Acro crab or bad guy?

[buruskeee]

I’ve seen the blue guys with the visor. How about this guy? It was in a wild acro.

 

[sgdnycct]

I’ve heard that if they have sharp pointy claws they’re a risk. Probably better to have that confirmed though.

 

[ISpeakForTheSeas]

I’ve seen the blue guys with the visor. How about this guy? It was in a wild acro.

Cymo melanodactylus:

The crab may be a Cymo sp. such as Cymo melanodactylus - they're obligate commensals with Acropora corals, and their relationship with them seems a bit mixed; they reportedly do very, very little damage to the coral, and they seem to help keep the corals healthy if the coral is diseased.

Looks like a Cymo sp., possibly C. andreossyi or C. melanodactylus (this one if it has black claws):

That said, there are also reports like the one linked below where the crabs may be responsible for killing coral colonies.

Are infestations of Cymo melanodactylus killing Acropora cytherea in the Chagos archipelago? - Coral Reefs

link.springer.com

However, the corals dying may have been caused by a few separate things:
-Overpopulation of crabs
-Disease
-Bleaching
-Etc.

If the corals were dying for reasons unrelated to the crabs, however, then the crabs may have been trying to protect the healthy colonies by consuming the sick/dying ones.

So, the crab is essentially typically a beneficial parasite; if the coral is small or struggling in your tank, then the crab could very well kill it, but if it's large and doing well, it should be fine to keep.

 

[painter1982]

Not sure the type but I was told if the tips of the claws are dipped in chocolate then get rid of it.

 

[jabberwock]

Danger, Danger, Danger! Gorilla crab. Eliminate with extreme prejudice.

 

[Troylee]

They like to eat polyps and do damage to Acropora in my experience.. the smooth skinned with a bandana look are the good guys.

 

[buruskeee]

I figured gorilla. Ive exiled it to the rubble sump. If it survives long and resurfaces when I look there in the future, I’ll have to hat tip it. Gotta give them a fighting chance.

 

[jabberwock]

I figured gorilla. Ive exiled it to the rubble sump. If it survives long and resurfaces when I look there in the future, I’ll have to hat tip it. Gotta give them a fighting chance.

They are tough as nails.