FAQs about Marine Crab (Including some Anomurans)
Related Articles: Crabs, Hermit
Related FAQs: SW Crab Identification 1, SW Crab ID 2, SW Crab ID 3, SW Crab ID 4,
SW Crab ID 5,
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 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,
& 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
Amphipods, Copepods, Mysids, Hermit
Banded Coral Shrimp,
Anemone Eating Shrimp,
Systems, Crustacean Feeding,
Uninvited Crab ID (Movie
My apologies for contacting you through Messenger. I wasn't aware that I
should be reaching out through a different channel and will do so in the
future. Thank you for responding anyway.
While I was on vacation, the LFS worker put this Crab in my tank and
told me it was an Emerald Crab. I was worried because of the black claws
and because it doesn't look like any Emerald Crab I had seen.
<You were/are right to be concerned... this is likely a member of the
family Xanthidae... "eat em up" crabs>
It was eating my Torches and I couldn't understand why they weren't
doing well. It was a nightmare to catch it but eventually I did and
brought it to the LFS. They couldn't identify it and I've asked around
and searched all over the forums and Google and no one knows what it is.
This morning I was searching again and I Came Upon Your article about
Crab identification. I couldn't tell if there was anything in there that
looked like this one so I reached out to you.
<Ahh! I was/am going to refer you to that portion of WWM to scan
through... starting here:
and continuing in the series (linked, in blue, at top)>
Attached is a video of it after I caught it. The other video I have of
it in the tank is of him eating the Torch. If you want to see a savage,
take a look at it. It doesn't fit in this email so I will send you
another one with it.
<Have just captured a still; which is sufficient>
Thank you very much for taking the time to help me, not just today but
whenever I reach out to you. I'm deeply grateful.
Looking forward to hearing from you,
<Cheers! Bob Fenner>
Tiny red crab 11/16/19
I spotted this very secretive crab a few night ago. I yanked it out of the
tank last night at 2 am when it was searching for grub on the bare bottom
tank. At first look I thigh too it was a Mithrax crab but I’ve not seen any
like this. Any ideas?
<Def. a Decapod, but doesn't look like something I've seen before. Have you
perused the Crab ID FAQs pages? Start here:
and the linked 21 other files above. Bob Fenner>
Hitchhiking Crab 7/28/19
Hi! Your forum is great!
Can you help identify this hitchhiker crab? Attached a vid and a pic.
Spiny, banded legs, blue dots on the face and claws... thank you!
<A beauty, though can't quite make out distinctive (Decapod family) features....
I shy on the side of conservativeness with crabs in marine systems; usually
urging folks to remove them. DO at least keep your eye on probable predation,
and bait/trap this animal (maybe put in a sump, refugium?) if it proves too
predatory. Oh, and take the long read through our Crab ID FAQs files:
and the 22 linked files above. Bob Fenner>
<Hi Marie, could you please resize your pix to just a few hundred Kbs and
Crab ID 6/18/19
<Hi Marie, thanks for resizing your files.>
I have had a hitchhiker crab since set up and sent you a email a few months back
for an ID but the pictures I had weren't great so didn't really get an ID.
I now have a fallow tank as I had to remove my fish due to white spot and the
crab has come out of the rocks fully and I was able to get some better pictures.
If I could get an ID on him if possible please that would be great and some
advice to whether he can stay in the tank or whether I should remove and place
in my sump.
<It appears to be a Gorilla crab, I wouldn't place it in the main tank.>
Thanks in advance,
Crab ID 2/2/19
I have written to you recently about a stocking list for my 75g and asked for a
snail I'd and now I'm asking about a crab id. I know crabs in general can't be
trusted but am wondering the potential size of the crab and if it is more safe
or less safe than most crabs.
<Certainly they can´t be trusted, especially as they grow>
The live sand and rock came from the Tampa Bay Area. I've seen about 6 or so in
the tank ranging from a quarter inch to about 1 inch and they look like the same
species. I did try trapping with some mysis in a glass but all I caught were
some Nassarius snails but no crabs. If it helps the tips of the claws are black
with spots on the body and a flattish head.
<Well, the crab on your image appears to be some species form Menippidae family,
probably Menippe mercenaria; they can grow to 7 inches. I wouldn´t trust it but
if you still want to keep it, watch its behavior closely, particularly if you
have small fish. >
<Cheers. Wil .>
Re: Crab ID 2/2/19
Thanks Wil for the reply and I think you may be right. I don't trust them and
would like to catch them but may be difficult since they are not falling for the
crab trap and the snails get all the food. Right now there are no fish in the
tank but will be putting in a shrimp/goby pair, Dartfish, Jawfish, and fairy
wrasse. Maybe I'll try a mediumish wrasse that may go after the crabs that I
could move at a later date.
