Another mystery creature in my aquarium!

September 1, 2006
From: Daniel LaRose

Dear Bill,
I wrote you last week about a slug in my reef aquarium, but didn't have a good picture for you to ID it with. From my verbal discription you said it was most likely a type of Elysia. Well I finally got a couple good shots of my Elysia and was wondering if you could further ID it as to which exact species. Also I have observed one or more Elysia babys hanging out in my mermans brush since I last wrote you.

Now this morning while scouring my tank looking for the Elysia I found this unknown rather large creature slithering slowly between rocks in my tank. It looks to me like some form of invert but as to what I have no clue. This is my first time seeing this thing in my tank, as said above its rather large and I have no clue where it came from or how long its been in my tank. It is about maybe 3-4 inches in diameter, has been slithering between rocks. I'm unsure of where the head is or the exact shape of it as I havent seen it in the open. I havent had much time to observe it before this message so I can't even describe any behaviors of it. Attached are going to be 3 pics, 2 of my mysteria Elysia and 1 of the new mystery creature. If possible could you let me know if this thing is reef safe, please.

Locality: Albany, less than 1 meter, New York, United States, home reef aquarium, 30 August 2006, reef aquarium, found on bottom under rocks. Length: 3-4 inches in diameter. Photographer: myself.

Thanks in advance,

LaRose, D., 2006 (Sep 1) Another mystery creature in my aquarium!. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Dan,
Thanks for these photos of the Elysia. For those of you who can't find Dan's earlier message it's not on the Forum. To speed things up I deal with some messages by email - especially impossible identifications. I am afraid Dan, that although I can tell they are of a species of Elysia, I can't say much more. One problem is that 'live rock' in aquariums can come from anywhere so we can't be sure where your animals came from. The 'mermaids brush' algae suggests that they are Caribbean in origin, but if you look at the various discussions on Caribbean Elysia on the Forum it's quite difficult to identify Caribbean species even with close-up photos and even live specimens. So I can't say anything more than a species of Elysia. If it likes your algae and you don't mind it eating them, then it certainly is worth keeping in your aquarium.

Your new animal is a flatworm. I wouldn't advise you to keep it in your aquarium if you want to have other small invertebrates like snails or even crustacea, as it will eat them. It may also start eating colonial animals like ascidians and perhaps cnidarians as well.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Sep 1). Comment on Another mystery creature in my aquarium! by Daniel LaRose. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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