Emerald crab eating SPS polyps?

[ThRoewer]

A few weeks ago I added a pair - or rather a male and a female - of emerald crabs to my tank in an effort to control bubble algae.
I found the male every night sitting in my big birdsnest coral but so far I haven't seen any damage to it.
Today I saw him picking on my Pocillopora.
When I inspected my other corals I found that two of my smaller Acropora frags are missing chunks of tissue.
I checked it under the microscope and it is not just affecting the tissue but also the skeleton. It looks like something was scraped off.
The best suspect for this is the emerald crab male.

I haven't seen the female doing anything to corals. She is only scraping algae.

Is it possible that the males are not reef safe?

 

[Tweaked]

I have about 4 dozen sps colonies and about 12 emeralds total give or take. I have great PE and have only ever seen them on my coral if there is a spot of algae to be had. I love emeralds and they are great at picking the hard to reach places.

 

[ThRoewer]

Are yours all female?
My female seems to do exactly what you describe, but the male is doing something. that seems not OK. There are no algae spots in that particular birdsnest and he is in it every night.

 

[JPMagyar]

This has been discussed numerous times before and some folks believe emeralds will eat SPS and some don't. I use emeralds extensively and I photograph and video extensively and I have never seen an emerald eat an SPS. I have seen emeralds sit in a birdsnest and pick and the same in a pocillipora but when I got out my mesoscope you could clearly see algae growing in the branches on dead tissue. If you have dead tissue it will grow algae and if you have algae on dead areas of coral the crab may pull off chunks of skeleton with the algae. I do not believe emeralds eat corals EVER, and I keep literally dozens of emeralds ALL the time in ALL my tanks and I have NEVER lost a coral to emeralds eating it.

I do not think your problem is your emerald.

Now having said that I will add the caveat that the aquarium trade continues to prove that there are countless species that look similar but are not the same. Without question there are some crabs that do eat SPS and it would not be hard for me to believe that there is a species of crab similar to emeralds that DOES eat SPS. All the emeralds I have ever had had flat rounded ends on their claws and the one video I saw of a crab that looked like an emerald that was eating SPS showed a crab with very sharp pincers. Could this be a different SPS eating emerald? Maybe.

 

[sahin]

This has been discussed numerous times before and some folks believe emeralds will eat SPS and some don't. I use emeralds extensively and I photograph and video extensively and I have never seen an emerald eat an SPS. I have seen emeralds sit in a birdsnest and pick and the same in a pocillipora but when I got out my mesoscope you could clearly see algae growing in the branches on dead tissue. If you have dead tissue it will grow algae and if you have algae on dead areas of coral the crab may pull off chunks of skeleton with the algae. I do not believe emeralds eat corals EVER, and I keep literally dozens of emeralds ALL the time in ALL my tanks and I have NEVER lost a coral to emeralds eating it.

I do not think your problem is your emerald.

+1

I've said it in many other threads. My buddy currently has 5 emeralds in an sps tank. I've always had them in my sps tanks due to bubble algae. Current tank don't have them because I started with dead rocks and QT everything and cut bases off.

 

[ThRoewer]

This birdsnest has no dead tissue in its crown where the male crab always sits.
Another birdsnest frag that has a dead tip with algae growing on it is completely ignored and the algae untouched.

The scrape marks on the damaged corals match the size of the males pinchers and there are really no other viable suspects.

Also, both, male and female haven't touched the bubble algae yet.

BTW, do you know if you have any males among you emerald crabs?

 

[Mael]

mine have never picked at sps, the bigger one did pick at BTA tentacles once but hasnt done it since. He also takes a swing at a fish occasionally but they are all too fast for him.

 

[Art13]

I've had emeralds that i've personally watched picking off skin on an acro and digi. I noticed missing tissue and that night i watch him do it. I don't know what set it off, possibly that the tank was sitting fallow and was fed a lot less. Having said that with my new set up and have two new emeralds in it along with sps and other corals and not one issue with them. It might have to do with available food, but all i can offer is what i saw.

 

[Tweaked]

Ahaha, the guy at the store was trying to tell me to sex them etc, I was like give me 8 as is. I already had 5 and love them.

 

[ThRoewer]

Lack of food would certainly explain it but mine has plenty of algae to munch on and is still doing it.

The statement that they never ever feed on corals is in my opinion quite foolish as there are simply no absolutes. Maybe it is rare or uncommon but it is still possible that individual crabs develop a taste for coral tissue. After all it's containing algae as well.

I will catch the male tonight and rather get another female.

 

[justthewife]

Lack of food would certainly explain it but mine has plenty of algae to munch on and is still doing it.

The statement that they never ever feed on corals is in my opinion quite foolish as there are simply no absolutes. Maybe it is rare or uncommon but it is still possible that individual crabs develop a taste for coral tissue. After all it's containing algae as well.

I will catch the male tonight and rather get another female.

Your idea of catching the crab is a good one. Although you may have a harder time of it than you expect, lol. I had to make a trap to catch mine. If you have a sump, you can put the crab in it for a while until you can see if the corals improve or it is another pest or problem. I have to tell you I had an emerald crab go rambo on my soft corals before and ripped the heck out of my ricodea. He was not eating them, he was just ripping them apart for no reason I could see. They are opportunistic creatures. Also all animals are different. They all have different personalities. Just like people. I have a Banded coral shrimp and he is a perfect citizen but I have seen others be sooooo mean.

 

[ThRoewer]

This is how you sex them:

The one at the top of the picture is the male, with a narrow abdomen and large pinchers.

The female on the bottom has a wide abdomen and significantly smaller pinchers.

I'm not sure if the color difference is just by chance or another distinguishing feature. I had 2 pairs and both males were light brown while both females were dark green.

 

[ThRoewer]

Since he is every night in the birdsnest coral he is easy to catch. I just need to take the whole coral out.

 

[eddiereefs]

The males seem to be the incorrigable ones. Never had a problem with my little female

 

[jimrawr]

I'm on my phone so it's difficult right now, but I started a thread some years ago about this. Caught an emerald crab red handed taking chunks out of my dps not only the polyps.. had pics and all

 

[dazza0685]

I have emerald crabs. Inc 2 large males. In my old 4x3 i often found 1 of them in my Stylo, he would scrape the polyps off the top. He killed one of the small branches due to this.. but has since left it alone and all other spots have recovered.
They can be a bit of a pain but i feel are an important part of my system so are going nowhere for now.

 

[swk]

I prefer the females as well. Stay smaller, and yes, I have seen the males climb and irritate the crap out of my sps. That was when my tank was lower in nutrients and perhaps there wasn't enough food for them

 

[Reefvet]

It's hit or miss with Emerald crabs which makes me believe there are a number of different crabs being sold as Emeralds. I swore 20 years ago I'd never buy another one.

Strawberry crabs on the other hand have done a great job of eating algae and never touched a single coral in the year that I've had a pair, a female and a male.

 

[CHSUB]

any crab well eat anything, it's just a matter of how much damage and risk one will accept?

 

[Arkayology]

I used to believe that emerald crabs wouldn't eat SPS flesh/polyps at all until recently. I added a couple very large emerald crabs to my tank and I noticed after the algae was gone that the bases on some of my SPS were showing odd bite marks. Well I watched one night and saw one of the emeralds munching on that very same coral.

+1 hit or miss, first one that has ever touched SPS in 5 years.