Glacos gaculos

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 sea slug in the family Glaucidae.

These sea slugs live in the pelagic zone (open ocean), where they float upside-down by using the surface tension of the water to stay afloat. In addition, they have a gas bubble in their stomach that makes it easier for them to float. They are carried along by the winds and ocean currents. G. atlanticus makes use of countershading; the blue side of their bodies faces upwards, blending in with the blue of the water. The silver-grey dorsal side of the sea slug faces downwards, blending in with the sunlight shining through the ocean's surface when viewed from below the surface of the water.