About two months ago I bought more live rock to add into my existing stabilized salt water tank. One night when I turned on the room light that it was in, something scurried across the bottom of the tank and for a weeks we couldn't find what it was. Today I finally saw it in full view. I attempted to take a picture, but my phone died. So I'm sorry I don't have a picture or maybe this would be easier. But it is a purple crab who must have come with the new rock (the tank has been devoid of fish and invert life since we moved a year ago). It has small red eyes set close to it's body. The legs are short and stubby, held close to it's body, same with it's larger claws in the front. The top of it's shell is solid purple color, but the texture is almost like a checkerboard, raised bumps in a checkerboard like pattern. Right not it is maybe 1" long. I've looked online and in what books we have and could not find an image of a crab even remotely like it. The red eyes are the most defined feature of it. Does anyone know what type of crab this is? I want to know how damaging it will be to a reef tank, we have feather dusters I don't want eaten. Thanks.
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I can not identify this crab
Replies (3)
Hi. It will be difficult without a picture (and sometimes even with one), however, I did find a picture online for you to look at. It is listed below. It is the Acropora animal question 9/20/05.
And, although these next two links are not pictures of purple crabs, they do have red eyes. The first is a good picture of the eyes, and the second is a picture of what it looks like out of the hiding spot.
http://mangrove.nus.edu.sg/pub/seashore/text/216.htm
http://www.wildsingapore.com/riablog/photos/040718-19sen/photos/sen04071814.jpg
Hope this helps.
Crab ID
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Phishie
"An ounce of prevention is worth a pound of cure."
Unfortunately neither were the crab I have. I have researched a little more and it seems to be possibly a purple xanthid. Sort of similar to this http://www.chucksaddiction.com/xanthid1.jpg
So far he hasn't damaged any coral or polyps, so I'm leaving him alone, for now. He's nocturnal and very skittish. Thank you very much for the help! If you know anything on xanthids, I wouldn't mind the information. 
I know more about fish than most invertebrates. I did take an invertebrate class, but we didn't go into specifics on all the species under the invertebrate classification. However, I consider myself very good at finding information on the internet. I found you one website on xanthids. I tried to find the scientific name for the purple xanthid crab, but I didn't have any luck, nor could I find anything on the purple xanthid. I would keep a close eye on him to be sure he isn't doing any damage.
Xanthids
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Phishie
"An ounce of prevention is worth a pound of cure."
