Elysia chlorotica, a bright green marine mollusk native to the East Coast of North America, blurs the evolutionary line between animal and plant. Early in life, the slug feeds on the intertidal alga Vaucheria litorea. Instead of fully digesting its meal, it selectively extracts intact chloroplasts and stores them in specialized cells lining its digestive tract—a process known as kleptoplasty.
These stolen organelles continue to photosynthesize, providing the slug with synthesized carbohydrates and allowing it to survive for up to nine months without eating another bite, relying solely on sunlight. Maintaining functional chloroplasts outside an algal cell presents a massive biological challenge because chloroplasts require thousands of proteins encoded by the host organism's nuclear DNA to repair photosynthetic machinery damaged by light exposure. Scientists discovered that Elysia chlorotica solved this through horizontal gene transfer.
Over evolutionary time, the sea slug captured key algal genes, incorporating them directly into its own nuclear genome. When light damages the stolen chloroplasts, the slug reads these acquired genes to synthesize the necessary repair proteins itself. This rare biological phenomenon demonstrates that genetic material can cross complex kingdom boundaries, creating a functional animal that operates as a living solar panel.