Why did my crab disappear out of my fish tank?

My crab i have had for a very long time & one day he went missing. Im the only one who opens the lid to my tank which i think is way to heavy for a crab to lift open to escape. I looked under my gravel, around & under decorations, in my lid (where i have found him a few times before) i have literally looked everywhere i could think of. Could one of my fish have eaten him? I have to small sevrums, a betta & 4 baby tiger barbs. What could have happened to my poor crab BooBoo? :( please help.