Acro crab identification?

[slythy]

Hey, this guy showed up on one of my acros. It looks like hes atleast cleaned off part of that leg. Black tips on the claws

 

[bluemon]

Looks a bit hairy.

Definitely not a Trapezia sp. “Acro Crab”

Just a random Gorilla crab, I would toss in the sump.

When you say it showed up with the acro, was it a wild acro colony that came with the crab? Or just a random crab from a live rock

 

[Matt L]

Gorilla crab. It’s already eating that coral. I’d get it out.

 

[slythy]

Looks a bit hairy.

Definitely not a Trapezia sp. “Acro Crab”

Just a random Gorilla crab, I would toss in the sump.

When you say it showed up with the acro, was it a wild acro colony that came with the crab? Or just a random crab from a live rock

Its a wild colony. When I got it a few month ago i did not notice it at all. Its in my frag tank so not a huge deal. I did dip the corals when I got them but im not super meticulous. I feel like most the time you dip you just kill the good things.

Gorilla crab. It’s already eating that coral. I’d get it out.

They are in the sump!

 

[ISpeakForTheSeas]

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.

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.

 

[slythy]

Cymo melanodactylus:

That was my first guess but its a relatively small colony and the leg hes sitting on died so hes in the sump

 

[pooootiqe]

Awfully cute for an acro killer…

 

[bluemon]

Cymo melanodactylus:

very interesting! Thanks for the read. Another SPS commensal crab I did not know about.

I feel like with these crabs, if they are in bigger colonies, they might be beneficial but with smaller colonies, I would practice caution. Anyone have first hand experience with these?

 

[bluemon]

That was my first guess but its a relatively small colony and the leg hes sitting on died so hes in the sump

Probably a smart move. The article states they are only found on injured corals.

Even if they aren’t actively feeding off acro colonies, they are definitely feeding off struggling or dying acros that could possibly bounce back in our tanks

 

[ISpeakForTheSeas]

That was my first guess but its a relatively small colony and the leg hes sitting on died so hes in the sump

Yeah, if the colony isn't able to support it while staying healthy, sumping it is a good choice.