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FAQs about Marine Crab Identification 1

Related Articles: Crabs, Hermit Crabs

Related FAQs: Crab Identification 2, Crab ID 3, SW Crab ID 4, SW Crab ID 5, SW Crab ID 6, SW Crab ID 7, SW Crab ID 8, SW Crab ID 9, Marine Crab ID 10, Marine Crab ID 11, Marine Crab ID 12, SW Crab ID 13, SW Crab ID 14, SW Crab ID 15, SW Crab ID 16, SW Crab ID 17, SW Crab ID 18, SW Crab ID 19, SW Crab ID 20, SW Crab ID 21, SW Crab ID 22, Marine Invertebrate identificationMarine Crabs 1, Marine Crabs 2Marine Crabs 3, Marine Crabs 4, Crab Behavior, Marine Crab Selection, Marine Crab Compatibility, Marine Crab Systems, Marine Crab Feeding, Marine Crab Reproduction, Marine Crab Disease, Micro-Crustaceans, Amphipods, Copepods, Mysids, Hermit Crabs, Shrimps, Cleaner Shrimps, Banded Coral Shrimp, Mantis Shrimp, Anemone Eating ShrimpCrustacean Identification, Crustacean Selection, Crustacean Behavior, Crustacean Compatibility, Crustacean Systems, Crustacean Feeding, Crustacean Disease, Crustacean Reproduction,

Hoplophrys oatesii in and amongst and with soft coral stuck on its head. 

Hi Bob,
I have noticed this little critter in my tank over the last couple of days (Image Attached), it is very secretive so it has taken me some time to get a good photograph for I.D purposes. Do you know what kind of crab this is? And if you do, is it safe to leave in my reef tank? I have a lot of shrimps and snails and I have heard that sometimes crabs can be a problem in reef aquariums.
Any advice would be great
Stuart Miller 

Hello Stuart,
Am out travelling, visiting in the Far East (Singapore presently for the Aquarama trade show), and hence away from reference works. I asked friend and fellow WWM Crewmember Lynn Zurik for her help:
Lynn... would you give the attached pic a look/see?
<Sure thing, Bob>
Do you have ref.s re what sort of Decapod this is?
<It looks like a Xanthid in the genus Paractaea possibly a fairly cosmopolitan species: Paractaea monodi (Monod's round crab):
Several photos half-way down the page and another good one at the bottom:
The next closest thing was another Xanthid: Eriphia ferox (aka the red-eyed reef crab). Its similar, but the carapace edges look jagged/spiny instead of smooth.
Thank you, BobF
<Youre welcome. Hope you're having a great time and enjoying your trip! LynnZ>

I also consider that this is indeed a Xanthid (member of the family Xanthidae) crab, and NOT at all reef safe and so would remove it to a sump, perhaps a more rough and tumble FOWLR or fish only setting.
Answer to a mystery query (FW crab) 8/26/05 Bob: <Actually, Sabrina with you, today> In answer to Sabrina's question about the "red crab" on http://www.wetwebmedia.com/swcrabidfaq3.htm from 8/13/05, I believe I have the answer. <My question??  Oh - OH!  I see.  Yes, that was in reference to the surrounding entries ("Mystery Crab") that I was helping Charlotte with.> I too bought what the LFS was calling a "fire crab" (or so it sounded with his slightly different accent from mine) for my nano. I tried Googling this and found nothing. Mystified, I posted a pic of him on reefcentral.com <I would love to see this image, if you can grab me a link.  If it IS an Uca/fiddler, I might be able to get it closer to a species, for yah.  Not necessarily likely, but there are some great references on the web.> and still everyone was baffled. It was when I went to a different LFS that I realized what it is I and others are buying:  A female fiddler crab (my LFS who I thought was saying "fire crab" was actually saying "fiddler crab" but pronouncing it "FIDE-ler" and I misunderstood him).  Some LFS call them red crabs.  Again: orange, 1", walks sideways, black eyestalks, burrows in the sand. <Does indeed sound like a fiddler/Uca sp., as far as burrowing goes.> Mine has proven to be totally reef safe, hiding about 99.9% of her life. I've seen her for maybe a total of 60 seconds in 3 months! The females do not have the typical fiddler chelipeded and so look like something else. <Correct/agreed.> The problem with this is that fiddlers are supposed to be given a land/water environment, as they live in muddy mangrove patches and near the shoreline. Unfortunately, a few seem to find their way into the reef trade, and unwitting people like me and Sabrina wind up with them. <Mm, again, 'twasn't me....  "I just work here" <grin>.  Though I do have a couple fiddlers....  but mine are in a large sandy terrarium with a 2g saltwater swimmin' hole.  They seem to be doing quite well.> I'm sure she'd be happier in a terrarium, but she does just fine it seems in my nano reef. <If possible, you might consider setting up a land crab system.  You wouldn't believe how much fun they are.> Hope this helps! <Thank you very much for this!  And again, I'd love to see an image of your crab, if possible.> Alex <Wishing you and your firefly FIDEler well,  -Sabrina>

