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X is for Xanthid crab

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I created this thread to consolidate my xanthidae crab notes and photographs. My husband found this crab when dipping his new coral acquisitions one night.At the time it was smaller than the end of a pencil eraser.I took it for a Acro crab and acclimated it to my main display.Where he promptly moved out of the acropora and disappeared.Weeks passed by and I didn't think he had made it.Then I found one of his molts.Apparently he was well and growing at a rapid rate! Once in a while I would spot another glimpse of him.Usually all that I could see was just a tiny claw at work. Over a period of a year I would take pictures whenever I got the chance.This was rather difficult because he would completely disappear whenever he noticed me.Eventually I was able to capture the photo below.In doing so I was finally able to identify him as a smooth spooners xanthide crab.




The very best picture I have available at this time.In this photo his head is upright and the claws(not visible) are tucked away.Notice the little "Last Airbender" arrow at the top of his head.Also, there is a standard plastic frag plug to the right of him for size reference.





Generally speaking the xanthid crabs are considered problematic.* They are not considered reef safe.Fortunately ,I have not noticed any remarkable damage.I do see him perch on corals.He will pick at the tiny polyps on SPS such as pocillapora and Montipora.However, I do not not believe that he is actually eating the coral.Rather,he seems to be foraging in a random fashion as the polyps retract.As for the safety of the inverts I may never know.I don't count my hermits,stomatella snails or brittle stars.The larger snails including the nassarius are accounted for.


He does have a good appetite for algae.So reliably that if I place a hairy frag next to one of his areas,it will be polished clean overnight.This leads me to believe that this variety is primarily nocturnal.Always secretive. Here all some of the numerous photos I took.Now most of these don't look that great.That said,they will illustrate just how elusive these critters are.So I have included them for educational purposes.















This guy is extremely agile and moves in a quick calculated manner.He has been seen clinging upside down in tiny crevices and at awkward angles.Very rarely do I see the full caprice.Often all that is visible is a rock or coral shifting as he wedges his way around.







I am pleased to have the opportunity to view this invertebrate in person.I hope for now I have shed some light on these notorious creatures.If you have any thought or experiences regarding these animals,please feel free to share them here.







Helpful link*



Edited by kireek
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Very cool, I wish there were more write ups like this on some of the more secretive reef life that I am missing. Sometimes I just overlook some of the little things. Thanks for sharing, and keeping a watchful eye over him/her, the crab could've been "sterilized" in the coral dip.

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I read somewhere that they might actually help coral out more than harm them. But can't find where I saw that....

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This behavior sounds like all my emerald crabs. I put them in my tank and they were around for a few hours and I only see them when I am moving rocks around. I actually thought they were all dead because I hadn't seen them for many months. Yesterday I seen one in a little hole in a rock I have. I bought 5 and I've found one empty shell so I could have 4 or only the one I seen.

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