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FAQs about Emerald Green, Mithrax/Mithraculus Crabs: Identification

Related Articles: Mithrax Crabs, Arrow Crabs, Crabs

FAQs on: Emerald Green Crabs 1, Emerald Green Crabs 2,
FAQs on: Mithrax Behavior, Mithrax Compatibility, Mithrax Stocking/Selection, Mithrax Systems, Mithrax Feeding, Mithrax Disease, Mithrax Reproduction,

Related FAQs: Marine Crabs 1, Marine Crabs 2Marine Crabs 3, Marine Crabs 4, Marine Crab Identification, 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,

Mithrax forceps,
Red-Ridged Clinging Crab. Family Majidae. Tropical west Atlantic. 1/2-1". Here clinging to a piling at night off Bonaire.

Red Emerald Crab; ID, comp.      9/15/15
So I will begin with the disclosure that I know emerald (Mithrax) crabs have a dubious reputation as reef tank inhabitants.
<Mithraculus are opportunistic; like other Decapods>

I don't hold opportunistic behavior against them but one of them is heading for a new home. My inquiry is in regards to your opinion of the traditional emerald crab versus the, sold by online vendors and my lfs, red emerald crab. He(big red)is described as being identical to a traditional emerald crab save for coloration. Do you have any knowledge of this crab variation or his possible variation in behavior?
<.... some folks purport this is also Mithraculus sculptus>
There are three Mithrax crabs in my 65 gallon reef, one is to tiny to be the culprit, and one each of a large red and large green Mithrax. Just trying to make the best decision, without hard evidence, as to who is tearing my green star polyps off from the base. I did consider the health of the coral and definitely seems to be some clawed species is clipping the polyps off leaving the purple base intact. Any input as to the potential difference in culpability. I did search for these red Mithrax variants and found little evidence of them on your site. Hence the email and appreciation of your opinion.
<Both, all are suspect. I'd remove them. Bob Fenner>

Mithrax question     11/20/14
Hi guys. Love your site. I know these guys are not always reef safe. Do you think this is an emerald crab, and is he to big?
<Too? Not yet... and appears to be Mithraculus>
Would you put him in your reef tank.
<Not I; no. See my opinions, observations re this Decapod on WWM>
He's pretty bold. Could he take down a fish.
<Only very small for no... in future?>
I can always move him into the refugium. Let me know your thoughts as I truly do value what you guys teach me.
<Help yourself. Bob Fenner> 


Taxonomic info    12/11/12
Hi WWM crew,
I came across this page,, and thought to send a few taxonomic corrections to you:
Condylactis passiflora is an unaccepted name; the accepted name is Condylactis gigantea
Mithrax cinctimanus was moved into the genus, Mithraculus by H. P. Wagner, 1990, Zool. Verh. (Leiden) 264:48-53. Accepted name: Mithraculus cinctimanus.
Although not on that web page, the Pederson's cleaner shrimp, also a symbiont found with C. gigantea and Bartholomea annulata (ringed or corkscrew anemone) has moved from the genus Periclimenes to the genus Ancylomenes by Okuno and Bruce (2010) Designation of Ancylomenes gen. nov., for 'Periclimenes aesopius species group' (Crustacea: Decapoda: Palaemonidae), with the description of a new species and a checklist of congeneric species. Zootaxa 2372: 85-105. Accepted name: Ancylomenes pedersoni.
Nancy Sheridan
Florida Fish and Wildlife Conservation Commission
Fish and Wildlife Research Institute
100 8th Ave SE
St. Petersburg, FL 33701
<Thank you for these updates. As principally a hobbyist site, we do tend to "lag" (e.g. not recognizing the addition, movement of genera, species in Veron's recent Scleractinia works), be more of lumpers than splitters taxonomically... but it is important to provide current systematics. Will post your input here re these species. Bob Fenner>
Re: Taxonomic info, Condylactis conservation     12/11/12

