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FAQs about Marine Crab Identification 14

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Pls forgive me 6/30/09
Lynn... have placed yet another very generic "whatizit" invert. query in your in-folder...
Hi Bob!
Heheee! No worries, I'm working on it right now. One looks like a sea spider...the other crab(s) I can't quite tell. Right now I'm manipulating the photos every way I can think of trying to determine whether there are 3 walking legs on each side or four and whether there are antennae just distal to the eyes. It's not going too well so I'm about to call it a day - or at least a crab!
Take care,
<Mmmm, yumm, I wish we were having spicy crabs in Bali right now... Oh, the mess! Worse than any/all Olde Bay incidents! BobF>
<<Heeee - now that's something! Just eating blue crabs with all the Olde Bay seasonings, etc on them is a gigantic mess - I can't imagine trying to eat crab covered with a thick spicy sauce! What a mess indeed, but I bet it's well worth it! -Lynn>>
<Oh yesssss! Then you can eat the juice et al. as a sort of soup/compote at the end! BobF>

Re: Aiptasia? Now, ID's: Sea Spiders and Crabs -- 6/30/09
Hi Bob,
<Hi Peter, Lynn here today>
Thanks for the help ID'ing those.
<On behalf of Bob, you're very welcome!>
Since my peppermint shrimp isn't going for them, I may try some Joe's Juice or something of that sort.
<Aiptasia-X reportedly works well along with the standard Kalk/water method (Google for more info at WWM: http://www.wetwebmedia.com/Googlesearch.htm).>
Wondering if you could help me out with ID'ing these things and crabs from my new 46 gallon tank.
<I'll sure try.>
The tank is about three months old and I have some new hitchers I haven't seen before. There are some tiny spider-like animals. I have at least three of these. They are about 1/4 inch across. Not sure if they are small arrow crabs?
<I don't think so. I can't quite see the body, but overall they look more like Pycnogonids/sea spiders to me (carnivorous marine arthropods). Please see the following links for more information: http://www.wetwebmedia.com/pycnogonids.htm
http://en.wikipedia.org/wiki/Sea_spider >
There are also some larger crabs, at least three, all with the same coloration. The largest of these is an inch across.
<Gotcha. Unfortunately, I can't quite see enough detail to ID but with your help I think we can narrow down the field of possibilities. First of all, do the crabs have 3 walking legs on each side or four (not including claws)? Also, do you see any long antennae just behind the eyes (they may be fairly clear so look closely)? Those two factors alone can help us determine how safe these crabs will likely be around your fish. Generally speaking, if it looks like a crab but has three pairs of walking legs + long antennae positioned behind the eyes, it's a false/'porcelain' crab (Family Porcellanidae). These are primarily filter feeders but they also scavenge. The good news is that they don't pose nearly the risk to fishes that true crabs do. True crabs have four pairs of legs (most adapted for walking, some species' for swimming) and fairly short antennae placed in front of (between) the eyes. These are opportunistic creatures that will, if given the chance, make a nice meal of any fish, fellow crustaceans, etc. This risk increases with size, competition/lack of sufficient food, and ease of access (especially small bottom dwelling or sleeping fish, crustaceans in molt, other benthic invert's). For more information on porcelain crabs and true crabs, please see the following links (as well as associated links at the top of each page):
http://www.wetwebmedia.com/galatheids.htm >
I guess I was wondering if they will be hazardous to my 1-inch ring-eyed goby or yellow clown goby. There are the only fish in there so far.
<See above. If those crabs have 4 legs on either side, I'd get them out and find them a new home. Small Gobies and crabs are not a good combo!>
Thanks again for the previous early response!
<Again, on behalf of Bob, you're most welcome!>
<Take care, LynnZ>

