Are emerald crabs known to be aggressive?

msavoy · Nov 20, 2020

The other day I bought a goby paired with a pistol shrimp. At the same time I also bought a tiny emerald crab. Anyway everyone seemed to be doing fine in the tank that was until this morning I noticed one of the pistol shrimp’s claws in the sand. I read online that they tend to molt their shells when stressed so I just figured maybe that’s what happened. However upon further investigation I found the pistol shrimp dead in his burrow with both claws removed. I was a bit puzzled as to who in the tank could have done this because the only other things in there that possibly could be capable would be my cleaner shrimp but I ruled him out right away because he doesn’t really have any claws. I thought maybe the emerald crab but he’s so tiny. That was until I saw my cleaner shrimp swim past the emerald crab and the crab reached out and tried to grab him in his little claws. So now I’m almost certain the emerald crab killed the pistol shrimp.

so I’m wondering is this normal behaviour for an emerald crab? The guy at the fish store didn’t say anything to me about this maybe happening. Maybe I’m not feeding him enough so he’s turning to other animals in the tank. Or maybe I just got a psychotic crab and I should get rid of it before it harms other animals in my tank. Let me know what you guys think. Thanks.

JGT · Nov 20, 2020

msavoy said:
The other day I bought a goby paired with a pistol shrimp. At the same time I also bought a tiny emerald crab. Anyway everyone seemed to be doing fine in the tank that was until this morning I noticed one of the pistol shrimp’s claws in the sand. I read online that they tend to molt their shells when stressed so I just figured maybe that’s what happened. However upon further investigation I found the pistol shrimp dead in his burrow with both claws removed. I was a bit puzzled as to who in the tank could have done this because the only other things in there that possibly could be capable would be my cleaner shrimp but I ruled him out right away because he doesn’t really have any claws. I thought maybe the emerald crab but he’s so tiny. That was until I saw my cleaner shrimp swim past the emerald crab and the crab reached out and tried to grab him in his little claws. So now I’m almost certain the emerald crab killed the pistol shrimp.

so I’m wondering is this normal behaviour for an emerald crab? The guy at the fish store didn’t say anything to me about this maybe happening. Maybe I’m not feeding him enough so he’s turning to other animals in the tank. Or maybe I just got a psychotic crab and I should get rid of it before it harms other animals in my tank. Let me know what you guys think. Thanks.

Sure it’s an emerald crab? Could be gorilla crab. Pic to confirm would help. They are typically herbivores and do a great job on algae.

ReeferWarrant · Nov 20, 2020

Crabs in general are opportunistic scavengers, emeralds tend to go after algae but given the opportunity they will eat meaty items. I'd be willing to bet the pistol died on its own means and the emerald saw an easy meal. A healthy pistol would be able to defend itself.

Textech · Nov 20, 2020

Crabs always get a bad rap. Your pistol shrimp prob died and then the crab was doing his part cleaning up the mess. Emerald and most crabs will wave their claws at passing fish, its more defensive than anything. Thats not to say there arent crabs that are jerks, but I think emeralds are more known for nipping at coral polyps than nipping at fish/shrimp.

msavoy · Nov 20, 2020

JGT said:
Sure it’s an emerald crab? Could be gorilla crab. Pic to confirm would help. They are typically herbivores and do a great job on algae.

Yes that’s a good question. I’m not exactly sure. I asked the guy for an emerald crab and like you told me, he said they’re good at eating algae. It’s not emerald color though it’s dark, almost black. He’s hiding at the moment so I’m not able to get a pic of him. So do emerald crabs not eat fish food at all? Because this guy definitely reaches for pieces of shrimp that float by when I’m feeding my fish.

msavoy · Nov 20, 2020

ReeferWarrant said:
Crabs in general are opportunistic scavengers, emeralds tend to go after algae but given the opportunity they will eat meaty items. I'd be willing to bet the pistol died on its own means and the emerald saw an easy meal. A healthy pistol would be able to defend itself.

Oh really you think? Any ideas as to why he might have died? Are they easily stressed when moved to a new tank? He seemed to be doing great for the first 5 days. Digging little burrows all over the place.

