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FAQs about Emerald Green, Mithrax/Mithraculus Crabs

Related Articles: Mithrax Crabs, Arrow Crabs, Crabs,

Mithrax, reef safe?
I noticed that the Mithrax Crab will eat bubble algae. Are they safe for a reef tank and how many is necessary for a 90 gal tank with 90 lb. of live rock?Thanks.........Lin Smith < Of all crabs, this genus is amongst the best for reef use, though not totally "safe" (they can eat coralline/red algae, might try your other animals if hungry), but do often consume Valonia/bubble/pearl algae as well. I'd only use a couple... easier to remove a few if they've got to go than many.Bob Fenner>

Emerald Crabs, re: pagoda cup 9/10/09
<Hello.>
Thanks in advance for the answer. Is it going to hurt my pagoda cup if the emerald crabs crawl into it from time to time?
<Only if it is a constant problem.>
If so how do I stop them from doing that?
<I'm afraid you cannot.
Good luck,
Josh Solomon.>

emerald crab disease? 08/15/09
Hi,
<Hello there>
I could not find the answer to my problem on your website (hope I didn't miss it!). I currently have 2 false percula clownfish and I recently bought an emerald crab from my LFS and did not notice it was missing two legs until I got home. I didn't think it would be a problem so I put it into my tank and it immediately hid under a rock. Later on at night, the crab walked around the tank and as I was looking at it I noticed little white "dots" crawling around his front claws. I was hoping you could tell me what they are. Are they some type of disease? Will it kill my Clownfishes?
<Mmm, likely another smaller crustacean species... not harmful to your fishes>
Please help I don't want my Clownfishes to die!!
Thanks in advance!!
<Welcome. Bob Fenner>

Black Mithrax Crab HH, incomp. 8/7/09
Good morning crew,
<Hi Jessy here>
I have a few questions about a hitchhiker. I've actually been trying to catch it for a month or so, but I've never had any problems with it until now. I have identified it as a black Mithrax crab. it looks and feeds like
an emerald but black with very hairy legs. (That's kind of a "duh" statement isn't it?) I recently had a pistol shrimp and a black ray goby pair up and make a burrow underneath my live rock. I came home one day to
find the shrimp gone and his goby hiding. Later the goby abandoned the burrow and the black Mithrax emerged from their den. The next day the goby returned for a while, and that was the last I saw of him again. The Mithrax isn't bigger than the goby, but could he have eaten them both?? I would think the goby would have protected his buddy, but maybe I'm confused. If this crab did kill them both, how do I trap the SOB?
<Jake, unfortunately that is probably exactly what happened to your poor pair. Those little devils of a crab are bad. To trap them, I've heard many things. They are much like mantis shrimp...hard to catch. I've heard of everything from a small glass jar being buried neck up in the sand for the crab to crawl into with food as bait, but not be able to get back out of.
I've heard of outright spearing the little buggers, but that seems a bit extreme and skilled. The last way I can suggest is figuring out what rock the little guy stays in and removing that rock from the reef to be flushed
out in fresh water or prodded until you can better see the culprit. Either way, it is not going to be an easy process. I wish you luck!>
Thanks,
Jake Maenius
<Regards, Jessy>

Emerald Crab Vs Hawk Fish – 07/17/09
Hello,
I am in need of a cleaner pack for my 1 year old RSM.
<What makes you so sure? Of what composition?>
I have a falco Hawkfish and a perc clown. Will an emerald crab, that comes in the pack, get eaten by the Hawkfish?
<Very likely so>
Or will the crab eat my clown?
<Only if the Mithraculus is much larger than the Cirrhitid>
I need to order the pack, but I don't want to send the crab on a suicide mission.
Thanks
<See WWM re such cleaner upper assemblages. Bob Fenner>

Re: Emerald Crab Vs Hawk Fish – 07/17/09
I heard somewhere that your supposed to replace your old cleaner pack every so often. That's why I was planning on getting it.
<Ahh, the moments dancing on the adventure of learning... nearing the edge of uncertainty. Keep reading. B>

LTA wound pic + bonus, decapod spawn release 6/27/09
Hi Bob,
<D-ster!>
Once the lights went out I was able to get a shot of the injury I mentioned in my previous email.
<Good pic>
While I was at it I saw this too. Holy Stomatopod banquet Batman! Is that an M. sculptus spawning?
<Appears so>
It was standing there like that for about 10 minutes, shakin' its booty. If so, can I expect lotsa little crabs?
<Mmm, highly doubtful... may not be fertile, and even is so, the young going through instars/developmental phases aren't likely to survive recirculation, the skimmer... and what to eat?>
By the way, the mantis lives in a small hole just below the crab's leftmost foot - it didn't seem interested. There was a note on the hole that read, "out for lunch."
<Me too>
D
PS - I've got a 6-pack of Tiger here for you the next time you're passing through Kuala Lumpur. Shoot me an email.
<Dang! I was supposed to be there till 6/21... another story, time>
PPS - I hope approx. 200K is not too big a pic for your inbox.
Apologies if so, I wanted to sent fairly resolved images.
<No worries Darryl. G'night mate. BobF>

Mithrax molt 4/2/09
hi Folks,
Great forum, thanks for the effort
<Welcome>
My question is behavioral in my past experience with molting crustaceans it happens at night and I don't get to see the process. I have a large Mithrax that has been happy in a 12 G Fowler and a Kenya tree coral. Standard cleanup of Nerites, Cerith, Astrea, blue legs, 1 maroon clown,
<Needs much more room>
1 guppy and 1molly (Yea, I know, silly LB's)
all get along great and seemingly doing well.
I don't have all of my test kits with me as I am in the office on my day off but nitrates are >10ppm
I found the Mithrax hanging off of a faux coral (getting sillier) motionless (though I want to believe I see subtle flexing movements) and unresponsive, in the open. I'm thinking it's dead but don't want to remove it until I am sure. there is no odor to it and there have been no changes in the tank since the last water change 5 days past.
is this type of death like stillness normal just prior to ecdysis or is he maybe trapped in his shell alive and what can I do if that is the case ??
<Mmm, if you had a real concern that this animal was dead, AND another place to put it, I would scoop up the shell/carcass and move it elsewhere (to avoid possible pollution). As you hint, this may be just the old exoskeleton... If so, I'd leave it there for the Mithraculus to (eat) reincorporate. Bob Fenner>

Need to rid my reef tank of an Emerald Crab! A Suggestion 3/23/09
Hello,
<Hi there James, Mich with you.>
Great site by the way.
<Glad you like it!>
I made a terrible mistake and put two emerald crabs in my reef tank. Why a LFS would sell these things is beyond me
<They have their uses I suppose. But I personally am not a fan.>
but I was dumb enough to buy two, not one! I have found out the hard way that these crabs only eat coralline algae off of live rock, or better yet, coral polyps, and this is also based on other reefer's experiences with these crabs on other blogs. I tried your raw fish in a jar trap and the male went for it after a few hours and I got him out (yea).
<Yay!>
The female crab will not go for anything even if its a few inches away and stinks really bad!
<Heehee! Perhaps the females of the species are smarter than the males?>
Is there any other way to get this miserable creature out of my tank if raw meat doesn't work?
<Well I don have a suggestion but I'm not sure you'll like it.>
I think I would rather pour bleach in my tank
<PBITAWA???>
and deal with that than watch this thing eat every polyp off my corals, oh yea, and my coralline.
<Well here's my trick. It sounds like you know where she lives. You need to remove that rock. Take that rock and put it in a long under-the-bed type plastic storage container filled about a quarter of the way with water. Elevate one side of the container and put the rock in the area that is the deepest. Then gradually over the course of say 2 hours or so move that rock higher up the incline so the rock is in ever more shallow water.
Make sure you put some PVC or some small rubble in the deep end of the water so the crab has some place to hide. But the crab in my experience will want to stay in the water and you will likely find her hiding in the safety of the hiding places of the deep end. I know you are probably not very fond of her right now. But please, this animal is doing what she has evolved to do, have mercy on her, care for her responsibly, move her to your sump, return her to your LFS or find someone who can provide her the care she needs.>
My tank is a RedSea Max 250 (65 gallons). Thanks for all your great support.

James
<Mich>

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 Mithrax Crab, Mithrax sculptus, Eating Soft Coral – 12/5/07
Hello there!
<Hello, Brenda here>
Very helpful site!
<Thank you!>
But I have a quick question for you. Is it possible for an Emerald Crab to start eating soft corals?
<Absolutely!>
I have lost one all ready, and was trying to think what could have caused it. It had tear marks on the edges and middle. I have recently had a Toadstool Leather Coral and it has never really opened its polyps out fully, but today I found the Emerald crab sitting on it picking at the flesh of it. Could he be the cause?
<Yep!>
Odd as in my 50 gallon tank there is a fair amount of algae to eat. Please shed some light I've managed to catch him and I'm keeping him separated from the rest of the tank for the moment.
<All crabs are opportunistic feeders, and potentially predators. They can kill small fish, snails, and coral. Please find it a happy home. More information here: http://www.wetwebmedia.com/mithraxfaqs.htm. Brenda>

Pesky Red Flatworms, Injured Emerald Crab - 9/16/07
Hi,
<Hi there!>
I have a Mandarin in a 20 QT and I am getting ready to move him into my 180. <Good - nice big tank, hopefully mature, with lots of live rock!> Does the Mandarin eat flatworms? <If you mean the little acoel flatworms (sometimes called "Planaria"), yes, sometimes they will eat them.> Can he help solve my flatworm problem?
<He could potentially help, but if it’s a large population, he might not make too much of a dent in it. The problem is that even if you’re lucky enough to get a Mandarin that eats flatworms, they still don’t eat them exclusively. They also eat the little pods, etc, we see in our tanks, so the flatworms will only represent a percentage of the Mandarin’s diet. Please see this link (and those listed at the top of the page) for more info re: http://www.wetwebmedia.com/fltwmpredcont.htm>
Also I have an emerald crab I noticed is missing an arm <claw/cheliped?> and all his legs on one side. <Yikes, poor little guy!> I think the monster emerald got him. <Is indeed possible, especially if the two are in confined quarters. These guys don’t get high marks in the “Gets along well with others” category!> Can he live like this or will he regenerate them?
<The legs/cheliped will regenerate, although it will most likely take several molts. It’s going to be very important during this time that he’s well fed and has enough places to hide. If he can still get around (albeit awkwardly), he’s got a fighting chance. What most concerns me is that whatever attacked him in the first place, may well finish him off now that he’s vulnerable. The best option would be to isolate the injured crab so that he could eat/recuperate in peace. The next best option would be to make sure both are well fed and that the injured crab has sufficient hiding places. One last thing that I would add is that if it were me, I’d only keep one of these guys – even in a big tank. Good luck - I hope everything turns out okay for the little crab, and that your Mandarin eats those pesky flatworms! –Lynn>

Added Crabs, Shrimp are Gone; Mithraculus 8/22/07
Hey gang! Boy, if anybody can mess something up it's me.

