Animals Wildlife World's Weirdest Slug Is Shaped Like a Fish and Glows in the Dark By Bryan Nelson Bryan Nelson Twitter Writer SUNY Oswego University of Houston Bryan Nelson is a science writer and award-winning documentary filmmaker with over a decade of experience covering technology, astronomy, medicine, and more. Learn about our editorial process Updated February 24, 2021 An illustration of a sea slug shaped like a fish is a bizarre example of convergent evolution. Wikimedia [CC by 2.0] Share Twitter Pinterest Email Animals Wildlife Pets Animal Rights Endangered Species If you saw this bizarre creature swimming around in the ocean, you'd be forgiven for mistaking it for a fish. Although it's about the size of a goldfish and has a body shaped like a fish — including what looks like a caudal fin and even rudimentary dorsal, pelvic and anal fins — this is no fish. It's actually a sea slug, or more precisely, a nudibranch of the genus Phylliroe, reports Deep Sea News. Phylliroe's fish-like appearance is a stellar example of convergent evolution. Essentially, this basic body shape is particularly good for swimming in open ocean. So, at some point, its ancestors abandoned their sluggardly walk along the sea floor and took to swimming instead, and thus evolved a body design better suited for their new lifestyle. AMAZING FINDS: 8 bizarre newly discovered deep-sea creatures Its shape and movement aren't the only things about Phylliroe that are fish-like. It also hunts kind of like a fish, known to swim and stalk its favorite prey — jellies — from below. It's a fierce predator, and surprisingly quick in the water. Phylliroe also sports some pretty mean-looking horns, called rhinophores, which are actually sense organs that it uses to sniff out its quarry. As if Phylliroe wasn't eccentric enough, it's also transparent. You can actually see its internal organs churning away inside of it. Oh, and it glows. That's right, Phylliroe is a rare example of a bioluminescent sea slug, capable of producing its own light. It's like a little swimming sea lantern, undoubtedly a spectacular sight for anyone who has witnessed it in nature. Phylliroe's juveniles are known to be parasitic, attaching themselves to the bell of the hydromedusa Zanclea costata, slowly consuming the jelly until becoming large enough to swim on their own. In other words, this nudibranch might be a jelly's worst nightmare. As if it wasn't already weird enough, Phylliroe also poops from its side. So there's also that.