crab ID

[Markarian421]

First off, I didn't get a picture -- as soon as I grabbed my camera our huge left handed hermit scared this guy off, so I'll have to try again later. But I'm hoping you all can start pointing me in the right direction.

When I fed tonight with only the actinics on, I saw a crab (flat, not a hermit) that must have hitchhiked in to out tank long ago at a much smaller size. Since it was only actinics I don't know the color, and I only got a good look at the claws and arms. These were slender with the claws coming to a small point -- the claws were small and equal sized, think along the lines of a blue leg hermit's claws but smooth, longer, and with more pointy tips. The claws and arms were dark and smooth, the joints were much lighter, maybe white. The body and legs were dark but more than that I can't tell you. My guess is that the body is maybe the size of a quarter or a little larger.

In just scanning common crabs, the claws were not hairy like a gorilla crab. I don't think an emerald crab is quite right, the claws on those look larger and are not light at the joints.

Any wild guesses? Tell me what you think, I'll hunt down some pictures and see if it looks right. Thanks.

 

[mbbuna]

need pic

 

[Markarian421]

yeah . . . I know . . .

I browsed member gallery pics of crabs until ReefCentral cut me off, and I didn't find anything that looked like it.

The camera is next to the tank now.

 

[Markarian421]

Well, no sign of this guy, but in looking for it tonight we spotted a second crab!!! This one was much smaller, and brown/gray . . . two crabs in our tank that we've never seen before in 2 days.

Are there any good hitchhiker crab ID guides?

 

[eastshores]

A good rule of thumb is that if the claws are shaped like spoons, they are herbivorous, if pointed, get them out.

 

[Markarian421]

Got a not very good pic of the first crab. Its claws and "face" which you see here (well, mostly just the face) are smooth and black with white joints. The arms are reddish brown, the legs are light gray and hairy. Looks like this guy was grazing on algae when I took this pic.

I'll try to get some better pics of both crabs.

 

[Pike]

Where did your live rock come from? This may be a good place to start looking for an ID if it all came from the same source.

Can't tell anything from your pic though, sorry.

 

[Markarian421]

Oh, and this first crab, it never comes out of the rock any more that that, it reaches its claws out and that's it.

The second crab comes out all the way.

 

[Markarian421]

The live rock came from multiple sources, so just about anything is possible.

 

[Markarian421]

More really bad pics. It's hard to take pictures in a completely dark tank, but these critters go into hiding as soon as there is light.

This picture is very nearly worthless, except you can see the color of the back of crab #1 -- black and yellow stripes -- it's the thing in the middle.

Here's crab #2, in the center of the image, just below a patch of coraline algae. Mostly you can see the pattern on its back, gray with lighter gray blocks. It is compact but has round (spoon shaped?) claws and is small, maybe the size of my pinkie fingernail.

 

[Markarian421]

Ok, with the hairy legs and smooth claws and body, it looks like it might be some sort of mithrax crab. I don't see pictures of any this color, but the overall form looks about right, and I did see some pics of black mithrax crabs with the white joints.

Something else to add is that this crab is slow. The second crab vanishes in an instant, the black and yellow crab slowly lumbers away when I shine a light on him. No big hurry.

 

[redpyro]

hmmm the infamous blur crab

 

[Markarian421]

better pics of crab #1. So is this guy a mithrax or what? I've never seen it doing anything but grazing on algae or the food I gave it the first time I saw it.

A closeup of the claw:

Crab #2 is a different story, we saw it take a chunk out of our clam last night!!! Moved the clam, set a trap tonight, and he's checking it out. If I catch him I'll get a better picture!

 

[LFSmarineguy]

mithrax

mithrax

It looks like a mithrax of some sort from the shape of the claws, or atleast from what I think I see. It looks like the tips of the claws are flattened like an emerald crab yet the inner surface has spines for grasping "moving" prey. I have no positive ID but it would appear to be omnivorous just from the claw shape. I would keep close tabs on your livestock for any damage or missing specimens. It does look cool though and if I find a photo anywhere of anything closely resembling it I'll post ASAP.

 

[Dr4g0nf1y]

I'm quite interested in the outcome of this, as I too have just discovered a small black crab in my tank, however mine has some white markings around what appears to be the eye areas.

 

[eastshores]

I have caught several gorilla crabs in my tank using the little "claw" thing you can buy for moving things in the tank without your hands being in there.

Go to lowes and get a flashlight that has a red filter cap (they are pretty cheap) crabs can't see the red spectrum but you will be able to. That is how I spot them at night and snag them when they least expect it!

 

[Markarian421]

I'll have to try that, but the little bugger usually is way too fast. We still haven't caught him. He's not bugging the clam any more, but something has started going after the snails, and we suspect he's it.

The first crab still seems to be sticking to eating algae.

 

[Dr4g0nf1y]

Melev has this photo on his site. Is this the culprit?

 

[kappa]

I have a couple of those black crabs that Dr4g0nf1y has in the the palm of his hand. I hope it is a mintrax. Man, All of black crabs grew big fast!

 

[Markarian421]

Ah, I missed that post with the picture. Yeah, our one crab is a little more colorful than that, with lighter colored legs, but it looks more or less like that. And yeah it's growing fast. One of the claws has barbs but the ends are pretty spoon like, and it's very timid and slow, it runs from just about everything. I directly feed it nori and it's gotten a little more used to me and comes out at feeding time for that (until one of the fish or hermit crabs chase it away) so I'll try to get a better picture.

We did have one clown fish vanish, we have no idea how, so I'm a little suspicious. But considering how pitiful this guy is at fighting for something it really wants (like nori) I can't imagine it getting a fish, and lots of other things could have happened to the fish. I'd be more likely to suspect one of the larger hermits.

Not too long after this post the other more evil crab came to an end . . . I didn't want to just kill it, I wanted to catch it, but one evening when I almost had it, it ran under a rock which I bumped trying to catch it, and the rock shifted, and, well, that was it. The cleaning crew moved in. At least it's not bothering the clam any more. It nearly decimated one species of snail in our tank before that.