Black clawed mithrax crab?

[JoJosReef]

Checking for a local. At first was recommending sump or dump due to the black claws, but they sent a zoomed in photo and the claws look a bit too spoon-shaped for me to say "gorilla crab" with confidence.

@ISpeakForTheSeas any chance this is a mithrax crab or similar? I.e., reef safe-ish or bad guy? Thanks!

 

[littlefoxx]

He doesnt look very friendly… lol following cause Im curious what this dude is

 

[AydenLincoln]

No he’s a bad guy definitely a gorilla crab.

 

[ISpeakForTheSeas]

Checking for a local. At first was recommending sump or dump due to the black claws, but they sent a zoomed in photo and the claws look a bit too spoon-shaped for me to say "gorilla crab" with confidence.

@ISpeakForTheSeas any chance this is a mithrax crab or similar? I.e., reef safe-ish or bad guy? Thanks!

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.

The link below is about a closely related species, but gives a bit more balanced of an idea of the semi-beneficial semi-parasitic relationship of these crabs with their host corals:

https://aquadocs.org/bitstream/handle/1834/9351/Cymo%20andreossyi.pdf?sequence=1&isAllowed=y