Help IDing crabs

[Tecnomage]

I was hoping someone here would help my wife and I identify the crabs we bought from Denny's Pet World. They were sold as emerald crabs but for some reason (they're not green) I've been doubting it. They seem to mow down algae but tonight we saw them try to go after our Yellow Eyed Kole Tang. So if someone can help and let us know if we should try fishing them out we'd appreciate it.

Here's the best pic we could get of the underside:

Thanks in advance!
-Jim

 

[returnofsid]

Red Mithrax Crab. Same thing as an Emerald Crab, but red.

 

[Slate]

That is an emerald crab. The one I have in my tank was also red(pink) when I purchased him. He is just now starting to turn green after having him for a month. But it looks identical to your crab in shape.

 

[Tecnomage]

Thanks! That makes me feel a lot better, everywhere else I went had actual green emerald crabs so I was doubting my sanity and wondering if they were something else and harmful.

 

[Tecnomage]

Ok bonus picture, I don't need help IDing this one the anemones are a dead giveaway. But it's too cute not to share.

 

[coralcrab]

So there are several species of Mithraculus, the genus that the "Emerald Crab" is assigned to. Telling the difference between all of them is a matter of counting spines and bumps. Yours is most likely Mithraculus forceps (the emerald is Mithraculus sculptus). What matters for your purposes is to look at the shape of the chelae (claws). See how the tips are shaped like little spoons, or horse's teeth? That indicates that this crab, at least primarily, feeds by scraping algae. I wouldn't worry too much, they are probably doing right by you, but they still might go for a weak, or dying fish.

-coralcrab

 

[Tecnomage]

Thanks coralcrab, I have noticed them going after algae like mad so I wasn't too concerned when we first got them, it was just the weird behavior when the tang came near.

 

[LeslieH]

Not all emeralds are model citizens. It's not uncommon to find posts complaining about rogue emeralds. Keep it well fed by adding nori if the level of algae in your tank gets low. Also, the spoon-tipped claws aren't really a good clue to feeding behavior. Many of the little xanthids that show up in tanks are primarily algae eaters and they have very sharp, massive claws.

 

[Jesse_emel]

Ok bonus picture, I don't need help IDing this one the anemones are a dead giveaway. But it's too cute not to share.

the boxing crab or pom pom crab. I love these guys and have one myself. When a fish or predator get to close they will try to sting it with that little piece of anemone they hold. but mostly they just wave them side to side.

 

[WABlonde]

Our emerald never went after any of our fish but he'd raise his claws up if fish came to close to him.

but I have an emerald question of my own... Our emerald molted and became fairly large. He had started to turn white-ish (as he did when he molted previously) and then disappeared. We figured he was molting again.... but after a week, still no crab. Finally found his remains last week. My question - what is the life capacity of these guys?

Was trying to take pics of the corals from the Frag Swap and he made appearance front and center. He'd beg for food right along with the fish.

 

[LeslieH]

It's thought that they live a year or two in the wild. It would be interesting to hear from reefers who have kept them for longer in their tanks.

One problem with buying wild-caught animals is that it's hard to know how old they are when you get them. A critter could be 2 months old & last for 2 years or it could be 1 year & 10 months old & survive for just a few months.

 

[coralcrab]

Also, the spoon-tipped claws aren't really a good clue to feeding behavior. Many of the little xanthids that show up in tanks are primarily algae eaters and they have very sharp, massive claws.

While I wouldn't disagree that spoons aren't the only shapes used for algae, I think spoon shaped chelae almost always do indicate a diet that is mostly algae.

-coralcrab