Glaucus atlanticus is a species of sea slug that goes by many names, including blue angel, blue glaucus, blue dragon, blue sea slug, blue ocean slug and, in a strange twist of a name that doesn't reflect its sapphire coloration, the sea swallow.
1. That namesake coloration is for more than just show. The sea slug uses the coloration to its advantage as it floats on the ocean's surface. The blue side of their body faces upward to camouflage it against the blue of the sea, while the silver side faces downward to camouflage it against the bright surface of the water.
Predators looking down at it or up at it will have a harder time seeing the little slug thanks to those beautiful dashes of color.
2. This species of sea slug is not defenseless as it floats. If the pretty colors don't hide it well enough from potential predators, the sea slug's sting is its next line of defense.
The slug isn't venomous all on its own. Rather, it stores the stinging nematocysts created by the creatures on which it feeds, including venomous siphonophores and Portuguese man o' wars. These cells are stored and concentrated, so when it's touched, the blue dragon can release these stinging cells to pack an even more powerful punch than the man o' war hydrozoan.
3. All blue dragons are hermaphrodites, meaning they have both male and female reproductive organs. When two blue dragons mate, they both produce egg strings. According to The Cephalopod Page, "Because of the lack of a hard substrate to attach the eggs to, the eggs often float freely in the water or are placed on the remains of prey until the larvae hatch."
So not only do they steal poison from their prey but they also lay their eggs on them. (Talk about adding insult to injury.)
These beautiful but dangerous sea slugs can be found throughout oceans around the world, from temperate to tropical waters. So if you come across one, it's best to admire it's beautiful blue colors from a distance.