This sea slug photosynthesises using chloroplasts stolen from its food

Elysia chlorotica eats a specific alga and, instead of digesting the chloroplasts inside it, extracts them intact and embeds them in its own gut lining, where they keep converting sunlight into energy for months. A slug that has fed just once can then survive largely on photosynthesis alone, going green and basking like a plant instead of hunting for more food.

— Mary E. Rumpho et al., Kleptoplasty in the photosynthetic sea slug Elysia chlorotica — Proceedings of the National Academy of Sciences, 2008