Mystery Crab (Again) - 09/01/2005 Hi Sabrina! First off, sorry to you and Charlotte for confusing you! <Oh, no worries!> Here are two pics of my crab when he was in my 5g. Hope they help! <Mm, some, yes....  I still think this is an Uca (fiddler), and it *might* be Uca ecuadoriensis....  though I'm by no means certain.  A very cool little invert; thank you very much for letting us see the images!> Alex <Wishing you well,  -Sabrina><<Am moving this to the marine section, since fiddlers are marine....-SCF>>

Crabby Hitchhiker 7/6/05 Hi Guys <Lesley> I set up a second, small marine tank last year and it is running well.  I got some more live rock a few months ago which had a crab in it, which we couldn't remove so I took the crab with the rock.  We've been trying to spot it ever since to identify it.   It's very shy and hides under the live rock. <Typical> It hasn't caused any problems in the tank (i.e. no missing fish or shrimps!) but I did get a good look at it for a second or two before it ran for cover, for the first time this weekend. It's very beautiful. The closest ID I can come up with is a harlequin crab as it looks reasonably similar to the picture on your website. However, the "patchwork" pattern on its shell is made up of various pinks and reds and whites not just two colours.  It's about and inch in size, including legs, at the moment. Could it still be some sort of harlequin crab?   <Yes> It seems to have an appetite for small Triffid anemones as it had one in it's claw when I spotted it, so it's earning its keep!  I assume it's also eating whatever we feed the other occupants with but does it have any special dietary requirements? <Likely picking up what it needs from your live rock, substrate> Also, if it is not a harlequin crab and you can identify it from my vague description, can you guess at what type of crab it is and what its requirements are? <No my friend> As it is so beautiful, and apparently useful, I'd like to keep it if it's not going to pose a problem needs food I cannot provide it with. Look forward to hearing your thoughts on this and thanks again for your help! <Do enjoy this life, keep trying to make a photo... Bob Fenner>

Unwanted Crab Hello, I just recently discovered a crab in my tank. He won't come out, only his little black claws is all I can see. I just recently put in some coral from Florida and he must have come in on it. I can't really describe him but I am worried that he may hurt my corals and/or fish. I have feather dusters, brown polyps and green center polyps. I have two ocellaris, one damsel and one coral beauty. Is he something I should get out of my tank?  <Without seeing a pic of it, I'd have no way of knowing whether the crab you have would destroy anything. James (Salty Dog)> 

A/the west coast crab? I am a graphic designer and one of my clients wants me to draw a "West Coast Crab". The only thing that I can find on the internet on this type of crab is recipes. Not going to work... <What species is this?> Do you have a picture or drawing of a "West Coast Crab"? They were very specific about this type in particular. <Have no idea... there are several species of crabs that might be termed "west coast"> Thanks for any help you could be. Karen Anderson <Maybe ask your customer to look on the Net, identify what they want to species... Bob Fenner>

Crab ID 1/5/05 Hi guys.  Just wondered if you can ID the crab in this link: http://www.ultimatereef.net/forums/index.php?s=b7cc8850447121a3ea8bd0cad5bfe58f&showtopic=33964&st=0&#entry271590 (Thought it would be easier to send you the link, rather than the pictures!) Cheers! Baz <Id'ing these little rouge crabs is difficult if not impossible, but it doesn't matter.  I almost always take an "innocent until proven guilty" stance with critters, but not crabs.  I don't trust any of them (even supposedly "reef safe" ones).  I would capture and remove this crab as a precaution.  Best Regards.  AdamC.>

