Crab Id

[Marcus K]

Sorry I can not get a pic of him yet. He is a greyish blue colour with dark orange eyes. I found him while looking into the rock holes after seeing fine silt like mounds outside some holes in the live rock. My wife says he has furry arms and is about the size of a dime. Any thoughts as to what kind of crab he might be?

Thanks

Marcus

[IslandReefer]

I was informed by my local expert (who is famous at Canreef) that;
and I quote,
" The black ones with red eyes are eevil"!
So that makes yours a: Stylish, Not soo Bad....you know Urban biker wannnabe

For myself, I am a sucker for all the critters in my tank...soooo if the crab, isn't "EVIL" and stays small and well behaved, he stays, live and let live....but first offense...to the sump!
Please post a pic when you can, someone might have one like him or know the type.......I have one that sounds similar no probs sooo far

[Beverly]

Marcus,

Is the coralline algae being eaten around the holes? If so, that's probably all it's eating, though it may snack on stray foods you feed your fish.

I'm guessing that you found this crab after lights out while flashlighting the tank. If so, the best say to observe this crab is to shine the brightest part of the light near but not on the crab so only dull light is on it. Have a look at what it's eating then and let us know what you discover.

[Marcus K]

I noticed him only because my rock is starting to look like rock again. Hair algae is starting to clear out. I noticed small piles of silt around some holes so I searched the rock with a flashlight and found some scary red eyes looking back. I have had some snails go missing once in a while. I will spend some time after lights out to see if I can catch him. I can add him to the refugium if I can catch him.

I will let you know if I do.

[Quinn]

Remember that most crabs are opportunistic feeders and therefore if you have small, bottom-dwelling/sleeping fish (gobies and blennies in particular I've found), I wouldn't trust any crab with relatively large claws. And of course your corals could well be at risk*. According to Shimek a better indication of risk would be claw shape/colour, rather than eye colour. In fact I think eye colour is a poor indicator of anything in crabs based on what I've read, and is a myth that should probably be smothered in this hobby. As noted, the safest thing is undoubtedly relegating any hitchhiker crabs you find to your sump or a tank specifically set up for them. Like Bev said, most I've had seemed to stay around their little caves, eating algae (at any on time in my 150 gal I could find perhaps three little claws protruding from holes, happily ripping bits of algae off the rock around the opening of the hole). I had a 2"+ monster (Garfield) who seemed the same, I never noticed any damage to corals and he was within inches of my best Acropora sp. colony. I did have a goby who seemed to loose chunks of her tail fin on occasion but I'm not sure if this was a crab or just my female dog-esque female clownfish.

* Remember though that just because you have crabs in your tank, and you are losing snails, fish, corals, etc., doesn't mean the crabs are responsible. Correlation is not causation, and the average aquarium is a pretty poor experimental design, due to the myriad of third factors present.