World's Weirdest Slug Is Shaped Like a Fish and Glows in the Dark

This nudibranch has evolved to look and swim like a fish.

An illustration of a sea slug shaped like a fish is a bizarre example of convergent evolution. Wikimedia [CC by 2.0]

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.

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. 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. Once free-living, Phylliroe continues to hunt jellies, as well as plankton. In other words, this nudibranch might be a jelly's worst nightmare.

Phylliroe, like all nudibranches, are hermaphroditic, which means either one can be male or female when it comes to reproduction. Typically, the individuals approach each other with penises out, and the victor penetrates the other's body wall, making it the dominant male.

As if it wasn't already weird enough, Phylliroe also poops from its side.