The worst things about crayfish is that if they are big enough to find its way to the top of your tank and over the other side you will have a crayfish escapee. It happened to my indo blue cray Bluey in the night. It has been six months since Bluey settled down in his new home, and the thought of moving him to a larger container was in my mind just then. But he seemed as if able to read my mind and did just the thing I would have least expected him to do...take a tour out of his tank

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I didn't even hear him until the middle of the night when I heard a scraping sound, like nails on wood. I got up instantly and flashed my torchlite around my table and then to Bluey's tank. The inevitable horror surfaced as I discovered the tank was empty, and I began to look for him. I dragged all the wooden furniture out (carefully, of course), looked under the bed, all but one place; behind the shelf on which the tank was placed. I carefully removed everything and then saw him crawling out all covered in dustballs, but he still managed to give my finger a good pinch while I soaked him in a shallowtub of water to rehydrate his gills, for one thing crays do if they are out of water for a long time and suddenly you put them back in and the pressure fills their gills, they will likely drown.
As for Bluey, he managed to survive the short fall down to the floor and has moulted off his shell for a new one, probably disgusted at what was still stuck on it, but afterwards he ate it anyway
So it happens too, I think, to other cray keepers. Crayfish will take a walk if they have a chance, or if something in the tank is not right, now reminds me to check what's going on inside....but so far since two weeks already, he is back to normal....but one thing strange...he has abandoned the idea of getting out of his tank a second time.