He was already acting weird last few days, riding up to rocks and walls (and clam) and dropping on his back. I moved him back to a place with more sand for him to move around in, but he didn’t go under. He had a weird spot on his rear, which I found out this morning, wasn’t a spot but a hole. He has now lost all of his hairs and the cleaner shrimp was already busy with him, so it is officially over now for him. Cause of death is unknown, but my guess is that it has something to do with the Cyano Bacteria covering the sand now creating air pockets as well. This was also the situation when the previous urchin died, so it might be related.