I had/have an issue with Valonia (sp?) bubbles, it was really severe. I decided against the raccoon butterfly because of issues it might have with my inverts so I bought some small Mithrax crabs..
<Ok>
The Valonia bubbles are receding except for the big ones I can snag with a siphon tube but what disturbs me is the sudden disappearance/deaths of my shrimp.
<Uh oh>
In the past two weeks (since introducing the crabs) I've lost two very large skunk shrimp and one small peppermint shrimp. I took the deceased peppermint out, it seemed to be intact. I only saw the carcass of one of my skunk shrimp briefly this morning but it was gone before I could get it. No sign of the other.
<Dead things disappear very quickly in aquariums.>
Everything else in the tank (2 fire fish, 1 azure damsel, 1 royal Gramma, 1 orchid Dottyback, and assorted corals that have been in the tank since I started over a year ago plus one huge bubble tip anemone and it's large clown) are fine.
Is it safe to assume that the Mithrax have killed the shrimp?
<Definitely a possibility>
I wouldn't have expected it, I figured they'd feast on the bubbles.
<They eat whatever is in front of them.>
Should I trap and remove them?
<Could>
I have a large abalone I'd hate to lose but I am seeing some empty snail shells as well.
<Sounds like the crabs may be your problem but hard to say for sure. If something else is killing the shrimp and snails then the crabs may just be cleaning up the leftovers.>
Thanks for your kind assistance that you generously offer.
Lisa
<If possible separate the crabs into the sump if you have one or another tank and just move live rock in and out as they eat the algae.>
<Chris>

?Disabled? Mithrax Crab - 08/11/07
Hi guys,
hope you are all well and happy!
<Thank you>
I contacted you a while back and got some wonderful advice from Mich, so, with my new questions I thought I'd come straight to you as I am rather concerned for my Mithrax crab.
He/she, (haven't managed to work out which yet), had a molt the night before last. Unfortunately, he, (I'll use the masculine for ease!), managed to leave his main front left limb including claw, behind. I am hoping that this in time will grow back, but have been observing him since this morning and realised that his remaining, rather mean looking clawed limb appears weak and not fully operational. Because of this he does not appear to be able to feed. Poor old "crabby"... sad to say it, I am rather upset as he is quite a character. I managed to hand feed him this morning and this evening but it was rather difficult and I am not sure he has had anywhere enough to sustain him.
<Mmmm>
Do you have any idea if this weakness is just a passing phase and anything to do with the molt? And whether the missing limb will definitely grow back, and if so, any ideas how long this will take?
<I do have such ideas... these occurrences are typically associated with either "something" deficient in the way of water quality (biomineral and/or alkaline content) and/or nutritional deficiency... Please look up these terms (the search tool likely) on WWM and test for Ca, Mg, alkalinity, provide sufficient foods... for your Mithraculus>
I appreciate your time as I am sure does "crabby"!
Kirsty.
<Hotay! Bob Fenner>

Re: ?Disabled? Mithrax Crab 8/12/07
Hello Bob,
<Kirsty>
thanks for your prompt reply.
<Welcome>
My water parameters appear fine, including:
Calcium 480mg
<Is your Mg in balance?>
PH 8.0
<Mmmm, not the same as alkalinity...>
Since the crab has been with me, it has had a diet of natural algae from the tank, the odd bit of cockle, brine shrimp and Ocean Nutrition Formula Two flake food, so I would hope the diet wasn't a problem, but, I have only had it for approximately 2 to 3 weeks.
Would you say that everything I have done is ok?
Thanks again,
Kirsty.
<Please read here: http://www.wetwebmedia.com/marind5_5.htm
The tray on Arthropods... areas on Crabs... Health, Nutrition, Systems... and the linked files you'll encounter of interest embedded t/here. BobF>

Emerald Crab Death... Sm. SW set-up mis-mixed lvstk. – 06/28/07
Hello,
I'm Jarrid, I have a 10 gallon tank with liverock, a yellow tailed damsel, Talbot damsel,
<Damsels (one) need more room than this Jarrid>
2 snails, a coral banded shrimp,
<Is more than semi-predaceous... No fun for your damsels to be crowded with something that will constantly be trying to eat them...>
2 emerald crab and a curly cue anemone.
<All mis-placed here...>
Recently I found a Emerald crab claw floating on the surface and 3 days later found the shell completely eaten out. I can only assume my coral banded killed him
<Highly likely... or that it died from some other cause... dissolved or was consumed... or that it is a molt...>
but I haven't seen them fight or even pay any attention to each other. The other emerald crab is fine with no signs of a fight. The emerald that died has plenty of algae to eat and I have excellent water quality with bright colored active fish and a good colored growing anemone, I feed the animals a variety of frozen brine shrimp and squid, usually a different food each day, I make sure there is food left for the shrimp. What could have killed my crab?
<Aspects of water quality (lack of biomineral and alkalinity most celebratedly), the other Mithraculus (nee Mithrax) the CBS... You have an untenable, unsustainable mis-mix of life here. Please take the time, make the effort to read re each of these species on WWM... Their Systems, Compatibility in particular... You need a much larger system, or... Bob Fenner>

Many Questions! Tube or other Anemone ID, Missing fishes/Mithraculus 5/24/07
Hi, I am new to the hobby and bought an existing tank 120g, complete with LR and LS. It has been up and running for about 10 mo. now. I have some questions that I am not getting answers that I trust. I have researched your site often and find much helpful info. 120 g, 55 g sump and VHO lighting, G2 skimmer, 1.025, 79 degrees, ca 440, 8.1-8.3. 4 tube anenomes,
<Stop! Are these Cerianthus? If so, this is going to be real trouble. Please see here: http://www.wetwebmedia.com/tubeanem.htm
and the linked files above. If these are not Tube Anemones, You/I need to know what they actually are>
sm purple tang, md orange shoulder tang, 2 true perc clowns, 2 cleaner shrimp, 4 peppermint shrimp, 2 emerald greens,
<What's? Mithraculus? Please see WWM re>
(one pretty lg one) and many soft and LPS. (previously 3 Bartlett's anthias and mandarin). First question, I have had a beautiful tank since I started it and no algae outbreaks, can still see through my back glass. I am told I will have an algae outbreak it's just a matter of when. Is this true if my tank, stays in the condition it's in?
<? What? ... No, algae outbreaks are not inevitable... Again, time to search, read...>
I have bristleworms to die for! I understand they are not an issue, however I have such an abundance, I have been told something is wrong with my tank.
<What? Overfeeding?>
i do monthly water changes. I have my own trapping system and over the past 2 mo have probably caught 125+ of the little suckers and some very large ones. I am just trying to keep them under control. They don't appear to be bothering my corals.
<Likely not>
I have had several snails turn up dead and many around my tube anenomes.
<Need to ID these...>
 i have actually saw the tube anenomes touching some of them, including a turbo snail which soon died. I was told at the LFS that it is unlikely the anenomes would sting and kill the snails. Is this true?
<... depends on the species...>
I have coralline algae growing on the rock and overflows but not much on the glass, why?
<Read re on WWM>
Now my main concern...I bought 3 Bartlett's anthias about 5 months ago. They were all eating fine and very healthy appearing. Ate daily and sometimes 2x/day. After having them for 2 months, one disappeared. I had noticed another of them the aggressor and sometimes would chase this particular one, so I thought it stressed it out. Never saw the body. About 30 days later the aggressive one disappears without a trace. Now I'm wondering what the heck. I talked to the LFS owner and he was supposedly puzzled. Now I'm down to 1. Well about another 30 days later, he disappears and 2 days later he is in my LR about 30% eaten. All of these fish ate well and this was not the issue. I asked if the large emerald green could have killed them and I was told, not likely.
<Actually... all too likely. Maybe in concert with getting stung by the anemone/s>
I have many of those flea-like pods in my tank and my mandarin was shy and stayed in the LR mostly, he was very healthy and there is much for him to eat. About 10 days after the last anthias died, i noticed I haven't saw him.
<Note change of tense>
It has been several days now and I'm sure he is gone. He used to sleep out in the front where I could see him. What is going on here?
<You're not reading...>
My sister (who has a tank) says that anthias are just difficult and there is no rationale. I want some! I loved those fish and I want to know where I went wrong. Could it be the crab?
<Yes>
Bristleworms? Water changes?
<No, neither of these are likely... perhaps a lack of nutrition at play as well...>
I would like to get more, but I will not until I have resolution to why this happened. It wasn't a starvation issue for any of them. Are my clown fish next? Did I have too many fish in the tank? Thanks. --
Chris Young

Re: Many Questions! Tube or other Anemone ID, Missing fishes/Mithraculus 5/25/07
Thanks for the quick response. The anemones are Cerianthus. They are placed away from any other life,
<Doesn't actually matter how far...>
except of course moving things such as fish, snails, crabs, etc. They have been buried in the the sand for many months and only 2 of them have moved slightly, about 3 inches ea.
<Telling>
 I did read further and need you to clarify. These Cerianthus can release stinging cells into the water at anytime and therefore create death to other living organisms.
<Yes>
 They potentially can kill off my tank without ever touching anything?
<Yes>
Do the tentacles need to be damaged for this to occur?
<No>
I am now hesitant to add any further creatures to the tank.
<You are wise here>
 I knew they could sting others, I had no idea of the extent of damage/death they could do. I will get out the 2 Mithrax, it will be a job.
<Bait, trap them out>
Due to my ignorance of inexperience, I was unaware of their potential harm to fish/snails, etc. Do you feel it more likely the Mithrax got the anthias or the Cerianthus?
<Not possible to tell.. a combination likely>
Thanks again for your time.
Chris
<Glad to share, be myself. BobF>

Emerald Crab Spawning Event 5/7/07
I happened to see one of my emerald crabs releasing a large number of tiny little white "things." I'm thinking they were eggs but I've never seen this before. All of the fish were gathered around and wildly eating them as the crab released them. Were these actually eggs? Is that normal for the crab to release them that way?
<Yes sounds like a spawning event, a good natural food source for your aquariums denizens, enjoy.>
TCP
<Adam J.>

