IOTM:Crabs I- Emerald crabs

celacanthr

Active Member
Ok, this is the January IOTM, so get a drink or something to eat, because it is kind of long winded and um…boring, well at least if you are not very interested in the biology.
But, I do realize the need for a person to skip the science blabber, and get right to the meat of the subject…care. For those so inclined, you can just skip straight on down to the end of this article, where in bold it says “care”, where I shall attempt to outline what these guys need.

Mithrax sculptus
I will begin by showing their “family tree” to everyone:
Eukaryote
--Animalia
----Arthropoda

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Malacostraca

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Decapoda

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Majidae

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Mithraculus (Mithrax
)

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sculptus

(there are a couple of other species, if so requested, I wouldn’t mind doing some more research, and elaborating on these other species, it is just that they aren’t commonly seen in the aquarium trade, so I do not feel a need to really elaborate on them.)
Well, now that we got that out of the way, lets get in to behaviors, and anatomies of these vegan terrors.
Well, I am having a little trouble figuring out where to start, so I guess I will start with what they look like.
Description:

Emerald crabs, named this because of there very green color, hence “emerald”. On the ends of their claws (where the claws meet when closed) it is white. I have also seen and heard of the red on the inside of there joints. Mine personally never showed this red, so maybe the red is only in mature individuals or maybe only individuals of a certain gender?
Emerald crab legs can only be described as “hairy”. I am not sure the reason that they are hairy, but they just are. Something imortant about there legs is that at the end, they have very sharp/strong (won’t break easily) points. I do know why they have these. In the wild, they
can encounter very strong currents. These legs prevent them from being carried away by the current…or the aquarist.
 

celacanthr

Active Member
Geography:
The only sites that I have encountered with info about their range, are retail sites, and even at that they only say “Caribbean” (very vague). I personally have encountered them while snorkeling in the Florida Keys. So I guess according to my observations, and based on the retail sites (like saltwaterfish.com), I would have to say that you can at least find them from the Florida Keys to the Caribbean. Beyond that I have no clue, they may range MUCH farther than this.
Sexual Dimorphism:

Since I have seen no proof that males are more aggressive, or females get larger, there is really no need to --- an emerald crab, but it is something that may just be nice to do, and it is so easy that you could do it while you stare at your tank, or while doing your tank chores.
If you look at the underside of an emerald crab (you can use this method to --- most other crabs too), you will see basically 2 plates. The big one is actually the main body of the crab aka the carapace. The second “plate” is actually the abdomen, which has been folded under in all “true crabs” (Brachyura), and become nearly useless, except in mating. The abdomen (the second plate) comes in two shapes. It will look either like an obelisk (kind of, got to use your imagination, when you see the crabs, you can tell the difference), or the dome on the white house (you know, the dome, but a kind of sharp bump is at the top). If it looks like the obelisk it is a male. If it is like the white house dome, it is a female. Sexing may be more difficult in younger specimens, but I am not sure. I would like to add, that it might not be a good idea to stress the emerald crab. Wait until they show there underside, it is a lot easier than trying to pry a determined emerald crab off of a piece of LR
I will try and add pictures of what the abdomen could look like.
The abdomen is used in the mating process. The males gonopods, genitalia, 2nd pleopores, and anus are found on the underside (the side pressed against the carapace) of the abdomen, but the females gonopores are found on the carapace, but are covered by the abdomen, which also house the females anus, and a full set of pleopods.
I have not heard of any success in breeding M. sculptus, but then again, I haven’t heard of anyone who has tried to either. If you do decide to under take it, please keep us informed, and up-to-date!
 

celacanthr

Active Member
Feeding:
Emerald crabs, are ferocious algae and marine plant eaters. They are specifically prized for there wonderful habit of eating bubble algae, without popping them. They will usually pick at small bits and pieces of algae along your rockwork, unless you have a larger macro-algae, or marine plant.
If they encounter a larger piece of algae, they seem to cut (?) the algae into bit sized pieces. It is quite interesting to watch.
For those of you as crazy as me, who want to be able to keep macro-algae, AND an emerald crab (yes, crazy), I would suggest putting the macro on the sand bed. I did this, and only once did I find my emerald crab in the caulerpa patch. She also kept the caulerpa nice and short, because whenever the caulerpa was tall enough that it would flout close enough for the emerald crab to get it, she would start to cut the caulerpa up, and eat it. I am not saying this will work, just saying that it did for me.
I would also suggest not getting any macro or plant worth more money than you are willing to risk losing.
Care:

