By any name, they are a very pleasing addition to our planet. Glaucus atlanticus are a species of small, blue sea slug, a pelagic aeolid nudibranch — a shell-less gastropod mollusc in the family Glaucidae.
Nudibranchs likely date back as far as the Early Jurassic, some180 million years ago. This was around the time that the supercontinent of Pangea was breaking apart to form the modern continents and the Atlantic and Indian Oceans. The date is an estimate built upon the evolutionary lineages of their closest relatives, in part because the soft-bodied nature of nudibranchs means they do not fossilize well.
These sea slugs are pelagic — they float upside down by using the surface tension of the water to stay up near the surface where they drift along, carried by the winds and ocean currents. Glaucus atlanticus makes use of countershading: the blue side of their body faces upwards, blending in with the blue of the water. The silver/grey side of the sea slugs faces downwards, blending in with the sunlight reflecting on the ocean's surface when viewed facing upwards underwater, helping them avoid becoming a tasty snack.
Glaucus atlanticus feed on other pelagic creatures, including the Portuguese man o' war and other venomous siphonophores. This sea slug stores stinging nematocysts from the siphonophores within its own tissues as a defence against predators. Humans handling the slug may receive a very painful and potentially dangerous sting. Good on you little Dragon!
