When pictures emerged of a tiny sea monster that washed up on the shores of Queensland, Australia, the otherworldly creature looked more like a Pokémon than a slug. But that’s exactly what it was: Glaucus atlanticus, a sea slug more commonly known as the blue dragon.
Strange stripes on the sea slug’s skin are not just ornamentation: They serve as camouflage, concealing the blue dragon from both above and below. G. atlanticus floats on the ocean’s surface belly-up, displaying its vivid blue underside, which blends in with the surface of the water. The blue dragon’s back is striped with silver, which helps it vanish from undersea predators against the shimmering surface of the water.
The blue dragon’s bizarre appearance is just the beginning. G. atlanticus is small—the largest specimens can reach about an inch and a half in length—but it packs quite a punch. The scrappy sea slug goes after animals like the Portuguese Man o’ War, which is not only much larger than the slug but also highly venomous. Instead of being deterred by the Man o’ War’s nematocysts (stinging cells), the blue dragon eats them.
Some of the nematocysts will be digested, but G. atlanticus saves the most venomous cells for later, concentrating the toxin and storing it in its strange, finger-like appendages.
Taro Taylor, Wikimedia Commons // SD-BY-CC 2.0
Because it’s been concentrated, the venom in the sea slug’s fingers is extra-potent [PDF]. We may be many times larger than the blue dragon, but it can still deal us some damage.
Which is why, if you are ever lucky enough to find a blue dragon, you should never, ever touch it; reported effects include burning, hives, and dark, damaged patches of skin. It's not known what happened to the person holding G. atlanticus in the photograph below, but it likely wasn't pleasant.