Watchman Goby Beat Himself Up? 4/30/07
Bob,
I would like to share something unusual that happened with my Yellow Watchman Goby (Cryptocentrus cinctus) last week, as it may be helpful for others to see a picture of this phenomenon, and of course am curious on your take on his situation.
Last Saturday evening, I placed a new piece of liverock in my tank after it had been quarantined for a few weeks. On Sunday morning, Watchman popped out of his cave for breakfast, and looked awful!! He had a huge blood-red mustache! (coincidentally the same color of the vestiges of BGA that are still lingering in my tank - although definately declining - slooooowly).
So after determining that he did not in fact have a smear of algae on his face, we got concerned. He is a senior citizen in my tank, and I get nervous if anything is amiss with him, he has been such a fascinating fish.
He is 9 years old, and 10 years is the highest estimate I have seen anywhere for their lifespan.
He had no signs of disease, was behaving, eating, and breathing perfectly normally. Actually, he seemed braver and hungrier than usual, since he has been extra shy since adding the newest fishes a couple of months back.
Anyway, back to his bruises, bright red, under the skin, no scrapes or external damage apparent. You can see in the photo that his iridescent spots are still visible on top of the bruise. Just bright red blood pools above both sides of his mouth. It was a little darker on his left side, but very symmetrical in pattern. I could only surmise that he had bruised his face, maybe banging into the side of his cave as I moved the rocks above his roof.
 I could only find one reference to this possible problem at www.nationalfishpharm.com under "changes in color". They mention possible bruising around the mouth due to fighting or intensive spawn site cleaning.
Everyday his bruises have gotten less scary looking. The first couple of days they turned more brown rather than red and now have faded to a slight shadow, that would not be noticeable if I weren't looking for it. He continues to eat and act perfectly normal.
Do you think he could have banged his own head hard enough to do this?
<Possibly>
The only other possible culprit is the Mithrax crab who was marching around the perimeter of the tank uncharacteristically the same morning, also apparently disturbed by the new rock that I put right in his normal territory (again, right above the goby). I find it hard to believe that the crab could have pinched him so perfectly on both sides of his face, and think that a head on collision would create this pattern more likely. And really the crab's claw is not big enough to fit the goby's entire head into.
<Mmmm, well, Mithraculus spp., esp. at size, can be predaceous... perhaps it was involved... indirectly... in "scaring" your Goby... causing it to dash about injuring itself>
Thought this might be a good photo for others to see if they ever run into this problem.
Thanks!!
Alex
<Thank you for sharing. Bob Fenner>

Mithrax Crab and Eviota Goby Compatibility – 4/29/07
I just today acquired a small group of three Neon Eviota gobies.
<Pretty!>
Since a couple of months back, I've had a Mithrax crab to get rid of a lot of the hair algae buildup I've gotten in my 15 gallon tank (brought on by equipment malfunction). I got the situation under control but still quite a bit of hair algae to move through.
<It's frustrating, I know, but time and continued good husbandry will remedy this.>
I have, however, been hearing lately on some forums of people having some problems with emerald crabs attacking fish, so I was wondering if my new gobies are at any considerable danger?
<Potentially, yes. Not only because of their size, but because they inhabit the same areas of the tank. Both wander about on the sand and rocks, which gives the crab more opportunity for predation.>
I previously had a small maroon clown that I traded in and it went unharmed, but I'm a bit extra concerned about the new Eviotas since they are so much smaller.
<Personally, I’d be very concerned. Keeping the crab well fed could help discourage problems (target/spot feeding 3-4 times/week), but there are no guarantees. It’s a gamble.>
Best regards, Mark Forsling
<Same to you, Mark, and good luck! -Lynn>

Emerald Crab Injury - 4/6/07
Hey All,
<Hi George.>
I have a 55 gallon reef tank with 2 Clarkii Clown fish, a ton of Hermit Crabs and 2 Emerald crabs.
<Okay.>
All water parameters are with in acceptable ranges.
<That is good. but in he future we prefer specifics, we may see something that is overlooked by the tanks owner at times.>
One on my Emerald Crabs has a damaged arm and he (the crab) is in the process of trying to remove the offending appendage. Is there anything I need or should do to help him out or should I let nature take it's course.
<The latter, just feed a varied diet, include meats of a marine origin. Maintain pristine water quality and in time...with consecutive moltings the animal will heal. I would not stress/worry.>
BTW you site is great and has helped me out on many occasions.
<Thank you for the kind words.>
Thank You
<Welcome.>
George
<Adam J.>

Emerald Crabs Hate My Tank! 3/28/07
Hello Crew,
 Thanks for the informative site, you’ve made my short 3 months of reef keeping much easier… or at least, more understandable!
<Ah... in his best Simpson's "Mr. Burns" voice impersonation, "Excellent">
My specs are: 60 gallon glass rectangle, Hagen Aquaclear 110 w/ floss and carbon only, Remora Pro skimmer, 3x Maxi-Jet 1200’s, 2x Jager 150W heaters, 40lbs of medium grain Aragonite sand, 130 lbs of live rock (Fiji and Tonga). 10k and 460nm HO T5s plus 10k and 460nm regular output T5s. This tank 'cycled' in less than a week.
 Steady water parameters; 78F +/- 1.5, 8.2-8.4 pH (am to pm), Calcium 400, Ammonia and Nitrites 0, S=35, dKH 9.2 to 10.4 (currently using Kent’s Super dKH every two weeks). I’ve been battling Nitrates as I develop my feeding skills for a 60 polyp colony of Orange Sun Polyps, usually ranges from 5 to 15ppm, most often around 10.
<This/these are fine... and I take it you've developed a cover/cup approach to assuring each polyp receives food...>
5% water changes 2 times per week. RO water topped off daily.
 Livestock; Twinspot Hogfish, Engineer Goby, Pygmy Geometric, 2x Cleaner Shrimp, multi-colored urchin, assorted hermit crabs and 15 or so snails plus 3 Turbos, Torch Coral, Purple Oxypora, Bull’s-eye Mushrooms, Pulsing Xenia, Zoos colony, Devil’s Hand, Closed Brain, Spaghetti Leather and the Orange Polyps. All specimens are medium to small and spaced no closer than 8 inches at this point.
<Okay>
 Feeding a combination of foods… TherA, Mysis and Brine Shrimp (soaked in Selcon) as well as plankton. Have restricted food in an effort to get Nitrate levels down and all livestock (except geometric) go aggressively after the food when offered.

 The issue I am having is with Emerald Crabs. The 1st was 1 ½ inches or so in diameter and lasted just two weeks, there was no trauma evident when I pulled out the carcass. Recently got two smaller ones, the 1st lasted just 4 or 5 days, the other seems to be very lethargic at 10 days. Any ideas? Other livestock appears healthy, Shrimp is full of roe, and Sun Polyps are reproducing at an alarming rate…
 Thanks again!
 Steve
<Mmm, well... Mithrax/Mithraculus can/do sometimes "just die" for no apparent reason... considering the other life you have here, the measures you present... I do think these losses are anomalous. However... I do encourage you to skip trying further specimens... it may be that the Bodianus is involved here. Bob Fenner>

Emerald Crab Missing legs 3/14/07
I have had my new 30 gall. tank running for about 2 and a half months now.
<Good for you. I apologize for the lateness of my reply, my DSL went down for some reason. I was quite pleased to see that it was up today.>
I have a Yellow belly Damsel, Chromis, 14 blue leg hermits, and 2 turbo snails.
I also have A Snowflake Eel (who is a baby, I realize he will have to upgrade in tank size as he grows)
<He needs to come out now. He will eat your crustaceans.>
and a Emerald crab. Today when I came home I was shocked to see my Emerald crab was missing on entire side of his legs. My question is could the eel have done this (as he is only the size of a #2 pencil)
<Sounds like he tried for a snack and was only partially successful.>
and if he did should I bring one of the two back to the store.
<I would remove the eel. The guys are opportunity predators, and they mainly eat crustaceans. This is not to say that some of the smaller fish that you have might not look like an opportunity, because they very well could.>
My other question is, will the crab be able to regenerate his legs during a molting period?
<Yes. But he will need protection whilst he is molting.>
He is still walking around using his right claw as a form of legs for that side of his body, and seems to be eating normally.
<A good sign.>
I guess it boils down to me asking will he survive, and will he grow his legs back.
<Without the eel, he has a very good chance of making it.>
Thanks again
<No worries. Brandon>
Ryan

Re: Emerald Crab Missing legs 3/15/07
No worries, I understand computer trouble. I figured you would be saying something like that so I took the liberty to quarantine the eel before he caused anymore death in my tank!
<Good move.>
He is only in a 7.5 gallon tank if a small piece of live rock. I know this is much to small for him to live in forever but as I said before he is only the size of a
#2 pencil. Will he be ok in there until the weekend (the LFS that I found to take him is 2 hours away)?

Also while I was reading up on the eel and Emerald Crab I read that the crab can, and will eventually eat my fish.
<He won’t be eating anything without the other half of his legs. (;^D) I would not say “will” per say, but can is definitely accurate.>
If this is the case I would like to take him out also. If not then can you share a list of compatible livestock to live with the Emerald Crab.
<Mithraculus are omnivorous. I will not outright tell you that this is a reef safe species, because there is no such thing with crabs, I will say however, that I have had good experiences with them as long as they were well fed. A possibility is to move it to the 7 gallon tank, and supply the little guy with algae as this is what they most often eat. I have seen mine stealing Nori, Squid, Bloodworms, beef heart, Mysis, and other things that were intended for the fish.>
As I stated in my previous E-mail I only have a 30 gallon tank, so I can't have large fish to defend themselves against the crab. Does this mean the crab will eventually have an afternoon snack with my clowns/ Chromis?
<Clowns, most likely not. I would not be terribly concerned about the Chromis either. I have only observed them in the upper parts of the water column. I would be concerned with something like an N. decora, or other cave sleeping species. Really anything small that likes crevices, and has a disposition to being snuck up on.>
Thanks so much, and I always appreciate your feedback!!
<You are welcome, and thank you for the kind words. Brandon.>
Ryan

emeral <Bam!> crab on back... Writer in the dark - 03/12/07
I have an emeral crab that has been on her back for about four days now, but her belly flap is open and it is an orangish color to it and it looks like there are eggs floating around in there. Do they just lay eggs or do they take care of them until they hatch our on their belly?
<More of this latter>
If this is what it is. she is constantly fluffing her tummy and picking at it. I am just worried that it might be something else going on with her that would need attention right away and if they are eggs that are fertile how do you take care of little baby crabs?
Please help?
Thanks
Diane
<... There is something wrong health-wise with this Mithraculus... likely aspects of water quality and/or nutrition... You need... to read: http://www.wetwebmedia.com/marind5_5.htm
scroll down to marine crabs, this species... the articles, FAQs files. Bob Fenner>

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"... 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

Signal Gobies, and Emerald Crab Comp. 1/31/07
Hi there,
I have a 125 gallon reef ready system, that is about three months old. I have 140 pounds of live sand, and 100 pounds of live rock. I have 1 tomato clown, and I recently put in 2 signal gobies paired, and 1 1" Emerald crab. The second day they were in the tank the gobies had claimed a spot and started burrowing under rocks and finding hiding places. I notice one of them nipping at the other just playfully, and I watched a couple times as the crag
<crab?>
came close to them. They put up their fins in attempt to scare him off but he just continued on picking on the rock and wandering around, not seeming to bother them.
<Mmm, what constitutes "bother?">
 About three days later (yesterday) I noticed that one of them were completely missing, and the night before they were both doing their thing. It was nowhere to be found. Now today I cant find the other one.
<Mmmm>
I read that signal gobies spawn by first nipping on the male, then burrowing and the female lays eggs in a burrow and then the male gets sealed off in the burrow. It has been 2 days that the one is missing and just today that the other is missing. Does this sound like spawning or homicide by the Crab.
<Could be either, neither...>
 I'm thinking of moving the rocks to see if they are under there but if they have spawned I don't want to bother anything.

