I've been keeping a marine fish tank for just over 12 months and I have a selection of soft corals, polyps and fish. I have observed for the past six months an unidentified crab which I assume entered the tank 12 months ago within living rock. The crab is 1" across, brown and slightly hairy in appearance. It is mainly active at night and appears to feed on algae in much the same way as my hermit crabs, although its claws are somewhat larger. As it appears to be causing no harm I have left it alone, but I am concerned that it may cause damage in the long term as I've no idea how large it will grow, etc. Would you suggest I remove the crab or leave it be unless I have some evidence it is causing damage?
Thanks, Robert Wood, Derby
There are very few crabs that I would consider keeping in my aquariums and then only in certain environments. What you describe would not be one of them. All crabs can not fully be trusted. Some serve useful purposes, however, even the Emerald Crab, Mithrax sculptus, that feeds primarily on Valonia (bubble algae) will eat small fishes when available. If it has a claw, it's not to be fully trusted.