ReeferWarrant · Nov 20, 2020

@msavoy, there are a ton of variables that could factor into that. What your parameters are? how old is the tank? I feel like invertebrates as a whole tend to be the most fickle creatures in the tank, next to some corals.

JGT · Nov 20, 2020

msavoy said:
Yes that’s a good question. I’m not exactly sure. I asked the guy for an emerald crab and like you told me, he said they’re good at eating algae. It’s not emerald color though it’s dark, almost black. He’s hiding at the moment so I’m not able to get a pic of him. So do emerald crabs not eat fish food at all? Because this guy definitely reaches for pieces of shrimp that float by when I’m feeding my fish.

They will eat meat but they prefer algae. They are def a green color though so black sounds questionable.

msavoy · Nov 20, 2020

ReeferWarrant said:
@msavoy, there are a ton of variables that could factor into that. What your parameters are? how old is the tank? I feel like invertebrates as a whole tend to be the most fickle creatures in the tank, next to some corals.

Ok that’s good to know. I’ve had my cleaner shrimp for a while now he’s doing fine but maybe the pistol shrimp was more sensitive to my water parameters. Will the goby be happy without his shrimp or would you suggest I get another?

ReeferWarrant · Nov 20, 2020

I'd do some research on what they are sensitive to, best way to acclimate them, and introducing one to a new goby. I feel like emeralds and cleaners are pretty hardy, I've never had a pistol shrimp so not sure. I feel like if you have the goby you might as well get it one, but I would be patient in that introduction. I'd hate to see this repeat and be back at square one.

msavoy · Nov 20, 2020

ReeferWarrant said:
I'd do some research on what they are sensitive to, best way to acclimate them, and introducing one to a new goby. I feel like emeralds and cleaners are pretty hardy, I've never had a pistol shrimp so not sure. I feel like if you have the goby you might as well get it one, but I would be patient in that introduction. I'd hate to see this repeat and be back at square one.

Yeah that’s good advice, I’m gonna look more into why could possible be going on here before I get another one

ingchr1 · Nov 20, 2020

Textech said:
...Emerald and most crabs will wave their claws at passing fish...

I've gotten waved at by one of my emeralds!

Casket_Case · Nov 20, 2020

One tried to squish one of my zoanthids with his claw so I snatched him out

msavoy · Nov 21, 2020

Frigus Reef said:
One tried to squish one of my zoanthids with his claw so I snatched him out

What did you do with him? I’m wondering if my fish store will take mine back?

Sharkbait19 · Nov 21, 2020

Mine is a pain in the butt. From ripping clove polyps apart to bothering snails and crabs, he’s more a pest than a pet. Plus, I’m 90% he killed my firefish.
If you keep it well fed with a steady supply of algae, it’ll be less likely to eat everything.

KrisReef · Nov 21, 2020

The dark one may be possessed by the devil?

Individual behavior can vary. Watch and wait.

Do you have a sump?

BCSreef · Nov 21, 2020

I bought a few small emeralds a few years ago. One seemed a little different in color (gray/green), but I didn't worry. Fast forward a year and 3 nice crocea clams thriving in the sandbed. One morning, one shell was completely emptied. Two days later, all 3 shells were empty. I also noticed the grayish emerald really got excited during feeding and the green ones did not. Also noticed the claws were sharp "pincers" and not flat "pliers". It was not an emerald. It was a mis-ID by the seller. Caught it with a piece of shrimp on a string. Never any issues with any other emeralds since then.

Moral of story: Check their claws carefully?

msavoy · Nov 21, 2020

Sharkbait19 said:
Mine is a pain in the butt. From ripping clove polyps apart to bothering snails and crabs, he’s more a pest than a pet. Plus, I’m 90% he killed my firefish.
If you keep it well fed with a steady supply of algae, it’ll be less likely to eat everything.

Ok maybe that’s the issue I’m having then. I have a new tank so not a lot of algae yet. A lot of diatoms but I’m not sure if they like those. I got a tiny one thinking he’d find somethings to eat but maybe he’s not getting enough.