animal could produce from its exposed surface area would be negligible compared to consuming organic matter, making heterotrophy a far more efficient survival strategy. The temporary nature of kleptoplasty confirms this limitation, as the stolen chloroplasts inevitably degrade because the animal cannot permanently replicate or repair the missing algal components.