Mussels are shellfish that spend almost all their lives attached to rocks and filtering the passing water for food particles. But before that, in their larval stage, they are parasites, and grow up inside a fish that their mother has caught for them. Yeah, nature is metal. Some species of mussel mothers snatch up a fish for this purpose, while others arrange their eggs and/or larva into a sort of pod that's shaped like something a fish would want to eat, which achieves the same purpose in the end. There's no one better to explain this weirdness than Ze Frank in his True Facts series. I mean, admit it, you wouldn't be exploring the reproductive habits of a mussel if it were from any other source. There's a one-minute embedded ad at 4:30.