Distinguishing Glaucus atlanticus and Glaucilla bennettae

There are two species of Glaucus / Glaucilla that wash up on our shores and in some ways they are the stars of the blue fleet as they are not seen as much as the other members.

These float upside down, so what is visable when looking down into the water is actually the underside, while what is seen from being in the water looking up - usually just white / blue without the colour variations - is actually the dorsal, or top.

The sea swallow - Glaucus atlanticus can be identified by the two dark stripes with a silver strip inside them visable when they are floating in to water - this is the foot. G.atlanticus is larger, up to about 30mm, and also has a longer tail.

Glaucilla bennettae (old name Glaucus bennettae) has two dark stripes that are have a blue strip inside them, instead of silver . G.bennettae is smaller - about 12-15mm and the tail blends in with the "frilly bits". This species tends to have a "glitter look".

If you ever come across a washup take a close look at different Glaucus, as once you spot the difference they are easy to tell apart, even at a quick glance. Below is a photo with both of them so you can see easily the differences. What is showing is the underside of the creature, but this is what is seen from the top of the water.

Right: Glaucus atlanticus Left: Glaucilla bennettae

Most of Blue Fleet Welcomes Me Home is a journal post I have done about these.

Posted on February 08, 2019 02:50 AM by tangatawhenua tangatawhenua


This is an awesome guide!
Now we just need a quick field guide for identifying the differences between Physalia physalis and Physalia utriculus!
Should we assume all Physalia physalis observation from NZ are wrong and need changing to utriculus?
And how do the new sp.1 and sp.2 fit into the picture haha!?!

Posted by beaumasters over 5 years ago

Right at this moment @beaumasters while I am going through all of the Physalia obs I am writing the Physalia journal post for here, a Guide for Physalia and a page on "What to Photograph for Physalia ID" and the sp.1 sp.2 and sp3 will all be explained :) You will get a journal post notice on your dashboard when I add that here as you are a member of this project so stay tuned :D

Posted by tangatawhenua over 5 years ago

These are off-the-charts amazing critters. I hope to see one for real one day.

Posted by jon_sullivan over 4 years ago

The legend of the figure is erroneous (to this date). Glaucus atlanticus is on the LEFT and Glaucilla bennettae is on the RIGHT!

Posted by tommy_andriollo over 3 years ago

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