Glaucus atlanticus is the only known species of the genus Glaucus. It leads a pelagic life by floating thanks to an air bubble which it supports by means of modified waxes. Its natural habitat is in the Atlantic Ocean, but it has also been found in the Pacific Ocean, the Mediterranean Sea, and the Caribbean Sea.
Together with Glaucilla marginata it lives in association with siphonophoric hydrozoans, floating upside down with them, carried by the sea currents and the wind. It feeds on hydrozoans of the genera Physalia, Porpita and Velella.
Like many cladobranchs, it stores the cnidocysts inside the cerata for defensive purposes. Its length is 3 cm maximum. The body is blue and white in the ventral area, facing the surface, and silver on the back, facing the seabed, as a camouflage from possible predators.
The cerata, particularly developed, have support functions and are of the same color as the body, grouped in pairs. Similar to Glaucilla marginata, from which it is distinguished by the number of groups of cerata, grouped in several lines in G.
marginata while gathered in 3 pairs in the case of G. atlanticus. Glaucus atlanticus is able to swallow the venomous nematocysts from siphonophores such as the Portuguese man o 'war, and store them in the extremities of its finger-like cerata.
Picking up the animal can result in a painful sting, with symptoms similar to those caused by the Portuguese man o 'war. The symptoms that may appear after being stung are nausea, pain, vomiting, acute allergic contact dermatitis, erythema, urticarial papules, potential vesicle formation and post-inflammatory hyperpigmentation.
Glaucus atlanticus was recently found in the Humboldt Current ecosystem in Peru in 2013, and off Andhra Pradesh in India in 2012. This is in line with the known habitat characteristics of the species: they thrive in warm temperate climates in the Southern Pacific, and in circumtropical and Lusitanian environments.
Before finding G. atlanticus off Andhra Pradesh, these nudibranchs were documented as having been seen in the Bay of Bengal and off the coast of Tamil Nadu, India, over 677 kilometers apart. Glaucus atlanticus was also recently found off Bermuda in January 2016, and uncommonly washes ashore on east coast beaches at Barbados, Lesser Antilles.
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. In April 2022, specimens were found in the Gulf of Mexico along the Texas coast.