Pink tiny nudibranch and concentric rings

March 25, 2005
From: Francesca Riolo

I live in American Samoa and I just started a small aquarium to give a closer and longer look to the creatures I usually observe underwater. Three days ago I collected the first habitants including a tiny all pink nudibranch (4-5 mm).

Locality: Sliding Rock Beach, American Samoa. Depth: 0.4 meters. Length: 5 mm. 22 March 2005. Reef flat. Photographer: Francesca Riolo.

I am not an expert and I am not sure it is a nudibranch rather than something else (opisthobranch or some other invertabrate I am not aware of) though I am quite sure I can see the two front rhinophores and on the back the gills as I see on many nudibranch pictures (not visible on the attached picture). Are these ID keys that tells me is a nudibranch? The following day at night I noticed that he started producing some concentric pink rings beside its body...are these eggs? By the way I found it on the bottom part of a rock but already during transportation he choosed a piece of Halimeda (I think!) algae as its new home and never left it since then...

Today the rings are gone (maybe damaged by hermit crabs)...but I took a picture last night...i hope I have not been too long...I would like to know if possible what species this creature is and what those rings are and if they are eggs if there is something that might have induce it to produce them the second day of its aquarium life!( stress, halimeda algae...) Thanks a lot for your help!


Dear Francesca,
From your description of the rhinophores and gills in the middle of the back, your animal is a dorid nudibranch but without further information I can't say anything more. The 'concentric rings' are (or were) not concentric rings but a continuous spiral - and as you thought they were eggs. Dorid don't feed on algae so it was using the Halimeda as a place to hide, or wait for better times. Nudibranchs do often lay eggs when stressed but I can't say if this laying was a 'normal' activity or a stress-related event. Certainly other animals will eat egg ribbons. 

Good luck with your aquarium and observations
Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Mar 25). Comment on Pink tiny nudibranch and concentric rings by Francesca Riolo. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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