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FAQs about Emerald Green, Mithrax/Mithraculus Crabs: Compatibility/Control

Related Articles: Mithrax Crabs, Arrow Crabs, Crabs

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

Related FAQs: Marine Crabs 1, Marine Crabs 2Marine Crabs 3, Marine Crabs 4, Marine Crab Identification, Marine Crab Selection, Marine Crab Compatibility, Marine Crab Systems, Marine Crab Feeding, Marine Crab Reproduction, Marine Crab Disease, Micro-Crustaceans, Amphipods, Copepods, Mysids, Hermit Crabs, Shrimps, Cleaner Shrimps, Banded Coral Shrimp, Mantis Shrimp, Anemone Eating Shrimp Crustacean Identification, Crustacean Selection, Crustacean Behavior, Crustacean Compatibility, Crustacean Systems, Crustacean Feeding, Crustacean Disease, Crustacean Reproduction,

Naughty, naughty... A so-called reef-safe (though "safe/r when young, small), herbivorous Emerald Green Crab eating a soft coral
Not always safe w/ other crustaceans, snails, worms, Seastars, corals/Cnidarians of all kinds
Several groups of fishes eat them: Hawks, Triggers, Puffers...

Octiplegic Emerald Crab... Eat more than algae     2/21/17
I saw one of my Emerald Crab on its back with no claws.
<Oops; something/s going on here... predation, too little alkalinity, alkaline earths, iodide/ate, possibly iron>

I flipped him over in case he was alive. The next day, I saw a larger Emerald eating his legs so I thought he was dead. Three days ago I was looking at something on the substrate and I notice that his little nose feelers were moving. I immediately removed him and put him in my frag tank. I've been strategically placing him on top of food for three days now and he is eating. My question is, will he be able to molt without any limbs at all?

If not is there anything I can do for him when he is ready and, if I can help, how do I know when?
<The aforementioned necessary water chemistry>
Attached is a video of him eating.
<Nada attached. Best to post elsewhere and just send the link. Bob Fenner>

Duncan coral flash damage... flesh? Decapods         5/1/16
Hi first I would like to say thank you for the great information that you provide, I am new to saltwater and all that goes with it and I get a wealth of information from you.
<Thanks from the crew.>
I have two emerald crabs that are feasting on my corals I have tried feeding them more so they will not munch on the coral but their appetites seem endless so with that said I have caught them and they are jailed in my tank until my LFS opens today in which they will be returned and hopefully sold to someone that can appreciate them for what they are.
<Crabs in general are opportunistic little beasts who will go for almost anything they can (shred) and fit into their mouths. That said they are fascinating in their own right and worthwhile. Just be sure you know what you are getting into, but certain kinds have particular uses as anti-algae weapons vs. "bubble" algae and so on, as you will no doubt have read on WWM. Generally untrustworthy however.)
The emeralds have eaten a good bit of flesh off the base of my Duncan coral, will the flesh grow back?
<A healthy colony should have little trouble recovering from non-catastrophic damage. They are surprisingly resilient although there are no guarantees. I wouldn't lose sleep.>
should I dip it in coral RX or Iodine mix to prevent infection or should I just leave it alone and see what happens. Thank you again for the great service that you provide.
<I'd watch it, leave it alone, and see how it progresses before moving onto disruptive treatments. If it seems to be going downhill (melting, discoloration) then the dip might be worth looking into. A "wait and see" approach would be my approach...should recover fine, just monitor it, keep things clean in the system, keep it fed. Some target-fed meaty foods will be appreciated. -Earl>

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

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

Emerald crab and Lubbock Wrasse... Decapod omnivorous-ness     12/3/14
Hello WWE Crew
I read your site threads on emerald crabs and wrasse and did not see my question so here it is.
I have a 55 gallon tank nitrates, nitrites and ammonia are all 0 PH PH 8.3. Water parameters are in check. The tank has been up and running for 3 years. It has live rock, it's inhabitants include a pajama cardinal , a tiny chalk bass and a rather large Emerald Crab.
<Like all decapods, Mithraculus become opportunistic omnivores>
It also had a pair of Lubbock Wrasse.......the Wrasse seem you have disappeared.
<Are great escape artists... Cirrhilabrus species... check the carpet, smiling cat/s>
I removed my live rock and can find no trace. I was gone for a day and my husband said he had not seen them since I left when I asked him.
Could my Xlarge emerald crab have caught them while they were sleeping and eaten them?

