Click on the image to read the article.



Long story short: this slug, called Elysia chlorotica, only needs to eat algae during its youth in order to acquire their chloroplasts and incorporate them to its own cells. Afterwards, it can live on sunlight (through photosynthesis) just like plants do.

I learned about this on another forum and felt it was weird/interesting enough to share it. There are more online sources (just search for "elysia chlorotica" on your favorite search engine).