Under a microscope, Elysia chlorotica looks like a leaf that learned to crawl.
The green color comes from chloroplasts. The sea slug steals that solar machinery from Vaucheria algae.
After a meal, the slug tucks chloroplasts into its gut lining. Those borrowed parts keep making sugar for weeks.
But the slug has not become a plant. Kleptoplasty is theft, not transformation; inside the slug, the borrowed chloroplasts still wear out.