I didn't see the female wrasse 2 days before I left. I thought she was in the rock work hiding. The lid was tight but I checked the
floor and behind and under the tank stand just in case. Still there is no trace of either one.
<Crab bait>

The emerald crab is about 3.5 inches wide from one end of his carapace to the other from end to end not eyes to backside. I am tempted to give him to my puffer fish in a different set up as an early Christmas dinner.
<Shall I sing a song?>
The wrasse were beautiful. Any thoughts as to if he or she is the culprit?
Do emerald crab normally get this big?
<See my comments on WWM... the butler/crab "did it". Bob Fenner>

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


Emerald crab - keep it?/Mithraculus sculptus/Compatibility 4/21/12
Hi WWM Team -
<Hello David>
First of all, thank you for the wealth of information on your website -
<You're welcome.>
I have a question about my tank stocking plans.    I had quite a bit of algae growth after the cycle, so I picked up some hermits, peppermint shrimp, various snails and 2 emerald crabs.   They have all done a great job on the algae, the emerald crabs especially, but i wish i <please cap "i's" in future queries.>
read a little bit more about them on the Mithrax FAQ pages before buying them.    The inverts have been in the tank for four weeks, and there are 5 Green Chromis in QT that will be joining them in the display next week.      After the Chromis, I was planning on getting a Goby/Shrimp pair, but am concerned that the Emerald Crabs may try to eat them.    The tank is 110 gallons, and the Crabs have not exhibited any aggressive behavior towards the peppermint shrimp, but one has molted and seems to have grown quite quickly since.
Do you think i should try to remove the Crabs (back to the store or the sump) before getting any bottom dwelling fish, or will they be fine if they have enough to eat?  I do enjoy watching them.  Also, if it is prudent to remove the Crabs, could you recommend a more peaceful but interesting crab for my tank?
<Care needs to be taken to ensure an ample supply of food is available for the Emerald Crab. My advice is to supplement the Emerald Crab's diet with dried seaweed and chopped meaty foods, such as shrimp.  Quality pellet fish food such as New Life Spectrum is another option and less messy. 
If underfed, this opportunistic feeder may turn to corals, invertebrates, or small fish for a food source.  If well fed, most Emerald Crabs are very tolerant of their tankmates and compatible in reef environments.>
 Thank you for your time and expertise!
<You're welcome.  James (Salty Dog)>
Re Emerald crab - keep it?/Mithraculus sculptus/Compatibility 4/21/12

Thank you so much!  Apologies for the punctuation/capitalization.
<You're welcome.  James (Salty Dog)>

Coral beauty growth    11/26/11
I am going to attach a picture of a growth on the right lower fin and under belly of my coral beauty. i noticed it on his fin first about 3 weeks ago and thought it an injury from a crab,
<IS a physical injury>
because like the cleaner shrimp he likes to come up to the emerald crab to get "cleaned". The last two days he has not been coming out from his cave that much to graze. I have actually never seen him eat, only graze. I offer him an assortment of algae sheets, my sis, krill, brine and flake and pellet. Can you please identify the growth so I know how to proceed with treatment?
<See WWM re such>
I have a 55 gal. Reef tank with two clowns, assorted crabs, snails, and shrimp.
<We don't accept files larger than a few hundred Kbytes. Bob Fenner>

Subject: Coral beauty growth    11/27/11

Deleted as well
I emailed earlier today about a growth on my angel. Here is another picture
to help identify. The growth is eating away the fin and forming a "hole" in
his under side by the fin.
Thanks for the help

Re: Coral beauty growth    11/27/11
Thanks for the reply bob. Sport about the size so you think it is a physical injury?
<Almost assuredly... Remove the Mithraculus>
I would assume from the crab. I will read up on out however the last two days he has been acting different. <ly> As I said out and about not as much and not grazing. Does the injury get infected?
<Can; easily at times... there's no real cure/treatment for such other than good quality and nutrition... BobF>

Mithrax sculptus & Stichodactyla haddoni/Emerald Crab Behavior 3/28/2011
Good on you for making yourselves available to myself and others.
<Thank you.>
I have a Mithrax sculptus (Emerald crab) that uses a Stichodactyla haddoni (carpet, saddle anemone) as cover. The crab walks along the rocks and squeezes itself under the anemone, part of which is spread out and in contact with said rocks. It will move around between the rocks and anemone, and also rest there. I've yet to witness the anemone react to the crab. Will the crab irritate or damage the anemone this way?
<Should not, and is not unusual to see this.><<RMF is not so sure>>
Thank you.
<You're welcome. James (Salty Dog)>

