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FAQs about Marine Crab Identification 13

Related Articles: Crabs, Hermit Crabs,

Related FAQs: SW Crab Identification 1, SW Crab ID 2, SW Crab ID 3, SW Crab ID 4, SW Crab ID 6, Marine Crab ID 7, Marine Crab ID 8, Marine Crab ID 9, Marine Crab ID 10, Marine Crab ID 11, Marine Crab ID 12, 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 identification, Marine Crabs 1, Marine Crabs 2, Marine 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 Shrimp, Crustacean Identification, Crustacean Selection, Crustacean Behavior, Crustacean Compatibility, Crustacean Systems, Crustacean Feeding, Crustacean Disease, Crustacean Reproduction,

Re: Crab ID - 3/20/09
<Hello again>
The crab came in my live rock whenever I got it. My live rock is premium Fiji rock.
<That's great, thanks.>
The crab is about an inch from legs to legs.
I have a few pictures of his claws and I'm still trying to get the top view picture but he looks just like a pebble.
<Yep, that shot can be a challenge when the crab is still in the tank. Sometimes you get lucky, though.>
You can't see his legs or claws from the top.
I've attached a few different angles of his claws.
<Thanks, he's a feisty little thing isn't he! The good news is that the new photos show a bit more of the body and legs, which gives us a few clues. Given the theory that form follows function, those heavy claws appear to be well adapted for crushing. That is, this is likely a crab that includes various bivalves/clams, other sessile organisms, crustaceans, possibly gastropods/snails, etc in its diet. Thankfully, it's not a crab built for speed, so it's not as much a direct threat to fish as say, a Sally Lightfoot (Percnon gibbesi). That's not to say, though, that it's no threat -- especially if/when it gets larger. If the opportunity arises for it to nab a sleeping fish, it's hasta la vista baby.
Unfortunately, I'm still unable to narrow the ID down to species level, but I'm thinking what you have is a Xanthid of some sort (family: Xanthidae). I've got a terrific link for you to look through which may enable you to narrow things further. It lists tropical crabs of the central Pacific region/French Polynesia. Fiji is a good bit west, but it's the best I can do as far finding online photos of crabs in that general area of the world. The main thing is that it will give you a good opportunity to see and compare various genera within the family Xanthidae. Look through the photos and see if you can find features in common with your little crab. I saw several that looked similar, but I'd have to have the crab right in front of me to take it any further. Just keep in mind when you're looking through the photos that color can vary quite a bit. The main criteria to look for include carapace shape/depth/texture (is it smooth/rumpled/hairy), whether the edge of the carapace is smooth, wavy, or if there are any spines/protrusions present (if so, how many), eye spacing, length/shape of claws (are there any spines/other protrusions present), leg shape and how hairy (if at all), and overall body posture (how and where the legs/claws are positioned/held in relation to the carapace). As you can see, there are quite a few features to look for! I'm hoping though, that you'll be able to determine at least the genus of your little crab. Here you go: http://decapoda.free.fr/search_result.php?submit32=Submit&spname=&auname=&yename=&gename=&faname=
xanthidae&ioname=&medium=&Substrat=&ile=&vertical=&pmin=&pmax=&auteur= >
Thank you
<You're very welcome. Take care and good luck! Lynn>

Re: Crab ID - 3/20/09
<Hello again!>
Thank you so much.
<It was a real pleasure.>
I love the wetwebmedia.com website. It is the most informative site I've come across.
<On behalf of Bob and the crew, past and present, I thank you!>
I use it all the time.
<Me too!>
The closest thing I found to the crab is an Eroded Crab.
<Neat. The common name Eroded Crab, or Eroded Mud Crab, is usually attributed to crabs in the genus Glyptoxanthus, a Xanthid with an all over rumpled/bumpy texture. Even the legs and claws are heavily textured. It's an odd but neat looking little crab.>
Again thank you for all of your help, it's much appreciated.
<You're very welcome. Take care, Lynn>

Crab ID please if possible 3/1/09
Hi folks,
Long time reader of your fabulous site and as I am starting to go bog eyed searching for info on a crab I've found, I decided an email was worth a shot.
In my coral tank, we found the attached crab and would like to know essentially if its safe to go back in there, or leave in the sump, or transfer to my fish only.
Coral tank ( with live rock from Fiji I think) (400 litres) has various SPS and LPS corals in, pair of common clowns with a Malu Anemone, mandarin dragonet, yellow tang, coral beauty, saffron coral goby, yellow goby with shrimp friend, magenta Dottyback, cleaner shrimps, blood shrimps, peppermint shrimps (with an allergy to Aiptasia!), 2 Sandsifter starfish, 2 emerald crabs and 2 scarlet leg hermits. Plus of course a few snails.
We haven't lost anything and the little stripey crab (carapace is about half an inch across) just seems to graze but I'm aware of the potential for problems if it isn't really suitable for the tank. I'm also slightly concerned the emeralds are a future problem waiting to occur. Any advice on this and any clue on my newly found crab?
<Only a guess... to the family... the body and claw-shape... looks like a Grapsid to me... only time can/will tell whether it gets too big, too adventurous for you to allow in your main display>
Oh my Fish only (with live rock) (500 litres) has 2 bubble tip anemones, 2 tomato clowns, majestic angel, blue cheeked goby, purple tang, blue spotted puffer fish, 2 hovercraft cow fish,
<These last two might well consume this crab during molting>
long nosed butterfly (allergic to Aiptasia...), regal tang, sally lightfoot, bristle starfish, blue banded hermits, a huge rock hermit, 2 sand sifter starfish.
Many thanks in advance,
Steve Witty
<I'd take a look, see, enjoy approach here with this Decapod. Bob Fenner>

