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FAQs about Marine Crab Identification 9

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 10, Marine Crab ID 11, Marine Crab ID 12, SW Crab ID 13, 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 Shrimp Crustacean Identification, Crustacean Selection, Crustacean Behavior, Crustacean Compatibility, Crustacean Systems, Crustacean Feeding, Crustacean Disease, Crustacean Reproduction,

Crab ID Needed -- Acropora Commensal! 1/19/08 Hey, guys and gals -- 3rd letter this week! <Yay! What can I do for you today?> I bought my first piece of Acropora coral and I was pretty excited. <Understandably so, they're beautiful corals!> Well, guess what? A little buddy hitchhiked his way on the coral! He's pretty much all off white with a bit of blue-ish purple on his shell. <Yep, I've seen this little guy's kin before. They're neat little crabs.> He looks nice and was snuggled in the branches of the Acropora frag but I have no idea if he's a good guy or bad guy. If he's bad, I would guess it'll have to be the sump for him until my refugium gets built! Here are some pictures to help with the ID. Hope they are clear enough! <They are, thanks! I'm happy to say that this little crab is a keeper. Ninety-nine percent of the time, when people write in about hitchhiking crabs, we have to warn them about the potential risk to their livestock. This, however, is the one percent that makes for an exception. What you have is a little Acropora (commensal) crab, in the genus Tetralia. They stay small, and are actually good for the coral. Interestingly enough, in the wild, this crab protects the coral from the voracious Crown-of-Thorns sea star (Acanthaster planci). It pinches the tube feet of the star and "discourages" it from dining on the coral! I have a link for you to read through for additional information, as well as several others with photos: http://www2.hawaii.edu/~delbeek/afmjan97.html http://home2.pacific.net.ph/~sweetyummy42/hitchcrabs.html http://home2.pacific.net.ph/~sweetyummy42/coral_crabs-2.html .> Thanks, Jon <You're very welcome. Enjoy your new additions! --Lynn> --
Jonathan Philpot

Hitchhiker Available For Food 1/14/08 Hey there. <Howdy> Enjoy the pics of this guy I caught over the weekend! With those red eyes, probably nocturnal, coming out to creep when lights out? I think it must be over 3 years old, since that's the last time I added any live rock. He's currently being fed & held in a container in the tank. Hoping someone local may need some puffer or mantis food or want it for a sump. CRAB PIC 1 <http://stonyreef.com/blog/wp-content/uploads/2008/01/crab.png> CRAB PIC 2 <http://stonyreef.com/blog/wp-content/uploads/2008/01/crab2.png> <Gee thanks for trying to share with us, but a copy/paste of the links leads me to error pages. Best to attach photos to the email. James (Salty Dog)

Hitchhiker Available For Food 1/15/08 Attached :) <Got them now, thanks for sharing.> Thanks!
<You're welcome. James (Salty Dog)>

Can you help ID this crab? Mithraculus? 1/13/08 <Hi there> Found this hitchhiker in a piece of live rock. Took me quite a bit of time to coax him out with some food. Is this crab reef safe? <Depends on what you have in your tank, but crabs in general are not a good idea in a mixed reef tank. They're mostly opportunistic omnivores, and 'pickers', that may start out small and cute, but can become a problem later on (nabbing fish/other inverts, picking on desirables, etc).> What kind is it? <Not sure - would need to know where it was from, as well as a shot from above showing all legs/shape of the carapace, as well as one showing the claws. Judging by what I can see in the photo though - hairy walking legs and what appears to be the potential for blunt/clubbed claw tips - it looks somewhat like a Mithrax/Mithraculus sp. This is the same genus as the commonly offered 'Emerald Crab' (Mithraculus sculptus) that some use for algae control. Most have more pointed "faces", but some do not. Next time you see him out and about, take a close look at the tips of his claws. If they're blunt/spatulate instead of sharp/pointed, then it likely is a Mithrax. However, it also looks like it could be a Pilumnus species (Family Xanthidae), although it's hard to tell in the photo if the carapace/claws are hairy, and if so, to what degree? Xanthids are, unfortunately, some of the more destructive candidates for reef tanks. For more information, please see these links (as well as related links listed above at the WWM link): http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm http://www.reefkeeping.com/issues/2003-12/rs/index.php> How big will/can it get? <Can't be sure without knowing which species we're dealing with.> Any input is much appreciated. <You're very welcome. Keeping a crab in a reef tank is a risk, but if you decide you'd like to let him stay, keep an eye out for damage and trap/remove if/when necessary. Take care. -Lynn>

