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Blue Sea Slug

Glaucus marginatus


We found these two tiny nudibranchs on our local beach this afternoon, washed up with a heap of bluebottles (Portuguese Man'O'War). Each is under a centimetre long, probably closer to 7-8mm. We scooped them up in a shell for these pics. These little creatures float upside down on the surface tension of the water. They are the only species in the genus Glaucilla, but are closely related to the larger (and showier) Glaucus atlanticus ( Update 5/10/14 - This species appears to have been reclassified. I have updated the scientific name to Glaucus marginata accordingly.

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47 Comments (1–25)

HazelMarley 8 years ago

I never knew those existed

hamzee413 9 years ago

Absolutely beautiful! They're like tiny dragons~

ShannaB 9 years ago

Thanks Neil. I got stung by a bluebottle once but it didn't affect me too badly - I think I was just lucky! Mostly, as a born and bred inlander, I just avoid bluebottle stings by staying well out of the water... ; ) I feel really lucky to have found these slugs when we did, I haven't seen them since... I'm desperate to find a Glaucus atlanticus too, but no luck. Actually that's what I was hoping to find the afternoon a couple of weeks ago when I came across that baby turtle!!!

Neil Ross
Neil Ross 9 years ago

Great series, Shanna. I'd forgotten all about these little beasties. So beautiful. Haven't seen one since I was a kid. These would wash up, and blue bottles weren't far behind. I always seemed to find the blue bottles whilst in the surf. Developed good immunity, I think. I was certainly stung enough.

RayTung 10 years ago

Blue dragon! amazing shot!

ShannaB 10 years ago

They certainly are, Tic! : )

TicThapanya 10 years ago

very beautiful organisms

Brian Muñiz
Brian Muñiz 10 years ago

I actually do, they look so similar to one another that their differentiation may have happened recently by evolutionary standards. They have the same blue coloration on the top to hide from predators from above and the silver below to hide from deepwater predators, and the fact that you mention the man o war is a great indicator that they are very similar, at least were I live there are some seasons where you cannot go to the water because man o war's arrive, as well as other deadly jellyfish and many that just hurt alot.

ShannaB 10 years ago

Thanks Scott. : )

ShannaB 10 years ago

Thank Gilma! Brian, thanks for the link. The info all seems to be about Glaucus atlanticus... there's hardly anything on Glaucilla marginata. Do you think the same info applies to both species? (Glaucilla marginata really seems to be the poor cousin!!)

Brian Muñiz
Brian Muñiz 10 years ago

Here's a link about their diet:

WOW!! Lovely!! Never seen them before. Great pictures.

ShannaB 10 years ago

It sure is, Dixie! : )

Dixie 10 years ago


ShannaB 10 years ago

Thanks Jason, Braulio and Adarsha!

Adarsha B S
Adarsha B S 10 years ago

Wow ! what an amazing creation!

Braulio Rivas Tapia
Braulio Rivas Tapia 10 years ago

Just amazing!

Jason Alexander
Jason Alexander 10 years ago


ShannaB 10 years ago

Haha Sheikanah, I see what you mean! But definitely not a fighting fish. : )

shekainah d. alaban
shekainah d. alaban 10 years ago

amazing creature, thought t'was a fighting fish.

ShannaB 10 years ago

Thanks Yuriana, Yuko, Ali, Holly, Yasser, Brian and Seema. Brian, I did read somewhere that they eat physalia and take their stinging cells but I couldn't find a reliable source!! (And we didn't know this when we found them... and we picked them up. Fortunately we didn't get stung!)

Seema 10 years ago

very unusual shape,,,,

Brian Muñiz
Brian Muñiz 10 years ago

Beautiful little guys, the pelagic sea slug actually feeds on the physalia or portuguese man o' war which is why you found them with it. They also take the stinging cells from the host for protection so I wouldn't touch them haha. Never seen one in person but they are interesting little nudibranches.

Yasser 10 years ago


HollyCline 10 years ago


Spotted by

QLD, Australia

Spotted on Jan 6, 2013
Submitted on Jan 6, 2013

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