Found some info @ reefcentral..
From : LeslieH
http://mail.reefcentral.com/forums/showthread.php?p=16406873
True zebra crabs - Zebrida adamsii - are obligate commensals on urchins. Fire urchins in the genus Asthenosoma are the most common hosts although they occasionally show up on Mespilia globulus & others. They create a living space for themselves by clipping off the pedicellaria & spines in a small area & feed on the epithelial tissues. It's unknown whether they will eventually kill their hosts & move onto another or if a healthy urchin can keep regenerating the tissue without problems.
From : finsurgeon
http://www.reefcentral.com/forums/showthread.php?t=1692124
You need 2 or 3 urchins for a crab. They would definitely be "reef-safe", but urchins will eat coralline algae, scraping the purple off your rocks. It will grow back (possibly faster because of the processing of the urchin and pooping the spores), so don't freak if you see scraped trails. Urchins also can knock things over, so if you have corals or frags or anything just "set" in place, it will get knocked down. Frag glue or putty solves that. Also, you might start with one diadema and something smaller at first. A diadema without crab will grow fast, and could quickly outgrow your tank, whereas one that is less than 2 - 3 inches may not be big or resilient enough to host yet. If you are worried about tank size and urchins, you can feed urchins by hand, putting nori under their spines or on a clip where they can reach it, or put macroalgae from a fuge into the display. They love any greens they can get much more than anything they have to work harder for. And it seems that 2 crabs on 1 urchin will fight. But neither the crab or the urchins that are recognized as reef safe will harm any other occupant. (The only nonreef-safe I can think of is a pencil urchin.)