Red Mithrax crabs Hi, I wanted to know a little bit more about red Mithrax crabs or some  times called ruby crabs.  I cannot find any info about them.  << In my opinion they are just like green Mithrax crabs, only harder to find. >> I saw  someone on eBay selling them for a good price.  They look exactly like the  emerald crabs but red instead.  I have had emerald crabs in the past that  died.  Also, does anyone know how big they can get? << I'll say about two inches across.  They eat algae, and lots of it. >> I have been told  that they eat algae is that true? I really like the way they look and I think  they would be a great addition to my tank. << They are awesome tank mates, but do eat algae... maybe too much algae. >> Thanks, Mo <<  Blundell  >>

Swimming Crab Pics
Never found any ID on this little guy...he was SO cute.  He came in some maiden's hair algae & was the exact same color as it.  I just bought some more maiden's hair and have at 2 or 3 more in there...they're too shy to come out yet, though.  He only came out when the algae went sexual and died off.
He wasn't afraid of anything in the tank, including me.  Got some short vid of him swimming too...took some effort since he'd rather sit on my hand than
actually swim!
<Interesting... and can use video. Bob F>
Let me know if you want video too.

Looking for key to Persian Decapods dear professor I'm study about fresh water crabs of Iran I need a key for identification pls help me faculty of science Tehran university Iran <Mmm, would have to research if there is such a reference in a college library. Not sure you're familiar with such bibliographic searches. Please see here re: http://www.wetwebmedia.com/litsrchart.htm  Do you have access to BIOSIS on the internet? The Zoological Record? I would start with their databases. Bob Fenner>

Stocking/id questions Hi WWM <cheers> question on stocking. I have a 50gal FOWLR, and a few polyps. presently I have 2 Percula clowns, a bicolor blenny, 2 peppermint shrimp, an emerald crab, various snails and hermits. I want to add 2 Firefish, a cleaner shrimp and a few more snails. do you see any problems with this?  <seems reasonable... some chance with Firefish aggression to each other> the Firefish will be mail ordered, should they be put into a quarantine tank, or just give them a fresh water dip to avoid the stress of moving from 1 tank to another?  <all fish should be quarantined without exception and Firefish really should be purchased locally if possible. They are notorious bad shippers> on another subject (thanks for your patience) I have finally caught my hitchhiker crab. attached is a picture. any idea what it is? is it safe or should I remove it? also attached is a picture of hitchhiker snail. looks like a whelk to me, should it be removed?? the actual snail is a light green colour. Thanks Barry <Barry and all WetWebMedia friends... please send any such pictures scaled down to a smaller 'Net sized image and never zipped (problems here). It absolutely crushes our mailbox with the amount of mail sent. Please resend my friend. Anthony>

Predatory Xanthid crab  3/5/03 Was wondering if you could ID this crab (small pic attached). I have three of them in my reef tank and I believe they are eating my fish.  Thanks for the help. <I cannot ID this crab by species with certainty but merely can say that it is a Xanthid species. Like most crabs, they are omnivorous with a strong inclination to be carnivores. I have no doubt that this creature would eat fish given the chance. Avoid most all crabs in reef aquaria and systems with small fishes. Kindly, Anthony>

Crab Hitchhiker Crew: I have discovered what appears to be a Mithraculus crab in the Indonesian LR in my refugium. Should I be concerned about negative effects? Because of the layout of my system, any attempt to remove it will almost certainly kill it. Thanks, Steve Allen. <If it is truly a Mithrax crab no problem, esp. in a refugium.  Craig>

Re: unidentified crab sans QT 2/26/03 Thank you for your help...I know that quarantining for the rock is a necessary thing for the health of our tank...and we have been a bit lax in doing it lately (hanging head in shame.. G), as over a period of 2 years (and several QT periods) we have had no problems with live rock from this shop (our tank has thrived and grown tremendously)...however, that being said...that is no excuse for not being more careful... <all correct my friend> thank you for the reminder that just because we have not had problems so far...doesn't mean that we couldn't have problems in the future.     <I must admit that I learned my QT lesson the hard way too <G>> As far as the crab holding his claws in front of his face...we have seen no evidence of that.   <perhaps not Calappa then... rather conspicuous> He (or she) seems to keep them either tucked to the side or slightly out and down of the body.  I got a really great look at it's back last night (o.k. it was a 2 AM peek at the tank.. G) and it's back was not as smooth as we had thought...it's very slightly bumpy...and it's body is  squarish in shape as well.  I will send in a pic when I can get one (looks like another 2 am tank peek...LOL) and send it to you.    <indeed the best way for us to help ID for you> We did notice one thing that may or may not help...around where it was at...was a bunch of what appeared to be algae in little balls around it.  Also...I might not have been to clear. but it's  back legs are hairy. but it's pinchers are smooth....if that makes a difference.   <unfortunately not...  a couple hundred possibilities/species> Thank you for answering my e-mail...and I must agree with what a lot of people have wrote about you guys. you are awesome!!! <thanks kindly! It really is a labor of love> Thanks again!  Will start re-searching the shame faced crab today. Lee
<best regards, Anthony>