 I don't even see any body parts anywhere if the were attacked by the crab.
<Mmm, you wouldn't likely... if they had been consumed>
Is it possible for them just to disintegrate, or dissolve.
<Actually, yes... they may have even "just jumped out"... or be hiding still... Again, I urge patience here>
I'm really concern. Just wandering what your thoughts were.
Thank You
Aaron
<Oh, and possibly removing the questionable crab... See WWM re. Bob Fenner>

Mithrax Crab Pestering Bubble Tip Anemone – 1/15/07
Dear Crew,
<Hi Billy! Brenda here with you today.>
Thank you for any suggestions you can give me. I recently added a rose BTA to my 75 gallon. It is still rather small (2.5" in diameter) and I wanted to let it grow before giving clowns a chance to host. <Good choice.> It has been in the tank two days and seems to constantly be harassed by the three Mithrax crabs in the tank.
I do not really underfeed or overfeed the tank. <Good> I assume it is because these animals are searching for food as it started to increase after I fed the BTA a small slice of silverside. <Yes> I kept the crabs away with tongs while it ate only to have it regurgitate several hours later. <Likely trying to get rid of what it couldn’t digest, bones, or two big of portions.> Is this pestering typical of these animals? <It is not typical, but it does happen. Mithrax Crabs are opportunistic feeders. If food is not available they have been known to eat corals, small fish, and invertebrates.> Should I attempt to remove them? <You can try feeding your crabs prior to feeding the anemone. You can also try placing an inverted strawberry basket over the anemone while it eats. If the crabs continue to disturb the anemone I suggest separating them.>
Should I not feed the anemone until I get a pair of clowns to host that will protect it? <Definitely continue feeding your new anemone with meaty foods such as silversides, krill, raw shrimp, or mysis shrimp, even after the addition of clownfish. Also, there is no guarantee that your clowns will protect or host your anemone.>
Any advice would be much appreciated; I have planned this tank for this animal and would hate to provide an inadequate environment.
Thanks you,
Billy
<You are welcome! Good luck with your new anemone! Brenda>

Update on Mithrax Crab Pestering New Bubble Tip Anemone 1/16/07
Thank you for the advice Brenda.
<Your welcome>
As an update the largest of the three crabs continued the pestering which I found somewhat odd considering it had never harassed any corals or invertebrates previously. I think it learned from the BTA regurgitating that if it pinched the anemone it would become stressed and vomit food which the crab could in turn eat. While the crab was too quick to catch during the day I was able to trick him at night by sinking a small Tupperware with a slice of silverside in it. The crab followed the rest of the inverts to the scent and was easily caught and placed in another tank. I look forward to the anemone having more peaceful days ahead!
<You have made a wise decision. A crab pinching an anemone is never good. Your anemone should be much happier now!>
Thanks again,
Billy
<Good luck to you! Brenda>

Mithrax sculptus Eating Coral 10/31/06
Hi Crew
<Hello>
Over the past few weeks my Acropora sp and Sinularia spp (finger leather corals) seem to have become a food source. <Uh oh>
Sections of the Acropora polyps are being nipped off, and large tip sections of the Sinularia go missing every few day's
I'm guessing it could be the banded coral shrimp, or a Mithrax Sculptus who has just shed his shell, and grew exponentially.
I have endeavored to catch the culprit with to no avail. Do either of these inverts in your experience eat coral?
I appreciate you help.
Regards
Damo
<Could be either, most crabs are opportunistic feeders, although the Mithrax is generally considered fairly safe. The Coral Banded shrimp is also a known coral sampler. May need to removed both and see if the situation improves.>
<Chris>

Re: Mithrax Sculptus Eating Coral 10/31/06
Hi Chris
<Hi Again>
Thanks for your response. I will try your suggestion. It should be fun trying to catch them.
Regards
Damon
<Some good ideas on traps available on the different message boards, give them a look, will hopefully save a lot of effort.>
<Chris>

Emerald Crab Problem 9/13/06
Hello - In the past few days, I have noticed that our 2 inch emerald crab is trying to capture and eat our 4 inch chocolate chip star fish. <Happens> Am I going to have to remove one of them from my tank? <Probably> A few weeks ago we had a fish die and found the crab eating him. <Most likely after death, did not cause death.> Maybe he has acquired a taste for fish now? Thanks for any advice you can give!
<Crabs are opportunistic feeders. Generally anything dead or slow will become a meal. Most likely will keep going after the star since it is a relatively easy meal.>
<Chris>

Emerald crab and cleaner shrimp - 09/01/06
Hello! <Hi Christy, MacL here today.> Thank you for all of your help from the past, present and future! <You are so kind and we all appreciate it.>
Three days ago I added an Emerald Crab to my 25 Gallon saltwater tank, 1.022 spg, ammonia 0, nitrite 0. I bought him to control my Bubble Algae. My current residents are a Scarlett Cleaner Shrimp, 2 Blue-Green Chromis, 9 Purple Mushrooms, 2 Red Leg Hermit Crabs, 8 Turbo Snails. Before I purchased the Emerald Crab, I researched and came to the conclusion that large Emerald Crabs could possibly pose a threat to small fish or crustaceans if it was hungry, but most likely a small Emerald Crab would do no damage to the other residents of my tank. <Unfortunately Christy, small ones grow to large ones and to be honest I have seen them cause problems at all sizes.> I haven't seen any sign of aggression between the shrimp and crab. My shrimp molted last night, and I didn't notice his antennas being any different earlier today, I'm not sure if I just hadn't noticed or not, but now tonight.. I've noticed that all of his antennas are considerably shorter than before, except for one which is the same length. <It is definitely possible for something to have occurred during the molt which caused the antenna length to change.>
I'm having a difficult time believing that my less than one inch Emerald Crab could have done that. I also can't find one of my Chromis anywhere! I'm now worried that I've got a little green goblin in my tank! My question is could anything have happened to my Shrimp's antennas other than the crab getting hold of him? While molting, could he have lost his antenna length? <My philosophy on this is pretty simple, if things were good before I add some creature then I start having problems after I add them, then I usually take the new creature out.>
On a different note.. I have a question about my shrimp's molting
regularity. He molts almost every week the morning after a water change. Is it bad to molt so regularly? <In my experience he's molting way too frequently. Are you adding supplements to the water like iodine? That can cause them to molt more frequently. There is a great section on molting in general in shrimp on site, I'd encourage you to take a look at it. MacL>

Emerald Crab getting sick? - 08/26/06
Hi,
<Mornin'>
I have a small (30g) marine tank that I've had running for about 4-5 months now. I fishless cycled the tank and have had fish in the tank for only 2 months now. I put an emerald crab in there one month ago and he seems to be suffering slightly. He always seems to scramble for food and has been fading his green colour to a very pale green/white.
<Hard to keep in small volumes... too change-able in chemical/physical make-up>
I was just wondering if this is because of poor water quality (more calcium?
<Possibly this, or alkalinity, or...>
I keep pH, ammonia, temp very consistent) or possibly a lack of food. There seems to be quite a bit of algae growth and I thought he would have enough food. Please help.
Jamez
<Please read here: http://wetwebmedia.com/mithraxfaqs.htm
and the linked files above. Bob Fenner>

Mithraculus comp. with Dottyback 8/6/06
I put an emerald crab in a 55 gallon tank with live rock, sand. The emerald crab was killed and/or eaten by the only fish I have in the tank; a strawberry pseudo. I was using him to rid the tank of excess bubble algae. It is the first time I have ever kept one of these fish, and found nothing to indicate the emerald crab would be in danger. Is this normal behaviour for this species? (strawberry pseudo) Thanks, Eddie
<Can be trouble here. Take a read: http://www.wetwebmedia.com/dotbkcompfaqs.htm
http://www.wetwebmedia.com/mithraxfaqs.htm
BobF>

Emerald Crab murder? By a Polychaete? 7/30/06
Hello crew.
 I'd like to preface with the "obligatory" first-timer praise/brown-nosing: Your site is a veritable treasure trove of knowledge and shared experience that I and countless others greatly appreciate.
<And here's the mandatory courtesy acknowledgement response: "Welcome">
I have spent many a night delving into the FAQ's for the answer to one query, only to find that multiple hours have slipped by and the FAQ I'm on has nothing to do with the one I started on... it's all so darn interesting. So on to my question...
 I have read in several of the FAQ's about various bristle worm spp. that are known to engulf snails, clams, etc. in a mucous "blob"
<Really? Have never witnessed this... generally errantiate polychaetes just engulf, swallow whole...>
which kills them, leaving them as a tasty meal for the worm. I actually witnessed such an interaction between a worm and a Cerith snail I had just put into the tank that evening. Not knowing better, I thought the worm may have been just checking things out. However, the next morning the snail had not moved, was pulled way back into its shell - i.e., no foot contact with the rock it was on -, and was surrounded by the telltale mucous. It was indeed dead, as a red-legged hermit confirmed by spending the rest of the day picking at its innards and then exchanging shells with it later on. But I digress. On to my question... for real this time...
 Are emerald crabs ( Mithrax sp.?)
<Now the genus is Mithraculus...>
subject to this same kind of predation?
<Can/could be>
This morning I found my lone emerald crab sitting atop a hunk of live rock surrounded by the cloud of mucous that I have come to fear. I decided that if it were from a bristle worm, the crab would be fast enough to free himself and not succumb to its suffocating effects and so I blew away the mucous with a turkey baster. But alas, the poor guy was most certainly deceased. The work of a bristle worm?
<Again... not necessarily... will get "fungussy" with death, time going by... Aquatic decomposers are by and large much "quicker" than terrestrial settings>
Should I suspect something else, as of yet unseen?
<Mmm, could>
I can see snails falling victim to such tactics, but its tough to picture an emerald crab just sitting there and allowing a worm to engulf it. Any insight?
 <Most captive marine crabs, snails are killed/die from unsuitable chemical circumstances, vacillations thereof... of a few genres: "Lack of biomineral and/or alkalinity", nitrogenous waste accumulation, variable specific gravity, "medicine" application... These are outright stated, many examples proffered on the various biological groups "Disease/Health" and "Systems" Subfaqs files>
 Thanks in advance. And a thousand thanks to the whole crew for your time, effort, and patience.
 -Jason
<As many welcomes. Bob Fenner>

Emerald Crab Turned Decorator - 05/19/2006
Hello! I hope you don't mind me emailing you but you seem to know a lot about Emerald Crabs, and their like.
So I was hoping you might have seen this before, or can start me down a path to finding out the answer.
I have an Emerald Crab that has what appear to be sponges growing on it.
Small white sponges, I can see their little hole. It is on the main body as well as the legs. I have been looking around for information on it, but this is the only site that I have seen that has any information on crab diseases. The crab is not dead, or seems to be dying, but I am curious about it, and if it might be detrimental to it in the future.
<Would not worry too much about whatever these are. Once the crab molts, the critters will go with the molt.>
Thank you so much for your time.
<You're welcome. James (Salty Dog)>
James

Mithrax crab disease or molt? 4/30/06
Hello,
I have searched WWM.com and cannot find an answer to my question regarding white patches on my Mithrax crab/emerald crab. These patches which are on
her legs and carapace look like small chunks of fluffy matter or like patches of salt build-up. I caught her having sex with the larger Mithrax
crab and read on another website that she may molt soon.
Could this be signs of molting or is this a disease?
<Could be either, neither>
 If it is a disease,
<Not likely>
do you know what it may be and what I can do for it or where I may search on the web to find such information.

