Behold, Hollywood, the sea slug Elysia chlorotica. This emerald green slug grazes on algae. As it breaks down the algae, it preserves their photosynthetic structures, called plastids. The plastids are shipped to the surface of its body, where they can continue to photosynthesize. The slug thus makes a living as plants do. And it turns a beautiful shade of green along the way.

Recently, some scientists discovered that the sea slug is even more plantlike than previously thought thought. They wondered if some genes from the algae the slug ate had become incorporated into their own DNA. This movement of genes is called horizontal gene transfer. It’s common among bacteria, which swap genes for antibiotic resistance and such. It’s not as