Glaucus Atlanticus

Glaucus Atlanticus (common names include the blue sea dragon, sea swallow, blue angel, blue glaucus, dragon slug, blue dragon, blue sea slug, and blue ocean slug) is a species of small, blue sea slug, a pelagic (open-ocean) aeolid nudibranch, a shell-less gastropod mollusk in the family Glaucidae. These pelagic sea slugs use the water's surface tension to stay upright while they float upside down, where the wind and ocean currents carry them along. In order to blend in with the blue of the water, G. atlanticus uses countershading, turning the blue side of its body upward. When viewed from above underwater, the silver/gray side of the sea slugs blends in with the sunshine bouncing off the water's surface.

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 defense against predators. Humans handling the slug may receive a very painful and potentially dangerous sting.