tmkx3- brittle stars have been known to form a sort of tent, lifting it's central disc off the substrate and standing on it's arms. Then it waits, for a tasy meal, slow swimming fish, sleeping fish and such. Also, no evidence of body present anywhere in the tank. I would have seen it as it wasn't missing long enough for it to have died of other reasons and the crabs to pick it apart, leaving a chewed up carcass floating around.
They do something that looks kind of like this..
Oh and btw
- I traded it in once it got to be 13" across!!!!!