Glaucus Atlanticus – The Blue Dragon

Quick Facts

Class: Gastropoda
Other Names: Sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug
Found: Atlantic, Indian and Pacific Ocean
Size: Up to 3 – 4cm long
Weirdest Features: They live floating upside down on the surface of the water, when they eat poisonous animals they use the poison for their own defense, plus they look a bit like real-life Pokémon.

How They Look

Glaucus Atlanticus, or Blue Dragons (which is a far more fun name), usually grow to be around 4cm long and have 3 pairs of arms, each one shorter than the last as they go further down it’s body. On these arms are single rows of growths called cerata which can hold stinging cells – there can be up to 84 of these on a single Blue Dragon.

Blue Dragons actually spend most of their time upside down so the blue, upper surface is actually their foot. This colouring helps with camouflage – they’re blue on the upper-facing side which blends in with the sea water, protecting them from flying predators like birds, while the under-side (which is actually the top of them, I know, it’s confusing) is a silver–grey colour to blend in with the sky when see from under the water. The blue colouring is also thought to reflect UV rays, which protects them from burns as they float on the surface of the water.

Poison Stealers

Blue dragons are pelagic; they live in the open ocean and instead of swimming anywhere they just float along, happy to be carried by the wind and natural ocean currents. Of course, this has its disadvantages, which is why they sometimes end up washed up on beaches. They are usually found in temperate and tropical water including surrounding South Africa, Europe, Australia and Mozambique.

While these sea slugs usually just float along, there are times when they feel the need to swim (which is what their funny-looking arms are used for!) – this is usually to catch prey to eat. Blue Dragons feed on a range of animals; mostly hydrozoans like the Portuguese Man o’ War, however they have also been known to be cannibalistic and eat other Blue Dragons, especially when kept together in captivity.

While the Portuguese Man o’ War is usually venomous, the Blue Dragon has a sneaky way of avoiding being poisoned! The Blue Dragon first releases a protective mucus inside itself and then will eat the entire organism, including the poisonous stings. It then selects and stores the most poisonous nematocysts (which are the organs which store the capsules of poison, they can eject threads to cause sting) in specialised sacs called cnidosacs at the end of their cerata (remember these are the bits which look a little like fingers on their arms).

What’s really clever is because the Blue Dragon can collect this venom from a number of animals, it can concentrate it and produce a poison which is more deadly than the Portugese Man o’ War’s and its other preys’. This means they can also sting humans if they’re picked up!


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