i'd love to upload a picture but unfortunately the little b*gger wont pose for me. he moves pretty quick hiding in all the crevices. i've tried using the books i have to identify him but nothing really looks like him (or her) to describe i would say it looks like a standard crab. claws are proportional as are legs and body. he is very dark, black almost, but the legs have a bit of a brown stripe in them, maybe two or three per leg, although the main claws seem to be a solid black. there are also hairs on the legs. hope that describes it well enough. i'm slightly concerned at the growth rate of this one though. i remember seeing one of it's claws sticking out of a hole in my live rock and it looked like a very small brown worm at the time. now maybe 4 or 5 weeks later the claw size (coz thats all i usually see) is nearly 5 or 6 times bigger. it's body size which i saw today briefly would be the size of a £1 coin or so, but a bit deeper.
now i've lost a starfish, 1 leg a night until basically a stump was left, which i'm certain is down to this crab. the rest of my tanks inhabitants are 2 x clown fish, a blue legged hermit crab, and just 1 cleaner shrimp (there were 2 and 1 x fire shrimp but these have died over the last10 days which i don't know if that is just coincidence or down to the crab)
any help or recommendations on what i should do with it would be greatly appreciated.
If I was you I would get rid of it, it sounds the same as the crabs I had in my tank. My LFS called them Red Eyes and said they would grow quite large and could easily take fish down, which I'm sure is what happened to my little Koran, it was found half eaten one morning. The LFS said they are wicked devils to have in your tank and should be removed asap. I took each piece of live rock out of the tank and poured soda water over it until I thought they were all out and then took them into my LFS who put them into their sump. I did find the odd stray but fished them out by swiping out the rock they were on and shaking it furiously until the crab was to sick to hang on anymore..... It worked. Remove it, you may regret it later if you don't.