Mystery crab hitch-hiker -12/22/2007 Hello gang. Thanks for such an incredibly comprehensive site! Thanks to you, I've given up searching the rest of the web most of the time anymore. <Haha... thanks.> On to the question: I'm looking for an ID on this crab, if possible. He showed up magically in my refugium-to-be a couple days ago. The refugium was set up about 2 weeks ago in preparation for an order I had coming from Floridapets.com. Although the tank itself hasn't been up and running for all that long, nearly all of it's contents came from a 10 gallon and a 5 gallon tank that had been going for about 3 months previously. It's a 29 gallon with deep sand bed, powerhead, whisper internal filter (rated for a 10 gallon, but mostly just used for circulation), and a cheapo style Berlin skimmer (air stone type). My plan is to let all the recent arrivals be in that tank for a month or two before I hook it up to my 20 gallon nano reef as a refugium. I'm monitoring the water quality closely, especially since this tank is so recent to be having livestock in it. I keep the ammonia & Nitrites at 0, and don't let the nitrates get over 10 or 12 mg/L. Usually less. <Sounds like things are going/cycling well/as expected.> Anyway, back to the crab - I got a ball Chaetomorpha, and 2 pounds of live mineral mud from florida pets, and he must have hitched along with that somewhere. He's about the size of a dime, including his legs, and he moves sideways. He holds himself kind of like a boxing crab, but he obviously doesn't have the coloring of one. He seems to prefer to dig himself a pit under a rock or shell, that he can back into and hide, and he's about the color of the sand, so he's pretty well camouflaged. I've seen him sorting through the sand, I assume looking for food. Also, a friend gave me some Hikari "crab cuisine" she had for her turtle, and this little guy eats those with great relish. <Hmmm... crabs are notoriously difficult to ID (especially from photos). It looks like some kind of Mithrax or Xanthidae crab to me.> Of note, maybe, is that he only has a claw on the left side (hopefully the photos show it kind of clearly). I don't know if that's because he's a lefty by genetics, or by chance. I haven't had him long enough for a molt, so it's hard to say for sure. <It's most likely he lost the right claw in a fight or in the journey/shipment from wherever he/she is from. It should grow back with time.> His tankmates include: 1 coral banded shrimp, about 15 hermit crabs (dwarf yellow tipped, and red-legged), a tiny frag of a yellow gorgonian, Nassarius snails, keyhole limpets, a small Aiptasia anemone (keeping a close eye on that one), and a feather duster worm. I'd like to know what this crab is, so I can keep him alive, happy, and out of trouble (and everyone else safe, if appropriate). <Unfortunately, I can't honestly tell you much for sure. But given his/her overall appearance and your noted observations, my GUESS is that he's your typical scavenger/picker. If you want to keep him, I'd just keep feeding him what you're feeding him. Sometimes even the "bad" crabs can be kept more or less "well behaved" if they're a relatively smaller species and if you keep them well fed so that they don't have to start getting aggressive/adventurous for food.> Thanks very much in advance, -Heidi
<De nada,
Sara M.>


Mystery Crab ID: Huenia spp. -- 12/20/07 Hey Crew! *waves* <Hi Christopher! *waves back*> I wrote to you about a week or two ago, but unfortunately never received a response or was able to find one on your site. <Oh, I'm so sorry!> So I'll try again :) <We'll try again!> First off, you'll be happy to know that I removed the porcupine, dog face, and clown trigger from my 55 gallon tank and brought them back to the LFS. <Yay!> Now have 2 Perc clowns, 1 yellow tang, and 4 Chromis. Whewph glad someone pointed that out for me about overstocking! My LFS seems to be more interested in $$$$ than :) Everyone in the tank seems really happy now, thanks :) <Great to hear.> Now to get back to my question, the one I couldn't find a response for. I found a crab who was a hitchhiker in my tank and am super curious what he is! <He's a neat little crab, that's what!> He's lime green, quite small, and seems to have what looks like a plant for a head. <Heee, like in that commercial where people have flowers growing out of their head?> I'm attaching a picture. If you can provide any info or a link to read on what he is it would be much appreciated. I cannot find anything! (Try typing in crab with plant for head, it doesn't work well) hehe. <LOL I bet that was interesting! What you have is an algal commensal called Huenia heraldica, commonly called an arrowhead, Caulerpa, or Halimeda crab. Please see these links for more information/photo comparison: http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm About two thirds of the way down the page at this next link - and would you look at that...one appears to have Halimeda growing out of its head! It's a Halimeda hat! http://www.starfish.ch/collection/spidercrabs.html > Thanks again! <You're most welcome!> #1 Fan Christopher <Woohoo! Thanks! -Lynn>

