Acro crabs

[JT3069]

does any one know how large these crabs grow to be (the good one )

 

[Toddrtrex]

All of mine have stayed fairly small - no bigger then a dime.

This one thinks it is a tough guy.

 

[JT3069]

So i could have one in a piece of coral 4 inches as long as it is branchY

 

[Jim Z.]

I do have an example in my tank where red bugs have left an Acropora after 2 Tetralia began hosting within the branches. These crabs move around the Acropora at night, but it is not known whether they consumed the red bugs or altered the coral's attractiveness to red bugs. This particular Acropora has been free of red bugs for almost a year. I have tried to get additional Tetralia crabs for other Acropora in the tank, but they are very picky on where they set up shop. I do not treat for red bugs presently--new SPS are dipped in Revive and I let nature take its course. Red bugs are present in my system, but do not cause the mortality observed in other reef tanks. I would like to see some controlled studies to see whether my observations are merely a fluke or are an actual benefit of commensal crabs. Initial observations are encouraging but getting one or 2 crabs to live in each and every acro in my captive reef system is like herding cats.

Dzhou, yes Trapezia will occasionally be found in Acropora branches as you have discovered, but it is not a very common event. Host selection by these crabs can be due to many features of the host or surrounding environment including host odor, branch arrangement, host abundance, and or plankton availability for example. Again an enterprising grad student may unlock this particular mystery through controlled experimentation.

McCaulleys online article at http://reeftools.com/news/guardians-of-the-acropora/# provides an overview of guardian crabs.......................Jim Z.

 

[dzhuo]

I would like to see some controlled studies to see whether my observations are merely a fluke or are an actual benefit of commensal crabs.

That's the problem. There is no such study done. AEFW, for example, has just been properly classified earlier this year and we still don't know much about red bugs. We do know a little about the crab's ability to defend certain types of acro predator such as crown-of-thorns starfish (COT) and vermetid snail. One study has revealed COT is very selective to pick on corals without the crab:

The crown-of-thorns starfish Acanthaster planci (L.) is well adapted to feed on a wide range of different corals, but often exhibits striking preference for a small suite of available prey species. Numerous theories have been forwarded to explain its feeding preferences, but many of these theories have not been tested. In this study, I test whether coral symbionts significantly affect the feeding preferences of crown-of-thorns starfish by removing symbionts from replicate colonies of 6 different coral species. Crown-of-thorns starfish had a clearly defined hierarchy of preference for the 6 different corals when these contained symbionts (Acropora gemmifera > A. nasuta = A. loirpes > Seriatopora hystrix > Pocillopora damicornis > Stylophora pistillata). However, when coral symbionts were removed, then the starfish readily consumed all 6 corals and did not exhibit any significant selectivity. Further manipulation of symbiont assemblages showed that the trapeziid crabs (Tetralia and Trapezia species) were the most effective of the various coral symbionts in deterring starfish from feeding on their host colony. Moreover, those corals that were least preferred by crown-of-thorns starfish contained the largest and most powerful species of Trapezia (T. cymodoce), whereas the most preferred corals contained only very small Tetralia spp. crabs. Further experimentation is required to assess the generality of these results, but for the 6 coral species tested, it is clear that coral symbionts land particularly trapeziid crabs) do have a marked influence on the feeding preferences of the crown-of-thorns starfish.

There are numerous studies on the effect on how the crabs defends the corals against vermetid snails but only on certain types.

The crab also remove sediment from the host coral and this new finding is done by US Santa Barbara in 2006:

The scientists showed the importance of trapeziid crabs by gently removing crabs from sections of the two species of branching corals on a coastal reef. This resulted in 50 to 80 percent of those corals dying in less than a month. By contrast, all corals with crabs survived. The nature of this common symbiotic relationship had not been recognized until this study. For surviving corals that lacked crabs, growth was slower, tissue bleaching was greater, and sediment load was higher.

However, there is also study that suggest the initial settlement of these crabs actually stress and slow down coral grow but the benefit is clear and long term.

Other than that, I would love to see more published study on them because I have never seen one on red bugs and AEFW so we shouldn't be carry away and start making rumors that these crabs would control these 2 pests.

Again an enterprising grad student may unlock this particular mystery through controlled experimentation.

Hopefully.

McCaulleys online article at http://reeftools.com/news/guardians-of-the-acropora/# provides an overview of guardian crabs.......................Jim Z.

Just some basic generalization.

 

[HiggyBrew]

Cool, I found one on a frag, here is a quick video:

http://www.ustream.tv/recorded/19645042

I have a live cam on him now, you'll see him picking around eventually:
http://www.ustream.tv/channel/higgyreef

 

[JT3069]

awesome

 

[Reef_Noob]

Those Teddy bear crabs sucks. They eat SPS. The other smooth hairless ones just hang around the SPS branches and does no harm.
From Personal experience....