The blue dragon slug (also known as Glaucus or the blue sea dragon) is a genus of nudibranch.

The blue dragon slug 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.

Glaucus includes five distinct species, though nearly visually identical, thus forming a species complex.

Glaucus - From Wikipedia, the free encyclopedia