I already have those
I put some pics up about a month ago asking what they were too They're another reason i dont like touching the live rock But they're only tiny - The crab has big feet thingys
LOL nearly all crabsa wont harm you at all.. they just shrink backinto the t rock and wont arm you. put your hand on a 3ft bristleworm and you WILL know all about it! And 3ft is just the average size... some get as large as 7ft! and these monsters were still never even known to be in the tank at the time!
Sorry.. i dont want to scare you but i just want you to realise that crabs really arent the danger you think they are and although they look ferocious they are nothing like that.