Are emerald mithrax crabs 'reef safe' with SPS ? NO !
 

I recently added two fairly small emerald crabs (males with large claws) to my 250g tank to eliminate a few bubble algae I've seen. Next day I noticed a few branches of an SPS coral (A. millipora) with complete tissue loss and exposed skeleton. Thinking I had either a case of a RTN-like disease or an predator nudibranch I stayed up late last night and frequently checked on it with a flashlight. Well I find one of the new crabs on this coral, picking flesh with its claws. The other was on the adjacent coral (A. Sarmentosa) doing the same thing, with a small area of skeleton exposed right next to where he was eating. These corals are healthy and did not have algae growing on them, or excessive slime. Like most of my other SPS these two corals came from Divers Den. Some have arrived with small symbiotic crabs which I have generally left alone. I did find a predator nudibranch on a klyxium leather coral I bought from them, which I removed before it did much damage.

Anyway since these buggers were on the corals, and I had not yet glued them in place, they were easy to remove. There has been no progression of tissue loss since. I'm not sure the damaged areas will recover, but the healthy/uneaten branches should be fine I think.

By the way I don't have an excessive cleanup crew i.e. hermits and snails, just enough to do the job. There was plenty for these emerald crabs to eat on the 400+ lbs of live rock in the system. I actually put one back in the tank and watched him climb right back up to the A. sarmentosa and resume his feast... he was subsequently placed into the high flow-chaeto-sediment trap section of my sump/fuge and introduced to the giant hermit crab (smuggled back from Bora Bora) I keep in there. The other was put in the low flow - plankton generation section of the sump/fuge. If I need him to mow down some valonia in the future I can probably find him in there.

Thanks for sharing your experience.

To this day, I still am not brave enough to introduce any crabs (specifically arrow and emerald) into my tank, except for a few scarlet hermits.

That's interesting. I had two emerald crabs at one point in my 90 all sps tank, and never had an issue.

Maybe it's just hit or miss.

No problem with my sps and mine fairly large at about 3 inches. All he does is take food from the lps. To be honest I think my hermits do more damage. Most of the time I see my guy chewing away at hair algae

IME around 1 or 2 out of 10 will do that and most will do just fine with SPS. You just have to keep an eye out for the bad ones. Sounds like you got unlucky and got 2.

Chris

[QUOTE][i]<a href=showthread.php?s=&postid=11309729#post11309729 target=_blank>Originally posted</a> by fishdoc11 [/i]
[B]IME around 1 or 2 out of 10 will do that and most will do just fine with SPS. You just have to keep an eye out for the bad ones. Sounds like you got unlucky and got 2.

Chris [/B][/QUOTE]

I totally agree. I've had one previously that never bothered any coral and it quickly eradicated the handful of valonia in the tank. I may try another at some point but in the future will plan on staying up late for a few nights with the flashlight to babysit them.

I bought a couple a while back....after I realize they were safe I went back and bought the rest.

I have 5 now....one of them is red. A red, green emerald crab :D

i introduced a green one that was a medium size about a year ago, he went directly to a green slimer colony and killed it overnight. after that he went into the sump.

i currently have several emerald crabs in my tank now, and i've seen them hanging out in sps corals but i have yet to see any damage.

i think its pretty much hit or miss.

Makes me wonder whether some vendors are collecting similar-looking crabs + lumping them with.

I too have had good success with them, but know others who have not. I would go with Fishdoc's statement - probably 10-20% seem to turn out problematic - though I've not had one of those yet in a number of years.

[QUOTE][i]<a href=showthread.php?s=&postid=11311405#post11311405 target=_blank>Originally posted</a> by MiddletonMark [/i]
[B]Makes me wonder whether some vendors are collecting similar-looking crabs + lumping them with.

I think I remeber reading a thread that spoke about two similar species that were sometimes mixed up. Similar to the whole peppermint shrimp, camel shrimp thing.

[QUOTE][i]<a href=showthread.php?s=&postid=11311405#post11311405 target=_blank>Originally posted</a> by MiddletonMark [/i]
[B]Makes me wonder whether some vendors are collecting similar-looking crabs + lumping them with.
[/B][/QUOTE]
I don't think you honestly need to wonder about something like this. Most stores/vendors will market just about any green crab as an "emerald" and be done with it.

I'm done with them myself, even the ones that were for sure emeralds didn't do squat for valonia or other algae, and my latest beast, is some big sucker almost 3 inches across that only comes out at night, moves like a speed demon, and has very hairy legs... so yeah I'm trying to get this guy for the past month or so to toss him in my sump.

[QUOTE]I think I remeber reading a thread that spoke about two similar species that were sometimes mixed up. Similar to the whole peppermint shrimp, camel shrimp thing.[/QUOTE]

I got a couple rogue camel shrimp once...sold as peppermints.

Well the A. millipora that the crab was eating developed RTN. Not sure if this is from the predation, or if the process had started for some other reason without me realizing it and the crab was picking away devitalized tissue. Once I saw progressive tissue loss from the bottom up I removed it from the tank and fragged a few of the tips. Superglue was used at the cut sites. I'll keep an eye on the frags, if they start losing tissue also I'll throw them out.

The other coral the crab was eating (A. sarmentosa) still seems okay. But I'm keeping a close eye on it as well as my other SPS. Carbon is running.

So is there anyway to tell an emerald crab from some other green crab with hairy legs ? I've assumed all the green crabs in the LFS tank labelled emerald crabs were these supposed "reef safe" mithrax species but perhaps not. I did have one emerald crab before which as a great algae eater, and he did eliminate scattered valonia in the tank. Apparently only the males (with proportionally large claws) eat valonia -- if you pick the smallest ones they are probably females and won't touch it.

I can't tell the difference visually between the "good" and the "rogue" crabs. There may be a difference if one looked a bit closer....I don't know.

Chris

I have two 3" Emralds in my propagation tank, and they took down my hair algae, believe it or not, they literally would use there claws to grab it off the glass, removing from the glass; I was blown away, will I put them in my main tank (NO) not worth the risk, they never touch my SPS in the prop tank, but they do try to grab my unidentified species of Pseudochromis, which I hope they do not get, that would be a bummer.

I guess i kinda got lucky myself then, i have 13 em crabs, 3 over 2" and no problems........yet

ekovalsky -
I was trying to figure out why you put a male emerald in your tank in the first place and then found my answer later in the post...you have been missed informed. Females eat valonia, not males:)

I also have multiple in my tank.
I've had problems in previous tanks but so far so good in this setup.

i guess like almost everything that's "reef safe" hit or miss.

i had an emerald at one time and he did a fantastic job of clearing all algae out of my tank so much so that he started to taste other things in the tank until he found my finger leather to be most scrumptuos, i tried isolating him and returnign a few days later and it immmediatly went back to it even with supplimental feedings of flake food

[QUOTE][i]<a href=showthread.php?s=&postid=11325845#post11325845 target=_blank>Originally posted</a> by fijiblue [/i]
[B] Females eat valonia, not males:) [/B][/QUOTE]

So how do you tell the difference? I've had a dozen or so over the years, the last one I got perched in my A. selago and proceeded to take the tips off of the 40 or so branches of the colony. He is now in the sump as well.

FWIW I've seen crabs that look like females and males eat valonia. Where did that data come from?

[QUOTE]Where did that data come from?[/QUOTE]
I forget where I read it, but it was one of those online encyclopedia sites. I will look for it today if I get a chance

Males have a straight and narrow marking on their abdomen. Females have a wider marking.