Disposable Slug Penis would be a good band name. Japanese researchers were surprised to discover a sea slug in the Pacific Ocean whose penis drops off after mating. But what surprised them wasn’t that the creature’s penis falls off—which apparently is totally normal in certain parts of the animal kingdom—but that it grows back, allowing the slug to mate again. The rest of the slug’s sex life is just as complicated. First of all, every Chromodoris reticulata has both a penis and a female organ, and both slugs donate sperm to each other at the same time. Then their penises fall off. Also their penises are “equipped with spines.” Scientists aren’t sure why the slugs mate this way, but one researcher speculates that it’s to clear out rivals’ sperm.
Nature Oxford Scientific, via Getty