Any idea of the cause?
I don't know really, but it had not eaten anything in about 2 weeks so I'm assuming it had become senescent. The crabs I had put in there at least 2 weeks ago are still in there, alive. Every time I offered shrimp or krill it would take them, but discard them a few minutes later. At first when I would play tug-o-war with it, it would at least come all the way out of its hole (not very far) but the past few times all I got was an arm or two sticking out from the rocks, very reclusive. It was slumped over a rock in front of its den this morning when I got to work and while the fish were curious about it, none of them were picking at it. Judging by the appearance and lack of smell, I don't believe it had been dead long. I didn't pull the rock out to determine if there are any eggs or not but it wouldn't surprise me if there are.