Re: Mystery Crab ID: Huenia spp. -- 12/20/07 <Hi Christopher!> Wow you're good! I'm impressed! Thanks sooooooo much! <Well, we can both thank Bob for that one! I knew I'd seen that little guy before, and sure enough I had -- at WWM!> Have a great day and hope the holidays go well for you and yours :) And Merry Christmas if that's ok to say :) <It's absolutely ok to say, and I thank you very much! A very Merry Christmas and a Happy New Year to you and yours as well :-)> Christopher

White Spot Mystery Crab 'not enough information -- 12/17/07 Hey crew! <Hey there!> I was thinking of getting a hairy legged crab that my LFS has for my 30 gal. FOWLR tank. When I researched him he is said to be peaceful. <Which species?> He does have the white spot markings on his leg that the "white spot crab" has. <The closest thing I can find to a 'White Spot Crab' is a hermit crab, Dardanus megistos, commonly called a 'White Spotted Crab'. This species does have hairy legs, but I'm not sure if this is the crab you're referring to.> Are they the same crab? Or has my LFS perhaps misnamed him? <Sorry, but based on the information given, I have no way of knowing. I don't know which species you're referring to, and have no photos/knowledge of the one at your LFS.> Because when I looked the white spot crab up, it says it will eat smaller fish. <Then I'd err on the side of caution and choose a more suitable addition. Otherwise, you might end up with a COWLR instead of a FOWLR!> Any help is appreciated. <Happy to help. Good on you for researching before bringing home! Good luck -Lynn>

Florida Crab ID, need more information -- 12/17/07 <Hi Grant, Lynn here this evening.> I was wondering if you could help me identify this crab? <Will sure try!> I got my live rock off Florida and sent up to Alaska where I live. <Yowza, that's a long trip!> I've had the rock and the tank setup for about 5 months now and I've never seen this crab until the last couple days, making me think it either was in egg form or else very, very, very, small when it arrived at my place. <Likely very small. Crabs hatch into a larval/planktonic form that wouldn't stand much of a chance outside a specialized system.> The rock was left in a 40 degree Fahrenheit warehouse for at least 12 hours before alerting me it had arrived, <Yikes!> I figured most everything on it was dead, in fact there were plenty of dead crabs and snails and such in the bottom of the bags and I saw no life besides macro algae. The crab kind of looks like an emerald Mithrax crab on its "back end". In other words, all of its legs except its two front pinchers are covered in a fine hair/algae type looking material. However, this one is stark white and it's shell is different. If you're familiar with the Dungeness crab in Alaskan waters, <Heeee! I live outside of Seattle, so I'm very familiar with those - yum!> that is what the shell reminds me of. A lot broader in the front end than the back. It would be like taking a Mithrax crab, looking down on it from the top and stretching the front out about 20% on each side. And bleaching it white :) Hope you can figure out what type of crab this is for me! <I sure wish I could! Unfortunately, I can't see enough of it in the photo to be much help. What I'd like to do is refer you to some links for comparison/more information. This first link has some great information on crabs in general/ID help: http://www.reefkeeping.com/issues/2003-12/rs/index.php See the photos at this link, along with other 'Galleries' linked at the top: http://www.nhm.org/guana/bvi-invt/bvi-surv/crab-g06.htm This final link is a good site for identifying critters, etc, on Florida rock. Scroll to #47 'Crustacean: Unknown White Crab'. It doesn't list the species, but it's from Florida and it's stark white. See if it looks familiar: http://www.tampabaysaltwater.com/thepackage/organisms.html> Sorry for the quality of the pictures, this guy hides a lot (obvious since I haven't seen him in 5 months) and I've only recently noticed him when I feed. <Believe me, I understand. I've had a small (genus: Lissocarcinus) crab hiding in my tank for over a year and I've never been able to photograph it. Even with the best photos, ID'ing can still be an uphill battle. There are a lot of crab species out there! Most times you're doing well to narrow it down to family!> If they are too low quality, I can upload the much bigger file size originals. <Sorry, but in this case (since that little crab is so camera shy!), I think the best thing to do is check out those links listed above and try to narrow things down a bit. By the way, most crabs are opportunistic omnivores, so depending on what you have in your tank, you might want to keep an eye out for any problems down the road. That's especially true if/when it gets larger!> I read your site all the time, this is the first time I've felt the need to ask my own question though :) <Well, hey, welcome to WWM! Please let us know if there's anything else we can do for you :-)> Grant
<Take care -Lynn>