Crab ID Must of came on live rock any ideas? <Mmm, no... e pluribus unum! Bob Fenner>

Crab Id please? Hey Crew.. I think I've found the clicking source from my tank. <nope... but I'm glad to hear your optimism :) You still will hear the clicking. Quite likely a small pistol shrimp from the live rock or sand> Think this is it? <Nope> Some kind of crab? <yep... this image shows a Mithraculus (AKA Mithrax or Emerald Green crab). They are good algae grazers but dubiously reef safe in the long run> Thank you!
<best regards, Anthony>

Re: any idea what this crab is? Hello,   I caught this guy in my reef tank after he was pushing around a coral.  He has also eaten two small clams.  I won't go into details about him eating the clams, unless you would like me to. His shell is about 1.5". <Nice photo. Don't know the species, but it's likely a member of the family Xanthidae. Please see here: http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm and the linked FAQs (in blue, at top) beyond. I would remove this crab. Bob Fenner> thanks in advance,
Keith MacNeil

Re: any idea what this crab is? Bob,   Thank you for your response.  I did remove him and put him in a specimen cup with a lid and holes in my grow out tank.  Now I just have to figure out what to do with him. thanks again, Keith <Maybe your local fish store will take "him" in... and find a home that's suited. Bob Fenner>

- Mystery Crab - Hello WWM <Hello, JasonC here...> I yesterday I emailed you guys about a crab that I had found as a stowaway on some live rock I recently purchased. <I do recall.> I just wanted to send you some pics of him so you can maybe help me identify him. <Well, it's safe to say that is a crab.> Cuz I have no clue what he is. <And neither do I.> Maybe you can help me. Thank you for your time. Matt
<Cheers, J -- >

Killer (Xanthid) Crab Good morning Crew, Can you please try to I.D. this white little monster that's making a meal of my blue starfish. I never see him during the daytime, but last night, I placed a few silversides in a nylon stocking & set it next to his hole. Within minutes, he pulled the bait into his hole. I ended up playing tug of war w/him & was able to retrieve the bait. Any ideas on how to catch him. I think he has been killing/eating my smaller fish (Firefish, Bluespotted jaw & small chalk bass). Thanx in advance for your time Craig <This is a Xanthid (Black-Finger Crab, family Xanthidae) of some species... and it should be removed, post haste. I would bait or trap this predator out. Insight on how to do this is posted in a few places on our root web: www.WetWebMedia.com. Please either use the Google Search tool there (on homepage or indices) or go to the Marine Crabs FAQs sections. Bob Fenner>

Crab ID and trapping I just got an id on a crab that hitchhiked on my LR from Tampa Bay SW, Gorilla Crab.  I can't find anything on this site, virtually nothing on the web.   <that's because somebody made up that common name, bud... and there are no popular references to it commonly.> Do any of you know the scientific name of this brute?   <you'll need to send us a photo, my friend. Something more than a common name> I know he's a bad guy and I want to get him out of my main tank, <exactly> but I'm having a hell of a time catching him. Thanks, Mike <try leaning a small glass jar (like for pickles) in the aquarium against the rockwork with weighted meaty bait in the bottom. The crab will scurry down into the jar for the food at night but to be able to scurry back up the slick glass. Best regards, Anthony>

Crab ID Hi guys, I have an ID problem with a true crab. I’d love to send you a picture but I can’t. This guy is about 3/4" across the carapace, very hairy/bristly, coloration seems to be purple and white especially on his legs, banded in these colors. He is a hitchhiker on my FL Keys LR. I have several other crabs, all much smaller (half that size for the biggest) which appear to be of the "Sally Lightfoot" variety, although I’m not convinced that this ID is accurate either. Anyway, as for the big hairy guy, his claws are disproportionate with the left being dominant. His claws look just like the Stone Crab claws, which you may have eaten. (I may grow this crab to maturity just to harvest a claw. :) I plan to eventually have a couple of seahorses in this tank. Is he (he IS a he) safe for my horses (4"-5") or should I remove him to my refugium to fight it out with the Mantis Shrimp (also a hitchhiker) that lives there? <I would remove.> BTW, all have of my crabs have molted since I've had them, 2 months. I'm assuming that's a good thing. <It means they are growing.> Any idea on the ID? <No, but better to be safe than sorry.> Sorry, I know it’s vague. All these crabs seem to be algae eaters; but I know that all crabs are opportunists. <Yes, very opportunistic and seahorses are slow and would be easy prey.> Thanks, Mike <You are welcome. -Steven Pro>