Missing Shrimp 9/27/10
<Hello Dana>
I used to have 5 Peppermint Shrimp and 1 large cleaner shrimp. I am now down to 2 peppermints, but have never seen any parts of the missing shrimps. Also in the tank (29 gal) are 1 Six Line Wrasse, 1 Lawnmower Blenny, 1 Yellow Watchman Goby, 1 small Emerald Crab, 1 Chocolate Chip Starfish and a few Nassarius Snails and hermits.
<In future queries, please cap proper nouns such as names of animals before sending. We just do not have the time to edit queries before posting.>
Do you think something is eating them? The cleaner shrimp was pretty big and it's hard to imagine one of the other guys eating him.
<Although Emerald Crabs are considered reef safe, they are opportunistic feeders as most crabs are, and when hungry, can and will eat shrimp. I'm guessing the crab is the likely culprit. The other possibility is that the shrimp are not getting enough food, die, and are eaten by other inhabitants.>
Thanks for your help.
<You're welcome. James (Salty Dog)>

Emerald crab eating soft coral picture    9/17/10
Hi guys,
I do not have any questions for you today (its a wonder!) but I did snap a picture of an emerald crab hitchhiker eating one of my Zoas.
<That little bugaboo! Caught in the act!>
I had this hitchhiker in my tank for a longer time than my mantis shrimp but figured he would be food for the mantis (when it was added) so I never really bothered to fish him out. After adding the mantis, I didn't see much of him so I figured the crab was breakfast already. Today, to my surprise, he was out in the open munching on my newly added Zoa frag that I had just added about 10 minutes ago.
Guess he liked the colors so much on that one he finally went out of hiding to eat it. I never had any evidence of him eating softies before.
<Mmm, Zoanthids aren't Alcyonaceans (Soft Corals)>
I guess now I know the reason why my LR coralline algae was "bleaching." I always thought it was my lights. He's in my FOWLR tank now with my other emerald. Guess he could help my other aquarium stay clean without eating my softies.
<Have oft mentioned that Mithraculus species are largely misunderstood...
are not "cleaner uppers", but eater uppers like most decapods>
Anyways, I figured you guys might like the picture and may want to use it for future reference when people ask about emeralds eating softies and other reef
compatibility issues.
Happy reefing!
Peter T
<And you, Bob Fenner>

killer in the tank   9/15/10
Dear crew;
It seems I have a killer in my tank; my previous losses (hermits, snails, and one small damsel) were separated by months, but in the last week, a skunk cleaner shrimp and a fire shrimp have gone, and today my serpent star is missing most of two arms. Clearly the killer is growing in size and aggressiveness! I know it's possible to trap the culprit, but unless it's a Mithrax crab, who is much larger than when I got it a year ago, (approximately 2" across now), I may have never seen it. If I do capture it, I'd hate to get rid of the Mithrax only to find it's not the culprit...
So my actual question is, a) do you think it more likely the Mithrax is the killer or some other yet unseen hitchhiker such as a mantis shrimp?
<Could be... either, even an Alpheid>
And b) if I can capture him, would he survive in an unlit refugium?
<Oh yes>
(I have a Red Sea Max 250) I have seen the Mithrax at night with a red led flashlight many times eating algae, and of course there's none in the small refugium built into the tank. So what do you think? Also, if you have a favorite way to try to capture the culprit, what would it be?
<Traps... Please read here: http://wetwebmedia.com/mantiscompfaqs.htm
and here: http://wetwebmedia.com/swcrabcomp.htm
As always, tanks for your great website and advice! Regards, Seth.
<Welcome! BobF>