 I really would like to save her as I have had her almost a year and she is cute and causes no problems. Plus my larger, male emerald
seems to like her as well since he chases her around the live rock and cuddles her!
Thanks for any help you can give me. I know she just a crab but she's family now..
Geri in Newton, MA


Dead Emerald Crab...Leave In Tank? - 03/22/06
Dear Crew,
<<EricR here...>>
After several weeks of reduced activity and increased 'hiding' it appears that my Emerald crab has passed. I have located him in a tiny pocket underneath lots of live rock with my flashlight.
<<Ok>>
His shell has turned a creamy white and it has not moved at all in several days even when food floats right in front of him (which, in the past, any amount of food in the tank would result in frantic stuffing of his mouth with sand!). I cannot reach him without un-piling all of this rock and felt I could probably just leave him to the 'elements' where his shell would provide calcium back to the tank like when he molts.
<<Indeed...and likely those "elements" have already dealt with the crabs flesh/organs.>>
Is there a downside I should be aware of in this situation or is this OK?
<<A moot point by now I'm sure...nothing to worry about. The micro-fauna, as well as other macro-fauna in your tank will have quickly dealt with any decaying matter.
Mark
<<Regards, EricR>>

Emerald crab chances against eels ... Pizza/RMF analogies 1/18/06
Hi! Another counseling request. As previously stated in my first email part of my aquarium residents is a snow flake eel. This eel have been a well behaved one. It doesn't bother any of the small fishes that I have (ocellaris clown, blue damsel, cardinal) neither the 3 or 4 hermit crabs or snails that commonly pass by his nose I can't say the same from my harlequin tusk).
<Both will consume them>
Based on that and because of a bubble algae problem I'm having in the last 2 month I acquired a small emeral
<Bam!>
crab to add to the tank. For the first 2 or 3 days everything was ok, or at least that I thought, but last night I saw a situation that, although interesting and educating in terms of predator/prey relation is not my main interest in my aquarium. I was looking for my brand new crab in the live rock when I finally found him feeding on a crevice in the rock but to my surprise the snow flake was actively looking for something very close to the crab. Suddenly the eel turned his head to the crevice and took the crab in his mouth. At that moment I though I lost the crab, but after a couple of minutes of chaos under a rock I kept looking and discovered that apparently the crab escaped from the mouth of the eel and was clamped to the live rock. The eel constantly try to bite the crab but was not able to break it loose from the rock but kept trying with a fierce and aggressive intention not normally seen in my eel.
<Like me and a pizza... when one's around, or round>
To just finish this drama I fed some krill to the eel and when he was satisfied and moved to the other side of the tank I retrieved the crab and put it in my refugium. From what I saw its just clear that it will be no easy task for the eel to feed on this crab but I wondered why so much interest in the crab if the eel never did the same with the hermits or snails or other residents of my tank.
<Less shell... to make a further analogy, I prefer thin crust, but will, if not available, consume a thicker crust pie>
Do you think I should put the crab in the main tank again and just fed the eel more frequently? Actually I fed the eel 2 or 3 times a week.
<Uhh, will be consumed... Bob Fenner>

Re Emeral (Lagasse?) crab chances against eels 1/19/06
Nice analogy! And logical, thanks as always. I will let it live in the refugium(2nd host, the first one was a cleaner shrimp that still roam in there).
<Real good. Bam! Bob Fenner>

Question about emerald crabs
Date: Mon, 31 Jan 2005
Hello gang!
<Hello>
I want to first say thanks for all the information you have given on your site. I have read a lot of the info you have posted which helped me get my tanks set up. My girlfriend and I currently have a 20 gallon tank with a porcupine puffer, lionfish and a stingray.
<A humongous amount of fish for a 20>
They are all juvenile and have got along great since I purchased them 5 months ago. I am currently setting up a 150 gallon tank for them..
<now you're talking>
... which I finished all the plumbing for the wet/dry filtration this weekend. The cycling process has begun and I am hoping to get my damsels in 3 weeks..
<Ah, more food for the lion>
... to start the "real" cycling before the main guys go in. Anyhow, to my main question: I also have a 12 gallon NanoCube which has about 15lbs of live sand and a large piece of live rock, probably about 7lbs. I have a Maroon Gold Striped Clown, rose bubble tipped anemone for the clown, one cleaner shrimp and the 4 damsels that cycled the tank in the beginning. They all get along great and the water quality is always zero (knock on wood). I had a small, but annoying problem with brown diatom algae and brown hair algae. After this began, I purchased my cleaner shrimp. I also started using RO/DI water, so the brown algae problem is 95% taken care of. My slight green dot algae is another issue, but not enough to worry about. I noticed during a water change that my LR had a green bubble on it, I poked at it with my planter prongs, not knowing what it was and it popped.
<This is something you don't want to do, do not poke or squeeze bubble algae, you just increase the spreading of it.>
Needless to say, I have a small chunk of that stuff growing on my LR now, a patch maybe the size of a thumb nail, with a couple other bulbs here and there on the rock. I have read the Emerald Crab will take care of this...
<Most do, some don't>
... but I am concerned about my other inhabitants of the tank. I read in your FAQs that he may go after the fish and cleaner shrimp, will he go after the anemone as well? If the chances are more likely that he will try to eat my livestock, I won't even take the chance of getting one.
<There is no danger of the crab bothering anyone in your tank>
All of our fish and shrimp eat out of our hands and we are pretty attached to them and don't want to risk losing them. And yes, I do feed the lionfish (he eats krill) carefully, that is one poke we don't want to experience. I'll just try to remove the algae by hand if possible. What do you suggest?
<As before, start with the emerald crab, don't overfeed the tank, any excess nutrients promotes all algae growth.>
Thanks for all the help!
<You're welcome, James (Salty Dog)>

Emerald Crabs and Predatory Behavior/Nano Reefs 12/5/05
I learned too late that might happen, and it did.
<Sorry to hear that.>
My question is, do I remove the crab or add some sort of different snail.

I'm running a tiny nano reef at the time, planning to upgrade after Xmas. My other question (due to space constraints), how small a salt water set up can I realistically get by with?
<Well you said upgrade so I’m assuming you want something larger than that, honestly I would go with a tank as big as you can. The larger the water volume the easier the tank will be to keep. Seems like you are still trying to go the way of the nano, I like the 15 gallon standard for that application…good surface area.>
My teensy 6 gallon has done just fine with 6 pounds live rock, a yellow tail damsel, a peppermint shrimp, an emerald crab, and a Cerith snail, till the crab ate the snail. So, how much bigger should I go and still be small?
<See if this answers some of your nano questions: http://www.wetwebmedia.com/nanoreefsysart.htm , Adam J.>

Valonia and Mithrax (3/13/04)
 Greetings and a huge thanks to all you in the crew for providing me and others such invaluable guidance, in the year I've been reading the posts daily, my tanks are UNBELIEVABLE!! Everything is growing and thriving at exponential rates. <Good to hear.> Which brings me to my question. I am getting a ton of green bubbles popping up all over the live rock in my 40 gal reef. I know nutrient control is the issue, and I've greatly cut back my feedings, tweaked my Remora for optimal skimming, and do 2 gallon water changes twice a week religiously. I have been hearing more negative than positive comments lately about Mithrax crabs being destructive and difficult at best to remove when larger, so I'm hesitant at putting one (or any crab) in my system. <Wise to be cautious.>
 Additionally, it seems to be the general consensus that in trying to remove Valonia (various species) one has to be very careful not to puncture the "bubble" so as to not release spores into the water column. My question is: if I do put a Mithrax crab in, isn't that exactly what they do, puncture the algae, and consume it (if they have a liking for it) so why does it not spread under these circumstances? <Some risk I suppose, but does seem to work for some.>
What are your thoughts on just slicing the bubbles open as soon as I see them appear (i.e. real small) with a razor blade? <Vacuuming them off would probably do better. Anthony & Bob's book (Reef Invertebrates) recommends attaching a toothbrush to the end of the siphon. That way you can suck up spores as the bubble bursts. Another option would be to remove rocks that are heavily infested and scrub all the Valonia off in a bucket of saltwater before returning the rock to the tank.> Doing this persistently, would the situation get worse, or better, not allowing any to reach a size greater than 1/8th inch? I was thinking doing this, the skimmer and other inhabitants would consume/remove any spores that may get released.. <Will help to some degree.>
 Other than that my tank is looking awesome, and in less than 3 months running, the ENTIRE back face of the tank is totally encrusted in what appears to be at least 4 colors of coralline algae, and the fauna is exploding! ((I chose not to add any fish until 6 mo. to a year) <Your patience will be richly rewarded. Keep up the good work. Choose your fish carefully and quarantine them.>
Thanks again for all you've done to assist! keep up the good work.
Blair <A pleasure. Hope this helps.>

Missing Cleaner Shrimp
Hello again, Hope you are doing fine. I have a 29 gallon tank with 25 lbs of live rock. It has been set up for about 2 months. One of my white striped cleaner shrimp ( Lysmata amboinensis) has disappeared, I think overnight, a couple of days ago. There is no trace of him anywhere, no body parts or anything. The only other animals in the tank are another white striped shrimp, 2 peppermint shrimp, a small (size of a dime) emerald crab and some Cerith and Nassarius snails.
<The Emerald crab is your most likely suspect from the list given.>
Does this sound like there could be a mantis shrimp in the tank?
<No>
I have checked the tank at all times both day and night and never have seen a mantis. I even use a magnifying glass and flashlight and check all the holes in the rock, and have never seen a mantis. I was going to get a couple of captive bred seahorses in about a week, but should I hold off to be certain that I don't have a mantis lurking about?