Crab id question 2/22/09
I'm hoping to find out what type of crab this may be. He probably came in with live rock from around the Barrier Reef and has been seen in the tank for the last 2 - 3 months. In that time he has grown at least 50% bigger and is now a size where his main body excl legs is about 30-35 mm across.
<I see>
The tank is 100 litres and has a few fish, Trochus snails and smaller hermit crabs (which he definitely regards as edible!).
<Oh yes>
Apart from that nothing has gone missing, although something is nipping at the Xenias. He is very shy, generally he moves about slowly picking at the rocks and does not like to interact with the fish. He is fond of shrimp pellets, and he can move really fast when anything approaches (so he's very hard to photograph, sorry about picture quality). As he grows his main claws seem to be fattening up and going blacker on the tips.
<I see this as well>
Overall he's a kind of beige/yellow but his (or her) body is covered with sort of little tufts of darker hair or spines. Haven't seen its eyes. I have a concern that as he grows it could become a problem but at present there's no huge problem...just some nervous hermits!
Regards and thanks, John
<This appears to be a Round Crab (family Xanthidae) of some sort... by the carapace shape and darkened claw tips... I'd keep your eye on it... will become more predaceous, opportunistic with time. Bob Fenner>

Crab ID: Mithracid - 2/13/09
<Hi Jack, Lynn here this evening.>
The attached pics are of a crab that came with a piece of live rock. It is less than an inch in size and looks to me as the same shape as an emerald, but as you can see it is red.
<Yep, it's fellow clinging crab in the same family (Mithracidae), most likely Mithrax hispidus or Mithraculus forceps. Please see the following photos for comparison.
M. hispidus: http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/Mithrax%20hispidus.jpg
M. forceps: http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/Mithraculus_forceps_S806.jpg
http://www.wetwebmedia.com/Arthropoda/CrustaceanPIX/SWCrabs/SWCrabs2.htm >
Is this one that I should get rid of? I have mostly a fish only tank but I do have some soft corals and some Zoanthids.
<I'd leave it, unless/until you see evidence of damage/predation. No crab is completely reef-safe but Mithraculus/Mithrax spp. have a better track record than most. He'll probably spend most of his time picking at bits of algae and leftover food here and there on the rocks and about the tank. You can also supplement the food supply though, with meaty bits of marine origin or sinking pellets. Keeping the crab well fed will help lessen the chances of him getting into trouble. For more information on these crabs, please see the following link: http://www.wetwebmedia.com/mithraxfaqs.htm >
Any information will be very appreciated.
<You're very welcome. Take care, Lynn>

Marine Crab ID -- 1/30/09
I've searched through all 20-some-odd pages of marine crab ID on WWM and can't find these crabs anywhere, so I thought I'd give you a shout. Can you help me ID these crabs?
Big Crab:
Little Crab:
<Mmm, my best guess is the Portunid: Leptodius sanguineus, but let's see
what Lynn comes up with. Bob Fenner>

Marine Crab ID -- 1/30/09
<Hi, Lynn here this evening>
I've searched through all 20-some-odd pages of marine crab ID on WWM and can't find these crabs anywhere, so I thought I'd give you a shout. Can you help me ID these crabs?
<I'll sure try. Your 'Big Crab' looks a lot like a Xanthid called Atergatis floridus. Please see this link for comparison: http://www.flmnh.ufl.edu/reefs/guamimg/crustacea/xanthidae/Pages/Image2.html
More photos at the following link (shows variation in color, etc):
The 'Little Crab' could be a coral commensal, perhaps something in the genus Tetralia. See examples here: http://calphotos.berkeley.edu/cgi/img_query?stat=BROWSE&query_src=photos_fauna_sci-Invertebrate&where-lifeform=Invertebrate&where-taxon=Tetralia+cinctipes&title_tag=Tetralia+cinctipes
These are my best guesses. Unfortunately, that's about as close as I can get without knowing where the crabs are from or having them right in front of me to see all the little details. There are just so very many species of crabs out there that details make all the difference! Take care, Lynn>

