crab ID

Markarian421 · 03/16/2005, 10:45 PM

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 · 03/16/2005, 11:29 PM

need pic

Markarian421 · 03/16/2005, 11:54 PM

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 · 03/18/2005, 01:09 AM

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 · 03/18/2005, 01:22 AM

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

Markarian421 · 03/18/2005, 01:34 AM

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 · 03/18/2005, 01:39 AM

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 · 03/18/2005, 01:40 AM

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 · 03/18/2005, 01:45 AM

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

Markarian421 · 03/18/2005, 02:12 AM

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 · 03/18/2005, 03:12 PM

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 · 03/18/2005, 03:36 PM

hmmm the infamous blur crab

Markarian421 · 03/19/2005, 11:35 PM

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 · 03/19/2005, 11:48 PM

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 · 04/07/2005, 10:47 PM

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 · 04/09/2005, 01:50 AM

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 · 04/09/2005, 01:56 AM

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 · 04/09/2005, 08:54 AM

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

kappa · 10/18/2005, 12:13 AM

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 · 10/18/2005, 12:29 AM

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.

Dr4g0nf1y · 10/18/2005, 12:48 AM

To give credit where credit is due, the pic is not of me holding that thing. That pic was taken from melev's site.

Markarian421 · 05/02/2006, 11:28 PM

OK I know I'm reviving a long dormant threat, but here's an update.

This guy did a great job of eating algae, and has grown a lot. But recently when we got some chromis, the started to vanish one by one. So we're now holding this guy -- it took us a month to catch it without hurting it.

I can't get any reasonably sized/quality pics here, so I posted these on flickr:

And yes, it now looks more or less just like the one on Melev's page (but a little bigger).

Dr4g0nf1y · 05/03/2006, 07:31 AM

Nature provides the clues you need. Look at the claws, they don't really appear to be the type that can be used for pinching algae off rocks. They have those serrated edges which probably are designed for holding prey.

Markarian421 · 05/03/2006, 10:39 AM

I'll try to get a better pic, but the claws are dual purpose. In the second one you can see it a little, the ends of both claws _are_ really spoon like, and this crab eats any algae it can find. But it also has the serrated edges on the inside of the right claw. It doesn't on the left claw, it's smooth with just a rounded bump or two. So I guess it has to catch fish with its right claw.

When our chromis started to vanish, it became a suspect mostly because it started hanging out at the base of the coral they slept in, and from watching it eat algae, we know it's really good at picking things out of tight spaces. It's also gotten much much faster as it's grown. But we've never seen it eat anything but algae, and this guy really helped clean up an outbreak of hair algae we had -- it can pick algae out of places snails and hermits can't reach. So it gets the benefit of the doubt, but we didn't replace any chromis until we caught it!

It's in a holding pen now, and we got new chromis. If they are all still alive by the weekend, we'll try to find a new home for this guy (a coral only tank or tank with large fish -- it has coexisted with our larger fish for a year with no problems). If our sump had enough room we'd put it there, but I can't figure out a way to make that work.

spankey · 05/04/2006, 10:06 PM

I have that same crab in my tank and its eating my digi's.. Riping the skeleton right off the colony right infront of my eyes.. How can we catch this pest and remove it?

Anyone?

HELP!!!

03/19/2005, 11:48 PM	#14
LFSmarineguy Registered Member Join Date: Feb 2005 Location: Long Island, NY Posts: 198	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.

10/18/2005, 12:13 AM	#19
kappa Registered Member Join Date: Jun 2004 Location: Huntington Beach, Ca Posts: 83	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! __________________ "Success is 99 percent failure"

05/02/2006, 11:28 PM	#22
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	new pics OK I know I'm reviving a long dormant threat, but here's an update. This guy did a great job of eating algae, and has grown a lot. But recently when we got some chromis, the started to vanish one by one. So we're now holding this guy -- it took us a month to catch it without hurting it. I can't get any reasonably sized/quality pics here, so I posted these on flickr: And yes, it now looks more or less just like the one on Melev's page (but a little bigger). Last edited by Markarian421; 05/02/2006 at 11:47 PM.

