One of my two urchins bulldozed a recent frag off it's mount today, so I was arm-in-tank trying to remount the frag.
The urchin was kind of in the way so I was gently nudging him to "move along...move along". They are quite reactive to touch, btw...if you touch any spine, it will quickly orient additional spines your way.
Anyway...so I'm pushing him along and thinking to myself.."these spines aren't really that sharp..." Oh Yeah?...ZAP!!!
I don't know if they actually contain some sort of venom or something gets injected or not...but let me tell you that you do NOT want to step on a black spiny sea urchin. I was losing a lot of blood and it hurt quite a bit. I would compare it to a wasp sting....except for the bleeding part.
5 minutes later...pain free and you can hardly even see where he got me.
Here is the offender...

__________________
400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436