Chiton or slug?

November 27, 2002
From: Dee Ryden

Dear Dr. Rudman,
I have enjoyed your site very much and have learned a great deal, the amount of information is amazing! I was hoping that you could help me out with an identification problem I'm having with a former tank

Here's what I know about him from my observations. He seemed to be primarily a nocturnal creature who was very peaceful with the rest of the tank mates. Save for the purple coraline algae. Which was what prompted me to remove him, he nibbled incessantly on the coral. His texture is that of a super tough muscle, very firm but not hard like a shell. He had little pointy things around the top portion of his body and these three colourful circles that seemed like some kind of plates. The top 1/4 of him was all I had ever seen until his removal and I was surprised that the rest of his body didn't have those similar colourful plates. His underside was like that of a snail, one big long foot and a mouth at one end. From all the searching I have done on the web, the closest I could come to was Acanthochitona hirundiformis. But he still didn't look like that completely. Is he a Chiton? if so which one? or who is he? Most Chitons seemed to be much more circular and uniformed with the plates on their body. Well from what I could tell any way. :) I now noticed that I have another one, but it seems to be a juvenile, just barely a half an inch. Still a nibbler though! I really hope you can help me out cuz this has been driving me crazy! So any information you can give me would be most appreciated!
Thank you for your time Dr. Rudman.
Dee Ryden

Dee Ryden, 2002 (Nov 27) Chiton or slug?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Dee,
Your animal is indeed a chiton. Most chitons have 8 very noticeable shell plates, but in a number of families there is a tendency to either reduce the size of the plates, or to hide much of each plate in the skin. it is not just snails that have tried so many times to become slugs. If you look at your photo there are 8 plates, but the ones on the right (posterior end) are considerably separated from one another. I'm afraid I'm not a chiton expert so I won't attempt to name your animal. It could be an acanthochitonid, as you suggest, but it looks very like a species of Cryptoplax to me. Whatever its name, I'm sure it won't do much damage in your aquarium.
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Nov 27). Comment on Chiton or slug? by Dee Ryden. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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