Photosynthesis is the process by which organisms convert light energy into chemical energy, fundamentally associated with plants, algae, and certain bacteria. This process uses sunlight, water, and carbon dioxide to create sugars and release oxygen, forming the foundation of most food webs. While the animal kingdom overwhelmingly relies on consuming other organisms for energy, a small number of specialized creatures incorporate photosynthetic capabilities into their lives. These rare exceptions do not possess the inherent biological machinery for photosynthesis but instead borrow or host the necessary components, challenging the conventional division between animal and plant life.
Kleptoplasty: Stealing and Using Chloroplasts
The most direct and arguably most complex method for an animal to exploit photosynthesis is through a process known as kleptoplasty, which literally means “stealing plastids.” This mechanism is famously demonstrated by certain sacoglossan sea slugs, such as the eastern emerald sea slug, Elysia chlorotica. These slugs feed on specific species of algae, such as Vaucheria litorea, but instead of digesting the entire cell, they selectively retain the functional chloroplasts within specialized cells lining their digestive tract.
The stolen chloroplasts, called kleptoplasts, are then integrated into the slug’s cells, where they continue to function and produce sugars for the animal for weeks or even months. The slug essentially becomes solar-powered, able to survive on light and carbon dioxide long after its last meal of algae. The impressive longevity of these stolen organelles presents a significant scientific puzzle because chloroplasts require thousands of proteins to be repaired and maintained, and most of the genes for these proteins reside in the original algal nucleus, which the slug has digested.
One hypothesis suggests that the slug has acquired the necessary repair genes from the algae through a process called horizontal gene transfer (HGT). Research has indicated the presence of at least one algal nuclear gene in the sea slug’s chromosome, a gene that codes for a protein needed to maintain the chloroplasts. This genetic integration would allow the sea slug to create the proteins necessary to keep the borrowed machinery working, making the phenomenon a unique example of a non-plant organism partially controlling a photosynthetic organelle.
Mutualistic Symbiosis with Photosynthetic Organisms
Another common method animals use to harness solar energy is by forming a mutualistic symbiotic relationship, which involves hosting an entire, living photosynthetic organism. In this arrangement, the animal provides shelter and necessary compounds, while the hosted organism, or symbiont, performs photosynthesis and shares the resulting nutrients. This differs from kleptoplasty because the animal is hosting a whole organism, not just a stolen organelle.
The most widespread examples are found in marine invertebrates, particularly reef-building corals, sea anemones, and giant clams. Corals host microscopic algae called zooxanthellae within their tissues. The coral provides the algae with a protected environment and carbon dioxide, while the algae produce sugars and oxygen, supplying the coral with a substantial portion of its energy needs.
A remarkable example in vertebrates is the spotted salamander, Ambystoma maculatum, which forms a unique association with a species of algae. Algal cells colonize the salamander’s egg casings shortly after they are laid, and the algae then penetrate the cells of the developing embryo itself. The algae utilize the nitrogen-rich waste products from the embryo. In return, the algae’s photosynthetic products, such as oxygen and sugars, are thought to accelerate the salamander embryo’s growth and development. The symbiotic algae release simple molecules like maltose and glycerol, which the host can readily use as an energy source.
Evolutionary Barriers to Animal Photosynthesis
Despite the existence of these specialized exceptions, true, inherent photosynthesis remains absent across the vast majority of the animal kingdom. The primary reason lies in the immense genetic and metabolic complexity of the process. A functional chloroplast requires the coordinated expression of hundreds of genes, most of which are located in the nucleus of the plant or algal cell, not within the chloroplast itself.
For an animal to evolve its own innate photosynthetic capability, it would need to acquire and integrate this large, complex suite of foreign genes into its own nuclear genome and develop the sophisticated cellular pathways to manage and maintain the chloroplasts. This is a massive evolutionary hurdle that has not been overcome.
Furthermore, photosynthesis is not an energy-dense process, and the energy it provides is often insufficient for the high metabolic demands of a mobile animal. For a large, active animal, the energy gained from a photosynthetic surface area would be negligible compared to the energy obtained from eating, making the maintenance cost of the photosynthetic machinery not worthwhile.
Animals that engage in forms of photosymbiosis are typically sessile, slow-moving, or have a simple body plan with a large surface area-to-volume ratio, making light capture more efficient. The inability to easily pass these acquired traits to offspring also acts as a barrier, as most of these symbiotic traits must be re-acquired by each generation from the environment.