Hmm. I still think there was something wrong with your urchin prior to the crabs feasting on it and you only had him for 5 days. Maybe there was something wrong with it when you bought him. I have a couple small ones and two larger ones. If they are healthy then when you poke them the spines will go stiff as to deter you from hurting it. I have a short-spine urchin that I can't grab because the spines are so sharp. There are also several larger, not full grown but still large emerald crabs that don't touch them (shortspine and pencil urchins).
Come to think of it my neighbor has a 125 gallon reef with aggressive fish. He had a puffer (notice I say had ) in there with a couple damsels, a lionfish, a grouper, and a niger trigger. He had put in some urchins but the puffer took out there spines and they were killed. Ever seen ur puffer go near the urchin?
I dont really like the setups I always see at doctor's offices cause they always have bad matchs ups and usually too many fish. My barber shop has a 55 gallon that has a yellow tang, a puffer, a trigger, two o. clowns, a couple damsels and something else. You can see that the tang is starving and the tank has fake coral decorations, and there a diatoms all over the decorations. It is sad. I also saw a maybe 60 gallon at a doctors office that had a huge tomato clown, niger trigger, yellow tang, foxface lo, and something else. It was just too much for such a small tank .
