my blue tuxedo urchin died, what happened

[pufferpoison]

ok got this thing from the LFS and acclimated it (drip) for over an hour and he was doing fine cruising all over the tank and then about a week later he was at the bottom of the tank next day, no spines and I picked him up and only the shell was there with a hole in it and nothing inside.

ammonia 0
nitrite 0
nitrate 0
phosphate 0
copper 0
mag 1300+
calcium 418
Sg 1.025
temp 80
KH 9

RO/DI water with snails, crabs, yellow tang, regal tang, lawnmower blenny, Anenome RBTA, and corals. what could have happened?

 

[Nitelife]

I am thinking maybe was dinner for something(crabs)...or died over the stress of the move to your tank,and was picked clean by brisel worms..Just my guess of corse....Maybe you could help me with my royal?Not to take over your thred or anything but i'm a little worried....It's got a spot in the middle of his side,,looks like an eye,black center with green edge and white outline is the last..hope that makes sence....he is new to the tank and seems to be doing ok besides that,,any ideas puffer?Thanks for any help..