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FAQs about Emerald Green, Mithrax/Mithraculus Crabs: Selection

Related Articles: Mithrax Crabs, Arrow Crabs, Crabs

FAQs on: Emerald Green Crabs 1, Emerald Green Crabs 2,
FAQs on: Mithrax ID, Mithrax Behavior, Mithrax Compatibility, 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,

Ostensibly for (green) algae control

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 would worry a bit about making it spread more. Your nutrient control approach is vitally important to successfully controlling this algae.> 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.>

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

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>

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 >

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>

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>

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