I have hear that they eat and spead coraline. I have a pencil urchin in my prop system and he is a one man wrecking machine, turns over everything and plants himself where he wants. Coroline has inproved greatly since I added him but, I'm setting up an semi FOWLR that will his eventual home. I'm tired of turning over frags and colony's because he likes to roam and knock stuff over. Two smaller spiney urchins have not been a problem in my main display.