<Good idea about the medium size wrasse, or, since there´s no fish in the tank
yet…. you could borrow a triggerfish from a friend or LFS; and it will eat the
crab if left hungry for a few days. >
<Glad to help, Jason. Wil. >
Possible to get an ID on crab 1/28/19
<Good morning, Marie.>
Just wondered if you could give me an ID on a crab I have recently spotted in my
tank please. Had a marine tank running since October and have only just noticed
it, I assume he came with the live rock.
Asked on a Facebook group and someone thought it could be a Xanthid crab and
that I should maybe remove it as it could prey on sleeping fish. They suggested
your website to ask for an ID before I took him out as I liked the idea of
<It could be some sort of Xanthid (family Xanthidae); I´d remove it too.
Please see here:
http://www.wetwebmedia.com/swcrabidfaqs.htm and the linked ones.>
Any advice would be appreciated.
Thanks in advance.
<You´re welcome. Wil.>
Unknown crabby 12/21/18
Hi guys just purchased a new tank with some rock. This guy fell out of it... the
guy said he had emerald crabs and hermits but this crab claws looked black and
oversized. Is he an emerald? Or a gorilla? Thanks!
<Hi, judging by the black tips on its claws, it could be a Gorilla crab, How big
is it? is it hairy? could you tell me more about your system, live stock? Wil.>
Re: Unknown crabby
Hi, just got another picture... his claws are quite enormous for his size.
One is bigger than the other. He is about 4.5cm, his legs are pretty hairy.
System is full of live rock. The guy purchased a huge piece when he first
<Ah yes, Xanthidae sp.; a predatory species that I would NOT keep, as it
can/will consume your livestock. Wil.>
Re: Unknown crabby 12/22/18
Thanks guys. I have relocated him to my sump. Lots of live rock and algae in
there. He is quite cute so I will be making sure I throw some food in for
<Good move, Greetings. Wil>
Hairy crab in my refugium 12/7/18
I found this little hitch hiker crab in my HOB refugium today. I'm assuming he's
a "bad" crab since he's hairy, but I wasn't sure how bad...
<Hmm... depends on how you define "bad." Just about all crabs have at least some
capacity to be destructive, even if just by virtue of roaming about, knocking
things over. There are many, many species of crab which appear "hairy" and they
are not all of the same genus or even of the same family.>
moment he's about the size of a nickel. Can you ID it?
<I can ball park it. It looks like something in the Pilumnidae family and within
the pilumnus genus.>
If it helps, the live rock was from Florida.
<Oh!! In that case, I think it must/might be pilumnus sayi or "spineback hairy
crab." See here:
One forum member said it was a Xanthidae crab but that seems to cover a pretty
broad category of crabs.
<Xanthoidea is a "superfamily." Thus, such a label does not narrow the ID down
The display tank is a FOWLR with no fish yet, just cucumbers, hermits, snails,
and one sea urchin that hitch hiked in. It sounds like there's a few mantis or
snapping shrimp of some sort that hitch hiked in as well but I haven't been able
to catch any yet with my pest trap (only caught one hermit crab 2x in one night
when the trap wasn't even baited).
<Haha! Good luck!>
How much damage would this guy do in the display tank?
<Hard to say - depends on how big s/he gets!>
Is it safe to leave him in my HOB refugium, or would I risk him making it into
<If he gets hungry/restless, he might it out and into the display.>
If I left him in my refugium (or maybe moved him down to my sump), would he
wreck my pod population?
<I don't think so, but these sorts of crabs tend to be opportunistic
predators/scavengers. So, if there isn't anything else to eat...maybe.>
He's such a cool little dude I'd hate to get rid of him...
<So keep him - feed him. Or, set up a separate system for him.>
But since I have a goby, a few wrasses, and a mandarin on my eventual stock list
I need all the pod generation capability I can get!
<Cheers! - Sara/Sihaya>
Crab ID please 3/19/18
Hi, I’ve been trying to catch this little crab for a few week now and last night
finally caught it and it now lives in my sump. Can any of you help ID it for me
please. It’s only small, about the size of my finger nail on my little finger.
<Umm; can't make out much here. Need a much better resolved pic. Let's have you
and the 22 linked files above in the series. Bob Fenner>
Thanks in advance
Tiny hitchhiker crab
I've been unable to find anything on this, so I'm hoping you can help. I had an
Acro frag that wasn't growing and coloring up as it should have. On closer
inspection I found a small area near the base where the skeleton was
exposed. Fearing AEFW I used a jewelry magnifying glass to look for worms and
eggs and found a VERY tiny little crab. It was probably 2-3mm including legs.
It had a black body and transparent legs. I don't have any photos.. I cut off
the tip and did a Bayer dip. I've inspected all my other frags and haven't found
anymore, but want a little peace of mind by knowing what that
guy was and if I should be worried about finding more.
