Mithrax Crab ID

[kase1084]

Finally I caught this guy!

He’s been very elusive, hanging out in tight nooks and crannies and doing the bolt as soon as I try to grab him. I’ve only ever managed to get photos of his moult. When I’ve seen him, he appears to have been grazing the algae on the rocks though. Either that, or eating pods hanging around the rocks.

Today I pulled some of my live rock out to do a deep clean and vacuum and he was hanging off the back of the rock and I managed to grab him! The fugitive crab has been apprehended and remanded in the sump for now. If he is a baddie, he stays in the sump, but if he’s nice I can return him to his home.

I think from googling, he appears to be a black Mithrax crab. Sump him or repatriate him?

 

[twentyleagues]

Those black claw tips are a dead giveaway not good, not a mithrax crab.

 

[kase1084]

Ooh, I’m glad I snaffled him up if you think he has danger mitts. What do you think s/he is? Forgot to mention I’m in Sydney, Australia. So the live rock he came from is from Australian waters, making him an Aussie crab of some sort. Also the way he menacingly waved a claw at me in a very “streuth, mate!” manner.

 

[Macdaddynick1]

I’ve had an exactly the same one living in my reef for about 5+ years. He would very OCCASIONALLY (like once every month, snip a tip of my acropora, I think it was by accident, and I’ve had a ton of acro frags. He was otherwise a good citizen, he ended up becoming huge too, and was very reclusive. Never ate any of my fish, I would sometimes even feed him directly. I’m not sure why he died, but I do miss him. This was one of his molts.

 

[twentyleagues]

I’ve had an exactly the same one living in my reef for about 5+ years. He would very OCCASIONALLY (like once every month, snip a tip of my acropora, I think it was by accident, and I’ve had a ton of acro frags. He was otherwise a good citizen, he ended up becoming huge too, and was very reclusive. Never ate any of my fish, I would sometimes even feed him directly. I’m not sure why he died, but I do miss him. This was one of his molts.

I am pretty sure this is a mithrax crab yes the claw tips are dark but so is the rest of the body. If you look at ops posted pic closely you can see a distinct line through the base of the claws where they actually become black. Also on yours you can see the cupped claw tips that also helps identify it as an "herbivore". The pic the op posted does not have a good enough angle to properly see the claw tips. Theirs do not look cupped though.

 

[twentyleagues]

Ooh, I’m glad I snaffled him up if you think he has danger mitts. What do you think s/he is? Forgot to mention I’m in Sydney, Australia. So the live rock he came from is from Australian waters, making him an Aussie crab of some sort. Also the way he menacingly waved a claw at me in a very “streuth, mate!” manner.

Haha! I am not good at actually identifying their species. I know enough of what to look for in overall body structure. If @ISpeakForTheSeas is around they may be able to identify it.

 

[kase1084]

I am pretty sure this is a mithrax crab yes the claw tips are dark but so is the rest of the body. If you look at ops posted pic closely you can see a distinct line through the base of the claws where they actually become black. Also on yours you can see the cupped claw tips that also helps identify it as an "herbivore". The pic the op posted does not have a good enough angle to properly see the claw tips. Theirs do not look cupped though.

I’ll try to grab him and take a clear picture of his claw, but it looks pretty pointy.

 

[twentyleagues]

I’ll try to grab him and take a clear picture of his claw, but it looks pretty pointy.

You dont need to if you dont want to. I am positive its not a mithrax crab.

 

[ISpeakForTheSeas]

Those black claw tips are a dead giveaway not good, not a mithrax crab.

Haha! I am not good at actually identifying their species. I know enough of what to look for in overall body structure. If @ISpeakForTheSeas is around they may be able to identify it.

Ah, in this case it does appear the crab is a "Black Mithrax" crab, but that common name is a misnomer, as the crab is not actually a mithrax crab at all; it's technically a *mostly* reef-safe Xanthid crab.

OP, it looks like you have a Chlorodiella sp., possibly C. nigra or C. cytherea (I don't know how to differentiate them offhand, and I don't remember if the sources I've read through explain how to - I'm on my phone or I would check):

I'm fairly confident you've got a young Chlorodielline crab of some variety there, probably a Chlorodiella sp., possibly C. nigra, but I'm not entirely sure.