Follow-up: Re: Aiptasia? Now, ID's: Sea Spiders and Crabs -- 7/15/09
<Hey there Pete!>
Thanks for the help!
<You're very welcome!>
These are four-legged so I guess they are true crabs.
<Uh-oh. I knew the chances were slim, but I had hopes that these would be of the safer Porcelain crab variety.>
After reading your article it looks like a Grapsid.
Not sure which I like better, the crabs or the gobies!
<Heeee! It's just unfortunate that crabs like to snack on gobies!>
But I think I will take your advice and maybe put these in my refugium.
<Well, just keep in mind that once you put them in there, it's no longer a 'refuge'. It'll be a nice little hunting ground for the crabs, but not such a safe haven for everything else! You could always set up a nano tank for the crabs. That way you'd be able to see more of them. It doesn't have to be anything fancy. Just make sure they have plenty of hiding spaces and that they're well fed so they won't get cranky and kill/eat each other!>
Thanks again,
<It was a pleasure.>
<Take care, LynnZ>

Crab Identification: Spider Crab from New Jersey Beach -- 5/30/09
<Hello Eleanor, Lynn here today.>
I've been searching all over the web and would really appreciate your help in identifying the crab in the attached photo.
<Sure thing>
He has such slender claws and an interesting snout.
<I'll say!>
I found him on the shore of Long Beach Island, NJ.
<It looks like Libinia emarginata, a scavenger commonly referred to as a spider crab (family: Majidae). For more information/photos, please see the following links:
http://www.biology.lsu.edu/webfac/jbelange/completed_files/dreamweaver/Libinia.html >
Please email me back when you get a chance.
Thank you,
<You're very welcome. Take care, LynnZ>

I Need help with an ID, please 05/27/09
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). It's similar, but the carapace edges look jagged/spiny instead of smooth.
Thank you, BobF
<You're welcome. Hope you're having a great time and enjoying your trip! LynnZ>
<<Thanks much Lynn... have forwarded the resp. to Ultramarine Mag/UK for their posting, with credit to you. BobF.>>

Can you help ID a small crab for me? 4/25/09
I'm sorry to bother you with this, I'm sure you get these way too often, but this little guy has me stumped. I have not been able to get even a semi-decent picture of him in the water, and am hoping most of a shed will be okay. I am unsure where the rock that he came on is from, he was at one of the local stores where they get in multiple types and rock from tanks that customers are breaking down. The crab is still quite tiny, and is grey with a black stripe down his back, black claws, black eyes and hairy legs. Most of that would usually indicate a "bad" crab, but he also seems to have the claws of a Mithrax family crab, with spoon shaped tips.
<Would be my best stab too, not that these are good crabs either!>
He tends to stay tucked away in a piece of rock while the lights are on and comes out at night to munch on algae that is growing on the rocks.  I'm sorry I could not get better pictures, but I hope that they are good enough (for reference, the holes in the canvas in the box are about 1/2") Besides trying it and hoping it does not fail, is there any way to tell if he would be fish-safe?
<Sure, it is a crab, they are opportunistic feeders. It will eat fish given the chance.>
My tank is a nano(10 gallons) and I would like to get a clown goby for it, assuming I can find one that is eating at the store to add some "movement" to it. So far in the year the crab has been there, I have not experienced and coral loss, my solo hermit crab is fine and I have not seen any damage to the hitchhiking sponges or feather worms(a few are recognizable by their markings) that are near his lair.
<Clown gobies and these potentially large for aquarium crabs rarely do well together, especially in cramped quarters.>
I also have a question about the goby. It seems like one of those would be about the best fish choice for a tank my size. I have read that they tend to nibble on Acroporas, but have not seen any information about other species of SPS.
<A negligible concern.>
I have a plating Montipora in my tank now and am looking for a Seriatopora and possibly an encrusting Montipora. Would the clown be likely to bother any of those, or is just the Acroporas that would need to be worried about?
<Well, if the particular fish is an SPS nibbler it will not differentiate.
But I really would not worry about this with this fish.>
Thank you for being there to help people like me to not kill everything in
our tanks!
<Welcome, Scott V.>

Re: Can you help ID a small crab for me? 4/26/09
<Sure, it is a crab, they are opportunistic feeders. It will eat fish given the chance.>
That's what I figured, unfortunately. I really would hate to kill the little guy.
<Me too, don't!>
Do you suppose if I could find a powerhead small enough for it I could keep him in a 2.5 gallon tank that is
dedicated to just him and possibly some macro algae and a few low light corals?
<Hmmm, well you could...do search WWM re "Sharks in the living room". It is one of my personal fave queries. Basically you could, but it does not mean that you should! Better to just meet some fellow aquarists and find it a more suitable home! Scott V.>

