Dear Bob

About 4 months ago I bought a Large Yellow Acropora frag inside the frag there where two small crabs ( Dark reddish brown with white eyes and about 6mm across the carapace ) the coral seems fine but I have noticed that the polyps don't come out all the time when the crabs are feeding.

My worry is, are they feeding on the polyps? The reason I ask is I just got a blue Acropora and one of the crabs was on it. The next day it was back on the yellow one but the blue one looks patchy.

Mmmm, well, there is documented proof of some of the commensal crabs being a bit more than just space hitchhikers. That is, they are known to nibble at living tissue of their stony coral hosts. But on the positive side, these crabs, particularly the better-studied Trapeziids, are known to protect their living coral homes from predators and sediment accumulation. It's not lost on me that the relationship twixt these two is so intimate that the crab species aren't found elsewhere'¦ and even if their symbiotic relationship trends toward the decapods being a bit parasitic, all the instances I've encountered in the wild have shown both parties to be in good health. I suspect that as long as conditions are propitious for them both (a bit of food provided for the crabs daily) that your Acroporas aren't in imminent danger. Reef coral studies have shown that the few polyps lost are a well-worth trade-off for protection and house cleaning services. I would leave these two together.

Acro Crab? Possible Trapeziid or Juvenile of Undetermined Family -- 3/28/10
<Hi Mike, Lynn here today.>
Just found this little crab on a new shipment I just received.
<Neat! What kind of shipment -- coral (Acropora, Pocillopora, Stylophora, etc.?), rock, or something else? Where was the shipment from? What's the size of the crab (carapace width)?>
Hoping you can confirm whether of not it's an Acro crab?
<That's a tall order! Did it hitchhike in on an Acropora colony? It could be a coral crab, or it could simply be a juvenile of some other variety. It has the same large eyes, roughly triangular carapace, and claw length/shape that you see in many Trapeziid crabs (family Trapeziidae -- aka 'coral crabs' or 'coral guard crabs'), but the color/pattern of the carapace is atypical. That is, it doesn't fit with what photos I've seen of Trapeziid crabs. The problem is that there are an awful lot of crab species out there, not all of which are available as photos on the web or in my research books. What you have may be a coral crab that in its juvenile phase has a completely different, more cryptic coloration, than its adult counterpart. This is not at all unusual in crabs, or other animals for that matter. Unfortunately, I haven't been able to find any photos of juvenile Trapeziid crabs for comparison. Again, there's the distinct possibility that this little crab could simply be a juvenile from any number of other families. Juvenile crabs typically have the same large eyes that you see in Trapeziids. The bottom line here is that unfortunately, all I can offer are a whole lot of maybes and no concrete answers. Time and behavior will tell with this little fellow. I can tell you one thing though. Trapeziids are generally found in the Pacific, either the Indo-West or tropical Eastern regions. If this little crab came in on a shipment from say Florida or the Caribbean, chances are it's not a Trapeziid. Please see the following link for an example of a juvenile Callinectes sapidus, aka 'blue crab' (see last photo). This is definitely not what you have (and not a variety of coral crab), but you can see the similarities: http://www.serc.si.edu/education/resources/bluecrab/lifecycle.aspx
Here are some examples of coral crabs, along with more info (see bottom of page): http://www.chucksaddiction.com/hitchcrabs.html
More info at WWM: http://www.wetwebmedia.com/Arthropoda/CrustaceanPIX/SWCrabs/Crab%20IDs/SWCrabs3.htm >
And good/bad?
<Time will tell. This could be a relatively harmless coral crab, or something else entirely. Since I can't confirm either way, I'll offer the standard crab warning. That is, crabs are omnivorous opportunists and scavengers that, if hungry enough, can and will eat pretty much anything they can grab with their claws. You have the choice of keeping, observing, and removing if/when the crab becomes a problem or playing it safe and putting it elsewhere until you have a better idea what it is and how much risk it poses to other livestock. Either way, do be sure to keep it well-fed with meaty scraps of marine origin (shrimp, clam, squid, fish, etc.) or perhaps sinking pellets.>
Attached a pic for ID.
<Thanks, he/she certainly appears to be an itty-bitty thing!>
Thanks!!
<You're welcome!>
Mike
<Take care, LynnZ>
Re: Acro Crab? Possible Trapeziid or Juvenile of Undetermined Family -- 3/28/10
Lynn,
<Hi Mike>
Thank you for the response.
<You're very welcome.>
He was attached to a "purple monster Acro" from either the Solomon Islands or Fiji, my supplier only specified vaguely where all our corals came from, one or the other.
<Thanks, that helps. I just wish that it was enough to be able to offer you a positive ID! As it stands right now, I'm not sure what type of crab this is. As was evident in the photo of the blue crab (see link in previous response), appearance can change significantly from juvenile to adult. We should know more as this little fellow matures. Honestly, it could be one of many different genera (not just in the family Trapeziidae) that are associated with Acroporids. It could also be an accidental hitchhiker; that is, not a coral crab at all. Perhaps it was ousted from its normal habitat during collection and simply took refuge within the coral's branches.>
I've attached a not so macro pic for a naked eye view as well. He is tiny, maybe 3-4 mm.
<Yep, that is one small crab!>
Thanks again!
<It was a pleasure! I'm just sorry that I couldn't give you a more definitive answer.>
Mike
<Take care, LynnZ>