I have one of these. They grow quickly. Just a few words on what I have experienced with my spiny sea urchin:
-Mine ate the crown of a feather duster....feather duster grew back but just a heads up.
-Mine also finished off a snail that died
-He hurts ya if you run into him. I accidently poked myself with his spines while cleaning the tank.
Other than that, I really like him. He has grown quite quickly, moves fast, and eats a lot of algae.
As above, interesting little guy.
"Fish are friends, not food"