Glaucus atlanticus, Forster, 1777 © Taro Taylor
The family Glaucidae has just one genus, Glaucus and two species: Glaucus atlanticus and Glaucus marginatus.
The cerata hold stinging cells (nematocysts), which are taken from the Portuguese Man o' War and other hydrozoans they eat and are stored in special sacs called cnidosacs.
This species closely resembles the only other species in the family, Glaucus marginatus. Both species float upside down under the water surface so that the blue colours are actually the ventral surface.
Glaucus marginatus, Bergh, 1860 © Taro Taylor
The main differences between the two species are: