Glaucus atlanticus (common names sea swallow, blue glaucus, 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. This is the only species in the genus Glaucus,[1] but is closely related to Glaucilla marginata, another member of the family Glaucidae.