Thanks again for all your help, Kevin
<You are welcome. -Steven Pro>

Getting rid of unwanted Mithrax crabs
Dear Everyone:
I really have to congratulate one of your readers. Her suggestion of putting food in a tall slippery glass jar worked like crazy for me. I put a couple of oysters in a glass jar about 8" tall and surrounded it with live rock. In about 10 minutes, no longer, the crab showed up and tried twice to haul himself over the edge, which was slippery. I'll be darned if he didn't make it on the second try and he landed in no-crab's land along with the oysters. He is now out at sea via the flush system. Just had to pass this along to you as it was so easy. And to thank whoever suggested it in your crabby mail. Connie (still painting)
<thank you for sharing! Anthony>

Mithrax, reef safe?
I noticed that the Mithrax Crab will eat bubble algae. Are they safe for a reef tank and how many is necessary for a 90 gal tank with 90 lb. of live rock?
Thanks.........Lin Smith
< Of all crabs, this genus is amongst the best for reef use, though not totally "safe" (they can eat coralline/red algae, might try your other animals if hungry), but do often consume Valonia/bubble/pearl algae as well. I'd only use a couple... easier to remove a few if they've got to go than many.
Bob Fenner>

Have you ever heard of this happening?
Hello Bob,
I've written in the past to ask questions about the upkeep of my nano reef, but this time I have more of an anecdote to relate. Night before last, I spotted my happy, well-fed and well-adjusted ocellaris clownfish, which was about 1.5" long, apparently 'wedged' between two rocks. Thinking (naively) that he had swum into a spot from which he could not escape, I tried to use a feeding prong to ease him free. After several minutes of prying, I almost had him free, when a frustrated green Mithrax crab reached out of his crevice and pulled him back in! It appears my emerald crab had caught and killed the clownfish, and was in the very early stages of consuming him. I know I caught him right after the fact, since the fish's eyes were clear,
his color was good, but his gills were still when I finally freed him from the tank. Have you ever heard of such a thing happening?
Thanks for your time. Josh
<Yes. Unfortunately, Mithrax/Emerald Green Crabs are not entirely the benign, strictly algae-eating creatures some folks make them out to be... They will indeed eat fishes when they are large, hungry enough to catch them. Only small crabs of this genus are to be tried, trusted. Bob Fenner>

Crabs and lighting
Bob
Thank you for all your help I really appreciate having some one to ask all these questions to. I have two questions for you. I deal with Two LFS and I am getting conflicting information from them.
<If you keep asking me, you'll likely have a dozen!>
One store tells me that emerald crabs are reef safe and will not eat corals, polyps, or fish the other store says that they will.
<Hmm, here we go... Most of the time, small Mithrax crabs won't harm sessile invertebrates... But there are many cases of larger ones doing so...>
So if you could resolve that question for me I would appreciate it. I have three in my 46 gal tank now so I need to know what to do.
<This is a bunch in a small system... You, they will likely be okay for a good long time... as they shouldn't grow that much, that fast in such a setting>
My other question is I have a 36in JBJ compact light with 4 36 watt 10K bulbs on my 46 gal tank. One LFS tells me that is plenty light for a brain and candy cane coral the other store says it is not enough and that I need metal halides. You are going to be the tie breaker please let me know your thoughts.
<You have sufficient light... the shop has a hankering to sell you a chiller... Bob Fenner>
Thank You
Jason

Emerald crab snack box?
Hi,
<Good morrow>
I have an emerald crab that I have noticed is cracking his shell open frequently and digging around inside. I figured it was just molting but this morning he was a little closer to the glass and I could see that he was pulling out what looked like Mysis shrimp (which I feed regularly to my copperband butterfly) and eating them. Am I just seeing things or does this little guy have a built in snack box where he can store food for later?
<Not as far as I know>
Also, I have a jawfish that has totally wigged out. He is acting all freaked out and hiding, when just a few days ago he was absolutely fine. My water chemistry is all excellent,
<Need values, not opinions re... in particular, given the data you present, am suspicious that you may have an imbalance in alkaline earth materials (calcium, magnesium) and alkalinity>
temps are fine, etc., all my other fish
and reef animals are doing well and show no signs of stress. Is this normal behavior for a jawfish or do I need to find a good aquatic psychotherapist?
Thanks, I love your site ad have learned so much from it!
Rob
<Need more info. to respond more... Bob Fenner>

Condy and emerald?..... Nemo and Dory?
Hi WWM. I have a question for you guys. Is there a connection between Condy anemones and emerald crabs?
<Evidently so: http://www.tmbl.gu.se/pdf/TMBL_pdf/Library_and_databases_pdf/examensarbeten_pdf/Lisbeth20p.pdf >
I introduced my Condy (since it was in a in-tank refugium) into the main part of my 55 reef and the emerald crab shot right to it. Always staying right by it like a clownfish with anemone. Perhaps it needs comfort? No predators in my tank. I don't know if you guys have heard of this relationship but it kinda seemed weird to me. This is not the first time this has happened to me. Different emeralds and different Condys. What are your thoughts? Keep up the good work. Teddy
<Will try, am trying. Bob Fenner>

Re: Frustrated by some opinions, cont'd
Thank you Bob.
<Welcome James>
If I had years of experience and the confidence there from it wouldn't be so frustrating. I just want to make sure that I am doing well by the living little friends that are now dependent on me and my decisions for their care. You can 'fix' a broken car but you can't bring the dead back...

Aggressive behavior is another good reason to avoid a smaller tank. The Royal Gramma is quite territorial over 'his' little areas and the snails and yes the Emerald Mithrax crab (no fish store wants him and I can't just off him) get chased and bit at regularly. I have seen this little tyrant knock the snails off the rocks and bite at the crab.
<Mithraculus are largely misunderstood... like most crabs they are "eater uppers", not cleaner uppers>
Sincerely,
James Zimmer
<Bob Fenner>

I read your book: "Conscientious Marine Aquarist" - has been so helpful
Hello Bob.
<Hi there James>
First off I wanted to say thank you.
<Welcome>
I was given a small tank (5 gallon mini-bow) from a friend this Christmas.
<A nice gift, but very small for a marine system>
It is nice and I figured that it would allow me the luxury of minimal loss if my lack of experience caused a disaster. Well, so far so good. I followed all the setup parameters outlined in your book. Anyway the tank was running with just the pre-mixed and aged water for a week to get an idea of temperature fluctuation and evaporation rate. I suppose since there are only 5 gallons I am particularly cognizant of how little leeway I have.
<Well-stated>
Tank readings are both good and consistent. I have liquid test kits and the test strips and the levels seem to coincide well.
Temp: 75-76F
SG: 1.0225
Alkalinity: 300ppm
Nitrate: 40ppm
<A bit high... would be better kept to half this maximum>
Nitrite: 0ppm
pH: 8.2-8.3
Having had the live rock in the tank for just over a week my wife and I noticed three very small star fish. Yesterday I noticed some hair-like (algae?) that is reddish brown in color and decided to get some cleaners earlier than the two weeks recommended in your book. I picked up two bumble bee snails a Nassarius snail and an emerald Mithrax crab (the little eating machine picked the plant life that I did like clean within minutes of finding it!) What do you feed these eating machines and how much do they need to keep from devouring everything else in sight?
<Please read here re: http://wetwebmedia.com/mithraxfaqs.htm Your system is actually too small for this species>
Being inexperienced I am particularly concerned with unintentional overfeeding. Would I be better off to remove the little guy and trade him for something else?
<Ah, yes.>
From what I read on the emerald Mithrax crabs (www.WetWebMedia.com) it seems I may have been premature in this acquisition. Since I have only seen them sitting in tanks I never had the opportunity to see just what these eating machines are capable of. He is kind of cool but no biggie to part ways with. The next step is a larger tank. We are waiting to resolve some power and air conditioning issues in the apartment before I invest in a larger system. The 5 gallon will then become my quarantine tank. I just need to be sure that the power won't kick off and come home to a dead tank and lost pets.
<You are wise here. There are "back up battery" units that can keep small systems running for several hours>
Oh, as a final question the guy at the fish store said I was making a mistake by buffering with baking soda. I saw this in your book and used a ¼ tsp mixed with tank water and then returned and the pH has been wonderfully consistent. What is the mistake about this method of buffering that according to him you can get away with in fresh water systems?
<... a bunch that could be stated here... sodium bicarbonate IS the principal ingredient in both marine and freshwater pH buffering products commercially... Methinks the dealer is wanting to perhaps sell you something>
Sorry for the length of the email.
Thank you.
<No worries. Thank you for writing. Bob Fenner, still out of the country>
James A. Zimmer
Garfield, NJ
<My mother in-law lives on Jewell across from the park in Garfield!>

- Emerald Crab Problems -
Hope all is going good with you tonight.
<Well... it's morning now... things seem ok.>
I have a problem I can't find on your site. I have a green emerald grab that has some bugs on him. They are like a clearish red color with a white spot on the back of them. They resemble an appearance of a regular mite. They are very small, about the size of a the tip of a needle. He also has a small hole in his left claw right on top. Its not getting any wider, just deeper. He is in a QT tank although I don't know of anything to treat him. I'm sure most things would kill him, even a freshwater dip. So do I let nature take its course or what?
<Yes... is really your only option for these animals. Make sure the water quality is tip-top.>
Thanks for your help. Josh
<Cheers, J -- >

Mystic Mithrax
Dear Bob: I have a 155 gal tank started in Oct. 99. I have lots of live rock and also the live sand. I really haven't had much growth on it except some brown grass and my Mithrax crabs ate that. I think they are eating so much that nothing is growing. I got 10 of them. I have feather dusters, two anemones, some different corals(7) arrow crab, a clam, two metal halide lamps, skimmer, 350 magnum pump with charcoal, temp. 80%, water tests good, but: I just lost a large coral (star coral) that had been doing really good, before that a carnation coral, before that an elegance (2). It seems that the inhabitants I have now do well but new ones slowly die. I just purchased some plants from ff. and I'm hoping this will help. I also really like them. Can you help? I lovvvvvvvvvvvvvvvvve this hobby. Temp. 78 - 80
Brenda (thanks and I really appreciate your diary.)
>>
Yikes, ten Mithrax Crabs? You really should try to remove all but a couple or three of these... more are too many... a tough job I know.
Is the Magnum (canister filter) all your filtration? Not sufficient for a 155 gallon system... Hopefully you have left off some gear, circulation mention.
And the Carnation and Elegance choices were poor... they generally don't live...
New animals slowly dying is a bad sign... Poor water quality, some sort of poisoning from inside (dead, dying organisms) or outside (supplements... what is your history here?)... The macro-algae ("live plants") you mention getting a step in the right direction... but do read over the Marine set-up and maintenance pieces stored at my site: www.wetwebmedia.com for insights into what's been done so far that is the cause of your losses. Once we get your system set-up and running properly, it will be far more fun and easier to maintain.
Bob Fenner

Emerald Crabs
Thanks for the response James! I ended up getting one of the emerald crabs at a LFS and he has since rid my little nanocube of all but 2 pieces of bubble algae. Unfortunately, he injured one of my 4 stripe damsels in the tank along the way. There was a small hole in the live rock that the damsel used as his little hiding/sleeping spot that the emerald crab decided to take away. The crab pinched away a large portion of the scales on the left side of his body, to the point where one of the stripes was almost completely gone. The little guy has since died after a 2 week struggle to heal (which seemed to be healing nicely) and the emerald crab has pinched off half of one of my cleaner shrimps antennae. Hopefully his whisker will grow back in his next shedding.
<It should>
Good news is that my 150G tank is well into its cycling, and the levels are starting to lower now. Maybe another water change and it will be ready to go.
<Good!>
My California spotted stingray has stopped eating the last couple days, which made me look through your FAQs for possible reasons. I have added iodide into the tank for a small lump that seemed to be on his belly, which I thought was a goiter, which turned out to be his last meal actually. I also tested my levels and found the pH has dropped to 7.8! That obviously will affect the critters' eating habits (well besides my puffer since he eats everything in sight). The supply water that I have been using to drive my RO/DI system has low 7.0 pH levels, so my top off water really affected my tanks. Word to all those out there, watch the top off water. I'm working on stabilizing the pH in my tanks now, hopefully it's not too late.
<Shouldn't be, it's not drastically low>
Thanks again for all the help you give to the readers out there, I for one really appreciate it!
<You're welcome, Mike. James (Salty Dog)>
Mike
P.S. I would like to give a stamp of stupidity to someone that was in my LFS that asked the clerk "how long will that starfish last in my freshwater tank if I bought him?" I'm glad to see the store would not sell him the animal. Some people are amazing ridiculous, to put it nicely.
<We get similar questions here.>

