Glaucus Atlanticus are up to 3 cm long and weigh between 3-100 grams
Their heads are blunt with tentacles near the mouth.
The slugs are silvery grey on their bellies to help them blend in with the surface when observed from below.
They are blue on their backs to help them blend in with the reflective surface of the ocean.
Glaucus Atlanticus have 3-4 clusters of papillae, or round protrusions, off the sides of their bodies.
Within all the clusters, there can be up to 84 individual papillae.
Off the papillae, there are 'fingers', or cerata, and at the end of the cerata, there are sacks called cnidosacs which are used for their defense.
Glaucus Atlanticus collect the venom from their prey's nematocysts, the barbed or venomous coils in jellyfish, and store this venom in their cnidosacs.
The Glaucus Atlanticus are protected from their prey's venom by the hard disks inside their skin and a protective layer of mucus.
Depending on the amount of venom stored inside the cnidosacs, the Glaucus Atlanticus can produce a more deadly sting than the Portuguese Man-O'-War, which has a sting described as feeling like hot knives on skin.