will this crab kill a reef tank?

Possibly, looks evil. Better safe than hungry

If it's furry, get it out. General rule of thumb.

Put it in a fuge or something.

tahts teh dexil!!!!

He's probably fine IMO.

I'm pretty sure that's a Mithrax Sculptus, or mixthrax crab (otherwise known as a Gorilla crab). Like most crabs, it's probably harmless and beneficial, and probably won't bother anything in your tank (95% harmless). I have two in my tank, and like them (although they're a bit shy and thus kind of boring). I say throw him back in :-).

Looks like a Gorilla Crab to me...General rule around here is that they are fine as long as you are okay with a crab that will grow up to eat all your fishies.....

Depends on the nature of your tank. If you don't mine a little bit of opportunistic feeding on his part, then by all means leave him in. There is a chance he could knock over frags, eat smaller fish, etc. though.

He's fine as long as all your frags & live rocks are glued together, those crabs loves to knock off new fragments/rocks.. I have 2 of em in my tank and I named them "Samson Crabs" because of the way they push the rock against their back, they investigate if I put a new frag/rock in my tank.. and knock it off at night. I moved them in my QT when one morning I saw my Accro frag fell over my Sabae anemone... XD

But they are very effective in removing hair algae & scavenging left over food.. I like them, but they tend to be destructive on rockworks and danger to lazy fishes.. they are my prime suspects for the disappearance w/o a trace of my 2 watchman gobies.

Thanks all...he's captured and I don't know what I'm gonna do with him. No matter what...I've decided he isn't going back in the tank. Too much risk.

It would make a WONDERFUL addition to a sump or fuge if you have one separate from your display....

Looks like Harry the hairy crab I used to have. Did alright for me, never really bothered anyone. Wasn't too fond of the emerald crab though. Check out the early pages of my 1/2 10 thread for pics of my guy. Looks just the same.