The Blue Dragon - A Beautiful Sea Slug and Nudibranch

The blue dragon - dorsal surface on the left
and ventral surface on the right
Photo by Imtorn,
CC BY-SA 3.0 License

The blue dragon or blue sea slug (Glaucus atlanticus) is a beautiful ocean creature that reaches a length of about three centimetres. The slug has a silver dorsal (or upper) surface and a blue ventral (or lower) surface. Two black stripes run down each surface. The slug also has six appendages which extend from its body and end in finger-like structures known as cerata. 

Despite its small size, the blue dragon is a predator on larger organisms, including the poisonous Portuguese man-of-war, or bluebottle. The slug is not only immune to the prey's poison but also stores the poison in its cerata. As a result, the slug can give a human a very painful sting. This sting may be more dangerous than the sting of the Portuguese man-of-war because the poison is more concentrated.

The blue dragon also feeds on the blue button (Porpita porpita) and the by-the-wind sailor 
(Velella velella). Both are floating colonies of other organisms, like the Portuguese man-of-war. If it can't find enough prey, the slug becomes a cannibal and eats other blue dragons.

The blue dragon
Photo by Taro Taylor,
Creative Commons Attribution 2.0 Generic

The slug swallows air to enable it to float on the ocean surface, which it does with its body upside down. The blue surface faces upwards and the silver surface faces downwards. This position provides countershading for the slug. Viewed by a bird from above, the blue surface of the slug blends with the blue ocean surface, making it hard to see the slug. Viewed from below, the slug's silver color blends with the color of the ocean as it's hit by sunlight, making it hard for slug to be seen by predators from below.

The blue dragon is found in the tropical parts of the Atlantic, Pacific and Indian Oceans. Like land slugs, it's a member of the phylum Mollusca and the class Gastropoda. It belongs to a gastropod group known as the Nudibranchia, which is variously known as an order, a suborder, a clade or a subclade. However it's classified, the blue dragon is a very attractive creature.