Crab identification Greetings Bob, A follow up to a previous email you answered regarding a stowaway on a piece of LR I recently purchased: here are 3 of the best pics I could get...he is a shy one. The next pic shows the yellow tips and "hairy" legs, claws are the same left and right neither is larger than the other). you can get a decent idea of them from the picture above. a correction to my last email, the tank has been up for 2 months but the crab arrived with a 29lbs chunk of LR 2 weeks ago. As far as I can tell he only seems to be eating detritus from the sand, but I haven't been able to observe him with the lights off. Any help in identification would be greatly appreciated! <the crab cannot be identified by species but the photo clearly depicts this specimens large crushing and hooked claws (in contrast to small pinching/algae cropping claws). As they say... "Form Follows Function". This crab is clearly an omnivore leaning toward predator with the hooked claws. It may behave for 10 days or 10 months... but rest assured that it will attack something desirable in time in the confines of an aquarium. Do remove (to sump, QT, refugium, or elsewhere)> one more thing, in the first picture you can see a discoloration on the anal fin of the Flame Angel. It is actually a clear spot on the fin about 1.5mm in size...almost completely transparent. The Angel has been in the tank 2 weeks, with 2 weeks in a Q tank before that and seems very happy. Any clue to what this may be? <no clue from the photo but does not yet sound pathogenic. Feed well and observe for changes> Thank you very much, Emerson <best regards, Anthony>

Crab identification Hi there! <Howdy!> Just when I thought it was safe to start adding new live rock to my tank another crab showed up last night. UGG! Just the body is 1", there are eight legs and two claws (of course claws *chuckles*). Anyway, I just walked in to look at the tank and there it was. ACK! Okay, break down half the tank and now have it quarantined. <very wise> My original base rock (that's been such a pain) is from an eco-conscious place in Florida with a lease 1-2 miles off shore. (All my other hitchhikers came from this rock, which is why I assume Florida.) I was wondering if you could suggest some good links for identifying Florida crabs or might have some idea what this crab is.  <yep... Humann and DeLoach "Reef Creatures"> I've been searching the net since last evening but am not having any luck in finding the type of aquarium crab links I need. <in terms of "reef safe"... no research is necessary: no crabs are long-term safe in reef aquaria. With only rare exceptions, most all are opportunistic scavengers... even "safe" crabs like Mithrax have been observed killing fishes, "reef" hermits killing snails, etc. I'm not one for absolute rules... but "No Crabs" is close to being valid> The crab seems to be more of a mottled dark brown than black, on a white background. Its body is oval shaped, smooth and the legs are very compact to the body. I've attached some pictures that I hope will help. <quite frankly... the pictured crab does not look especially threatening to me. Large smooth carapace, small claws, etc... indicates a likely nice guy to me. Still... now that its out of the tank... leave him out. Perhaps remit it to a refugium or sump. You can also tempt it with meaty foods to see how much of a carnivore it is and offered algae at the same time. Best regards, Anthony> Any help much appreciated!! I actually don't mind the way this crab looks, but after past hitchhiker crab experiences in my reef call me a bit paranoid. :-) CJ
Le Crab Fantastique! Dear Crew: Please see the attached jpeg. I found this crab in my refugium. Can you tell what it is? Is it a problem? Thanks, Steve Allen. >>Hi Steve.  Not knowing it's size I can only venture a guess.  It appears to be quite small, and from what I can make out the pincers are small (relative to body size) as well.  Since it appears to be covered with some kind of algae or sponge (maybe even detritus) I would treat it as a small hitchhiker who will likely remain small.  At this point I would consider him a benefit, but certainly watch for nipped sessile inverts and such.  If the thing seems to grow at any appreciable rate, then I would remove him to the refugium (assuming you have one). >>Also, I know it's about impossible to make an I.D. without having a "crab bible" handy (it's why I can't make an I.D., though someone else in the crew might be able to), but I will link you to our invertebrate library at--> http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm >>Good luck!  Marina

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