Thanks for the reply and you're welcome for the info. Also, in case you haven't heard, a three-year prohibition on the recreational and commercial harvest of Condys in Florida state and federal waters went into effect Nov. 1, 2012. For more info, please see - http://myfwc.com/news/news-releases/2012/october/24/marine-life/.
<Ah, I thank you for this note as well. Have a long-standing disregard for the use of Actinarians in the ornamental trade... not worth their extraction in my view. Cheers, Bob Fenner>

Emerald Crab...black? 09/01/08 Hello WWM Crew! Just curious what this crab is that I seem to have. He came along unexpectedly with one of my corals that I purchased about 6 months ago. He was quite tiny - about 1/2" or so - but just looked like a little emerald crab - same shape, hairy legs...but seemed a little darker in color. I didn't see him again in the tank and figured he didn't make it...until about 3 or 4 weeks ago when low-and-behold...there he was! MUUUUCH larger now...about the size of an adult Emerald Crab....only pitch-black in color. Green Emerald Crabs are quite common in every fish store I've ever been in, but I've never seen a variety that is black. Any idea what I have? Just an odd-ball colored variety of the Emerald Crab? <It is likely some species/variety of Mithrax crab. Sorry, but without a pic I honestly can't tell you much more than that... :-/ > I'd send a picture, but the only times I've seen him he's hiding between the rocks. Thanks, Chris <Do keep an eye on him... Best, Sara M.>

Emerald Crab ID??   3/11/07 Hey guys, <Brian> I went to a LFS to buy an Emerald Crab about 2 months ago, when I got there they were all tan/beige colour. The LFS said that it was because of the algae they were eating in the tanks. <Mmm, could be, I had a pizza last night and this morning I was yellow.  Do not believe this is the case.> I bought him anyways, my last Emerald Crab was bright green so I figured the new one would turn green eventually. He just molted today and now is gigantic compared to the original size, but still hasn't turned green. Do you think that maybe they pulled the wool over my eyes and that its not an emerald crab? It looks the exact same as my last green one, same carapace, same hairy legs, same claws but he's just beige instead. What do you guys think? <I'm not aware of any beige Emerald Crabs, the name certainly implies the color.  Bob?> <<Not all are green, nor stay green. "Follow the Yellow Brick Road"... Might be Mithrax forceps... see WWM, the Net re. B>> Thanks <Brian, in future queries, do cap beginnings of sentences, proper nouns, and "I's".  We just do not have the time to correct text before posting on the dailies.  James (Salty Dog)> Brian

Mithrax Crabs    Hey fellers, <Hey Hey> I was just reading your postings on bubble algae. I have a small patch of the stuff which I vacuumed up once but it came right back. I saw your recommendation for Mithrax crabs.  Are they reef safe? <They are supposed to be, but every crab has its own personality.> Do they have a common name or are they only known as Mithrax crabs?    <Around here (Northern California) they are usually labeled Emerald or Green Crabs.> One last; could aging lights be partially responsible for this pesky stuff?  I've never had troublesome algae before. <Can happen with other types of nuisance algae, but not usually the bubble algae.  As the bulbs get older they can drift towards the warmer end of the spectrum and encourage nuisance algae.  Be sure nutrient export is good, manual removal of the algae may work best. -Gage>

- The not-so-Emerald Crab - <Greetings, JasonC here...> I went to my LFS tonight and found an Emerald Crab that was pink instead of green. I of course had to purchase him but am really curious as to why he's this different color. Is it common for these to vary this drastically or did I get an oddball? <Well... my guess would be that this is not an actual emerald crab [Mithrax sculptus] but perhaps a close relative - there are other crabs in the Mithrax family which have the coloring you describe. Another option is the crab's diet, which can also affect the coloration. Perhaps we should ask the crab... ;-) > My LFS said they originally thought it was because he was going to shed but they have had him for close to 2 weeks now and nothing has happened so they believe it's just his coloring. He is eating *very* well and shows no signs of disease. <Ahh good.> On another topic, when a Turbo Grazer Snail is bred, how long until they lay the eggs? <Oh... that is a good question, and one to which I don't have a very good answer. My guess is about a week perhaps less, depending on temperature.> Ronni <Cheers, J -- >

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>

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