Very Odd Creature ID Please?

Instant Tang

New member
I've seen this guy in my tank late at night after all the metal halides and actinics have gone off. It's sensetive to light, so I can't examine him with a flashlight for long.

He can change shape from being "half cylindircal" (what you see in the photo link) to really pancaked out. When he's flat like a pancake he looks mostly tan with a round white spot in the middle of the pancake and small white bumps around the edges.

When he's in his normal mobile mode he is white/tan, about 8mm across, and has three distictive "white diamond" shapes on his back that are rimmed in brown/tan colors. He's got many small white bumps on his head.

He appears in the same location when I find him. I've never seen his entire body, and there doesn't appear to be a shell really- so I can't tell if he's much bigger than what I've seen, but part of his body is definitely hidden always from my view. He seems to only be on my live rock- I've never seen him on a coral.

What the heck is this thing? I've looked on many sites and have yet to see anything that looks like him.

http://reefcentral.com/gallery/showphoto.php?photo=204678&size=big&papass=&sort=1&thecat=

Sorry for the fuzzy picture, but I think it reveals enough shape for him. I'll try to setup with a tripod next time, but the photo is a little distorted from the bowfront curved glass (next time, I'm avoiding curved glass fronts on any tanks!!!!).

Your help is greatly appreciated! I want to know if he's a bad predator and could be eating things I don't want him to be eating.
 
That is a Cryptoplax chiton. Like 99% of chitons it's a grazer on microalgae on rocks and aquarium walls. Some people welcome them; others don't because they may eat desirable corallines or may scratch acyrlic as they graze.
 
He's not segmented like a Chiton- are you sure about the ID? I've seen him flatten out like a pancake and even then, you can't see any segmentation on him whatsoever. Are some Chitons not segmented?
 
The diamond shaped things you see on the back are part of the segmented shell. The name Cryptoplax reflects the fact that the shell isn't much like most chitons.
 
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