An Animal Behaving Like a Plant
In the shallow salt marshes and tidal pools of the Florida Keys, a creature exists that is hard to define as an animal. Elysia chlorotica, the eastern emerald elysia, looks like a bright green leaf, typically growing to between 20 and 30 millimeters in length. This appearance is for camouflage; it is a direct result of a remarkable biological theft. This sea slug feeds on a filamentous alga called Vaucheria litorea. Using a specialized, scraping tongue called a radula, it punctures the algal cells and sucks out the contents.
Instead of digesting everything, the slug carefully preserves the chloroplasts—the tiny green organelles where photosynthesis occurs. It incorporates these stolen plastids, known as kleptoplasts, into the cells lining its own extensively branched digestive tract. The process, called kleptoplasty, effectively turns the slug into a solar-powered animal. Its flat, leaf-like body provides a large surface area, maximizing its ability to absorb sunlight. The slug's entire life cycle, which lasts about 11 months, is synchronized with its food source. The larval slugs must consume Vaucheria litorea to metamorphose into their adult form.
The Mystery of Sustained Photosynthesis
After an initial period of feeding, Elysia chlorotica can survive for the rest of its life—up to nine or ten months—without eating again. It relies on the stolen chloroplasts, which continue to photosynthesize and produce sugars, providing energy directly from sunlight. This long-term functionality is a scientific puzzle. In their native algal cells, chloroplasts depend on proteins produced by the alga's nucleus to repair damage from sunlight and maintain their function. Without these proteins, they should quickly degrade.
For years, the leading hypothesis was horizontal gene transfer: the idea that the slug had incorporated algal genes into its own DNA. Some studies reported finding algal genes, such as psbO, in the slug's genetic material, suggesting the slug could produce the necessary repair proteins itself. However, this has become a point of scientific debate. More recent and comprehensive genomic analyses, specifically of the slug's egg DNA, have found no evidence of algal genes in the slug's germ line. This suggests that genes are not passed down to the next generation and that another mechanism must be at play. The chloroplasts from Vaucheria litorea may simply be exceptionally robust, or the slug may have a yet-unknown method for preserving them. The mystery of how this solar-powered animal works is still under investigation.