My new companion (Phestilla)

March 6, 2000
From: Kiichi Masu Chiba

Dear Bill,

I have found a new companion in my small reef tank. It looks like a soft coral. However, it has pair of two tentacles, and moves slowly.
I guess that this is a family of sea slug.
I send you two pictures, pictures quality might be bad and I cannot still take the front view. The upper photo is back view of it when slowly moving, you can see pair of tentacles on his head, there is another pair of tentacles at his mouth. The lowr photo is totally back view.

The body length is one inch. And almost flat circle shape. This animal has bright yellow body, and has an innumerable tentacle (looks like soft corals) in the place where the central line of the back is excluded. The thing like the tentacle is purple color and bright yellow spot on tip.

He always be on the Christmas tree worm rock, He never come away from there. But he does not seem to be eating the worms or corals. I worry what he eats and I worry about the influence that he exerts on my tank too. If you have knowledge of it, Please kindly help me.

Very thanks,
Kiichi Masu Chiba

Chiba, K.M., 2000 (Mar 6) My new companion (Phestilla). [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Kiichi,
Your new companion is a species of the coral-eating aeolid genus Phestilla. Although it seems to be a little more heaviy pigmented than usual I think it is Phestilla lugubris which feeds exclusively on some species of the coral Porites. The 'rock' your Christmas tree worms are in is a living colony of Porites.

Another possibility is Phestilla sp. 1, which can have yellow-tipped cerata, but it in feeds on another coral. I suspect your animal was a very small juvenile when you got your coral colony and has been feeding and growing in your aquarium. Have a look for white patches on the Porites colony - they may be underneath or on the sides of the colony. If you find any it will be a sure sign that the slug is feeding on it. You will then have to decide whether you want the slug or the coral to die. I think the worms will continue to live quite happily in the coral skeleton even if the coral animal is dead.

The 'tentacles' you describe are special organs found in aeolid sea slugs. Have a look at the Cerata Page to find out about them.
Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Mar 6). Comment on My new companion (Phestilla) by Kiichi Masu Chiba. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Phestilla lugubris

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