ID Please: Likely Mithracid or Pilumnid Crab -- 1/24/09
Hello Bob
<Hello Harald, Lynn here this afternoon.>
My Name is Harald, I come from Germany.
Maybe somebody can tell me what crab this is?
<The flattened tips of the claws are typical of Mithraculus/Mithrax spp. crabs (family: Mitracidae). However, I've occasionally seen similar in the family Pilumnidae. Please see the following links for examples:
Family Mitracidae: http://www.nhm.org/guana/bvi-invt/bvi-surv/crab-g03.htm
Family Pilumnidae (towards the bottom of the page): http://www.nhm.org/guana/bvi-invt/bvi-surv/crab-g06.htm
Thank you beforehand.
Liebe GrüÃe
< Gern geschehen! Lynn>
Copyright issue with query photo
Hi Bob,
I just replied to a query with an attached copyright photo that does not appear to be that of the querior (Harald). The title is: ID Please: Likely Mithracid or Pilumnid Crab - 1/24/09. The copyright name on the photo is "rieros Miniriff". I looked the person up on the 'net and it appears to be a Thomas Tögel : http://riero1971.de/html/impressum.html ). It's possible that Harald received permission to use the photo, but who knows. Anyway, I just wanted to give you a head's up.
Thanks and have a great weekend!
Ahh, thank you Lynn. Will be sure to not post it/this. Cheers, BobF.

I.D. For Strange Hitch-Hiking Crab Or Spider -- 01/17/09
Thank you for your great site.
<<Quite welcome>>
I work at a LFS here in Hawaii,
but have only been keeping salt water tanks for about a year and 1/2. Recently this 8 legged crab/spider (?) showed up.
<<I think I've seen these in the rocks along the shoreline around Kona on the Big Island. It looks to be Percnon gibbesi, the Nimble Spray Crab (see here, down the page a little more than half: http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm ). And while not the true Sally Lightfoot Crab (Grapsus grapsus), it is often seen/sold in the hobby under this common name>>
His legs are banded with yellow and the body looks to be almost clear except for some fluorescent greenish-blue markings. The legs are 3 times as long as the body. From what I can tell he mainly stays on one live rock I have and probably came in on it.
I picked most of these rocks from near the shore here.
<<Mmm'¦ I didn't think it was legal to collect live rock in Hawai'i. Last I knew, one could only gather 'dry' rock from above the high-tide line, and then only take what a 5gal bucket will hold, per day>>
I looked all over your sight and also tried some other sites. I thought he might be a sea spider but he is about 2 inches in diameter already, I didn't get the impression from your site that they grew that big.
When I first noticed him, he was only about 3/4 of an inch. (He seems to mind his own business but once in a while when another one of the inhabitants gets close to him he does seem to move aggressively towards them.) Hope the pictures come through.
<<They did>>
Thanks for any help.
<<Happy to share>>
I often peruse your site for helpful answers to the questions the customers ask.
Any I.D. or info would help.
<<Take a look at the link and see what you think. Regards, EricR>>

I.D. of Crab -- 01/17/09
Hi WWM Crew,
<<Hey Mary>>
I just sent you an email a few minutes ago and was checking your website and I believe I found the crab.
<<Ah yes, I believe I just responded to that query>>
It looks like the image of Percnon planissimum (Flat Rock Crab) you have pictured.
<<Hmm'¦ I still think it looks more like Percnon gibbesi in the photo just above P. planissimum (compare the markings on the legs/carapace with those in your photo)>>
Could you tell me about them?
<<Opportunistic omnivores'¦ Large specimens can be a threat to small fishes>>
I have a FOWLER tank that contains a small coral beauty dwarf angel, a small raccoon butterfly, a bicolor blenny and a scooter blenny (also a Lysmata amboinensis cleaner shrimp). Anyway, is this Flat Rock Crab alright to keep,
<<I wouldn't (not a big fan of most crabs for their too opportunistic nature), but that is up to you>>
he is extremely quick.
<<It is probably not a big threat right now to anything listed here other than maybe the shrimp. But that will likely change as it gets larger. Some of the literature on the NET lists these as fairly small species (4cm), but I could swear I've seen them around the Big Island with carapaces that would measure two or three inches across>>
He seems to be getting plenty to eat and has probably doubled in size since I first noticed him 2 months ago.
<<Do keep an eye on this>>
Thanks again.
<<Cheers, EricR>>

Re: I.D. of Crab -- 1/19/09
Hi Eric R,
<<Hiya Mary>>
Thanks for your reply.
<<Quite welcome 'hope it was helpful>>
Yes I agree with you it does look like Percnon gibbesi.
And I think I will remove him the next time I do a major water change; which will be soon because there is one other boxy crab with black pinchers I want out.
<<Indeed'¦ Fascinating creatures, but'¦>>
Yes, most of the rock I collected one piece at a time over a one and 1/2 year stretch. Mostly in the upper tidal area around the shore line. I try not to pick up any that have anything alive on them. And even though the pieces have begun to dry out, sometimes after curing there is still live stuff deep in the rock. The owner of the LFS had a permit to make his own live rock from crushed coral and cement.
<<Even this requires a permit? Amazing'¦>>
So early on I seeded my aquarium and the rocks I picked up with that and so for all practical purposes it is now live rock. Again thank you for all your help and your advice. Wish me luck in catching that critter-they are fast.
<<Perhaps some bait in a glass jar positioned near the rock 'good luck!>>
Aloha & mahalo, Mary
<< A hui hou'¦ EricR>>

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