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FAQs about Pinnotherid, Pea Crabs

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Hitchhiker Crab Identification and Relationship: Pea Crab on Sea Apple – 5/30/12
<Hello Kevin and Amber, Lynn here this evening.>
Two days ago we adopted a Sea Rose, more specifically I believe to be Pseudocolochirus violaceus.  His tank is currently (and will remain) fallow. 
However, he appears to be distressed and we noticed a friend clinging to him and even appearing to move him about.  By distressed, he is shedding a mucous like coating and has some ulceration near the oral cavity.
<Not good>
My questions are first, can you identify the crab?
<Not to the species level, but I’ve seen this little fellow before.  It’s a Pea Crab of some sort, (Family: Pinnotheridae - many genera/species).  Please see the FAQ titled “Sea Apple Hitchhiker: Pea Crab - 3/21/08” at the following link for more information and a photo for comparison:  http://www.wetwebmedia.com/Arthropoda/CrustaceanPIX/SWCrabs/Crab%20IDs/swcrabid10.htm  >
Secondly, is this likely a symbiotic relationship of some sort
<I don’t believe so, no.  I think the relationship (in the wild and under normal circumstances) falls more under the heading of commensalism.  That is, one organism (the crab) benefits while the other (the Sea Apple) is neither helped nor harmed to any significant degree.>
…or is the crab most likely stressing him out?
<I think that at this point it might well be adding to the Sea Cuke’s stress so I’d separate the two.>
We have searched, read, and learned a great deal about this Cuke from your website. However, these questions remain. Of course, I am hesitant to move the crab until we can figure out what is going on.
<I can understand that but at this point I’d separate the two.>
<You’re very welcome and good luck.>
Kevin and Amber
<Take care, Lynn Z>

Re: Hitchhiker Crab Identification and Relationship: Pea Crab on Sea Apple – 5/30/12
<Hello Kevin, Lynn here again.>
The attached images were taken about 9 hours after the ones I sent in my last email. As you will note, there are some substantial changes and white areas.  Any thoughts?
<My guess is that it’s been injured and is deteriorating.  The fact that it’s happening quickly is not a good sign.  Hopefully, with good water parameters and getting the crab out of there (in case it’s picking at/irritating those areas), it’ll at least have a chance to recuperate and survive.   On the plus-side, at least the Cuke is in a tank by itself, so if it does decline quickly overnight or tomorrow, it won’t nuke any other inhabitants.>
Water parameters are 0 Ammonia, 5 nitrates, 0 nitrites.
Again, he was placed here about 2 days ago. The crab does not seem to have been near some of the areas that are white; however, we have not been watching constantly the last 48 hours <grin>.
<Heeee! You mean you haven’t watched it every second of every hour?  Seriously though, my concern with the crab is that it might be further irritating the Cuke when you’re not around.  It could be picking at the dying tissue.  At the very least, the last thing the Cuke needs at this point is further irritation.>  
<You’re most welcome.>
<Take care, Lynn Z>

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