Emerald-a the Mithrax
I just found your site while I was searching for answers to a problem. I have a small emerald crab in a 55 gal tank, LR filter. Bought the already running tank in Sept., and got the crab in Oct. Have run water tests with Saltwater Master test kit every week. Everything is very consistent and in normal bounds with exception of nitrates (40)...
<Cherie, one thing to keep in mind about testing for nitrates...some test kits read high level and some read low level. You have to determine what kit you have.>
... but always consistent. The crab was last seen moving and eating two days ago. This morning she was still. I picked her up and she moved feebly. I put her back down and she flipped over on her back. I righted her and she just doesn't move anymore. Have left her alone since. She has white spots on her back which I assumed were coralline algae. But when I examined her closer, they were 3-d and swirly. Could they be some kind of worms or parasites? I've got two other emeralds in a 75 gal tank but they're all connected with a common sump. The other female also has these spots but I don't see them on the male. Any ideas or advice would be appreciated.<It's quite possible your emerald may be in the molting stage. Keep an eye on it for a few days. James (Salty Dog)> Thanks, Cherie

Emerald-a..? What Happened?
The emerald crab that died had small white " 3d" swirls on her back, about 10-12 of them. Could they be a parasite? <Sorry Cherie, I really don't know without a picture of it that some members of the crew could look at. James (Salty Dog)>
Thanks, Cherie

Green Emerald Crab
Mr. Green was eating bubble algae and green grass till he devoured it. I had 4 damsels 1 domino one white and black striped and 2 blue with yellow tails. Removed all but one yellow tailed. 2 peppermint shrimp s sifting stars are still in there. I was feeding them frozen brine but backed off on the amt. of brine (enough to feed the tang) didn't want to pollute the tank. don't know if I should continue to feed more for the crab or what meaty things to feed him or the shrimp. Will get more fish (Gobi blenny and hopefully a tang eventually but I want to go slow and add more live rock. I will do the rock first so not to stress the fish out. <You should try feeding some Mysis and some other foods and stop the brine as most is over 90% water and has little nutritional value. Yes, you should be feeding the crab also.> I am taking my time whew patience is a virtue. next question for the duster I fed Mr. green real well and went to move the duster he is at the bottom of the tank but now I have another problem. He has seemed to attach the end of his tube to the rock. now what? Do I tug and remove it and move him or what? can anything be easy LOL? Please help thanks.
< You could try to gently cut it away but it would be best to move the whole rock. I don’t think you could just rip it off without doing some damage to the worm, as they attach themselves very well. Cody>
Misty

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>

Emerald Crab
>I noticed a few weeks ago (~5) that my emerald crab's pinchers were missing. It seems to have lost them during one of its molts. My Sally lightfoot  crab didn't lose it and it molts about 2-3 times a month.
>>You have a mix of two different crabs, both known to become aggressive, are you certain they were lost *because* of the molt?  It could very well be that the Sally Lightfoot (btw, there are at least two, if not three, other species of crab called "Sally Lightfoot"--ALL aggressive) managed to catch the emerald at its most vulnerable.  With such a mix this must be expected.
>It is still eating. It has learned to use it legs to get food. Will my emerald crab's pinchers ever grow back?
>>Yes.
>All the water parameters are zero and I add iodine to help with molting.
>>All parameters, even pH, temperature?  ;)
>Also what would a likely cause be for the disappearance of my peppermint shrimps? I had 3 but they slowly disappear.
>>Quite easily one of these crabs, or you could even have an unknown (and most unwelcome) guest in the form of a mantis shrimp.  If you ever hear odd clicking noises from the tank (especially at night), you may have a mantis.  These animals can decimate a tank.
>I have 4 mid-sized pieces of live rock, 2 pink skunk clowns, 1 humbug damsel, and 1 Nemo. Any info would be greatly appreciated.
>>There you have it, you have an aggressive (far too aggressive for peppermints) mix of inverts going on there, in my honest opinion.  Marina
Thanks

Emerald Crab
Greetings Bob and Crew!<<Greetings to you too!>>
I just recently bought two small emerald crabs to take care of some
algae in my tank. I know that they will grow larger and attack my snails and fish, but I'm hoping not for a while. When they do grow too large, I was thinking about buying a Marbled Cat Shark. I have a 100 gallon and in Scott W. Michael's Sharks and Rays (Bible) said that the Marbled cat shark can be housed in a 70 gallon aquarium. Do you think the shark will be able to take down the two crabs? And if not, how would I go about taking them out? I am not so worried because they are both less than half an inch wide, but still, I want to plan for the future. Could you tell me a good way of getting rid of them? Thanks Bob I really appreciate it!. Thanks and I love the site!
<<Catch them and trade them in at your LFS. I would not recommend keeping a marbled cat shark in a 100. Yeah, maybe a 70 for less than a year. Sharks get BIG, are messy, require massive filtration and skimming, streamlined/angled corners, no powerheads or stuff to knock loose, etc. Please read the shark info at http://www.wetwebmedia.com/sharkfaq.htm
Many letters regarding your interest. Many requirements you are unaware of to learn about here! Please look carefully before you leap! They require BIG systems. Craig>>

Mithrax
Bob, please stop me but I have more questions. I just found this website today. ;)
I was wondering on your thoughts regarding Mithrax crabs? Specifically I've seen (and caused) some debate on the boards recently as to whether these crabs are bad guys are not. Many people swear they were caught red handed feeding on corals. But many others have argued that maybe they were just cleaning damaged tissue. I challenged people to speak up if their crab was not only caught picking at the coral but that the coral showed tissue damage. No one really seemed to have any hard evidence that they had caused any real damage. But I saw on the website that these are known fish killers. I'd like to hear your thoughts on the issue.
Cheers,
Chris (aka newkie)
< Hey Chris, Gage here. The problem with crabs Mithrax included is that they are opportunistic omnivores. They will eat anything that they feel like eating fish, coral, snails, other crabs. If they think they can take'em they will have a go. Check out this link for some more info on Mithrax and other crabs.
http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabfaqs2.htm>

Harlequin shrimp/Emerald crab question
Greetings, JasonC here.
I've not actually heard that Emerald crabs can be a threat to Harlequin Shrimp directly. That being said, there aren't many crabs that can be trusted much farther than you can throw them. They're quite often too opportunistic for their own good, and the well being of other tank inhabitants.
I do know from my own Hymenocera that they will use their flat paddle-like forelegs to wave about and chase off the too-curious.
I would just keep an eye on the crabs.
Good luck,
J --

- 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 -- >

Not a minute-man! (Emerald Crab)
Hello, oh helpful ones:
<whassup, bubba?>
So the other day, I see my Emerald Crab isn't moving. I figure, okay, I'll wait a day and see if he moves. Nothing. So, I finally take the poor guy out and say goodbye.
<literally or figuratively on the "goodbye" part?>
Then, I figure, while I have the gloves on, I'll move some of my LR around a little. Lo, and behold, my Crab moves from under a rock. A few questions, numerically, if I may:
<rock on my brother...>
1. I guess he molted (this is a first for me)?
<indeed... a molt of the exoskeleton. Conspicuous in some lazy arthropods that leave it lying around... rare with others that eat it promptly (for chiton/protein recycling)>
2. Now I read that you're supposed to leave the molt in the tank. Is that true?
<true for above reasons>
3. I already got rid of it. Is that bad?
<no biggie>
4. Is 2 weeks considered fast?
<fast?!?! 15 minutes is pretty good for most of us... 2 weeks (!), you are a god. Your poor wife/girlfriend.>
5. What would influence the speed of a molt?
<diet, water changes and iodine for some/many>
As always, thanks for all your help. Rich
<no... thank you for giving our team a good name <G>. Best regards, Anthony>

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>

Mithrax crabs
Dear Bob:
First of all I have to tell you that your book is my bible. I keep it near my 60 gallon saltwater reef tank and as a result it looks pretty dog-eared. (It's had it share of salt water). I had a bad case of bubble algae and the fish store recommended Mithrax crabs.
<Very typical.>
My husband came home with one really full-sized crab and he couldn't do the job, so we got a bunch more, most of them small. I have four false Percula clowns, 1 pygmy angel and 1 royal Gramma, also a bunch of small crabs. I have a feeling I am missing some crabs, and now am frightened that I'll lose one of my clowns. They are all juveniles and one in particular is not a fast swimmer. The Mithrax crabs have done a good job but I now realize I have too many and would like to get rid of the largest ones and keep maybe one or two of the small ones. Question: How do I accomplish this without emptying out the tank.

Is it my fertile imagination that one night soon I'll lose a clown to one of these crabs?
<No, a very real possibility.>
And should I have any at all.
<One to two small ones.>
If I got a tang would that crowd my tank and would he eat the bubbles?
<Depends on species, but probably yes and no respectively.>
This is early Monday a.m. in California. I don't know where you are but I sure hope you check your email often. I think we need to take action within the next few days.
PS We put some ROWAphos in our pump along with the carbon filters and it looks as if it's helping with the phosphate level, which was never sky-high as I have a protein skimmer and conscientious about cleaning the substrate.
Thanks so much for your help. Connie Cavan
<Good luck! -Steven Pro>

Mithrax crab and Thalassia Seagrass questions
Hi WWM Crew!
Having just recently discovered your site, I want to tell you how much I have benefited from your answers. Thanks for sharing your expertise. Two quick questions: Will Mithrax crabs eat on a starfish?
<Definitely one that is damaged or sick in some way. Less likely a healthy one, but still a possibility.>
(My blue Linckia starfish appeared one morning missing an arm--and there are no other likely villains in the tank!)
<Possible>
Secondly, I am just about to start up a 15 gallon refugium as a part of a 75 gallon reef setup. I read with interest your recent answers indicating that turtle grass is superior to Caulerpa in a refugium. Any idea where I could obtain it?
<Check the links page on www.WetWebMedia.com for livestock e-tailers. I know one of them carries it, I just forget which one.>
Thanks in advance, Tim
<You are welcome. -Steven Pro>

Emerald Crab
Hello Bob, how are you today?
<Bob should be doing wonderfully, he is diving in Cozumel. Anthony and I are holding down the proverbial fort for the week with Barb's help posting all our answers.>
I bought a small emerald crab a couple weeks ago from the local fish store. He was missing one of his claws, due to a fight with one of the other crabs. They told me it would grow back quickly,
<Back yes, but quickly no.>
but after 2 weeks, I don't really notice anything growing yet (it is hard to tell, but there may be a stub starting to grow). I thought I'd ask you, since I know you all have lots of experience with
marine life. Will the claw grow back, and about how long should it take?
<Yes over the course of several molts. How often he grows and molts depends on your water quality and feeding.>
Thanks much, Kevin D
<You are welcome. -Steven Pro>

