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Creature Feature: Glaucus atlanticus

Creature Feature: Glaucus atlanticus

Article by Lou De Beuzeville

Glaucus atlanticus, or the blue dragon, is a type of planktonic nudibranch. Unusual for nudibranchs, it is neustonic (a new word for me, it means living at the interface of sea and air). They travel on the surface because they have a gas-filled bag in their stomach. They are closely related to the Glaucilla species such as Glaucilla marginatus below.

Glaucus atlanticus

Glaucus marginatus

These gorgeous blue nudis are stunning for their bright blue colour and the up to 84 papillae (fingerlike things) in 3 pairs of clusters that adorn their bodies. Although their dorsal side is a bright blue with a silver streak (unlike G. marginatus which is just blue), their underside is a light silver, allowing them to hide from predators above and below the water surface. They grow to about 3cm.

Glaucus atlanticus from above & below

G. atlanticus eat most of the “blue fleet” they travel with: Physalia physalis (blue bottles)*, Porpita porpita (the cute round blue button things), Velella velella (By-The-Wind-Sailors).

Members of the blue fleet that are preyed upon by G. atlanticus

They have some sort of protection from being stung themselves, yet take the stinging nematocysts from blue bottles and store them at the end of their “fingers” to use later on other prey. (I read that blue bottles can actually empty their gas-filled float to escape Glaucus!) Another thing I read (but not on a paper) was that if hungry enough they will eat their own kind.

G. atlanticus chowing down on blue bottle tentacles

Blue bottle tentacles

G. atlanticus are hermaphrodites. There’s a hole at the side for a long boy part (so that they don’t sting each other when mating). And another hole for the girl bit. Only it’s between two close together legs so I haven’t actually seen it.

Reproductive bits

So how to find them? Generally, look for them on the beach if there’s been a strong NE wind or swell and there are blue bottles around. They will often roll themselves up into a ball as below.

G.atlanticus rolled up

If anyone is interested in seeing them, there’s a new Facebook page set up for people to post sightings so others can know where to go. The group is named Glaucus & “Blue Fleet” sightings – Greater Sydney and can be found

*before anyone says we have utriculus, apparently we don’t. See Worms and a conversation on iNaturalist. They now believe there’s just one species of blue bottle.


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