Help? Pocos... decapod id   4/20/09
Background: I purchased two Pocos
<Pacus? Colossoma sp.?>
about three weeks ago. The first week or so, they were doing well. Then the green one started to go white at the base of where the arms
would branch off. Figured it was too high in my tank, so I lowered it. The pink was doing great. Suddenly the pink began showing similar issues.
<... What are/is this?>
Today I went to look at my tank and I saw this strange growth on the pink poco. It moved. Upon further investigation it was a crab-like creature and it was clearly picking at the coral. I removed it and placed it on a bucket lid to photograph.
It looked like a baby emerald crab with extra legs. MY wife said it looked like a spider. I didn't see any "claws" but it clearly used the fronts to scrape at the coral. The scrapers are on the raised part of the bucket in pics posted below.
Can anyone ID this ugly thing? Do they only attack SPS?
<... is a small crab of some sort... Might be a commensal that causes, can cause some damage. Bob Fenner>

Crab ID 4/14/09
I bought this crab at my LFS. They said he is reef safe, and a sponge crab.
After looking up a sponge crab, I realized that he is certainly something else... but can't determine what.
He has very small pinchers, like a shrimp might.
Any ideas?
<I concur that it is unlikely a sponge crab. With not much detail in the pics, my best guest would be a Stenorhynchus species, commonly known as decorator crabs, and they are not entirely reef safe. Like all crabs, they are opportunistic feeders and will eat what ever they can catch/find. Caution here
my friend. Since Mr. Fenner's second home is on the reefs, I will ask for his input here.><<My Standard Spiel here re Decapods period... Cave Crabem! (beware the crab). RMF>>
Thanks so very much!
<You're welcome. James (Salty Dog)>

Another creature from the deep   4/10/09
Hi there guys,
I want to thank you for your prompt reply the other day. You have really helped me.
Now, I have another quandary for you. I have found this crawling monster in my tank. It looks like a giant underwater louse. Surely there cannot be such a wondrous thing?
<It is not a louse; while there are numerous marine animals called lice, they're not insects but usually crustaceans of various types.>
It is about 2x2cm, flat, crawling rather slowly, at a snail's pace instead of a crab scrabbling away. It is camouflaged, but I'm not sure if that is its natural colour or if it is just so slow that algae started to form on
<It's surely one of the Majidae, the spiny spider crabs. Generally slow-moving scavenger/predators, famous for becoming covered in algae, sponges and other things that camouflage them. Hence aquarists often call them "decorator crabs". Mostly very small, though the biggest crustacean of all is a coldwater spider crab. Would expect your specimen to be largely harmless.>
Here is a photo of the Thing.
Can you please help me here?
Thank again.
<Cheers, Neale.>

Crab ID - 3/19/09
I need help trying to identify this crab. I've done hours of looking at photos and descriptions but can't seem to find anything that looks like this one.
<That's frustrating, I know, but understandable. There are an awful lot of crab species out there and not all are represented on the internet. Do you have any idea where this little crab came from, as far as locale? Also, what is the size?>
If you need more or better pictures, I will be glad to get them for you.
<Thanks, I appreciate it. A good photo taken from directly above the crab, showing the entire carapace and all the legs would be super if you can get it. Also, a good detailed shot of at least one the claws, would be helpful as well. I have to warn you though, even with new photos and more information, the chances of getting the ID narrowed to species level is unfortunately very slim. Most of the time, the best you can hope for is to narrow it to family level, much less genus. Just remember that crabs are opportunistic feeders and can potentially damage the livestock within a reef system. Small crabs pose less risk than the larger individuals, but they're by no means harmless/trustworthy. The best thing you can do (if you wish to keep this little crab) is to make sure it gets enough to eat. This will help deter the little critter from viewing its fellow tank mates as food. You can offer meaty bits of marine origin (clam, shrimp, etc), as well as sinking pellets and the like. Personally speaking, I like crabs. I think they're neat little creatures that add to the biodiversity of a system. However, it's imperative that you keep an eye on them and be prepared to remove, if/when you see any signs of damage.>
Thank you
<You're very welcome. Take care, Lynn>

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