The family Glaucidae has just two genera, Glaucus and Glaucilla and each of these genera have just one species: Glaucus atlanticus and Glaucilla marginata.Glaucus atlanticus:

• can be up to 3cm in size
• the species has up to 84 cerata - outgrowths on the upper surfaces of the body
• the cerata hold the stinging cells (nematocysts) which are taken from the jellyfish they eat and are stored in special sacs called cnidosacs

This species closely resembles the other species in the family, Glaucilla marginata. The main differences between these two species are:

• Glaucilla marginata has more cerata than Glaucus atlanticus, up to 137 in total
• the tail (metapodium) of Glaucus atlanticus is much longer than that of Glaucilla marginata
• Glaucus atlanticus is the larger of the two species - Glaucilla marginata only reaches 12mm in length