How many Emerald Crabs?
Twofold question: Got rid of a bunch of Mithrax crabs. Kept two. What is "small"? How big do they have to get before we get them out of the tank.
<adult size approaches 6" diameter! I'd feel comfortable with a carapace under 2" regarding "fish-safe" crab size>
I am left with one really small one but the other has awfully long legs and I worry about how long to keep him. His body is small but those legs!
<forget the legs... its the pincers the fishes have to worry about!>
#2 I have a 60 gallon reef tank with about 40# Fiji rock, 2+ inches mostly shells, etc.. Have a good skimmer and a Fluval pump that's supposed to be for 100 gallon tank. Tank is about 4.5 feet long. I have 4 clowns, 1 Gramma and 1 pygmy angel. They are all still juveniles, except maybe for the Gramma. Do I have room for a flame angel or anything at all.
<not likely with the other angel>
Have always figured a flame angel would be the premier fish in my tank, but the one I got didn't survive shipment, so I got the pygmy instead. Would there be WWIII or is there just not enough room?
The size of the tank has nothing to do with it... a 1000 gallon tank is still only x feet long and too small for a flame's territory. It works on rare occasions but I would never recommend it>
Long Question, but would really appreciate your opinion. The skimmer seems to be on under load.
Thanks again. I guess you know from all the mail I've read that you are really, really appreciated by many, many of us. Connie Cavan
<thank you kindly, Anthony>

Emerald crabs
Good Morning Bob!
Have a quick question for you. Would Emerald crabs eat Turbo snails?
<Yes>
I ask because I saw my large Emerald trying to pull at a Turbo last night. I managed to chase him away, the snail is ok. But now I wonder if maybe he is too big for my system now. Crab is app. 2 1/2 " across form one arm to the other, closed. I have a 75 gallon set up. Only 2 damsels, 2 Clarkii clowns, 1 cleaner wrasse, and 1 lawnmower blenny. Ass. hermit crabs, turbo snails, and 1 sand sifting starfish.

Thank you for your time. Kat
<You are welcome. -Steven Pro>

To Mithrax or no
Bob,
Thanks for all your past help. I've read your book, (loved it) and have all your FFE Q/A in one doc for easy searching. Here's my questions.
I have a 75 with 4x96 CF Bulbs (2 6700, 2 7100) bulbs. I run them 14 hours a day. I've read different things on CF, what is your opinion on how frequently they should be changed?
I've seen you recommend Mithrax crabs for reef tanks, and also seen the following quoted from Julian Sprung:
Avoid Mithrax crabs. They may do a good job when real small but they grow real fast and cause damage .
http://www.athiel.com/lib5/macna2.htm
What problems could they cause? What should I NOT have them with?
I am looking for a good sand stirrer. I had some white burrowing starfish, but they got eaten by my pacific or fire cleaner shrimp?
Thanks! Marty
< Good questions... ones that are tough to answer... Regarding the CF lamps, you'll have to check with your supplier...some have "effective lifetimes" about the same as (T-12) regular/old-fashioned fluorescents... on the order of 7,500 to 10,000 hours... and others up to 20,000 hours... And really, the only way to tell, irrespective of "doing the math" on how many hours you're running yours... that they need replacing is to use a PAR or lumen meter... IMO they should be replaced either at 80% of their calculated "effective lifetime" or at 80% of their output... The latter, the better judge.>
"Avoid Mithrax crabs. They may do a good job when real small but they grow real fast and cause damage", this bit lifted from the URL you posted. But also see:
From 'The Reef Aquarium, Vol. 1', Sprung and Delbeek, p279:
Mithrax crabs are good herbivores for large aquariums. Mithrax sculptus only grows to about 6.3 cm (2.5 inches), but is capable of capturing small fish. Some Mithrax species grow very large, and are therefore unsuitable for most Aquariums. Some species have been reported to eat Valonia. This crab
is from the Caribbean.
< So... I think these statements are confluent... that in the former Jules was referring to little reefs (ten gallons, like his), and in the book version of his opinions, larger systems.
Some aquarists have reported that Mithrax crabs will eat polyps if hungry enough. I recommend that you start with only one and keep your eye on it to make sure it does no harm.
Bob Fenner >

Question regarding Mithrax and Sally Lightfoot Crabs
It appears that either a newly added Mithrax or Sally Lightfoot Crab killed two of the shrimp in my 60 gallon tank. My guess is the Mithrax but I would appreciate your opinion.
<Yes, this does happen with both crab species... they are not "simple herbivores"... not nearly as opportunistic as some crab species... but still...>
Also is there a way to catch the offending crab without dismantling the tank.
<You can try baiting, trapping... per "crustacean" removal sections on our site: www.WetWebMedia.com>
Thanks for your input.
Mark Hill
<Good hunting my friend. Bob Fenner>

Mithrax Crabs... no carte blanche guarantees...
Hello Mr. Fenner
In your previous writing's on your site you have stated that the Mithrax crabs are good bubble algae and Bryopsis eaters. I very much would like to have a few in my 120 Berlin system. Will they be ok with my red leg, and blue leg crabs and all of my corals? How many would you recommend for my tank? As always , I am grateful for your site, books and advise. Thank you , and take care.

here... but no guarantees... there are numerous cases of this species trying out other crabs, even corals from time to time... Get one inch or so individuals... chances are they will leave your other life alone. Bob Fenner>

Emerald Green crab attacks!
Hi Bob,
I just got back from my favorite fish shops in Chicagoland. Realized I really do have more time than money. Any way, I saw something there I could not believe. I watched an Emerald crab reach out and snag a Saddleback Clownfish.
<This does happen... with size, hunger, Mithrax crabs will eat most anything>
It grabbed him and drug him down and held the poor thing fast while it started to try to eat it! I quickly got an employee and its life was spared. (maybe) Is this normal or just a case a very hungry crab? I was
thinking about picking up a few until that event.
Thanks for your input on the lights. I just purchased the CSL ABS 4-65 watt Power Compact unit online. I also decided to buy a 75Ga. and sell the 55Ga. More room (width) for the lights, filters, livestock etc.. Just plain easier all the way around is you ask me.
Oh, buy the way, do you need a 55Ga. tank. I know someone who is selling.
<Do post this on the Chatforum: http://talk.wetwebfotos.com/
Bob Fenner>
Thanks
Zimmy

Mithrax (emerald) crab
Dear Robert, I have a 25 gallon reef tank and thinking of getting a Mithrax (emerald) crab to help control hair algae; will it bother my Sally lightfoot crab? I have had her for close to a year, I would sure be sad if it were to be eaten.
<This is a possibility, especially in such small quarters... but not too large a one>
Also, I keep my tank from 74 to 78 degrees would an abalone do Ok and I do have a skimmer.
Any help is greatly appreciated!
<I would skip on the abalone. Please read over the mass of algae, control and related articles and FAQs stored on WWM: Index here: http://www.wetwebmedia.com/acidalkph.htm for much more on your choices.
Bob Fenner>

Mithrax Crab / Bubble Algae Question
Bob,
About a month ago, I wrote to you about a bubble algae problem I was beginning to have. Well, now it is a huge problem. I have been trying to remove it by hand, but this seems to be a losing battle. In your reply to my earlier e-mail you mentioned that Mithrax sculptus, the Green/Emerald Crab is known to eat bubble algae so I'm thinking of adding one (or more) to my tank. My questions are. a) will it bother the banded coral shrimp and 2 common cleaner shrimps I have and b) will it bother / eat any corals or clams?? Also, any other ideas for eliminating this pesky algae would be greatly appreciated. Thanks for your help again!
Phil in San Diego
<Very rare to have Mithrax crabs bother other crustaceans, corals, or clams... try one per fifty gallons or so... and start small... like one inch across the carapace in size. Bob Fenner>

Emerald crab larvae feeding 7/25/04
Hi, excellent web page - I read it very often...
<Thanks! Good to hear!>
I have noticed one of my 4 emerald crabs releasing larvae yesterday 1:00am night and I have collected some of them for further observation... I have no idea how to feed them so I would ask for your advice... Is it now more than 30 hours from the hatching moment and they are alive and kicking. <Many larval organisms feed on phytoplankton and then zooplankton (rotifers can be a fine substitute). Many have very specific requirements.>
So far I tried dropping some "Coral & Clam Diet" (semi-live
phytoplankton concentrate from Mariculture.com) I gave them
a little of live rotifers and some of freshly hatched Artemia. 
I do not see any feeding under my 10x magnifier glass.
Looks like the algae is to small particles and rotifers are too big, - not even mentioning Artemia nauplii...
Any ideas how to feed them? Any hope they might benefit from the phytoplankton mixture? Any idea when to expect first molting and how many molts they go through 1st week?
I looked into breeders registry but could not find any
breeding reports for this crab... Do you have any experience? Greetings from very hot Chicago, Przemek
<I have not heard of anyone breeding this crab. There are manuals for breeding and rearing Lysmata shrimp, and some of the techniques may be transferable. Keep in mind that some larval organisms live on their yolk sac and don't feed for some time. I am sorry for not having better info, and good luck with your pursuit! In the mean time, be grateful for the free coral food! Best Regards. AdamC.>

Emerald Crab Larvae
Unfortunately all larvae are dead. <So sorry to hear this.> I came from work about 6pm 7/23rd and saw them all dead. Last time I saw them alive and kicking was 1:00 am 7/23 - then I fed them with some drops of algae. The same morning, around 8:00 am I added other drops of algae and rushed to work little late without paying close attention to their health. There were rotifers and Artemia still alive in the container. <Really too big for the larvae at this point in their development I think.> Also, ammonia was almost not detectable, <If there was any ammonia at all it was too much for larvae.> no nitrites so I would say not the pollution was the fatal factor here. <Larvae are so delicate that you need to have pretty much pristine tanks with no signs of anything.>
Since I have not seen them eat any food I have provided, (algae concentrate, rotifers L, Artemia nauplii) I would assume they all starved. <Very possibly. I would think they would need nutrients similar to that raised in green waters.> No idea what could be good diet for them - I would guess shellfish algal diet was sufficient but it looks like it was not... <I think what you tried to feed them was too large. I believe what they would begin to eat would be very very tiny foods. I know people producing some in refugiums. And other people that are raising several types of green water and cultivating things like DT's to get tiny foods.> Other explanation would be that I feed too little algae and after introducing live zoo-plankton concentration of algae decreased even more... Maybe rotifers/Artemia have eaten all algae crab larvae supposed to eat? <Definitely a possibility. But the big thing is that the rotifers and Artemia are just too large at this point in their development.>
Next time I will have a chance to breed them I will hold with zooplankton until I will see some progress in larvae development fed on algae concentrate only. <You might talk to some of the companies developing these larval foods to see what's the smallest food you can find.>
Any other thoughts/suggestions?

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