I thought adding pieces of shells and smaller coral substrate was to help out my two pistol shrimps with building their burrows. Turns out the red tuxedo urchin has benefitted the most. It even likes to decorate itself with zoanthids that it pulls off the colonies. Once, I caught it carrying around two inch frag of M. digitata. I just recently added a Halloween Urchin, but it hasn't hoarded anything... yet.