<Mmm; would need a well-resolved pic to try to identify. I would leave this
little Decapod where it is. Not likely a mal-influence here. Bob Fenner>
Can you help me identify this crab like fellow
It was found in the bay area in Bonita Florida, by the gulf in Southwest
Florida, I have spent hours trying to research this to no avail. Thanks
much. Colleen Bolger
<... Might be a long/er range contaminant/recruit. I don't recognize this
Decapod either. Am out dive traveling and back tomorrow where my in-print
references are. Will look for then. Bob Fenner>
<I do think this is Aratus pisonii. BobF>
Having trouble identifying this Acro hitchhiking crab
I recently bought a small Acro colony and it had 3 crabs.
<Neat! Evidence of good handling>
I knew there was one in the colony, it's part of why I chose it. But I
didn't expect 3! This album contains the pictures I'm referencing, feel free
to host them on your site though https://imgur.com/a/5b3LK I was able to
identify the small one (3rd & 4th pictures) as a Hairy blue-eyed Acro crab
(Cymo andreossyi) which as I understand it will pick on
<All will/do to extents. I'd keep them all... not that damaging in most
On large colonies they're supposedly symbiotic, but dangerous to small
colonies, so I tossed him in with my P. ciliata mantis shrimp. I was able to
identify the other (last picture) as a Tetralia crab, safe and symbiotic,
and put it in the tank with the coral. But I'm stuck on the one pictured in
the 1st and 3rd pictures. The color matches the coral closely, and the body
shape reminds me of a Trapezia which makes me think that it's commensal.
<I agree on both counts>
It's legs are a little bit fuzzy though. One claw is slightly larger and a
little bit sharper, the other is smaller and has flat tips, clearly intended
for gripping and scraping. I've been Googling for a couple hours now and
haven't been able to get a good ID on him. In the meantime he's in a critter
keeper with an airstone. So, should I put him with the colony
or would it be wiser to let him test his fate with the mantis?
<As stated, I'd keep all>
<And you for sharing. Bob Fenner>
<Your uncropped pix are too poorly resolved for use. See, and in READ on WWM
re our image requirements, AND review the many FAQs files on Crab Id.
Hi need help on identifying a crab I found from a live rock. currently in my
sump but if it's reef safe I will put it into the main tank.
Re: Crab ID 11/14/16
Apologies hope these 2 cropped pix help .
<Nope; still poorly resolved, don't show diagnostic characters. READ where
you were referred to on WWM. B>
Re: Crab ID 11/14/16
Did my reading and I am leaning 30% its a Xanthid
<I tentatively agree... are the tips of the claws darkened?>
(stays in sump) and 70% its a Ruby Red Crab (could be added to tank) ?
<I'd leave it out of the main/display... Yes to keeping it in the sump.
Re: Crab ID 11/15/16
No the claw tips are white. been in my sump for 2 weeks and survived ?
<As long as dissimilar... a Xanthid trait>
Could it be a box crab - they are reef safe aren't they. I have a full soft
coral tank so the risk of a Xanthid causing destruction is lower ?
<Oh yes; SEE/READ on WWM. Don't write: read. B>
Possible Xanthid Crab? 5/22/16
Good evening Bob and Company!
Thank you so much for your fabulous website; you are an amazing resource.
<Glad to share>
I keep a 60 gallon seahorse and soft coral reef (primarily sun corals and
assorted sea fans) and always acquire all of my livestock (aside from the
seahorses, which come directly from the breeder) from a single local shop that I
Tonight I brought home a piece of live rock with mixed colonies of clove and
daisy polyps (it’s lovely) - but about an hour after introducing it to the tank,
I noticed it “breathing” - which, of course, neither of these species do. A
brief investigation with a flashlight and a pair of long tweezers revealed this
little guy hiding beneath the polyps. (Photos attached.)
<I see this Decapod>
His legs are much hairier than they appear in the photos - like the legs of a
Mithrax crab (but his pincers are clearly wrong for Mithrax).
I pulled the rock from the tank at once, nudged the crab into a bowl, and shot
these photos. After spending about an hour on your site, I suspect he’s a
Xanthid - and that it’s a good thing I removed him before he could cause any
havoc. Can you confirm his species, or at least let me know if I should keep him
away from the seahorse reef?
<It is a member of this family; and I would keep it out of your seahorse system>
(My husband thinks he’s cute, but I don’t think my softies and seahorses will
appreciate this particular variety of cuteness.)
<Mmm; if you have a live sump; I might place this crab there; where it can do no
Thank you for all you do!
<Certainly welcome. Thank you for sharing. Bob Fenner>
Saltwater Crab ID 3/7/16
I purchased a Seriatopora coral, and this cool looking fella came with! Is he
some sort of tetralia or not a good 'un?
I have looked all over but Tetralia matches him.
<Am out dive-adventuring in Roatan; away from my ref. materials. Looks to be a
Xanthid. Use your search tools to look up pix of this family from the geog. area
your coral was collected. Bob Fenner>