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>

Cyanobacteria 5/25/09
Dear Bob, Mr. Calfo,
<Hello, Scott V. with you today.>
I have big problem with Cyanobacteria which has spread out on my Acropora on peaks, I am trying to blow them with turkey baster everyday, but it seems it will not want to leave. When I am blowing Acropora's peaks-end also some part of tissue is damaged and coral is losing Zooxanthellae. I have tried to bath coral in tropic Marin pro cure but Cyano is retrieving back.
<This will not do it.>
Cyano was caused by my mistake that I have put china MH bulbs instead good ones. Please help!
<Well, in the end there are other fueling factors behind the BGA besides the light, see: http://www.wetwebmedia.com/bluegralgae.htm
There are other factors at play. As for your corals, you will need to figure out what is causing the BGA to be so competitive. Until then keep blowing off what you can.>

Re: Cyanobacteria 5/26/09
Ok, thank you so much, I will keep trying.
<Welcome, this battle can be won! This is a mild case, you have caught it early. Scott V.>

Pistol Shrimp Hitchhiker: Coral Commensal -- Coralliocaris spp. 5/24/09
Hi Guys, Janet here.
<Hi Janet, Lynn here today>
I bought a colony of Acropora last week and have found 2 hitchhikers in it.
<Neat!>
I know that Acropora generally will come with an Acro crab, but these little guys are shrimps.
<They sure are. They're small coral commensals, in the genus Coralliocaris (only around 8-9 species recognized at this time). Unfortunately, I was unable to find photos of each species for comparison, but I did find one that looks similar enough to be a definite possibility. This species, Coralliocaris graminea, varies in color, but has the same thin, longitudinal lines on the body, as well as orange-ish tipped legs and claws. Please see the following links for comparison/more info:
http://tolweb.org/onlinecontributors/app?page=ViewImageData&service=external&sp=26879
http://www.vanaqua.org/aquanews/field/images/yaqara.htm
http://www.seadb.univpm.it/en_Green-Acropora-commensal-shrimp-Coralliocaris-graminea_587.htm
Also, if you happen to have Helmut Debelius' book, Crustacea Guide Of The World (2nd edition), see page 189. Reportedly, this Acropora commensal reaches about 1cm in length and is indigenous to the Indo-West Pacific region.>
They were inside the Acropora colony, and I managed to "spook" them out and get them into a container for a picture and to make sure they were reef safe etc. I am attaching a picture of one of the two.
<Super, thanks>
The other one has no claws, but is identical other than that. The 2 of them must have had a tussle and one ended up with no claws out of the deal?
<Perhaps yes, or it could have been due to rough transit/handling when collected, shipped, etc.>
I currently have them in a 10g emergency tank. Do these guys eat Acro?
<Not according to what I've read. They're commensals that likely have little negative impact on the colony.>
It is even a pistol shrimp?
<No. Although these shrimps do use their claws to make clicking/snapping noises, they're not actually what we consider pistol shrimps (family Alpheidae). They belong to another family: Palaemonidae, subfamily Pontoniidae. This group of shrimps doesn't have the same formidable snapping ability of the Alpheids. Also of note is that the claws of Coralliocaris species are matched in size, whereas those of the Alpheids are markedly different. For more in-depth info on snapping claws, please see this pdf file: http://decapoda.nhm.org/pdfs/27238/27238.pdf . It's probably more than you ever wanted to know, but it's interesting!>
The Acro colony has had some die off which occurred after getting it home a few days later.
<Can happen sometimes>
I have another smaller colony of Acropora, but this piece is fine.
<Excellent>
Because of the die off I started taking a look at the colony to see if I could determine what may be happening to it. This is when I noticed these little guys. I actually managed to nab both of them in my 75 gallon tank without losing them and then moved them to the 10 gallon tank. As you can see by the picture of the shrimp in my hand, they are very tiny.
<Yep, that's typical for these guys. The genus as a whole generally ranges in size from 1-2cm.>
This is a picture of the larger one too. Can you provide any information as to what they are, and if they are reef safe or not. Do they prey on Acropora only or any Sps coral?
<I haven't come across anything indicating that they're parasitic, or harmful to corals at all, so you should be okay.>
Thanks, Janet
<You're very welcome. Take care, LynnZ>

Re: Tiny White Crab on Montipora capricornis: Gall crab - 4/8/08 Lynn, <Hi Andy> Thank you for the response. <You're most welcome.> Hmmm, hard to tell. The claws definitely were NOT noticeable. In fact, I couldn't see them at all--the only things I could see were the 4 legs on each side. It resembles the Cryptochirid, but doesn't look like an exact match. <It may be something else within the Cryptochiridae family. Unfortunately, those were the only two online examples I could find of the different genera within that family. Depending on where you look, there are anywhere between 6 to 20 different genera and beyond that, a whole slew of species so there are lots of possibilities.> I'm just glad it likely was not harmful to the coral. <Me too!> The marine world is a strange and fascinating one. <It is indeed! Take care, -Lynn>