Good guy or bad guy? (Crab in Acro)

So I noticed this blob on one of my acro's about a week ago and never really paid attention to it, but then today I saw it move! Upon closer inspection it is a crab. Ive looked up acro crabs and this one doesn't look anything like the ones I've seen, but LiveAquaria has four different variations to it. This one is very small, and is dark brown/gold colored.

Here are the pictures, they are pretty bad because my camera is not that great with close up underwater.

 

I'd coralRX it, just to be sure.

 

There are a lot of different types of acro crabs, personally I consider them good guys and a bounce if I get one on a acropora.

 

I would not kill it, it looks very similar to one of mine. It will hang out in the corals almost exclusively.

 

it has not moved from the acro at all and the acro is not showing any stress. I dont think I am going to take it out

 

Studies, such as this have shown trapeziid crabs are very important for acropora health. Not all acro crabs are trapeziid crabs though. If the crab seems to be eating the coral, then that is bad. You will easily notice the tissue loss though. If it is just picking at the coral with no tissue loss, it is probably just cleaning it and is a good guy.


Symbiotic crabs maintain coral health by clearing sediments
Hannah L. Stewart, Sally J. Holbrook, Russell J. Schmitt and Andrew J. Brooks
Coral Reefs
Volume 25, Number 4, 609-615, DOI: 10.1007/s00338-006-0132-7

Abstract
Stony corals are the foundation of coral reef ecosystems and form associations with other reef species. Many of these associations may be ecologically important and play a role in maintaining the health and diversity of reef systems, rendering it critical to understand the influence of symbiotic organisms in mediating responses to perturbation. This study demonstrates the importance of an association with trapeziid crabs in reducing adverse effects of sediments deposited on corals. In a field experiment, mortality rates of two species of branching corals were significantly lowered by the presence of crabs. All outplanted corals with crabs survived whereas 45–80% of corals without crabs died within a month. For surviving corals that lacked crabs, growth was slower and tissue bleaching and sediment load were higher. Laboratory experiments revealed that corals with crabs shed substantially more of the sediments deposited on coral surfaces, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues. The mechanism underlying this symbiotic relationship has not been recognized previously, and its role in maintaining coral health is likely to become even more critical as reefs worldwide experience increasing sedimentation.

 

Looks like a red mithrax (red colored emerald crab) I can't tell but the claws may look black tipped instead. I can't tell from those pics

 

that looks like a cymo andreossyi to me. this crab protects the coral but actually eats the polyps as well. some people consider them good and some people consider them bad. if it is a healthy coral, it should be able to handle the damage done by the crab. However, if the coral is struggling, it is probably not a friend.

here is a link

http://www.chucksaddiction.com/hitchcrabs.html

 

if you dip your corals and have enough flow in your tank there is no need for the crab..