id this crab plz

[marvinsae]

sorry i dont have a picture but its so tricky to take one...he's about 5cm wide and he is black. always hiding. i've seen him come out once... i THINK the claws are pointy (ive read somewhere that pointy = bad). it was hiding in my buddies LR that he gave me. he boughtthe crab from some store in mississauga that was kinda shady. he put it in with one of his lobsters and he attacked it and killed it the next few days. should i keep him in or take him out? if i should take him out, how would i catch him...