Ok, time for the “quick” answers:
Temperature:
Normal tropical reef temperatures, and probably all the way down to 70 degrees. The reason I make this assumption, is because I have seen them in nature in 70-degree water, and in 82+-degree water.
Salinity:
This has always been something I have wondered about. If the oceans water is 1.025-1.027 SG (34-35 salinity), then why do people insist on keeping salinities at 1.021-1.024? I believe, I have heard that it helps fish to evade disease. But I would think that a salinity to low would stress them… but I guess that keeping the salt so low in a fish only tank is okay, but IN A REEF TANK YOU NEED FULL STRENTH SALTWATER THAT MEANS 1.025-1.027! It is very important for inverts (and some chordates like tunicates) to have the proper salinity! Now, I will say that there are exceptions to this rule, like if you are setting up a marine biotope, and now for a fact that the area that you chose has a lower salinity a majority of the time, or if the tank is a brackish tank, or a freshwater tank, then you obviously need a lower salinity ;-).
Feeding:
discussed above, but basically algaes, and marine plants.
 

celacanthr

Active Member
Temperament and tank mates: Although they are algae eaters, they are fairly defensive. There are countless accounts of fish swimming over the crab, and the crab trying to protect themselves, or in the case of a seahorse, catch them. I also am aware of the many stories of the emerald not causing trouble, and I am not sure why the crabs were not aggressive in the situations, or were aggressive in the other situations.

I would like to say that I highly doubt that the crabs become aggressive because of hunger, since in many (almost all) the crab had plenty of greens to choose from. I am not even sure that if an emerald catches a fish, that the crab would eat the fish.
I assume that this action is probably a defensive instinct in nature, but once again, I do not have any scientific fact to back this theory up.
Tank size:
If you are talking about just the amount of room that they need to move, and the amount of room for the bio-load, a 10 would be fine. If you were talking about the amount of space needed for me to feel secure, and put the crab with fish, or other mobile inverts, that are slow, and could fit in their claws, I would say a 55. That is just my 2 cents, I am sure that I will have many who will oppose me on that.
Well, I hope you enjoyed! Please be forgiving on grammatical errors and typos, I am spellchecking right now!
All comments, questions, ideas, are welcome! If you see something wrong with the info here, please do say so, and I will try and get the article fixed.
 

celacanthr

Active Member
Thanx wax! I agree, it needed a pic, I am just not a very good pic person... the only pics I have are...bad...oh yeah, all other pics are welcome too!
 

fishmamma

Active Member
Though my emeralds always fall prey rather quickly I have had a few and noticed different temperments within them. I have had small fiesty crabs who would hide and jump out to snap at passing fish as though it were a fun passtime, and other larger specimins that were happy to slowly make their way around the tank sleeping and snacking ignoring the fish all together. Never saw any coral nipping.
 

celacanthr

Active Member
Thanx fishmamma! I kind of suspected that it depended on the individual.
So generally it was your smaller ones that were rowdy?
 

wax32

Active Member
This Kong sized one in my tank is a little rowdy, he doesn't bother corals though, I think he is basically just defensively aggressive, protecting himself so to speak. He swings at fish when they swim by and he knocks over the occasional Nassarius snail when it tries to crawl over him.
 

celacanthr

Active Member
hmmm...ok. Every account helps!
Ok, here is what we know! The big ones are aggressive...when they are not peaceful.
The small ones are peaceful...when they are not aggressive.
So basically we know nothing. :thinking:
Wax, are the big sometimes aggressive ones male or female?
 

wax32

Active Member
Ya know, I haven't looked to see what mine is, next time I can I will snatch him out and peek.
Their little scratchy hairy legs give me the heebies so I don't like to handle them much!
 

wax32

Active Member
OK, mine is a male. Dunno what effect that would have on aggressiveness, but interesting to note anyway.
 

celacanthr

Active Member
Well, maybe if we have more people with sort of aggressive males, and tame females, then that could be a clue. Or the other way around.
Thanx wax!
Hey, did you get the crab off of a rock to look at it, or did it just get in a position that you could see its underside from?
 

wax32

Active Member
It got in a good position. Like I said, I don't like to handle him. Besides the furry legs those claws look like they could deliver a nasty pinch.
 

celacanthr

Active Member
LOL, thanx!
Well, when you have had to get it out in the past, how do you pick it up? Like, get it off the rock that is. Every time I had to, I had to "persuade" her on to a small rock, then pull the rock out of the water, to get her to free up her legs. They are just to dang powerful!
 
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