I was looking into getting an emerald crab for my 20g and I was wondering if he'll be a threat to any fish or shirmp that might be placed in the tank? Most likely the shirmp will be cleaner shimp and the fish are damsels.

As long as the fish are bigger than it they should be fine, however, Mithrax crabs are scavengers and if they come across an ill or injured fish they will try to attack it. The same goes for shrimps, if the Mithrax can corner it or if the shrimps just moulted then it will become potential prey.

They do sometimes nip at corals as well if there's not a ready source of food so keep it well fed.

Thank you very much!

there great scavengers, just make sure the shrimp and fish are bigger
i have 2 in my tank and all they do is sit there eating algea.

In general, most mithrax crabs 'hot out of the lfs' will be very small, probably less than 3/4's of an inch in length. They are virtually harmless at this stage and will keep small for quite sometime. However, they can grow to up to 2 inches and at this stage they can become opportunistic if they are hungry. Personally, I like them in the cleanup crew and they have cleaned my tank of valonia. SH