This sea slug steals chloroplasts from algae and photosynthesizes on its own

Elysia chlorotica is a small sea slug that, when it feeds on algae, doesn't fully digest them — instead it pulls the chloroplasts, the structures responsible for photosynthesis, out of the algae's cells and incorporates them into its own gut cells.

The stolen chloroplasts stay active for months, producing energy-rich compounds from sunlight, oxygen, and carbon dioxide — exactly the way a plant does, even though the slug is very much an animal.

This lets the slug survive long stretches without eating, partly living off solar energy captured through organelles it stole from something else — a phenomenon rare enough to have earned the name "kleptoplasty".

It sounds like science fiction, but it's a fully documented biological phenomenon — one of the very few examples of an animal that can, in a limited way, "live off light".