Re: Elysiella pusilla

July 28, 1999
From: Eirik Eilertsen

Dear Bill,
The animal I've found looks much "taller" than the animals depicted in your forum, but I still think you got it right.

The algae, by the way, grew out of some live rock in my main tank about a year ago. I moved it to a "small critter tank" a few months ago, and I only recently discovered the slug. It was probably introduced with some red and green algae (including Halimeda) from Singapore, but it seems the old Halimeda is a much closer match than the new one, where both shape and color of the individual segments are concerned. Whenever I manage to spot it in the tank, I am amazed as to just how much it looks like the algae on which it clearly feeds. The color is an exact match (you don't mention anything about this species being able to change color) and the overall shape of the animal makes it virtually impossible to distinguish from the algae segments. The animal even has a distinct narrowing of the body just behind the head, giving the impression of two connected segments of Halimeda. Today I noticed that the color and overall texture of the animal seems different from when I first spotted it. It is now covered with tiny white appendages, which makes it look even more like the algae which is overgrown with other species of (micro) algae.

Eirik Mack Eilertsen


Eilertsen, E.M. , 1999 (Jul 28) Re: Elysiella pusilla. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Eirik,
For the Halimeda to stay alive so long in your tank, you obviously have a healthy aquarium system. If you look at the pictures on the second page you will see animals of Elysiella pusilla which have much more raised Elysia-like parapodia than the flattened animals on the first page.

Concerning them changing colour. Most sacoglossans get their colouration from their plant food. Only a very few have external skin pigmentation, of which Cyerce nigricans is a spectacular example. This means that the colour of most sacoglossans matches the food they are eating very precisely. I am not sure how they manage the mottled effect in large flattened E. pusilla, but it certainly matches the mottled white and green colour of the older segments of Halimeda, while smaller animals, with their uniform green colour, match the deeper colour of the young growing segments of the algae.

Interestingly, some animals on the more cylindrically-shaped variety of Halimeda have a colour pattern of whitish streaks.

I am puzzled by your mention of white papillae appearing on your animal. While the rhinophoral tentacles can have papillae I have never seen papillae on the rest of the body. Perhaps you should follow their progress and see if you can get a photo of the animal. It is always possible that you have a species of Elysia rather than Elysiella. My identification is based to a large extent on your animal being associated with Halimeda.

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Jul 28). Comment on Re: Elysiella pusilla by Eirik Eilertsen . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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