Blue Sea slug: Glaucus Atlanticus, Australia (unedited)

Glaucus atlanticus (common names include the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small, blue sea slug, a pelagic aeolid nudibranch, a shell-less gastropod mollusk in the family Glaucidae.

These sea slugs are pelagic: they float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents. Glaucus atlanticus is camouflaged: 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 silvery surface of the sea.

Glaucus atlanticus feeds on other pelagic creatures, including the venomous siphonophore, the Portuguese man o' war (the blue bottles). This sea slug stores stinging nematocysts from the cnidarian within its own tissues as defense against predation. Humans handling the slug may receive a very painful and potentially dangerous sting.

This species looks similar to, and is closely related to, Glaucus marginatus, which is now understood to be not one species, but a cryptic species complex of four separate species that live in the Indian Ocean and Pacific Ocean.

This species looks similar to, and is closely related to, Glaucus marginatus, which is now understood to be not one species, but a cryptic species complex of four separate species that live in the Indian Ocean and Pacific Ocean.

This nudibranch is pelagic, and there is some evidence that it occurs throughout the world's oceans, in temperate and tropical waters. It has been recorded from the east and south coasts of South Africa, European waters, the east coast of Australia, and Mozambique.

Although these sea slugs live on the open ocean, they sometimes accidentally wash up onto the shore, and therefore they may be found on beaches.