Sally light foot, and Stenopid comp.  7/12/10
I'm back with more reef questions...
I have a 180 gallon tank (empty but with water/rock) and I was wondering if a mated pair of coral banded shrimp and 1 sally light foot crab would be okay together.
<If they don't get too hungry>
Except for a fair amount of snails
<They may eat these>
as the only other invertebrates, these guys would be pretty much the only inverts in the tank. Fish-wise, there would be a school of green Chromis', 1 Pseudochromis, a Sixline wrasse, a algae blenny and 3-4 PJ cardinals.
Would this overall combination work, again a 180 tank [6'x2'x2'], or should I expect WWIII between one or more things in the tank?
Thanks in advance,
<See WWM re crab and boxer shrimp compatibility. B>
Sally Light Foot/Sally Light Foot Compatibility/Crab Compatibility, James' go 7/12/10
<Hi Tristan>
I'm back with more reef questions...
I have a 180 gallon tank (empty but with water/rock) and I was wondering if a mated pair of Coral Banded Shrimp and 1 Sally Light Foot crab would be okay together.
<I would not chance. When large, they are capable of catching and eating shrimp and other invertebrates.>
Except for a fair amount of snails as the only other invertebrates, these guys would be pretty much the only inverts in the tank. Fish-wise, there would be a school of Green Chromis', 1 Pseudochromis, a Sixline Wrasse, a algae blenny and 3-4 PJ cardinals. Would this overall combination work, again a 180 tank [6'x2'x2'], or should I expect WWIII between one or more things in the tank?
<Looks like a peaceful clan. The Sailfin/Algae Blenny (Salarias fasciatus) does better in aged systems where some algae is present. If not, supplemental algae feedings will be required. May want to read here. http://www.wetwebmedia.com/algaeblensart.htm>
Thanks in advance,
<You're welcome. James (Salty Dog)>
Re Sally Lightfoot... now Mithraculus Compatibility/Crab Compatibility 7/12/10 - 7-14-10

Hmmm...(thinking about your reply)
Both you and Bob said that the two would not get along, I didn't expect that they would, I was just crossing my fingers and thought I should ask, hoping the tank may be big enough. Would a handful of Emerald Crabs be okay
with the Coral Banded Shrimp?
<Generally safe, but they are an opportunistic feeder, and if food is not available to them, they may turn to supplementing their diet with corals or invertebrates.>
Otherwise what non-hermit invert would work; helping to keep the bits of macroalgae growing in check? I'm a bit reluctant to put in a urchin because I don't want it to knock stuff over and I would like to keep as much of my
coralline algae as possible. I'm at a bit of a loss as to what could work.
<The Blue Tuxedo Urchin (Mespilia globulus> would be one, as they are much more forgiving in terms of rearranging your rockwork. You may want to read here and related links in the header for methods of controlling algae.
http://www.wetwebmedia.com/algaeconMar.htm. James (Salty Dog)>

Sebae Anemone And Emerald Crab 1/11/10
<Hello Brian>
I have a had a Sebae anemone for several weeks now. He found a nice place in the tank, and seems to have some good days and some bad days - hopefully he is on his way to adjusting and thriving in my tank, but right now it <I> think it could go either way.
Anyways - I just got 2 Emerald Crabs. 1 found a nice place in a live rock "cave", but the other one has taken an instant liking to hiding under the Anemone. He actually moves the anemone around a bit and almost wears it like a hat - the Anemone is actually more opened up now, and looking a healthier. Could this be a symbiotic relationship, or is the crab hurting the anemone?
<No symbiosis here, the crab is just looking for cover. I suggest relocating the crab to a different area of the tank as to give the anemone a chance to anchor in. You may want to read/learn here as this is not one of the easier anemones to keep.
Thanks for any light you can shed on this strange relationship.
<You're welcome. James (Salty Dog)>

Emerald Crabs, re: pagoda cup 9/10/09
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.>

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!>
Jake Maenius
<Regards, Jessy>

Emerald Crab Vs Hawk Fish -- 07/17/09
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.
<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>

Mithrax molt  4/2/09
hi Folks,
Great forum, thanks for the effort
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
<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).
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 do have a suggestion but I'm not sure you'll like it.>
I think I would rather pour bleach in my tank
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.
<I hope this helps!>

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>

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

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 anemones, <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 anemones. <Need to ID these...>   I have actually saw the tube anemones touching some of them, including a turbo snail which soon died.  I was told at the LFS that it is unlikely the anemones 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 <I would read re the Mithrax/Mithraculus and remove them... and keep reading. Bob Fenner>
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 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>

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 definitely 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!!
<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 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)?   <I would imagine so.> 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

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 would hold off here for a week or more...>   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>

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 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 emerald <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 Emerald (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> <<Oh James. RMF>> 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 would remove the crab, some become quite predatory with age.> 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.> 

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? <I would remove the Mithrax crab.> 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

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> 

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!>
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...  <I know I can't> 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>

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 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? <<Yes. RMF>>
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 --

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. <I would try to trap they. Use the Google search engine on www.WetWebMedia.com looking for traps, mantis shrimp, etc. We have discussed several commercial and a few DIY traps.> 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 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. <I would remove him.> Thank you for your time. Kat <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>

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. <I would try four or so small individuals of the Emerald Crab <Mithrax> 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>

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