He plows into them and takes them off the rock but dosent seam to bother him. I've goten stung by the large one in the one pic and it wasent too bad. I think they might be immune to it because he's so big and his skin is thick but idk haha they are nice to have in the tank to show how the water quality in the tank. Plus they look cool on the rocks he hides under.Wow those diadema urchins are gonna get big FAST. Are you concerned about him getting stung? Or yourself for that matter? That's the only urchin I've ever been stung by. Luckily it happened when I was cleaning a tank in a doctor's office, so I was in good hands!
Not that they're dangerous or anything, just very easy to prick yourself on.