Crabs and Nudi/Slug? Crab and Sea Slug Issues: Removal and Id - 12/10/07 Good morning/afternoon, <Good morning, David!> I've done a little digging around in your website on this matter and it seems as though the general consensus when it comes to 'unidentified' crabs... is guilty until proven innocent. <Agreed> Here's my situation, My latest piece of live rock has pockets and holes galore and I wouldn't be surprised if it actually had a fairly large hollow chamber in the middle. I have noticed at a minimum, 5 different crabs, and at least 3 different species living on this single piece of liverock that is a mere 5lbs. <Crab central!> This is a 90 gallon salt tank with the following inhabitants: 3" Kole Tang <Will appreciate a larger/longer tank in the near future!> 1.5" Yellow Tailed Blue Damsel 2x Yellowheaded Jawfish cohabitating in the same burrow 1 large Coral Banded Shrimp Tuxedo Urchin Both blue and red legged hermits 1 Large brown black-banded serpent star Soft Coral Polyps Other unidentified soft corals (pictures are coming and I'll take some time to identify them myself if I can). <Okay> Back to the crabs... there is one HUGE crab in the mix. <Uh-oh. Did you Qt/cure the rock separately? That's a good time to check/remove any unwanted pests.> The main body is easily the size of my thumb from my 1st knuckle to thumb tip (approaching 2"). It's a glossy black with sandy/brown legs. There are at least two others of the same species that are much smaller in size. Furthermore, I have seen two smaller dead crabs of this species. <Not too surprising -- could be resulting from too much competition for food/territory, were preyed upon by other crab(s)/hermits/CBS, etc, or could have simply fallen victim to transport/water condition issues.> This large crab is of my primary concern. <My immediate one as well.> I have only witnessed the crab using its claws to apparently feed off the liverock or the encrusted algae on it. As well, I coulda sworn it was eating some hair algae. <'See, I'm a nice little crab! I'm just eating this bit of hair algae - that is, until you go to bed tonight, Hehehee!'> I've tried to catch the crab on 3 occasions but this fella is quick. <Yep, they have to be to survive!> I was afraid that it would prey on my fish, especially the smaller Jawfish <Understandable/valid concern.> or my damsel. However, just the other day the damsel was aggressively posturing towards this larger crab driving it back into a hole in the rock. The crab had every opportunity to reach out with it's claws and grab the fish... I think it was big enough to do so. <Those scrappy little Damsels! They're not easily intimidated, and indeed take on individuals many times their own size. I've had them come after me when I've gotten too close to their territory in the wild. LOL I have to admit, it's pretty funny seeing them having a little fishy hissy fit - that is, until one darts out and bites you!> I'm hoping to remove it and introduce it to Mr. Snowflake Moray in my other tank... however this porous liverock piece has some soft corals and feather dusters growing on it. <Murphy's law.> I don't want to freshwater dip the rock. Would you be of the same opinion that I should still work towards getting these varied crabs.. Especially this big black brute out? <Absolutely. There are several critter traps available on the market, but you might try the jar method first. This is particularly handy since crabs can't climb glass. Just take the jar, lean it up against 'Crab central' and partially bury it in the sand to secure in place. Put a stinky piece of shrimp/similar in the bottom of the jar. You may want to place/wedge some rock rubble around the mouth of the jar to keep the hermits/CBS out! This method is best done at night/after lights are out, and although it can take some patience, usually works.> I also happened to notice what appeared to be a very tiny crab... that swam across my tank. <Wheee!> I couldn't believe my eyes... It looked beige with red spots on it. It was no larger than the size of a pencil eraser. Any ideas? <Was Tequila involved? Sorry, couldn't resist! I need photos/more info for a glimmer of hope at an Id.> Lastly, I noticed something strange in my coral polyps. <Any apparent damage to the coral?> It looked like a worm of some sort and I thought it best be removed... so I did. Upon further investigation I'm thinking it's either a tiny Nudibranch or a sea slug/cucumber of some sort? <Likely an Opisthobranch/sea slug instead of a Holothuroid/Cuke).> It's under an inch long, brown with two antennae's at the head. The back has a number of tufts almost resembling tiny feather dusters and the underside is whitish kind of like a snake belly. <Yep, that confirms it.> I placed him in my 200gallon tank <Is this a fish only system? It's always possible that this little sea slug is of the coral eating variety, so if you have any in that tank - bad news. I would remove it.> and immediately both my Magnificent Foxface and my Harlequin Tuskfish approached it as a food item and made a hasty retreat. <I bet!> Nudibranch? <Definitely sounds like an Opisthobranch of some variety. Please see the following links for more info re: http://www.wetwebmedia.com/seaslugsopisthobranchs.htm (opisthobranchs) http://www.wetwebmedia.com/nudibran.htm (Nudis) http://www.seaslugforum.net/ > David Brynlund <Take care -Lynn>

