Glaucus atlanticus (commonly known as the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, a marine gastropod mollusk in the family Glaucidae. It is closely related to Glaucilla marginata, which is sometimes included in Glaucus.
This sea slugs feed on other pelagic creatures including the venomous cnidarian, the Portuguese Man o' War. Because the sea slug stores stinging nematocysts from the cnidarian within its own tissues, a human picking up the sea slug may receive a very painful sting. For more information on the effects of the nematocysts, see Portuguese man o' war.
G. atlanticus preys on other, larger pelagic organisms: the dangerously venomous Portuguese Man o' War Physalia physalis; the by-the-wind-sailor Velella velella; the blue button Porpita porpita; and the violet snail, Janthina janthina. Occasionally, individual Glaucus become cannibals given the opportunity.