03/16/2005, 10:45 PM	#1
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	crab ID 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.

03/16/2005, 11:29 PM	#2
mbbuna Moved On Join Date: Oct 2004 Posts: 6,872	need pic

03/16/2005, 11:54 PM	#3
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

03/18/2005, 01:09 AM	#4
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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?

03/18/2005, 01:22 AM	#5
eastshores Registered Member Join Date: Sep 2003 Location: Cocoa Beach, Florida Posts: 530	A good rule of thumb is that if the claws are shaped like spoons, they are herbivorous, if pointed, get them out.

03/18/2005, 01:34 AM	#6
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

03/18/2005, 01:39 AM	#7
Pike Registered Member Join Date: Jan 2003 Location: north carolina Posts: 683	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.

03/18/2005, 01:40 AM	#8
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

03/18/2005, 01:45 AM	#9
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	The live rock came from multiple sources, so just about anything is possible.

03/18/2005, 02:12 AM	#10
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

03/18/2005, 03:12 PM	#11
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

03/18/2005, 03:36 PM	#12
redpyro Moved On Join Date: Mar 2003 Location: Greenville, Ohio Posts: 3,504	hmmm the infamous blur crab

03/19/2005, 11:35 PM	#13
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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!

04/07/2005, 10:47 PM	#15
Dr4g0nf1y Premium Member Join Date: Dec 2004 Location: Shavertown, PA Posts: 621	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.

04/09/2005, 01:50 AM	#16
eastshores Registered Member Join Date: Sep 2003 Location: Cocoa Beach, Florida Posts: 530	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!

04/09/2005, 01:56 AM	#17
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

04/09/2005, 08:54 AM	#18
Dr4g0nf1y Premium Member Join Date: Dec 2004 Location: Shavertown, PA Posts: 621	Melev has this photo on his site. Is this the culprit?

10/18/2005, 12:29 AM	#20
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	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.

10/18/2005, 12:48 AM	#21
Dr4g0nf1y Premium Member Join Date: Dec 2004 Location: Shavertown, PA Posts: 621	To give credit where credit is due, the pic is not of me holding that thing. That pic was taken from melev's site.

05/03/2006, 07:31 AM	#23
Dr4g0nf1y Premium Member Join Date: Dec 2004 Location: Shavertown, PA Posts: 621	Nature provides the clues you need. Look at the claws, they don't really appear to be the type that can be used for pinching algae off rocks. They have those serrated edges which probably are designed for holding prey.

05/03/2006, 10:39 AM	#24
Markarian421 Premium Member Join Date: Dec 2003 Location: EastSFBay, CA Posts: 329	I'll try to get a better pic, but the claws are dual purpose. In the second one you can see it a little, the ends of both claws _are_ really spoon like, and this crab eats any algae it can find. But it also has the serrated edges on the inside of the right claw. It doesn't on the left claw, it's smooth with just a rounded bump or two. So I guess it has to catch fish with its right claw. When our chromis started to vanish, it became a suspect mostly because it started hanging out at the base of the coral they slept in, and from watching it eat algae, we know it's really good at picking things out of tight spaces. It's also gotten much much faster as it's grown. But we've never seen it eat anything but algae, and this guy really helped clean up an outbreak of hair algae we had -- it can pick algae out of places snails and hermits can't reach. So it gets the benefit of the doubt, but we didn't replace any chromis until we caught it! It's in a holding pen now, and we got new chromis. If they are all still alive by the weekend, we'll try to find a new home for this guy (a coral only tank or tank with large fish -- it has coexisted with our larger fish for a year with no problems). If our sump had enough room we'd put it there, but I can't figure out a way to make that work.

05/04/2006, 10:06 PM	#25
spankey Registered Member Join Date: Dec 2004 Location: Eastern Pa Posts: 2,703	I have that same crab in my tank and its eating my digi's.. Riping the skeleton right off the colony right infront of my eyes.. How can we catch this pest and remove it? Anyone? HELP!!!