The pattern on the legs may change a bit as it ages, and it should develop a somewhat rougher carapace, but it should stay small (adults of most of these species have a carapace size typically right around ~0.4"-0.75" full grown, so it really shouldn't pose a threat to most livestock - no guarantees with corals though) - there are claims that these can reach 8 cm/~3.14" full grown; I have seen absolutely no evidence to support that (even if you include the legs in the measurements).

I have seen one exceptionally large individual with a carapace width slightly less than 3 cm (it was ~1" carapace, and ~6 cm total counting fully outstretched legs); even at that size, it should be around the same size to somewhat smaller than a grown emerald crab.

I have also seen these for sale as "Black Mithrax Crabs" (no, they are not Mithrax crabs, nor are they closely related) - they're purportedly reef-safe, but I've also heard they may eat small inverts (like brittle stars).

They're safer than most crabs, but I'd say probably reef-safe with caution (not many people keep them, though, so that's from a small sample size to work with).

They're technically Xanthid crabs (which are generally not reef-safe), but these are generally very small, and at least one related species (Chlorodiella nigra) is sold occasionally as a "Black Mithrax Crab." I know at least one was kept without causing problems until it was full grown (then it reportedly started eating small inverts), but even emerald crabs are known to sometimes cause issues when larger.

 

[twentyleagues]

Ah, in this case it does appear the crab is a "Black Mithrax" crab, but that common name is a misnomer, as the crab is not actually a mithrax crab at all; it's technically a *mostly* reef-safe Xanthid crab.

OP, it looks like you have a Chlorodiella sp., possibly C. nigra or C. cytherea (I don't know how to differentiate them offhand, and I don't remember if the sources I've read through explain how to - I'm on my phone or I would check):

Thank you again for your attention and knowledge!
So I am right but not at the same time...lol. I feel all crabs are "with caution" because the ones labeled as "herbivore" are actually omnivores and can decide to go rouge at any moment. But typically, any crab that has pointed claws or the "tips" of the claws are black (also typically pointed) are something that should at the very least be thought of as a possible issue. I had some bubble algae pop up and I have long had the crabs are not friends mentality. Against my better judgement I got 4 emerald crabs and they were 100% emerald crabs. With in a couple days I had caught all 4 of them destroying my acans with a tank full of bubble algae which they are supposed to eat more than any other algae. So they got sumped and started working on mowing all the caulerpa down to little nubs and letting the long strands get caught on the egg crate that is meant to catch errant strands of algae. I wouldnt be so mad if they were at least eating it but no just snip it off and let go of it move on to the next one. Pain in the neck, crabs!

 

[ISpeakForTheSeas]

Thank you again for your attention and knowledge!
So I am right but not at the same time...lol. I feel all crabs are "with caution" because the ones labeled as "herbivore" are actually omnivores and can decide to go rouge at any moment. But typically, any crab that has pointed claws or the "tips" of the claws are black (also typically pointed) are something that should at the very least be thought of as a possible issue. I had some bubble algae pop up and I have long had the crabs are not friends mentality. Against my better judgement I got 4 emerald crabs and they were 100% emerald crabs. With in a couple days I had caught all 4 of them destroying my acans with a tank full of bubble algae which they are supposed to eat more than any other algae. So they got sumped and started working on mowing all the caulerpa down to little nubs and letting the long strands get caught on the egg crate that is meant to catch errant strands of algae. I wouldnt be so mad if they were at least eating it but no just snip it off and let go of it move on to the next one. Pain in the neck, crabs!

Haha, yeah - technically right, colloquially wrong.

Yeah, reef-safe true crabs (as opposed to porcelains crabs, for example) are few and far between (the current safest I know are Pitho crabs), and black tips and pointy claws are definitely red flags. Honestly, with these crabs, I would assume they're mostly safer than others due to their size anything else, but I may be wrong. With emerald crabs specifically, I have heard that females are safer than males, and the larger the crab the larger the risk.

 

[kjkszpj9]

They arent mithrax crabs but hell they sure do look almost exactly like one, ive had 2 in my rock works smaller ones you never see them out and about obly inside the rocks im unsure if mine are still alive but i liked seeing them when i shined my flashlight at the rocks.