Re: Crab and Sea Slug Issues: Removal and Id - 12/10/07 Excellent, you guys/gals are awesome! <Thank you David, it was a pleasure!> I was just setting up my tank and the new piece of liverock went in without (hahahaha, ya right) any critters (that I was aware of). <Heee! They were hiding, and plotting, within that rock!> I also had a mantis shrimp from the other piece of new liverock but luckily was able to catch the critter quite easily within about 5mins. <Excellent> He stayed nice and still for me and then climbed into a small piece of live rock which was easily put into a bucket... <That's something you don't read every day! Mantis shrimp have extraordinary eyesight and can be a challenge to catch/trap.> Once in the bucket he swam around... Anyhow, I think I will try the jar method for sure. It is a very nice looking crab... Glossy black with a reddish tinge to it. Looks like King Kong compared to the other crabs. <Heeee - King Crab!> This rock is positioned over both the Jawfishes who share a lair. <Yikes!> I think the opportunity was certainly there should he wanted to have preyed on them. <Yes, although he might have, later on, after the more readily accessible food was gone. I'm glad you're getting him out of there.> As well, the Coral Banded Shrimp and this brute have been side by side without harassing each other. <Yep, that's a fairly good size crab. I doubt the CBS would have a go at him. As for the reverse, they'd most likely get along fine until the crab saw the shrimp as a threat to his territory/food supply - or if he saw an opportunity when the shrimp was molting/vulnerable.> Even still, I'd sleep better at night with AT LEAST this guy out and into my 200g tank.... <Understandable, though I'd still keep an eye out for trouble as he grows. That is, if he doesn't get eaten first!> David Brynlund <Take care -Lynn>

Help ID'ing a crab: Need more detail - 12/10/07 Hi guys. <Hi Michael> I'll start off by saying how much I enjoy your site, it's by far the most comprehensive aquarium keeping site on the net. Keep up the great work!! <Thank you!!> I need some help ID'ing a crab that I recently found in my tank. He's a whitish, grey colour with small hairs on his legs. He has red/orange arms with a set of purple pinchers. Any ideas guys? <Sorry, but without additional photos/detail, I'm unable to narrow it down. I'd need to see the top view of the carapace/shell, and the legs. Even then, it could be a challenge! I can, however, refer you to several sites that should help: http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm http://www.reefkeeping.com/issues/2003-12/rs/index.php http://www.nio.res.in/crab/crabs/tex2.html > Does he pose any danger to my reef system? <Potentially, yes, because most crabs are omnivorous and opportunistic.> I've enclosed a picture of the crab in question (sorry for the crap image quality, even the best camera phones leave a lot to be desired) <Hey, not bad for a camera phone(!) but in these cases more detail, with multiple angles, would be needed.> Thanks for reading. Any info you can give me would be more than appreciated. <You're very welcome. I just wish I could have been more help!> Michael McLaughlin <Take care -Lynn>

 Re: Help ID'ing a crab - 12/11/07 <Hi Michael!> Thank you for taking the time to answer my question. <My pleasure.> The sites you pointed me towards were a great help and narrowed it down quite a bit. <Excellent!> I wish I could provide more pictures, but he's very skittish and seldom comes out of his hole. I actually had to entice him out with a piece of squid. <That's good to know when/if you need to trap him!> Anyway, here's a few more details about him. He has a trapezoidal shaped body, which makes me think it's possibly a type of Grapsid crab? <Quite possibly, yes.> The colour is a very light grey almost white with small hairs. He also has faint vertical maroon brown stripes on his body and legs. <Pretty little crab :-) I wish I could be more specific with an Id, but considering the number of species out there, I think you're doing well to narrow it down to a Grapsid. What's important is that most crabs are not to be trusted long term in a reef tank, so I'd either keep a very sharp eye on him/his tankmates, or trap and relocate.> Once again, thanks for the knowledgeable insight!! <You're most welcome!> You guys rock!! <Woo-hoo! Thanks, Michael!> Michael
<Take care -Lynn>

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